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How much does moving the cam affect peak rpm?

General engine tech -- Drag Racing to Circle Track

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Bill Chase
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Re: How much does moving the cam affect peak rpm?

Post by Bill Chase »

mt-engines wrote: Wed Nov 18, 2020 4:16 pm I've seen valve lash change rpm more so than specifically changing intake centerline.
Was this a hydraulic roller where you see the gains/losses?

I am using one of Mike's cams. nothing radical276/280 adv 228/232 @ 50 576/576 110 lsa installed at 107icl exhaust 113 icl. I just degreed it where the card said to, and set the lash 1 turn past zero lash with the delphi ls replacement lifters. Given the fact that I want this engine to be as strong as possible from idle to 6000 would looser or tighter lash help? new 385 sbc, afr 195 street ports and shaft rockers from scorpion racing. as is it started right up, is very quiet, got oil up top almost instantly when priming. I was actually surprised at how quiet it was at idle for a first start of a new mill.

Think going looser would help? I read a lot of conflicting info on these lifters being used in the gen 1 880 roller block. many lash at 1?2 turn past zero lash, yet others have went 1.25 turns past zero. and still others have said set preload for them the same as you would in the ls block. if running a little looser would help it rev easier off of idle, or tighter I would be all for that.
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Re: How much does moving the cam affect peak rpm?

Post by tenxal »

treyrags wrote: Fri Nov 20, 2020 6:34 am If anything that will favor keeping the peak below your target 6000 rpm and might boost your low range torque slightly.
It may or may not. I've seen where retarding the cam shows increased low end torque.
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Re: How much does moving the cam affect peak rpm?

Post by treyrags »

tenxal wrote: Fri Nov 20, 2020 7:42 am
treyrags wrote: Fri Nov 20, 2020 6:34 am If anything that will favor keeping the peak below your target 6000 rpm and might boost your low range torque slightly.
It may or may not. I've seen where retarding the cam shows increased low end torque.
Agreed. But most of the time it will go that direction. Unfortunately there is no rule of thumb that is 100%.
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Re: How much does moving the cam affect peak rpm?

Post by Orr89rocz »

Doesnt it depend on how close the cam is to being “correct” for the application and desired performance? If cam was spec’d right it should do the power and rpm you want and moving it a few degs shouldnt change much. If its out in left field moving it may show greater changes. At least thats what i have read and makes sense to me

Kinda like lash changes on a solid roller
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Re: How much does moving the cam affect peak rpm?

Post by Bill Chase »

CamKing wrote: Wed Nov 18, 2020 9:17 am Every engine combo is different.
Rule of thumb: if you want to shift the power down in the RPM, advance the cam in 2 degree steps, until it no longer helps If you want to help the top-end power, do the opposite. Make sure you have the P2V clearance to make the moves. Don't be surprised if the engine acts 180 degrees opposite of what you were thinking.
I keep thinking about this, and how it appear counter intuitive to what most neophytes like myself think will happen when you advance the cam. Not sure if I am wording this correct, but here goes.

CamKing, you mention the engine will do the opposite and actually enhance upper rpm power when advanced.. is this because of the simple fact that it allows more area/time to fill the cylinder at higher rpm? Or does momentum of the intake charge also come into play at rpm and you essentially shut the door while the charge is at a higher rate of flow?

Seems like a pretty loaded scenario, a lot of factors come into play. Just wanting to learn and understand better, if you have a minute and care to elaborate on this I for one would be very grateful sir.
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Re: How much does moving the cam affect peak rpm?

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Bill Chase wrote: Fri Apr 16, 2021 10:02 am
CamKing, you mention the engine will do the opposite and actually enhance upper rpm power when advanced.. is this because of the simple fact that it allows more area/time to fill the cylinder at higher rpm?
It happens when the cam is so far off from what's correct, that the change in events is bandaiding the issues.
Maybe the exhaust is way too small on duration and/or the Exhaust centerline has the exhaust opening too late, and at higher RPM's the pressure is so high at overlap, the exhaust charge is contaminating the intake charge. Advancing the cam will open the exhaust sooner, lowering the exhaust pressure at overlap.
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Re: How much does moving the cam affect peak rpm?

Post by Walter R. Malik »

CamKing wrote: Fri Apr 16, 2021 12:27 pm
Bill Chase wrote: Fri Apr 16, 2021 10:02 am
CamKing, you mention the engine will do the opposite and actually enhance upper rpm power when advanced.. is this because of the simple fact that it allows more area/time to fill the cylinder at higher rpm?
It happens when the cam is so far off from what's correct, that the change in events is bandaiding the issues.
Maybe the exhaust is way too small on duration and/or the Exhaust centerline has the exhaust opening too late, and at higher RPM's the pressure is so high at overlap, the exhaust charge is contaminating the intake charge. Advancing the cam will open the exhaust sooner, lowering the exhaust pressure at overlap.
YES ... In my testing over the years, if you advance the cam and make more power at the top of the RPM range, you probably have an issue with something to do with the exhaust.
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Re: How much does moving the cam affect peak rpm?

