Questions regarding George's 275/285 Custom Street Cam Profile

This sounds exactly what I want for my engine.
Thanks for helping make my cam choice so easy.

Jake

Doug,

You have experience with the IDA's from what I can see on all the posts you have made on them.
Care to recommend to me a good starting point on what the air/fuel jet values should be on the IDA's. Especially with this cam.

Engine spec: 351c 4v iron head 10.0 to 1 comp, 180 exhaust

I purchased my weber setup used, haven't looked at what is in it yet for jetting.

Jake

Yours is pretty simple.

37mm chokes.

Idle: 120 holder, 66-67 fuel. For that you need to use a 65 fuel and drill it out with a pin vise to get that size. 65 is too lean and 70 is too rich.

Main: 140 fuel. 160 air. F7 emulsion tube

Pump jet bypass valves: .55

You can run the mains down to 125 and up to 170 on the fuel. Try the 140's. They SHOULD be all you need even on a track day.

What is important is that you need to keep the proportion of the fuel jet vs. air jet the same as in the 140/160. 170f for instance would use a 195 air.

If you have an exhaust gas analyzer, throw it away. All it will do is confuse the Hell out of you. Webers are old school. They will not tolerate scientific analysis and will defy logic.

They also need a lot of initial timing. 16 degrees for a start, and like as fast of an advance curve as you can get octane for. Kind of difficult to get the car to idle at 650 with that much advance.

36 degrees total but they like it all in by 2,000 to 2,500 engine rpm. Most cars will ping really badly on that with pump gas these days.

Final suggestion. Iregardless of what you wind up with jetting on the carbs, they will have the tendency of fouling the plugs. Mostly from pumping the accellerator to start the car cold.

They can be difficult to start cold. Cold for Webers is under 70 F.

If it wasn't for starting a cold engine, you can actually run .9mm pump jet relieve valves but the car becomes very difficult to start cold. .55 is the best compromise.

Check what is in your carbs when you get them. Some have ZERO's, i.e., plugged up, in them new.
I recommend using the P-E Ignition with them. It is the only ignition that I know of that will fire a fouled plug. MSD will not.
It also retards the timing when starting the car so it is easier to turn over. A lot 10 or 12 degrees. Really helps on a hot engine too.

If you are running iron Ford heads and using an AF32 plug, go one hotter to AF42. It helps.

Doug and George,

Thanks for your wealth of knowledge/experience and your generous sharing of what you have learned with others.

Thanks,
Jake

I called Bullet Cams and talked to Mark. Talked about the engine use I envision, what changes I am making to the engine, what intake and exhaust I would use. Sent him the profile sheet and the only thing he recommended was to change to a 112 deg lobe separation angle. Said we were giving up some low end power with the 114 deg lobe separation angle.

Jake

I personally use a 110 with the Webers. My impression is that in using a 112 to 114 because of the Webers, you are giving up 80 to 100 hp.

I realize that wasn't necessarily your intent but it should be said.

A "Weber" cam is cut for only one reason. Because of the fuel reversion.

The fact is, regardless of what anyone tells you, you can't ELIMINATE that. You can REDUCE it.

To me it's no BFD. I'd rather not give up the 100 hp.

The other thing with Webers is you will NEVER find the perfect camshaft. You will always be feeling that it isn't just right.

If you want a "perfect" system, go to IR fuel injection. Classic makes one that looks just like IDA's.

Don't be surprised though if you are forever writing new programs for it, 'cause (psst come 'ere) it will never be perfect either.


If I were you I'd ask Mark to "define" what "a little" torque loss down low is?

I suspect maybe 15 to 20? If so, leave the cam selection alone and go with your first choice.

The 8 stack 48 ida's are picking up a little more than that all across the rpm range anyway, plus the solution to high performance camshafts which all tend to move the torque range up in the rpms, is to regear the car and not spend time in the range where it is lacking torque.

In cars this is relatively simple. The problem really is for trucks which need to pull hard right off of idle.

quote:

Originally posted by Jake:

... Said we were giving up some low end power with the 114 deg lobe separation angle ...

I strongly disagree with him because your motor is a Big Valve motor. I'll guarantee you he's wrong. With the big intake valve, and canted valve geometry the effects of overlap are amplified. You want to minimize overlap to optimize low rpm power, improve drivability, improve vacuum at idle. The lobes were chosen based on 114 degree LSA, and the way they interact at top dead center, in the overlap period. The lobes are pushing the limits for the boundaries I established. As far as I'm concerned, 112 degree LSA screws that up a bit. It will still work, but not the way he's telling you it will work. Expect drivability and low rpm power to be worse, not better.

