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Dear friends, my weber engine needs more power.

I have a standart 351c block

with a flat tappet cam with about .06 lift

on a ported closed 4v heads.

Heads use crane roller rockers .

So first of all , the mixture is much to rich.

It runs now at about 11:1 at 3500rpm @ about 100miles per hour ( le mans gear ratio)

150 main

200 air

F11 emulsion tube

and only 38 Venturis.

So my plane is to switch the venturis to 42 or 44 mm to increase top end power.

Car runs only 250  km/h Gps locked. When it is a cold day maybe 260 km/h

The engine only spin 7000rpm

from 6000 to 7000 rpm there is no more power aviable. 

In this range the air to fuel ratio is about 10 :1 at full throttle.

Is there someone, wich have any experience in change the venturis ?

The 4V head have verry big intakes so i think a small ventury make no sense.

I can not understand, why we restrickt the 48 mm to 38 mm only to reach more air velocity. I guess this high air speed is only in the carbs. When the air arrives at the intake valve , it will be slow again , because of the big ports from the heads.

What is your opinion ?

Should I try to use 42 mm Venturis ? 

I found a old race setup from a porsche :


it use 48 idas on 42 Venturis with a F7 tube , 165 main and 170 air correction  and a 5 inlet "valve"








Images (1)
  • Weber48 IDA
Original Post

It's not just about camshaft lift with Webers, more dependent on camshaft lobe separation angle and reversion.  Need a good Weber guy and dyno to measure the change impact on Air/fuel ratio and iterate it out or maybe reach out to Jim Inglese, he can design a system with the specs of your engine.

Or buy a nice 393C stroker with higher redline.......

140f/160a. F5. 42mm. Compcams solid lifter .606. 294 adv duration, 244 @.050.

You can try 170f/200a, f5's on the track. They work on the street too but I don't think you can use all that fuel streetwise? What I do like about the 170's is how hard the engine pulls off of idle with them but they are heavy. Plugs will read dark brown.

You need to maintain those proportions of fuel to air, up and down the scale. I tested as high as 175 fuel and kept getting power increases but ran out of air correctors to match them, so I stopped there.

1.50f/2.00a is a 289 set up. The Clevelands use a different jetting ratio of fuel to air.


I did also test emulsion tubes. I like the F7s better on the street. I think there is a little more power with them on the street. The 289's don't like them as much as the Clevelands do.

I did test the F11s as well. I didn't like them at all. I thought that I lost high rpm power with them.

I stopped at F5's and left it at that. I do have sets of all of them F1 through F13 so it isn't like I didn't consider them. All the combinations get very time consuming to test and there is not a lot of difference between the three that I narrowed it down to but F11's are not the way to go with my set up and usage.


You can run the mains down as far as about 125's (with corresponding air correctors) and that will clean up the plugs. The largest effect that will have is limiting upper rpm. So I would think the 170's will help top end.

Look. This isn't NASA here. I do this by substituting jet changes and testing as best that I can for power changes. I don't have a private race track to do this on like Shelby had at Willow Springs or Ken Miles to drive it. That team was there all the time in the '60s.


Idle: .60f/.80a. You can play with the AIR CORRECTORS A LITTLE, such as substituting .85 idle holders. The 80s can be difficult to find. Look out though. Idle can immediately go lean and backfire bad.

If you haven't done the carbs already, install the third transition hole. It will eliminate the transition flat spot at 3000rpm or so. The new hole goes in the middle between the two existing ones.

It isn't necessarily the main circuit causing the richness. It's the idle circuit AND the pump jets also, since they all add fuel to the mixture.


In fact I found in my testing that the 2.0 pump jets that come stock in the carbs are only necessary to start a cold engine. I worked those down to .5 pump jets and found no power changes, just cleaning up the plugs effects.

That caused my first embarrassing moment with the Webers. I couldn't get the car started the next day with those pump jets and missed my first meet with the car as a result.

I thought that it might be possible to use the 2.0 pump jets in just one carb and in effect start the cold engine on the one carb and keep the .5s in the other three. I have not tried that yet but eventually probably will.