Post by Tuner »

CamKing wrote: Fri Apr 16, 2021 12:27 pm
Bill Chase wrote: Fri Apr 16, 2021 10:02 am
CamKing, you mention the engine will do the opposite and actually enhance upper rpm power when advanced.. is this because of the simple fact that it allows more area/time to fill the cylinder at higher rpm?
It happens when the cam is so far off from what's correct, that the change in events is bandaiding the issues.
Maybe the exhaust is way too small on duration and/or the Exhaust centerline has the exhaust opening too late, and at higher RPM's the pressure is so high at overlap, the exhaust charge is contaminating the intake charge. Advancing the cam will open the exhaust sooner, lowering the exhaust pressure at overlap.
This should be easy to spot with Nitro2's TFX pressure transducer data recording system.
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Re: How much does moving the cam affect peak rpm?

Post by digger »

Walter R. Malik wrote: Fri Apr 16, 2021 6:24 pm
CamKing wrote: Fri Apr 16, 2021 12:27 pm
Bill Chase wrote: Fri Apr 16, 2021 10:02 am
CamKing, you mention the engine will do the opposite and actually enhance upper rpm power when advanced.. is this because of the simple fact that it allows more area/time to fill the cylinder at higher rpm?
It happens when the cam is so far off from what's correct, that the change in events is bandaiding the issues.
Maybe the exhaust is way too small on duration and/or the Exhaust centerline has the exhaust opening too late, and at higher RPM's the pressure is so high at overlap, the exhaust charge is contaminating the intake charge. Advancing the cam will open the exhaust sooner, lowering the exhaust pressure at overlap.
YES ... In my testing over the years, if you advance the cam and make more power at the top of the RPM range, you probably have an issue with something to do with the exhaust.
what sort of things have you found on the exhaust that were lacking that necessitated an advanced cam other than what Camking already mentioned?
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Re: How much does moving the cam affect peak rpm?

Post by Walter R. Malik »

digger wrote: Sun Apr 18, 2021 7:54 pm
Walter R. Malik wrote: Fri Apr 16, 2021 6:24 pm
CamKing wrote: Fri Apr 16, 2021 12:27 pm
It happens when the cam is so far off from what's correct, that the change in events is bandaiding the issues.
Maybe the exhaust is way too small on duration and/or the Exhaust centerline has the exhaust opening too late, and at higher RPM's the pressure is so high at overlap, the exhaust charge is contaminating the intake charge. Advancing the cam will open the exhaust sooner, lowering the exhaust pressure at overlap.
YES ... In my testing over the years, if you advance the cam and make more power at the top of the RPM range, you probably have an issue with something to do with the exhaust.
what sort of things have you found on the exhaust that were lacking that necessitated an advanced cam other than what Camking already mentioned?
Aren't those enough ...? A very restrictive exhaust system is one other.
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Re: How much does moving the cam affect peak rpm?

Post by Bill Chase »

Walter R. Malik wrote: Sun Apr 18, 2021 10:13 pm
digger wrote: Sun Apr 18, 2021 7:54 pm
Walter R. Malik wrote: Fri Apr 16, 2021 6:24 pm

YES ... In my testing over the years, if you advance the cam and make more power at the top of the RPM range, you probably have an issue with something to do with the exhaust.
what sort of things have you found on the exhaust that were lacking that necessitated an advanced cam other than what Camking already mentioned?
Aren't those enough ...? A very restrictive exhaust system is one other.
So basically 90% of the hot street/strip cars out there will see benefits from advancing the cam? The oems have got better about it, 2.5" duals seem to the the norm for 3 liter v6 stuff now, a God aweful amount of old school muscle cars out there with too small exhaust. And probably the wrong cam lobes too.
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Re: How much does moving the cam affect peak rpm?

Post by digger »

Walter R. Malik wrote: Sun Apr 18, 2021 10:13 pm
digger wrote: Sun Apr 18, 2021 7:54 pm
Walter R. Malik wrote: Fri Apr 16, 2021 6:24 pm

YES ... In my testing over the years, if you advance the cam and make more power at the top of the RPM range, you probably have an issue with something to do with the exhaust.
what sort of things have you found on the exhaust that were lacking that necessitated an advanced cam other than what Camking already mentioned?
Aren't those enough ...? A very restrictive exhaust system is one other.
i was thinking about either terrible low height exh port, exhaust system length and things of that nature
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