Wider LSA results in opening the exhaust valve earlier which also improves high rpm over-rev when mufflers are involved. This is why they say narrow LSA cams put more demand on the exhaust system, i.e. the system must be better designed and have low back pressure.

Talked with Mark from Bullet Cams again and we settled on a LSA of 113 degree. Split the difference so I can be happy with the valuable advice I have received from everyone and live with my choice. I know the cam is the heart of the motor and I hope my choice will have a long life in the Pantera.

Thanks,
Jake

quote:

Originally posted by George P:

quote:

Originally posted by Jake:

... Said we were giving up some low end power with the 114 deg lobe separation angle ...

I strongly disagree with him because your motor is a Big Valve motor. I'll guarantee you he's wrong. With the big intake valve, and canted valve geometry the effects of overlap are amplified. You want to minimize overlap to optimize low rpm power, improve drivability, improve vacuum at idle. The lobes were chosen based on 114 degree LSA, and the way they interact at top dead center, in the overlap period. The lobes are pushing the limits for the boundaries I established. As far as I'm concerned, 112 degree LSA screws that up a bit. It will still work, but not the way he's telling you it will work. Expect drivability and low rpm power to be worse, not better.

Wider LSA results in opening the exhaust valve earlier which also improves high rpm over-rev when mufflers are involved. This is why they say narrow LSA cams put more demand on the exhaust system, i.e. the system must be better designed and have low back pressure.

I'm not arguing. Camshafts are controversial at least.

If the engine was in a Mustang with a full length exhaust I think a 114 would be more important.

Even a stock Pantera has a less restrictive system than that. It could use better headers and the restriction in the stock Ansa's could be fixed.

Now if you go to a set of 180 headers and mufflers that are so unrestrictive then why not go even further with the cam?

Try the cam George recommended. See if you like how the engine runs. If not change it

quote:

Originally posted by George P:

quote:

Originally posted by Jake:

... Said we were giving up some low end power with the 114 deg lobe separation angle ...

I strongly disagree with him because your motor is a Big Valve motor. I'll guarantee you he's wrong. With the big intake valve, and canted valve geometry the effects of overlap are amplified. You want to minimize overlap to optimize low rpm power, improve drivability, improve vacuum at idle. The lobes were chosen based on 114 degree LSA, and the way they interact at top dead center, in the overlap period. The lobes are pushing the limits for the boundaries I established. As far as I'm concerned, 112 degree LSA screws that up a bit. It will still work, but not the way he's telling you it will work. Expect drivability and low rpm power to be worse, not better.

Wider LSA results in opening the exhaust valve earlier which also improves high rpm over-rev when mufflers are involved. This is why they say narrow LSA cams put more demand on the exhaust system, i.e. the system must be better designed and have low back pressure.

There is a lesson here for the unwary.

Many many mechanics, cam grinders, engine tuners, exhaust specialists, carb specialists ect have small block Chevy indelibly etched into their minds, Even if they are aware of the differences it seems hard for them to eradicate the Chev influences. So the 351C gets treated as if it was a 350 Chev.

This is a Catastrophic Error.

The 351C is a completely different beast to all other comparable engines and must be treated as such.

Make sure any experts you speak to truly understand the difference or performance will suffer.

I have come across this phenomenon often and seen the results as being a very "average performing" 351C and upon conversing with those involved discovered the build been viewed through the 350 Chev prism, this then led to the engine as being christened a "dud" because it did not perform to expectation when really the "duds" were those that did not understand the 351C.

Called Bullet Cam back up on this order and made them stick to the profile as George recommended including the 114 deg lobe separation angle.

Thanks for every ones feedback on this.

Jake

Well I've seen people buy entire engines out of another car because the guy drove the car and loved the way the engine ran.

Then install it in their car.

Next thing I know is the guy is pulling it apart because it went from "loved it" to "it sucks". I can't stand the cam.

So the safest thing to do with cam recommendations is not to get involved and let the other person discover on their own.

I doubt identical twins would like the same cam?

If a cam grinder tells me what the specs should be, who am I to argue? Unless that is the first cam he is grinding for some one?

Just my perspective on the cam thing.

