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Idle Rumble

C carloMember
Hi, I have a question for the engine experts.

Is the rough idle that is common to many V8s, particularly the older ones, caused by poor mixture quality at very low RPM or is it because of the camshaft design?

I compared the specs of the cam in my Cleveland with those of a '95 Winsor 302 HO and noticed that my cam has slightly more duration but considerably more overlap being 61° compared with 39° of the 302.

Modern V8s generally idle smoother than say, those of the 70's. Is this so because fuel-injection provides a better fuel mixture or is it because of a change in camshaft design?

Cheers,
Carlo
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George PGeorge PAdmin.
Its a combination of camshaft design (valve timing) and cylinder head design.

In terms of cam design it can be either the amount of overlap or how early the intake valve opens that create a rough idle. In terms of cylinder head design the shallow combustion chambers and large valves of the Cleveland engine produce a bit of a rumble at idle even with very mild overlap. This is because those aspects of the cylinder head amplify the effects of overlap. The 351C makes more top end power with 50° of overlap than most engines make with 65° to 70° of overlap.
jfb05177jfb05177Member
the effect of aggressive overlap results in exhaust gas going into the INTAKE at idle.

Back at the introduction of SMOG control, the primary device added was the EGR (exhaust gas recirculation). at high load, this valve would allow exhuast gas to be mixed with the intake to reduce combustion temp and NOXs.

While all the performance guys were blocking off the EGR, I replumbed the vacuum hoses so mine would open at idle, giving me the rough idle of a radical cam. all show and no go! (Charger 318 2bl)
R rrs1Member
Not to state the obvious, but lots of things cause a rough idle beside engine design. Carburetor adjustment, and internal condition, ignition timing, vacuum leaks, choke adjustment, where you set the RPM at idle, carbon build up, and more. Of coarse cars of the 70's relied on manual adjustment for all of this, which is never ending. Computers take care of most of it now.

There is nothing more gratifying than getting your engine dialed in without the help of computers! No matter that it is always temporary.
Last edited by rrs1
panteradougpanteradougMember
You can get a fuel injected engine to both idle smoother and run smoother across the complete rpm range because you can plot the fuel/ignition curve point for point vs. a carb which can only average that for certain points.

When using fuel injection you would also have different cam timing events specific to characteristics of that specific engine design.

Probably the most obvious example of these phenomenons would be an individual runner intake manifold with no common plenum, like a Weber 48ids system on an American V8.

With carbs you would literally SEE the fuel getting pushed back and with fuel injection because the fuel is squirted on a timed basis, there is no fuel to push back.

You will see and hear that in the idle AND the sound of the engine under power.

Ironically at some high rpm, maybe 6700 rpm, the Webers and the fuel injection will sound and work almost identically.

I think of it that the carbs are an analog system and the fuel injection a digital one, although granted my intellectual capacity for imagination may in fact be severely limited? (Alright, no agreeing with that here please?)

I had one customer say to me, "you know what your problem is? You're a genius and no one knows WTF you are talking about?"

Just another way of calling me an ass h ole, but I guess he was being very polite for sure? I saw through that and DID NOT discount his bill. Roll Eyes
ausfordausfordMember
Carburettors rely on airflow to initiate fuel flow and in a engine fuel flow effects engine operation which effects air flow. This is a feedback loop.

A bigger cam obviously effects air flow characteristics at idle in such a way as to make the airflow more intermittent and manifold vacuum more intermittent. That intermittent air flow adversely effects carburettor function making fuel delivery more intermittent and thus increasing the rough idle which then increases the intermittent nature of the air flow and manifold vacuum. This then amplifies the problem.

Fuel injection, analog or digital can be programed to ignore the wildly varying air flow and manifold vacuum caused by the big cam and inject fuel to a predetermined calculation.

This breaks the feedback loop that causes carburettors to amplify the idle roughness.

I have replaced carburettors on 351C's (that had very rough idles due to big cams) with fuel injection systems that did not sense airflow to calculate fuel at idle and the change in idle quality was astounding.

Once that feedback loop caused by the way all carburettors function had been eliminated the idle became almost as smooth as stock.
noquarternoquarterMember
"There is nothing more gratifying than getting your engine dialed in without the help of computers! No matter that it is always temporary." agree!

Rough'er idle when more duration and overlap is unavoidable, but a properly set timing and a nice 4-corner carb idle will go a long way. My Pantera with a "cam-from-hell" idled miserably at 1500 when I got it. I went through the steps in my book (sorry) and now it idles fine at 1000 also with A/C on.

BTW, temporary, in my experience a bronze colored Holley needs more frequent tune ups than silver colored Summit carb (can't remember English words for what material they are)
panteradougpanteradougMember
Well I don't know what the complaint is about? A bad idle means something different to me then a rough idle.

If a rough idle seriously bothered me I'd install a Tesla electric motor?

It's really the difference between rock 'n roll and a string quartet.

