Someone Teach Me About Cams... Single Pattern vs Dual Pattern

[andyf]

As a follow on I'll add that I currently have 3 race engines and they all use dual pattern cams. From what I've seen over the years, a little extra duration on the exhaust side will help extend the power curve on a race engine with open headers. But as soon as you put mufflers on the engine the dynamics all change.

A street engine will usually make more power on the dyno with extra exhaust duration but it might not work better in the car with a full exhaust system. The extra duration on the exhaust side can hurt more than it helps with a full exhaust system since the back pressure can force the exhaust gases back into the engine. So the final words from me on the subject are "it all depends" and "you won't know until you try it".

[plovett]

A street engine will usually make more power on the dyno with extra exhaust duration but it might not work better in the car with a full exhaust system. The extra duration on the exhaust side can hurt more than it helps with a full exhaust system since the back pressure can force the exhaust gases back into the engine. So the final words from me on the subject are "it all depends" and "you won't know until you try it".

That's interesting and I think I agree with it.  The "prevailing wisdom" over the last few decades has been that if you had a restrictive exhaust system, then you needed more exhaust cam duration to make up for it.   I think you're saying that adding more exhaust cam duration won't help if you physically can't flow it through the exhaust.   Okay, it may help a tiny bit as you are giving it a few more milliseconds of time, but the payback of the increased overlap at lower rpms is much more severe.  Something like that anyway

I asked Mike Jones a question like that a long time ago and the answer was similar, as I remember.  I was asking about camming engines with restrictive exhaust manifolds vs. engines with headers and free flowing exhaust sytems.   His answer was something to the effect that if you try to crutch a restrictive exhaust system with a lot of exhaust duration you essentially lose everywhere.   The engine still won't make good top end power and you will lose the power in the rpm range that the engine could have made good power at.  Basically he said concentrate on lower rpm power in such a situation, and likely use a single pattern (and smaller) cam. 

Sometimes when you try to have your cake and eat it too, you end up with neither.

On the other other hand, why did the 428CJ cam work so well?  It seems to follow the opposite theory.  Or maybe the 428CJ would have been even better with a single pattern cam?

just jawin'

paulie

[My427stang]

A street engine will usually make more power on the dyno with extra exhaust duration but it might not work better in the car with a full exhaust system. The extra duration on the exhaust side can hurt more than it helps with a full exhaust system since the back pressure can force the exhaust gases back into the engine. So the final words from me on the subject are "it all depends" and "you won't know until you try it".

That's interesting and I think I agree with it.  The "prevailing wisdom" over the last few decades has been that if you had a restrictive exhaust system, then you needed more exhaust cam duration to make up for it.   I think you're saying that adding more exhaust cam duration won't help if you physically can't flow it through the exhaust.   Okay, it may help a tiny bit, but the payback of the increased overlap at lower rpms is much more severe.  Something like that anyway

I asked Mike Jones a question like that a long time ago and the answer was similar, as I remember.  I was asking about camming engines with restrictive exhaust manifolds vs. engines with headers and free flowing exhaust sytems.   His answer was something to the effect that if you try to crutch a restrictive exhaust system with a lot of exhaust duration you essentially lose everywhere.   The engine still won't make good top end power and you will lose the power in the rpm range that the engine could have made good power at.  Basically he said concentrate on lower rpm power in such a situation, and likely use a single pattern (and smaller) cam. 

Sometimes when you try to have your cake and eat it too, you end up with neither.

On the other other hand, why did the 428CJ cam work so well?  It seems to follow the opposite theory.  Or maybe the 428CJ would have been even better with a single pattern cam?

just jawin'

paulie

I don't think the 428 CJ cam did all that well.  I think a 428CJ with a better cam would likely do better everywhere, but I can't prove it It's basically a truck/Cadillac cam by today's standards, low lift, slow ramps, wide LSA, retarded for the added compression.  It also was the off the shelf GT cam, not like it was engineered for the CJ.  The CJ was "better exhaust manifolds, better exhaust, better heads, bigger valves, better intake (by a margin) and a bigger carb"...oh and 38 more cubes   All of those made it run pretty hard (sorta) compared to the 390 GT, not the cam IMO

That being said, remember, exhaust moves out (and the intake moves in) due to a couple of things, negative pulse in the primaries, a piston being jammed upwards, overlap, and some momentum, I am sure there are others.  A little more exhaust duration will add overlap, which lets the exhaust do it's business a little better and it allows the piston to push for a little longer.  Way simplified, and even to this day I am trying to wrap my head around exhaust valve timing in relation to blowdown and scavenging, but it is there.  My hunch is, a restrictive set of manifolds likely will not benefit as much as a set of headers with a quite/small diameter intermediate pipe. So the "restrictive exhaust" IMHO doesn't mean bad manifolds, its more "bad mufflers and pipes behind the headers.