Engines with small valve/in-line valve cylinder heads rely on the cam to make top end power, because the size of the intake port is so small it skews the engines power band towards low rpm. So the cam and single plane intake manifolds are used to build high rpm power. The cams role in building a performance street motor equipped with that style of cylinder head is very important. The effects of overlap are not amplified by such engines, so you can get away with a significant amount of overlap without ruining drivability. This is the type of engine 99% of enthusiasts are accustomed to.

With the big intake valve and canted valve geometry of the 351C 4V the effects of overlap are amplified. This is a good thing for a race engine, overlap can be used to scavenge exhaust gases and get the intake charge flowing early which improves volumetric efficiency. Drivability (i.e. performance below 3000 rpm) isn't important to a race engine.

While amplifying the effects of overlap is a good thing for a race engine, its a bad thing for a street engine. We have to minimize overlap to optimize low rpm power and maintain drivability. Many journalists over the years have written "its easy to over-cam a 351C 4V", without really explaining why. I've just explained why.

The large port/large valve heads dominate the power band of 351C 4V engines having power bands that extend to 6000 to 7000 rpm. In one way the cam is just along for the ride, its job is mostly to open the valves. The cams role is not as important as it is with a small valve/in-line valve engine. But there are things you can do with the cam to make things worse, that's what you want to avoid: (1) opening the exhaust valve too late causes high rpm torque to fall-off like a brick, (2) too much over-lap softens low rpm torque, and (3) closing the intake valve too late causes low rpm reversion while lowering dynamic compression. Narrow lobe centers create conditions #1 and #2. Combined they have the effect of making the torque curve (power band) narrower and steeper. Free flowing mufflers or an un-muffled exhaust system can help alleviate problem #1.

Since the big intake valve and canted valve geometry of the 351C 4V amplifies the effects of overlap, a camshaft with 114° LSA has the same effect on a 351C 4V as the effect a cam with 110° LSA has upon an engine with small valve/in-line valve heads. In the same way 112° LSA has the same effect on a 351C 4V as the effect a cam with 108° LSA has upon an engine with small valve/in-line valve heads. This is an aspect people can't wrap their head around. The indoctrination people get via magazines, internet and television makes people think they are losing out on something with the wide 114° LSA. It just isn't so with the 351C 4V, not if drivability is important.

This is something I discovered in the 1980s, and the reason I began having my own cams ground back then. I experimented with 110° LSA cams a few years back, and didn't like the results ... not for a street engine where drivability is an issue for the owner.

Combined with the 4V cylinder heads the cam I've specified provides good drivability AND bitchen performance. This I guarantee. I've learned over the years to trust the heads to be the star of the show, they've never let me down.

Like I said, camshaft recommendations are a damned if you do, damned if you don't situation.

Maybe the crux of the situation is over the term "driveability" and exactly what that means.

Probably the most driveability the Pantera ever had was bone stock?

It is very difficult to have a "tear your balls off" performance and "driveability" in the same package. I can't think of even one vehicle ever built that had both at the same time, in the same package.

The Pantera certainly did not but it had "potential". It all depends on exactly what you want out of the car.

I will say this with absolutely no remorse. If you are putting Weber IDA's on the car and want it to be a "cruisemobile" then you are positively misunderstanding what the carbs are all about, and you are absolutely wasting you time and effort.

The set up was conceived of for road racing PERIOD. IF you should happen to be compliant enough in your concept of acceptances than MAYBE you can live with them? Maybe?

This crap about what camshaft to get for them is just bulls..t. If you are worrying about that now, you definitely are already over your head with these things. Just my very humble opinion on the subject, but it is just that. My opinion.

That and $7.50 will get you over the GW Bridge most of the time. Unless Chris Cristi doesn't like you.

No, no cruisemobile wanna be here.
But I don't, want a bracket racer for the street either. With the corresponding high rpm peak HP. Nice long torque curve I hope instead.
My original question was in curiosity about the cam profile George has often recommended here and how its street manners would be. I think it fits perfectly with what I am looking for. The fact that the webers will work with it and maybe even help the torque curve is great.
I want to scare the ricer boys and give myself some "Miles of smiles listening to the sounds coming from behind "
My radio delete plate will not be coming out of the Pantera.
I think I am going to have a ball with this build. Just want to get it right for what I want to do with it.
I understand the webers will be a learning curve and I am looking forward to it. If it turns out otherwise, so be it, I have a Blue Thunder intake on the shelf along with a nice 780 cfm carb. I'll put the webers under my glass coffee table for a conversation piece.

