Firing Order or Cam?

Route666 · Sep 7, 2004

Did a bit of a search, but since this isn't a problem it probably hasn't gone across anyone's mind.

I've heard different lumpy motor idles, and some sound chaotic, like a random idle, and some sound sorta like a harley, fire fire skip skip fire fire but with a V8 it's like all the cylinders at once, then a little gap, repeat.

Is it just different cam grinds that can give the same motor a random idle and a lopey (I guess) idle? I think the random stuff is extreme cam, built for astronomic revs, but I thought a less aggressive cam would just give a smoother idle, not a trump trump trump.

Also, I guess the plenum intake harmonics come into play with this, one cylinder sucking the gas one way, perhaps starving the next to fire if it is at the other end. I suppose that IS why lumpy cams sound lumpy.

Gawd I hope someone understands and wants to talk about this lol. Describing sounds in words isn't the smartest thing to be doing, but I guess it's not as bad as actually trying to make the sounds.

WORTH · Sep 7, 2004

Route666 said:
Did a bit of a search, but since this isn't a problem it probably hasn't gone across anyone's mind.

I've heard different lumpy motor idles, and some sound chaotic, like a random idle, and some sound sorta like a harley, fire fire skip skip fire fire but with a V8 it's like all the cylinders at once, then a little gap, repeat.

Is it just different cam grinds that can give the same motor a random idle and a lopey (I guess) idle? I think the random stuff is extreme cam, built for astronomic revs, but I thought a less aggressive cam would just give a smoother idle, not a trump trump trump.

Also, I guess the plenum intake harmonics come into play with this, one cylinder sucking the gas one way, perhaps starving the next to fire if it is at the other end. I suppose that IS why lumpy cams sound lumpy.

Gawd I hope someone understands and wants to talk about this lol. Describing sounds in words isn't the smartest thing to be doing, but I guess it's not as bad as actually trying to make the sounds.

brooooom,,,broooom,,,,

So you had some free time this weekend??

mustang70 · Sep 7, 2004

Yes, it's the cam that's making the engine idle like that. I know it has to do with the lobe seperation angle and duration. But I couldn't tell you how.

Ronstang · Sep 7, 2004

The cam does not cause the lumpy idle. I realize that everyone thinks it does but the true cause is the intake manifold design. On a regular single carb V8 the intake is feeding 8 cylinders all in a different phase of ingesting air and the plenum design will have a lot to do with how lopey and engine sounds with a given camshaft.

I have a 66 Shelby with four downdraft 48 IDA Webber carbs on it. The intake is of the individual runner type and each cylinder has it's own barrel of one of the carbs with a very short and straight intake tract that goes almost vertical and right into the head's intake port. With a pretty healthy camshaft this engine HAS NO LOPE whatsoever. It idles as smooth as a stock engine. The intake is what causes the lope....not the cam.

crushnut · Sep 7, 2004

Interesting

Route666 · Sep 7, 2004

How MUCH of a difference does an IR setup make Ronstang? Say you have a cam that would be (just) unstreetable in a plenum-based induction setup, how would it run with the IR induction? Smooth as silk, a little less lumpy, or a lot less lumpy?

WORTH: Yeh I've got a fair bit of free time, looking for another job at the moment.

TOM B · Sep 7, 2004

I hope you really dont beleave what you said ronstang, did you read that on a cracker jack box.

Route666 · Sep 7, 2004

It's a combination of both. The larger overlap causes the exhaust to pull back into the chamber slightly at low rpm as it doesn't have much momentum, but there is plenty of mass of air in the intake plenum resisting change of movement into the cylinder. On the IR setup, each cylinder only has to deal with a tiny stack of air that is easy to push and pull around - giving better idle with larger cams from better low rpm flow into the cylinders.

Also, because a plenum full of air has so much mass, if a cylinder sucks in from one end, and manages to get the whole block of air moving that way, then a cylinder at the other end has its turn, it not only has to overcome the bad vacuum caused by too much overlap at low rpm, but also the air is away and moving away from that cylinder, so it has to pull it back. This leaves not much going into that cylinder this time around. The next time may be different as the cylinder before it may starve as a result of the position and direction of the main bulk of the plenum of air.

It doesn't seem that the manifold is the culprit causing the idle, because it is only evident when you put in the lumpier cam. BUT if you change the intake to an IR setup, you also get a smoother idle. Which is it? The cam or the intake causing this? I can bet too, if you put one way valves in the header tubes, that you would find better idle with a plenum, as it would act as a smaller overlap at low rpm.

ratio411 · Sep 7, 2004

I won't get into harmonics this and pulse tuned waves that...
I know there are lots of variables...

One thing I will say for sure:
A split duration cam has a rough idle. Not lope, just chaotic rough.
A single pattern cam has a steady 'beat'... More duration makes the beat more obvious, less makes it fade into the backround.

Did that make sense?

Split duration: 'rough' lope
Single pattern: 'smooth' lope

Dave

Route666 · Sep 8, 2004

Yeh that makes sense, but what is a split duration cam? Different cylinders have different durations or is it for 4 valve heads - one in/out pair with one duration and the other pair with another?

rbohm · Sep 8, 2004

TOM B said:
I hope you really dont beleave what you said ronstang, did you read that on a cracker jack box.

actually ronstang is for the most part right. there are a number of things that go into a smooth engine idle, and cam timing, and intake design are just two of those components. with an ir design each cylinder is tuned individually as far as idle mixture goes, jets and such need to be the same on all cylinder for balance. electronic fuel injection can also smooth out a lumpy idle caused by a cam that is too large otherwise, again by tightly controlling the fuel mixture. there does however come a point that regardless of how things are tuned, that lumpy idle will be there, but then we are talking in the realm of pro stock or top fuel engines.

Route666 · Sep 8, 2004

Yep yep.

So Ronstang, how much of a smoothing will IR intake make? Like will it take a cam that is very grumpy with a plenum intake and turn it into a mellow idler?

Ratio, could you explain what a split duration cam is please.

Also, I assume the single pattern is like a standard performance cam where all cylinders get the same overlap, etc. Is that right?

ratio411 · Sep 8, 2004

Dual patterns are cams with different durations on the intake and exhaust.
The Comp Extreme Energy series for example...
Most splits are from 4 to 15 degrees. On SBFs they tend to be bias to the exhaust lobe because we are known to have weak exhaust ports. It is a way to make the exhaust flow on par with the intake flow. There are also reverse splits, intake with a higher duration, but those are mostly for blower/turbo apps.
If both intake and exhaust have the same duration, then it is a single pattern cam. Best example is the Comp Magnum series.
Dual pattern, or split duration, means that basically you have 2 different cams functioning in your engine. One cam for the intake, one for the exhaust. Since they both will sound differently lope-wise, you end up with a very chaotic sound.
Single patterns have a rythmic sound.
Dave

Route666 · Sep 8, 2004

Ahhh I get it now. Thanks!

Firing Order or Cam?

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