The best that you can tune it for is about 6,500 rpm. It will go to whatever rpm you want to provided the valve springs will take it. Those are usually only good to around 7,200rpm.

You PROBABLY can tune it for maximum power at a higher rpm but I don't know where you can do that since you are approaching 200mph with that? Maybe Lemans?


Tuning on the IDA's was and to some extent still is proprietary information and generally is not casually obtained. It isn't a video game or something that you can tune with a computer like you can with EFI.

There is some work to get the power curve where you want it with a system this old.

You need to ignore what the plugs look like as long as they aren't white or black and adjust the jetting for power.  If the plugs wind up dark brown as a result, it's just coincidental.  Throw  the A/F meters away. They don't work with the Webers. It is just conflicting information.


I went to A3 Motorsport heads. They are about 30hp (at least) over whatever you can get out of the iron 4v heads and you need to get good headers on it.

The set of heads that I have were out of a circle track race car and had already been worked. They are "not out of the box stock heads". They have been shaved and pockets cleaned up a little. They have no cast in reducer rings like the iron 4v heads have and were intended not as street replacement heads but full competition heads.

I only got them because the PO was upgrading to later version of the  heads. C302's. I know he wanted to buy them back so I would presume that he was disappointed with the newer heads?


For headers, Either Pat Michal under or 180's over. 2" primary pipes.


There is no camshaft that exists that will eliminate the reversion on an engine of your usage. On the street, you can reduce it with a "Weber cam" but essentially what it does is limit the overlap to 28 degrees. That will give away around 100hp.

It is this reversion that makes "Weber tuning experts" useless UNLESS they are experts on US V-8's.  It is the v-8s with individual runners that make it complex. The Porches with the Webers don't have the reversion issues to this extent nor do the 4 cylinder cars.

A Porsche jetting is not going to work on your Cleveland.Argue if you like but why waste your time?

A 351c withe Webers is a bit of a "Horse of a different color. There's only one of him, and he's it".

Last edited by panteradoug

Doug, thanks again .

Do you think, switching the 37 mm venturi to 42 or more makes any sense ?

Its your suggesed comp flat tappet comp cam inside.

I try the engine with a 850 holley and it have much more power. It spin easy to 7500 with the holley carb, but I switch back to the weber because of the look.

Also old lola F1 engines where equiped with 48 ida s and make easy more than 8000 rpm .  Maybe they use 46 or bigger venturi . I do not know how they do this back in the days.

problem is : my girl friend have a opel speedster turbo with arround 300hp and 850 kg flying weight. She is much faster than me ! Those little car runs easy 270 km/h and from start to the end , I have no chance with the pantera !  I need more power :-) :-)




Here's the problem you are running into. It is true that 48IDA's were run in factory sponsored comp cars in the '60s and early '70s.

It is also true that they exist due to Ford's financial investment in them with Weber.


Almost all of those Ford engines were 289 Fords. The "factory" racers used special 289 heads that are now referred to as GT40 heads. The Group 4 Pantera is one of the exceptions.

The camshaft they were using, I don't know specifically. It may have been a version of what we now call the 289 Lemans cam but likely the cams varied from team to team.

Those cams initially were ground for Shelby by Sullivan. There are several versions of them. Eventually Ford put them in their service parts and referred to their version as the Lemans cam.

It had/has 63 degrees of overlap. So where ever the intake valve close is, the thing is going to produce reversion. The plate over the top of the Webers on the GT40's is really a collection plate for the fuel droplets. That way they could control where the fuel dripped on the engine and avoid the hot headers.

My only remark about them is that by today's standards they are very tame with not a lot of valve lift, just more duration. Very disappointing in a street car.


With this configuration in a 289 (4.7L), 48mm throttles were found to be restrictive at around 6,500 rpm WITH 42mm chokes!

I do not know what 44mm chokes in a 48IDA will run like. Probably better at WOT and terrible on the street?