Jake

quote:

Originally posted by Jake:
No, no cruisemobile wanna be here.
But I don't, want a bracket racer for the street either. With the corresponding high rpm peak HP. Nice long torque curve I hope instead.
My original question was in curiosity about the cam profile George has often recommended here and how its street manners would be. I think it fits perfectly with what I am looking for. The fact that the webers will work with it and maybe even help the torque curve is great.
I want to scare the ricer boys and give myself some "Miles of smiles listening to the sounds coming from behind "
My radio delete plate will not be coming out of the Pantera.
I think I am going to have a ball with this build. Just want to get it right for what I want to do with it.
I understand the webers will be a learning curve and I am looking forward to it. If it turns out otherwise, so be it, I have a Blue Thunder intake on the shelf along with a nice 780 cfm carb. I'll put the webers under my glass coffee table for a conversation piece.

Jake

What you need to do, what everyone needs to do, is drive a car with Webers on it. See how you like it.

I wish you the best of luck with success in your endeavor and hope that you are ecstatically happy with the results.

I drove a Pantera with Webers and it drove very nicely. No hestation, no backfires, smooth acceleration, sounded great.

A decade ago it was the poster child for every conversation on the POCA forum when people said they were a PIA, difficult to tune, and unstreetable. The owner tuned them w/o a doctorate in Webers. Pantera ran great w/o issue. Whether he was maximizing the engine's potential, I couldn't tell you. Located in Texas, the owner was William King and the car was Seafoam Green. Eventually, he tired of fighting the naysayers, who said it wasn't possible based on their personal experience or what they had heard,and gave up defending the Weber installation. Unfortunately, his son passed away and Will disengaged. I do not know where he or the Pantera is today.

I guess we are all guilty of proselytizing with conviction our own experiences on these Forums despite others experiences to the contrary. But while a worthy dialogue, that's probably a different thread.

quote:

Originally posted by JTpantera:
I drove a Pantera with Webers and it drove very nicely. No hestation, no backfires, smooth acceleration, sounded great.

A decade ago it was the poster child for every conversation on the POCA forum when people said they were a PIA, difficult to tune, and unstreetable. The owner tuned them w/o a doctorate in Webers. Pantera ran great w/o issue. Whether he was maximizing the engine's potential, I couldn't tell you. Located in Texas, the owner was William King and the car was Seafoam Green. Eventually, he tired of fighting the naysayers, who said it wasn't possible based on their personal experience or what they had heard,and gave up defending the Weber installation. Unfortunately, his son passed away and Will disengaged. I do not know where he or the Pantera is today.

I guess we are all guilty of proselytizing with conviction our own experiences on these Forums despite others experiences to the contrary. But while a worthy dialogue, that's probably a different thread.

They are very street able and tunable but should be considered a "special needs" setup.

The issue really is that many people don't know what to expect from them and probably were over their heads with anything but a box stock car to begin with.

You don't normally start out with even an undergraduate degree but if you dial them in yourself successfully you will certainly be entitled to advanced degree credits at least.

It is interesting to compare the cam that George speced here to the special "Weber cam" recommended by many for the set up.

It isn't identical, but it is close enough to consider it the same cam.

Would you recommend this cam profile for a 3500lb car running primarily on the street.
AOD transmission, 3.73:1 rear. I like the idea of staying as close to a Ford design as possible. Iron closed chamber heads, Blue Thunder intake, Holley 780...
Great information that just is not availible elsewhere!

Yes

i have a cam spec comp cam gvl int. .520 exh. 531 duration .006 tappet lift 282 288 .050 int. open 5 btdc close 45 abdc exh. open 56 bbdc close 0 atdc specs for cam installed @110 intake centerline duraton@,050 intake 230 exh. 236 lobe lift intake .3010 exh. .3070 lobe seperation 114 multiport fuel injected. thoughts? i love the dicussion on cams it confuses me but very interesting. one of those never ending topics. on the side george do you have pictures of your new rims 18" and 17" i know different thread.

Carl, since there is 6° difference between intake duration and exhaust duration the cam you describe reminds me of a cam designed using one of the laptop dyno programs.