[plovett]

I don't think the 428 CJ cam did all that well.  I think a 428CJ with a better cam would likely do better everywhere, but I can't prove it It's basically a truck/Cadillac cam by today's standards, low lift, slow ramps, wide LSA, retarded for the added compression.  It also was the off the shelf GT cam, not like it was engineered for the CJ.  The CJ was "better exhaust manifolds, better exhaust, better heads, bigger valves, better intake (by a margin) and a bigger carb"...oh and 38 more cubes   All of those made it run pretty hard (sorta) compared to the 390 GT, not the cam IMO

That being said, remember, exhaust moves out (and the intake moves in) due to a couple of things, negative pulse in the primaries, a piston being jammed upwards, overlap, and some momentum, I am sure there are others.  A little more exhaust duration will add overlap, which lets the exhaust do it's business a little better and it allows the piston to push for a little longer.  Way simplified, and even to this day I am trying to wrap my head around exhaust valve timing in relation to blowdown and scavenging, but it is there.  My hunch is, a restrictive set of manifolds likely will not benefit as much as a set of headers with a quite/small diameter intermediate pipe. So the "restrictive exhaust" IMHO doesn't mean bad manifolds, its more "bad mufflers and pipes behind the headers.

I mentioned the 428CJ cam mostly in a rhetorical way as it has been portrayed as helping the 428CJ run so well despite the "good (okay) exhaust manifolds" and horrible exhaust system after that. 

The CJ cam has a huge split of around 206/220 degrees @ 0.050"  Something like that anyway.  The reason I brought it up was to point out that maybe the CJ would have run much better with the stock exhaust system with a very different cam.  I dunno.  Or maybe the CJ really came alive when fitted with headers and a modified (or no) exhaust system?

If true, has it really taken us almost half a century to figure this out?  Holy cow.

just jawin'

paulie

[My427stang]

I have a cam here going in a 396 FE, 280/230 intake, 284/236 exhaust, on 112, I plan to put it in the engine at 106, we'll see what it does LOL

It's a neat cam, hydraulic, would have liked to see it tighter LSA, but it's out of another Forum members engine, and is what it is

[andyf]

The guys who have this really figured out are the ones who run in FAST class. They have to run with stock appearing engines and full stock appearing exhaust. I know some of the tricks that they use but I don't know them all since I haven't done any of that stuff for years. But there is a fairly firm limit on how much duration you can run on the exhaust side when you have manifolds and a full exhaust system. Too much duration and you lose power everywhere. They compensate by running a lot of lift and a ton of compression but they don't run a huge amount of duration. I wouldn't recommend a FAST cam for street use, just bringing it up as an example that you don't want to get too excited with exhaust duration if you are using a full exhaust system.

If you want to see some crazy split cams take a look at what the Chevy engineers are using in the LS engines. The COPO 427 engines have 40 degrees more duration on the exhaust than the intake. Something like 230/270 for duration. I'm not sure how they came up with that. People always say "think outside the box" but I don't think I would've ever thought that far outside the box!  Here is a link to a COPO data sheet:  http://www.jegs.com/p/Chevrolet-Performance/Chevrolet-Performance-427ci-425hp-COPO-Crate-Engine-2012-2015/2804804/10002/-1

[plovett]

Yes, high compression is your friend with factory exhaust manifolds and exhaust.  Push it out hard and quick.

It also makes sense,  I think, to have a high lift, aggressive exhaust lobe in the factory exhaust manifold situation we're talking about.  Get done what you can while you can.

In most hot rod engines the intake lobe tends to be more aggressive (faster) than the exhaust.  This makes sense based on the pressure differentials in the intake and exhaust tract.  At least I think so.  There is more "pull" on the intake port when the valve is further open, so you want to open it fast to get it in the sweet spot longer.  The exhaust port has the highest pressure differential when the exhaust valve is just opening, so normally low lift flow is more important.  So exhaust lobes tend to be less aggressive.  In fact it is often the case that intake lobes become exhaust lobes as cam and valvetrain technology advances.  One generation's intake lobe becomes the next generation's exhaust lobe, as a new more aggressive intake lobe is designed.

BUT, in a very exhaust restricted combination, all this might not be true.  I bet those F.A.S.T cars have exhaust lobes at least as aggressive as the intake lobes.  And super high compression.  I don't know the rules.  It is interesting, though.