The 351c iron head numbers work out to requiring a 51mm throttle bore. That configuration naturally from Weber doesn't exist.

If you examine the throats of the Pantera Weber intake manifold you should note that they are in fact 51mm.

The 427's actually need 58mm carbs. It's thought that only four sets of those were made. I don't know why.


Now, you CAN get 51mm "Webers". They are made by EMPI and really are not Weber products. They are Weber lookalikes.

What EMPI has done is bored out the throats of 48mm carbs to 51mm and installed the corresponding throttle plates. Everything else remains the same.


I have seen the carbs and there is a caution on them. They are very thin at the gasket mounting surface. I personally know no one who is using them so I cannot speak to the reliability factor with them.

However, these would be the carbs that THEORETICALLY you need to run to make the power that you are looking for and you would use 44mm chokes with.


I would GUESS that the camshaft you have would be adequate BUT I suspect that you are going to need to go to a cam grinder for a more precise one.

I am using that cam, with 1.72 ratio rockers. You might benefit from 1.80 ratio rockers possibly without changing cams but you would need to experiment with that and there MIGHT be an issue with rocker arm studs at the rpm's you are talking about going to. This would give you .634 lift. That will help you with more rpm's.

Also unless you are limited by racing rules that are sanctioning your car, I think that you MUST go to the aluminum aftermarket Cleveland heads.

There are several here that can attest to "Clevelands" in the 800hp range using aftermarket heads.

The issue there is that even though you are able to run that engine on "the street", you are really running a race engine with things like Jessel rocker arm assemblies that are likely to need to be rebuilt around every 1,000 miles or so.

The horsepower is there but it is a very expensive maintenance engine to care for.


As far as a Holley 850 carb making more power, yes. Absolutely. You could consider a Holley 950cfm three barrel or a 1050 Dominator if you can find an intake for them.

The Dominator is huge and takes up most of the intake manifold so it has the outrageous looks you want.


IF you want that look of an 8 stack system looking like Webers then you need to go to the "Inglese" efi system. By the time you get the cpu and set up, that's going to cost you around $8,000 right now and is going to take you a while to dial it in.

Unfortunately to get that one right where you want it, you can no longer tune that on the street and you are looking at track rental and a team  to go with you to the track to test and tune it. So the power is there but there are other very serious considerations to get it.


You MUST do something with exhausts. If you are going under, talk to Pat Michal. If over, talk to Ron Mccall. Both are making step tube headers and you are going to need those to milk another 50hp or so out of the exhaust at those rpms.


I'm fine with my setup. I am a dinosaur. Frankly I don't think that I am capable of handling the car with more power. You would be entering into the vicinity of a professional race driver that practices day in and day out with the car and just drives it and doesn't maintain or repair the car themselves. That's not me.


Here's a pic of the Holley Dominator manifold to fit your iron 4v heads. Put a 1050 Dominator on it.

It's as impressive as the Webers are. This IS a full race manifold for that engine. I doubt you will have restriction issues with it.


My apologies for the long replies but there is no simple answer with a Weber 48 ida setup.











Images (1)
  • Holley Dominator flange  D1ZX-9425-FA_01 for Cleveland
Last edited by panteradoug

I agree with Doug about going to aftermarket aluminum heads. The smaller ports on the aftermarket heads have higher velocity than the 4V cast iron Cleveland heads, which in turn will make more power. If you insist on having the individual throttle bodies, then switching over to a good EFI system is the way to go. The fuel air ratio that you are stating is 11:0 to 1 at 100 mph and not full throttle is way too rich and you are losing power there.  A good EFI system with the car being tuned on a dyno is the ultimate way to go! None of these suggestions are going to be cheap, but speed costs money, so how fast do you want to go?



@gt4peter posted:

Dear George Pence, if possible delete the message from Gts and block him as member. Ashole ec. should not be the forum speach here. 

Also please, I would need his personal data. There are some personal attacks in his post. Thanks for helping. 



Thanks Joules and Doug for your help. 

I hope there was something there that I posted that is helpful to you.

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