The first thing I noticed is the cam has fast ramp lobes (52° lobe intensity) yet only 0.520"/0.531" lift. Seems as though whoever put this cam together placed emphasis on duration at 0.050". The 351C 4V heads are going to make peak BHP at 6000 rpm even if the duration at 0.050" is only 205°! That's the intake duration of the Cobra Jet cam. The 351C 4V power band is not as dependent on duration as it is with other engines, but to make power the valves need to be opened as high as possible (within the limitations of engine longevity) to take advantage of the cylinder head air flow potential. Lobes are available today allowing valve lift in the 0.550" range (or more) with reliability.

The valve event timing is just fine, the overlap is low (57°), it is more or less centered within the window it should be centered within, that window is the piston's "dwell" time at TDC. However due to the 52° lobe intensity, overlap at 0.050" is 5°. I'd like that better if it were closer to zero. The exhaust valves are opening and the intake valves are closing where they should be based on advertised duration. But I'd prefer it if the intake closing at 0.050 was 40° or 41° ATDC instead of 45° ATDC. Those are the various boundaries I try to stay within to insure good low rpm performance.

The cam should work fine, but if lobe intensity were 57° instead of 52° (225°/231° duration at 0.050) that would correct both the issues I cited. The lobes are symmetric, if they were asymmetric that too would correct the issue I have with the intake valve 0.050" closing event. Power would improve if the valves were lifted higher.

I haven't purchased wheels yet, I'm working behind the scenes with someone on a project related to that subject. I won't make a wheel purchasing decision until I know if the project is go or no-go.

Can this custom Cobra Jet cam be ground for use with solid roller lifters?
Danford1

quote:

Originally posted by Danford1:
Can this custom Cobra Jet cam be ground for use with solid roller lifters?
Danford1

I was going to make the same inquiry.
Also with the proper valve springs & titanium valves, would this cam have the characteristics to allow the motor to rev to 7,000rpms!
I need a solid roller cam that has decent idle & streetable driving qualities, yet will rev to 7,000rpms!!! I have 180 headers & WANT THAT Can Am SOUND!!!!!!!!!!!!....
Much like my 1991 Ferrari Testarossa!!!!...Mark
https://www.youtube.com/watch?v=gmUpLypFts0

another clip..car leaves dealership at 2:10 of video & drive-by at 2:26....notice the front-end lift under the torque of the flat-12.

https://www.youtube.com/watch?v=_UQkr_x-Y9k

quote:

Originally posted by Danford1:

Can this custom Cobra Jet cam be ground for use with solid roller lifters?

Hi Dan, the answer is yes. Comp Cams makes a series of camshaft lobes called "High Energy Street Rollers". There are a pair of those lobes in their catalog that should do the job fine. Would require using different rocker arm ratios between intake & exhaust. Net valve lift (0.020" lash) either 0.575", 0.585" or 0.610" depending upon the intake rocker ratio (1.7:1, 1.73:1 or 1.8:1).

quote:

Originally posted by 1Rocketship:

... Also with the proper valve springs & titanium valves, would this cam have the characteristics to allow the motor to rev to 7,000rpms!

I need a solid roller cam that has decent idle & streetable driving qualities, yet will rev to 7,000rpms!!!

Mark, with the "right" valve train any type of camshaft you choose can rev to 7000 rpm; hydraulic flat tappet, solid flat tappet, hydraulic roller tappet or solid roller tappet.

Here's the sound of a V8 equipped with 360 degree - 8 into 1 exhaust

http://www.youtube.com/watch?v=w1xvZ0c59Ic

http://www.youtube.com/watch?v=OxQAz_tJuWw

George, Do you grind the solid roller version? If not, who would we contact?
Thanks
Dan

I've always thought that in a 351-C, what cam duration etc worked well was also dependent on whether the engine used stock iron heads with their curled-up exhausts, or modern high-port heads with more flowing exhaust ports. Seem intuitive that modern heads would NOT need the extra cam timing traditionally used on 351-C exhausts to make power. Yes?

bosswrench that sounds logical. george i forgot to tell you that i'm using my old 4v quench chambered cast iron heads. would that make a difference?

quote:

Originally posted by Danford1:

George, Do you grind the solid roller version? If not, who would we contact?

I don't sell parts. I give you the information you need so you can contact the cam grinder yourself and have the cam manufactured. I don't like sending people to Comp Cams, but in this situation they offer the lobes you need.

For the intake lobe use catalog number 1498, ground on 112° ATDC mathematic centerline.

For the exhaust lobe use catalog number 1476, ground on 116° BTDC mathematic centerline.

I'd use a 1.80:1 ratio rocker arm for the intake valves and 1.73:1 or 1.70:1 ratio rocker arm for the exhaust valves.