Admittedly sometimes I need to cleanse my pallet of the racket but after all these years I'm still impressed when I come back since Rock n' Roll "never forgets". Lucky for me I guess?

Big Grin
V 4vandproudMember
quote:
Originally posted by Carlo:
Hi, I have a question for the engine experts.

Is the rough idle that is common to many V8s, particularly the older ones, caused by poor mixture quality at very low RPM or is it because of the camshaft design?

I compared the specs of the cam in my Cleveland with those of a '95 Winsor 302 HO and noticed that my cam has slightly more duration but considerably more overlap being 61° compared with 39° of the 302.

Modern V8s generally idle smoother than say, those of the 70's. Is this so because fuel-injection provides a better fuel mixture or is it because of a change in camshaft design?

Cheers,
Carlo


first thing comes to mind comparing overlap degrees is where the measurement is taken, some sources provide advertised duration numbers (seat to seat or near seat to seat) while other sources specify valve event times to a specified lift ie .050" or .020" for some mechanical lift cams. point is make sure you're comparing apples to apples. there's a way to convert differing measurements by math. 61* overlap is not necessarily a raggedy running beast by any means

the first 1/2 of the 70's cars came equipped with breaker point ignition, convert points to electronic and the difference is obvious, smoother idle and low end drivability, improved efficiency evidenced by decreased fuel usage

now add that any early 70's car still a running breaker point ignition likely has significant wear to the lifters, cam and valve faces / seats and that can easily explain a rough or poor idle

'performance' camshafts and their idle characteristics is an entirely different matter altogether
marlinjackmarlinjackThe Machinist #5723 AMARGOSA MACHINE 'Running on Empty- Middle of Nowhere'

...One person hit on it. The very START of All this, The Ignition!

Rough Idle...Bad Idle...No Idle.

1. Points System Worn/Bad. Bad Rotor and Dist. Cap. Burned Points or Bad Condenser and/or Coil, can cause a 'Bad' Idle. Yes, a Electronic Ignition Would Smooth this Out.

2. Wrong Spark Plug, Wrong Gap, Carbon Fouled (One Suggested a V8 running 6 Cylinders.

3. Ignition Timing and Advancing curves, and More.

...From there we go to Bad Fuel, dozens of Carburetor mis-adjustments. As George Pointed Out, CAM Specs and Cam TIMING.

I must leave it there, too much to List.

I run a Distributor-Less Ignition, 4V Iron Heads, 292/588 Cam, Air Gap Dual Plane 0n 91=95 Fuel. Dumping the 'Cap and Rotor', Improves a Lot by Tons!

I LOVE the Cleveland Lope Idle 950 RPM. At 85 MPH She Purrs.

MJ

P.S. The Smooth Running was on My Desert Trip. I was running a 600 CFM Holley, Tuned to Premium, from Experience. Now Back Home, I'am putting the 750 Back-On. I run Vac-Secs on the Street, with a Manual Choke. In My Opinion Mechanical Secondaries Are Best on a Racetrack. Which brings to mind, so are Single Plane Manifolds, that do Not 'Come-On' until 5000 RPM, Still another cause for 'Poor/Bad' Idle...the Problem of Fuel Distribution, where, On the 351 Cleveland, the #8 Cylinder can become starved of fuel. That would be a SINGLE-Plane Manifold with a Carburetor. Installing Fuel Injection Has Eliminated this Problem, as in I.R. Injection. The F.I. Experts, of Which, I am Not one of, may Confirm this.

And in that regard, Aftermarket DUAL Plane Manifolds deliver a more precise Fuel Distribution to all 8 Cylinders, when Induction is by 'Normally Asperated' Carburation. Edelbrock flow bench tests, their designs, their Manifolds Work!

...One thing I need to add, concerning a 'Breaker-Points, Cap and Rotor', Ignition System. Those whom have switched to a 'Coil at the Plug' ignition system, already know this. Take a good hard 'Look' at the Physics involved, within the Distribution of the 'High-Voltage SPARK/ARC', of the Cap and Rotor. The coil creates the Arc, Grounding through a Questionable Points/Condenser, in Time to be 'Switched' to the Correct Spark Plug. Though this system has been used for 100 Years, it is Purely Wasteful, as far as, How much of the Spark Intensity is Lost in the actual Distribution, before ever reaching the Plugs. Making all of the Rotor to cap, GAP, 'Burning', purely unnecessary, and lose of potency. Now talk, rough Idle.

My suggestion, to All, is, 'Come out of the Dark Ages, and into the Third Millennium'...Get rid of the Distributor, Go to a Distributor-LESS, Ignition...You'll Realize just what I am Describing, a Hotter Fired Plug, a Highly Smoothed-Out Idle, and I'll even go further and add, Improved MPG from more efficient Fuel Burning. You will Never go Back!

Of-course, this is all 'In My Opinion', but Proven on the Highways, Best by Test!

Thank You.

MJ

Last edited by marlinjack

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