JMO,

paulie

[scott foxwell]

The guys who have this really figured out are the ones who run in FAST class. They have to run with stock appearing engines and full stock appearing exhaust. I know some of the tricks that they use but I don't know them all since I haven't done any of that stuff for years. But there is a fairly firm limit on how much duration you can run on the exhaust side when you have manifolds and a full exhaust system. Too much duration and you lose power everywhere. They compensate by running a lot of lift and a ton of compression but they don't run a huge amount of duration. I wouldn't recommend a FAST cam for street use, just bringing it up as an example that you don't want to get too excited with exhaust duration if you are using a full exhaust system.

If you want to see some crazy split cams take a look at what the Chevy engineers are using in the LS engines. The COPO 427 engines have 40 degrees more duration on the exhaust than the intake. Something like 230/270 for duration. I'm not sure how they came up with that. People always say "think outside the box" but I don't think I would've ever thought that far outside the box!  Here is a link to a COPO data sheet:  http://www.jegs.com/p/Chevrolet-Performance/Chevrolet-Performance-427ci-425hp-COPO-Crate-Engine-2012-2015/2804804/10002/-1

When you have an intake tract as efficient as that, coupled to a full exhaust system and "OK" ex port, that's what it takes to make power. Lift is almost irrelevant on the ex side pother than to maintain a certain valve path. 90% of the work is done the second the ex valve cracks off the seat, not unlike your favorite beverage when you pop the top. The remainder is just "time" (duration) to get the rest of the exhaust out. Intake to ex duration is directly related to the ratio of the flow numbers. The closer you can get to real running conditions, ie: flow the heads with intake, carb, headers and exhaust system bolted on, the better the resutls will be with the cam. A bare head sitting on the flow bench can be WORLDS different than one with everything bolted on it. Not just in mean flow numbers, but also the ratio/percentage of intake to ex. but in the end, of the flow ratio heavily favors the intake side, you'll see a healthy split with duration favoring the exhaust. These duration numbers aren't a "guess", either. They're derived from math.

[427HISS]

As a follow on I'll add that I currently have 3 race engines and they all use dual pattern cams. From what I've seen over the years, a little extra duration on the exhaust side will help extend the power curve on a race engine with open headers. But as soon as you put mufflers on the engine the dynamics all change.

A street engine will usually make more power on the dyno with extra exhaust duration but it might not work better in the car with a full exhaust system. The extra duration on the exhaust side can hurt more than it helps with a full exhaust system since the back pressure can force the exhaust gases back into the engine. So the final words from me on the subject are "it all depends" and "you won't know until you try it".

I'd like to ask for more information about exhaust systems and cam choices. If you have a small engine bay, say like a 60's Mustang with a big block and have unequal length headers, that you need to be creative on making heads and not loose power, versus a car like mine, a 427 Shelby Cobra replica, where the header tube angle couldn't be straighter, and the flow into the side pipes with no real muffler.  I would only assume, that this is one of the easiest systems to expell exhaust flow from the heads.

So, with the Mustang vs 427 Cobra's exhausts, does that mean the cam & exhaust port's would be way,....different for best performance ?

[scott foxwell]

As a follow on I'll add that I currently have 3 race engines and they all use dual pattern cams. From what I've seen over the years, a little extra duration on the exhaust side will help extend the power curve on a race engine with open headers. But as soon as you put mufflers on the engine the dynamics all change.

A street engine will usually make more power on the dyno with extra exhaust duration but it might not work better in the car with a full exhaust system. The extra duration on the exhaust side can hurt more than it helps with a full exhaust system since the back pressure can force the exhaust gases back into the engine. So the final words from me on the subject are "it all depends" and "you won't know until you try it".

I'd like to ask for more information about exhaust systems and cam choices. If you have a small engine bay, say like a 60's Mustang with a big block and have unequal length headers, that you need to be creative on making heads and not loose power, versus a car like mine, a 427 Shelby Cobra replica, where the header tube angle couldn't be straighter, and the flow into the side pipes with no real muffler.  I would only assume, that this is one of the easiest systems to expell exhaust flow from the heads.

So, with the Mustang vs 427 Cobra's exhausts, does that mean the cam & exhaust port's would be way,....different for best performance ?

Most guys will never flow their heads let alone their exhaust systems. Induction flow factors are a very large part of good cam design. I know when Chris designs a high end cam, he tells the customer to get flow numbers with intake, carb and headers attached if they want the absolute best results. It really doesn't matter what configuration you're working with as long as it becomes a "known". If you don't have the ability to flow the exhaust system on the heads, then we have to make assumptions and "best guesses" based on experience...which Chris has a lot of. So to answer your question, the cam would most likely be different if you had the ability to flow everything in place. If the engines were the same and going to operate in the same rpm range and target the same power, then the cams could possibly be "tweaked" if one ex system was radically different from the other but at that point you're also picking pepper out of fly poop. Not sure if it's in your question or not, but I will tell you that any exhaust system is going to kill power, no matter how good the cam is. We ran the best dyno tested exhaust on our big block Cobras (which was the Loback) and it cost 60hp.  The ex ports would (should) not be any different. There are tuned headers available for the 60's big block cars and the difference between those headers and your Cobra headers, which are most likely not tuned, are probably insignificant.