If you need more info than that let me know.

Thanks George.
Do you know anyone that had one ground already? If so then Comp might have the info on file.
Since the cam would have a 112 ICL would it need to be installed straight up?

Thanks for the help thus far. If I need more info, yes, I will be asking :-)

I went to Comp's lobe profile catalog and used those lobe numbers.
1496 268HER-2 224 @.050" .610
1476 288HER-2 244 @.050" .613
That is a 20 degree spread intake to exhaust.
Is that correct?

Danford1

quote:

Originally posted by Bosswrench:

I've always thought that in a 351-C, what cam duration etc worked well was also dependent on whether the engine used stock iron heads with their curled-up exhausts, or modern high-port heads with more flowing exhaust ports. Seem intuitive that modern heads would NOT need the extra cam timing traditionally used on 351-C exhausts to make power. Yes?

Not really ...

Forgive me Jack if I explain things too basically, I don't mean to insult your intelligence, but there are many readers of varying levels of understanding, and I want to make this explanation as understandable as possible to the greatest number of readers.

Between the "piston dwell period" at BDC and the "piston dwell period" at TDC there is about 120 degrees of crankshaft rotation for the piston to "push" exhaust gases out of the cylinder, no amount of additional exhaust duration can change that. BUT that is a moot point anyway because if a cylinder hasn't purged itself of exhaust gases by the time the piston begins moving upward during the exhaust stroke, it will fall on its face. This is due to the fact that the engine will have to work harder "pushing" the exhaust gases out as the piston rises on the exhaust stroke. The problem worsens as engine speed increases, the torque curve will fall-off like a brick at higher rpm. There is at least 100° of crankshaft rotation between the time the exhaust valve opens and the time when the piston begins moving upward during the exhaust stroke, this is the period in which most of the exhaust gases must be purged from the cylinder ... without the aid of piston motion.

Keep in mind that the exhaust valve is not fully open until about 115° BTDC, which is more than 65° too late! This reality is reflected in the trend these days to use low-ratio rocker arms (~1.6:1) on the exhaust valves of race engines, yet the intake valves are equipped with high ratio rocker arms (~1.8:1 to 2.0:1). The performance of the exhaust port when the valve first comes off the seat is most important, not air flow at peak valve lift. The 351C 4V exhaust port's performance is aided by that big 1.7" exhaust valve which exposes more valve curtain area for a given amount of lift than cylinder heads equipped with smaller valves. Cam timing must take into consideration not only the low-lift performance of the exhaust port itself, but also the scavenging performance of the exhaust system. You cannot consider them separately, they combine to make a system.

With a race engine the "early scavenging" of the exhaust gases is accomplished with a low back pressure, free flowing, and properly tuned exhaust system that presents a negative pressure wave at the exhaust port just as the exhaust valve opens. Engine designers try to grind the cam so the exhaust valve opens in harmony with the arrival of the negative pressure wave. However, the same is not true of a street engine equipped with mufflers, small tail pipes, long tail pipes, etc. The exhaust system is not going to provide a strong scavenging pulse when the exhaust valve opens. More than likely there will be some amount of pressure in the exhaust system (i.e. back pressure), and that pressure shall rise as engine speed rises. It is important to understand that as a piston descends within a cylinder during the power stroke the pressure within the cylinder decreases. In order for exhaust gases to begin flowing from the cylinder and into the exhaust system the exhaust valve must open early enough that the cylinder pressure is greater than the pressure in the exhaust system. Since exhaust system pressure increases with engine speed, the higher you want a street engine to rev, the earlier the exhaust valve must open. Opening the exhaust valve earlier is accomplished either by changing the exhaust lobe's centerline or by employing an exhaust lobe with longer duration.

I have good luck with most applications getting 351C 4V street engines to rev freely to 7000 rpm by opening the exhaust valve around 80° BBDC, these are engines equipped with headers and decent mufflers. But in most applications if the exhaust valve is opened just 5° later, around 75° BBDC, the engine will tend to loose steam after 6000 rpm.

That's the high rpm reason for a longer duration exhaust lobe. There's a low rpm reason too.

Overlap softens low rpm torque. The big intake valve and canted valve geometry of the 351C 4V increases the interaction between the exhaust port, the combustion chamber, piston motion, and the intake port during the overlap period. To put this another way, the big intake valve AMPLIFIES the effects of overlap.

Piston motion provides the energy during the overlap period that causes gases to flow in directions we don't want them to flow. There is a window however at top dead center in which piston motion virtually stops, its called the "dwell period". I've learned empirically that in regards to the 351C this window is about 60° wide, extending 30° on either side of TDC. If I can keep the overlap period within that window, overlap shall have less effect upon low rpm torque. If a cam has 50° of overlap, we have 10° of "wiggle room", but if a cam has 60° of overlap we have no wiggle room. With performance cams ... as overlap increases, it becomes more important to center the overlap period at TDC, if we wish to keep low rpm performance and drivability at its best. To put this into understandable numbers, a cam with 286° of "average duration" ground on 112° lobe centers has 62° of overlap.

If I time a 286°/286°, 112° LSA, single pattern camshaft to open the exhaust valve at 80° BBDC (for reasons I explained above) and to close the intake valve at 70° ABDC (for reasons I haven't explained), the overlap period shall be centered not at TDC, but at 5° BTDC.

By subtracting 5° duration from the intake lobe, and adding 5° duration to the exhaust lobe, the intake duration shall be 10° less than the exhaust duration but the resulting dual pattern camshaft (281°/291°) shall have the same average duration as the single pattern camshaft. The lobe centerlines of the dual pattern camshaft are "adjusted" to again establish opening the exhaust valve at 80° BBDC and closing the intake valve at 70° ABDC. With the lobe centerlines thus adjusted the overlap period shall also be centered on TDC, thus minimizing the effects of overlap and holding on to as much low rpm torque as possible.

Centering the overlap period on TDC is the other reason for a longer duration exhaust lobe ... and keeping the overlap period thus centered works very well. In terms of the "Cobra Jet" cam spec (the subject of this thread) combined with the 351C 4V cylinder heads the result is an engine that operates from 700 rpm to 7000 rpm; exhibiting good low rpm power, factory-type drivability, strong mid-range acceleration and a willingness to rev.
________________________________________________________________

For those of you who like to experiment, or do things on your own, here's some math:

The camshaft for the M code version of the 351C 4V had 274° average duration (268°/280°), it was ground on 118.5° lobe centers and thus had 37° overlap. The exhaust valve opened (EVO) at 81° BBDC and the intake valve closed (IVC) at 70° ABDC. Thus the overall valve event duration encompassed 511° of crankshaft rotation. With roughly 10° difference between intake duration and exhaust duration (12° to be precise) the overlap period was very well centered on TDC.

In order to open the exhaust valve at 80° BBDC and close the intake valve at 70° ABDC a camshaft's overall valve event duration must encompass at least 510° of crankshaft rotation. To arrive at that figure is as simple as adding 80° + 360° + 70°.

A camshaft having 280° average duration, ground on 115° lobe centers, is the smallest cam we can have ground today that meets this 510° valve event criteria. Cam grinders cannot or will not grind a cam with lobe centers wider than 115°. However in some cases it is difficult, if not impossible, to get a cam grinder to grind a cam with 115° lobe centers. Therefore I've adopted a self-imposed limitation of 114° lobe centers. Based upon that limitation a cam with 282° average duration and 114° lobe centers is the smallest cam that achieves 510° overall valve event duration. Centering the overlap period at TDC results in a camshaft having approximately 277° intake duration, 287° exhaust duration, 54° overlap, and 114° lobe separation. Such a "street camshaft" should employ lobes having no less than 54° lobe intensity, thus overlap as measured at 0.050" lobe lift shall be zero or less.

Here's a formula for determining a camshaft's overall valve event duration:
overall valve event duration = (advertised intake duration + advertised exhaust duration) - overlap

Computing overlap is easy:
overlap = average advertised duration - (LSA x 2)

and average advertised duration ...
average advertised duration = (advertised intake duration + advertised exhaust duration) ÷ 2

Have fun with this ...

quote:

Originally posted by 73 l:

... george i forgot to tell you that i'm using my old 4v quench chambered cast iron heads. would that make a difference?

No, not if its a street engine. I hope my explanation to Jack's question explains why.

quote:

Originally posted by Danford1:

Do you know anyone that had one ground already? If so then Comp might have the info on file.
Since the cam would have a 112 ICL would it need to be installed straight up?

You and Mark are the first people who have inquired about grinding this spec with solid rollers.

I have considered using a solid roller tappet for my own engine, which is why I knew about the lobes.