[machoneman]

I'll just add that on every car I knew of or actually worked on, dumping the cast iron manifolds and having dual exhausts and fairly efficient mufflers (some hardly muffled!) made a big difference, seat-of-the-pants wise. This included Boss 302's, Windsors, SBC's and BBC's (sorry!) some Chrysler 340's, 383's and even a 392 early Hemi. 

My point is with a muffled street engine, once the exhaust side is well thought out, the difference between a single versus dual pattern cam, given they are off-the-shelf cams, is IMO minimal. Now, open headers and a race optimized engine IS a whole different story.   
 

[scott foxwell]

I'll just add that on every car I knew of or actually worked on, dumping the cast iron manifolds and having dual exhausts and fairly efficient mufflers (some hardly muffled!) made a big difference, seat-of-the-pants wise. This included Boss 302's, Windsors, SBC's and BBC's (sorry!) some Chrysler 340's, 383's and even a 392 early Hemi. 

My point is with a muffled street engine, once the exhaust side is well thought out, the difference between a single versus dual pattern cam, given they are off-the-shelf cams, is IMO minimal. Now, open headers and a race optimized engine IS a whole different story.

I will 100% disagree with that.

[66FAIRLANE]

While we are on camshafts, frankly I have never understood this (taken from another site & much proliferated);

"There is a very easy way to tell if the camshaft has been advanced by the cam company when the cam was machined. Look at the cam timing card and compare the lobe separation angle (LSA) with the intake centerline number. If the numbers are the same, then the camshaft has not been advanced.

When we look at the cam card, the intake centerline is listed at 107 degrees while the LSA is listed at 112 degrees. The numbers tell us the intake centerline has been advanced five degrees from 112 to 107 degrees after top dead center (ATDC)."

Never got this. Maybe I'm just stupid or missing the obvious.

[blykins]

We were covering this very thing on another thread here.

When the LSA = ICL, it's called "straight up".   When the ICL is smaller than the LSA it's considered "advanced" and when the ICL is greater than the LSA it's considered "retarded". 

So, a 110 LSA with a 110 ICL is considered straight up. 

A 110 LSA with a 104 ICL is considered 6 degrees advanced.

A 110 LSA with a 112 ICL is considered 2 degrees retarded.

The cam company can grind the cam with any amount of advance or retard built into it.  Most of Comp Cams' off-the-shelf cams are usually 110 LSA on a 106 ICL, so they are all ground 4 degrees advanced.

The key is, that you can move that relationship with a multiple keyway timing set, and advance/retard it to wherever *you* choose.  So, just because the cam company put a certain amount of advance/retard into it, it doesn't mean that you have to run it like that.

[My427stang]

While we are on camshafts, frankly I have never understood this (taken from another site & much proliferated);

"There is a very easy way to tell if the camshaft has been advanced by the cam company when the cam was machined. Look at the cam timing card and compare the lobe separation angle (LSA) with the intake centerline number. If the numbers are the same, then the camshaft has not been advanced.

When we look at the cam card, the intake centerline is listed at 107 degrees while the LSA is listed at 112 degrees. The numbers tell us the intake centerline has been advanced five degrees from 112 to 107 degrees after top dead center (ATDC)."

Never got this. Maybe I'm just stupid or missing the obvious.

Not stupid, it is hard to visualize



If you look at the picture we used in a different thread, when a cam is neither advanced or retarded, when compared to TDC (which centerlines are measured against) the cam is installed with each centerline equidistant from crank position at TDC.

If you advance it or retard it, compared to TDC, the LSA stays the same (ground into the cam) but the distance from TDC changes for both lobes, the cam can be viewed as "leaning" one way or another using the picture.

I wish there were drawings that showed them in a different order.  Cam events will happen in this order, maybe this makes more sense (or confuses more LOL)

1 - Exhaust reaches max lift at exhaust centerline
2 - Piston reaches TDC
3 - Intake reaches max lift at intake centerline

- The distance in degrees between 1 and 3 = LSA
- If a cam is installed without advance or retard, those centerlines are equal distance from #2.  ECL=ICL=LSA
- If an intake lobe has a centerline less than LSA, the whole cam was rocked toward TDC (advanced) and that also moves the ECL, opposite is true too