Yes the cam is specified to be used "as ground". Of course, they never get the cam indexing "spot-on" you always need to check timing and correct it with a multi-index crank sprocket.

quote:

Originally posted by Danford1:

I went to Comp's lobe profile catalog and used those lobe numbers.
1496 268HER-2 224 @.050" .610
1476 288HER-2 244 @.050" .613
That is a 20 degree spread intake to exhaust.
Is that correct?

The intake lobe is the wrong number, it should be 1498 (276° ... 12° spread). The "8" in the on-line catalog looked like a "6" to my eyes, I had to zoom-in to see it clearly. I'm glad you caught that mistake.

This ia all fascinating stuff.
Do the same EVO at 80 BBDC and IVC at 70 ABDC events pertain to other engines such as the Ford 302?
It has a 3.00" stroke so the dwell time would probably be shorter.

I have another car (66 Fairlane) with a 302 that I'm currently building for the street. I'm at the cam selection state so this thread has excellent timing.
I want it all, torque and power just off idle on up to 6000 rpm or more, works with 2000 stall converter C4, 2.80:1 highway gears, has a lope at idle and gets great highway fuel economy. I'd be using Ford hydraulic roller lifters, not solid.
The engine will have home ported Windsor Jr aluminum heads with 2.02" and 1.6" valves, 58 cc chambers about 9.5:1 compression, Weiand Stealth intake and 600 Hooley carb with 1 5/8" full length headers and 2 1/2" full exhaust.

If the timing events apply to this engine I'll start a search for a cam that has them.
I look forward to your reply.
Oh, if this is hy-jacking this thread just say so and I'll start a new thread for the 302.
Thanks.
Danford1

Lets add another wrinkle. What is the difference in your recomendations if you have the Boss domed pistons?

quote:

Originally posted by Danford1:

... Do the same EVO at 80 BBDC and IVC at 70 ABDC events pertain to other engines such as the Ford 302 ...

Yes and no. How's that for a concise answer?

Theoretically the answer "should be" no. The 302 has smaller valves, in-line valve heads, different combustion chambers, different stroke, different rod length. But practically speaking, I've seen a lot of factory performance engine cams from several manufacturers with about the same timing. For instance, the Ford 351 Cobra Jet cam (1971 specs) opens the exhaust valve (EVO) at 82° BBDC and closes the intake valve (IVC) at 72° ABDC. Those same specs were used for the 390 GT, the 1968 hydraulic cammed 427 and the 428 Cobra Jet. The 429 Police Interceptor camshaft specs were very similar, EVO = 86°, IVC = 73°.

Perhaps more pertinent, Ford camshaft #C9OZ-6250-C, was a hydraulic tappet performance camshaft for the 289/302/351W, its specs were EVO = 84°, IVC = 74°. The earlier 289 performance camshaft, C4OZ-6A257-A, specs were EVO = 78°, IVC = 72°. If you advance that cam by 2° you'd have 80/70. That cam was described as having a "reasonable idle", a "strong" power curve that extended into the 6000+ rpm range.

So yes, you can use that spec for the 302, or something close to it.

quote:

Originally posted by Danford1:

I want it all, torque and power just off idle on up to 6000 rpm or more, works with 2000 stall converter C4, 2.80:1 highway gears, has a lope at idle and gets great highway fuel economy.

While you can expect a wide flat torque curve, don't expect the same wide power band as the 351C with 4V heads ... the 302 will run out of steam sooner than 7000 rpm.

Its going to be hard to "have it all" because of the gears you plan to use and the torque converter stall speed. The Cobra Jet cam normally goes along with 3.50:1, 3.70:1, 3.91:1, or 4.11:1 gears, and/or a 3000 stall converter.

The highway gears and 2000 stall converter point at using a mild cam, the mild cam will definitely help low rpm power too, but it may impact the high rpm pull. I guess we'll find out how good those Windsor Jr. heads are. Fuel economy is directly related to overlap, but so is a lopey idle. To have both there will need to be some compromises there too.

Crane offers two different series of hydraulic roller cams for the 302, depending upon if its an older engine that wasn't equipped with a roller cam, or if its a newer engine that was equipped with a roller cam (firing orders are different). Tell me which type of engine you've got, and I'll take a look at the Crane catalog and make a recommendation.

quote:

Originally posted by captaintobeys:

... What is the difference in your recomendations if you have the Boss domed pistons ...

None

Hi George, Thanks for the simple formula ! That is very interesting...
I have a newer roller block. Should have the same firing order as our beloved Clevo's :-)
Danford1