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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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70GS455
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How much does moving the cam affect peak rpm?

Post by 70GS455 »

If it's dyno day and you need to move hp/tq peaks one way or another a few hundred rpm, how much does moving the cam relate to the rpm change? Say 2 or 4 deg change in advance/retard equals x rpm difference. This would be for a typical BB street/strip 550 ish hp N/A build.
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Re: How much does moving the cam affect peak rpm?

Post by Walter R. Malik »

70GS455 wrote: Tue Nov 17, 2020 5:53 pm If it's dyno day and you need to move hp/tq peaks one way or another a few hundred rpm, how much does moving the cam relate to the rpm change? Say 2 or 4 deg change in advance/retard equals x rpm difference. This would be for a typical BB street/strip 550 ish hp N/A build.
When the exhaust is deficient, I have seen advancing the cam actually help raise RPM and power upstairs.
That is backwards of what is usually preached.
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Re: How much does moving the cam affect peak rpm?

Post by rustbucket79 »

Played with my small block, moving the cam really didn’t change the powerband, mostly just traded torque and horsepower.

Since every engine is unique in it’s assembly of components, I don’t believe altering cam position will have the same effect in every situation. If you have the time it’s worth testing to see what your engine likes, and may I suggest looking for average numbers rather than peaks.
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Re: How much does moving the cam affect peak rpm?

Post by Bill Chase »

rustbucket79 wrote: Tue Nov 17, 2020 10:35 pm Played with my small block, moving the cam really didn’t change the powerband, mostly just traded torque and horsepower.

Since every engine is unique in it’s assembly of components, I don’t believe altering cam position will have the same effect in every situation. If you have the time it’s worth testing to see what your engine likes, and may I suggest looking for average numbers rather than peaks.
I recall reading an article in hotrod magazine years ago (for what that's worth) But they were saying to advance cam until max cranking compression was achieved and button it up, that for most street cars , assuming the cam is specced right, or close, that usually that's where they will perform the best. the context was the typical 10.5 to 1 23* sbc build.

Seems to me that If I want performance I will buy a custom cam, trust my cam guy and just install it where he tells me to. I did use the cloyes quick cover and a hex-a-just set though, so come chassis dyno day I can change it quickly if need be.
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Re: How much does moving the cam affect peak rpm?

Post by travis »

Bill Chase wrote: Wed Nov 18, 2020 4:10 am
I recall reading an article in hotrod magazine years ago (for what that's worth) But they were saying to advance cam until max cranking compression was achieved and button it up, that for most street cars , assuming the cam is specced right, or close, that usually that's where they will perform the best. the context was the typical 10.5 to 1 23* sbc build.

Seems to me that If I want performance I will buy a custom cam, trust my cam guy and just install it where he tells me to. I did use the cloyes quick cover and a hex-a-just set though, so come chassis dyno day I can change it quickly if need be.
If the intake valve closes at 60* ABDC, wouldn’t max cranking compression be achieved with 60* advance?
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Re: How much does moving the cam affect peak rpm?

Post by tenxal »

70GS455 wrote: Tue Nov 17, 2020 5:53 pm If it's dyno day and you need to move hp/tq peaks one way or another a few hundred rpm, how much does moving the cam relate to the rpm change? Say 2 or 4 deg change in advance/retard equals x rpm difference. This would be for a typical BB street/strip 550 ish hp N/A build.
You simply won't know until you try it.

Unless it's a real bread and butter deal, not moving the cam around on the dyno is like taking a shower and putting your dirty underwear back on. :shock:
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Re: How much does moving the cam affect peak rpm?

Post by CamKing »

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

Post by panic »

advance cam until max cranking compression was achieved
The person who wrote that is still in grade school.
Every engine will give maximum CCP with intake closing at BDC.
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Re: How much does moving the cam affect peak rpm?

Post by Bill Chase »

panic wrote: Wed Nov 18, 2020 10:27 am advance cam until max cranking compression was achieved
The person who wrote that is still in grade school.
Every engine will give maximum CCP with intake closing at BDC.
I didn't quote it verbatim. What they basically meant was install straight up, or as the cam grinder indicated. Then advance or retard within 2-4 degrees. that was the jist of what they were saying and within those confines whatever gave max cranking compression (and p t v clearance) was probably going to be best for the street sbc they were referring to. and 15 years later that is my recollection of what they were saying. I will see if I can find the article. I think Marlin Davis wrote it; but do not quote me on that.

It made sense though. Have a cam that is right for the application and play with cam timing + or - a few degrees and see what gave the best cranking compression.
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Re: How much does moving the cam affect peak rpm?

Post by mt-engines »

Most of the time I see it gain or lose torque. As far as rpm raising or lowing it rarely changes 100 to 200 rpm unless the cam was in left field. I've seen valve lash change rpm more so than specifically changing intake centerline.

Like Mike and Walter mentioned. I have seen engines with large heads do exact opposite of common belief with advancing and retarding. Where advancing the cam picks up power up high rpm.

Bottom line is, there is no set number, you just have to try it to see what the engine wants.
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Re: How much does moving the cam affect peak rpm?

Post by Nut124 »

Bill Chase wrote: Wed Nov 18, 2020 3:49 pm
panic wrote: Wed Nov 18, 2020 10:27 am advance cam until max cranking compression was achieved
The person who wrote that is still in grade school.
Every engine will give maximum CCP with intake closing at BDC.
I didn't quote it verbatim. What they basically meant was install straight up, or as the cam grinder indicated. Then advance or retard within 2-4 degrees. that was the jist of what they were saying and within those confines whatever gave max cranking compression (and p t v clearance) was probably going to be best for the street sbc they were referring to. and 15 years later that is my recollection of what they were saying. I will see if I can find the article. I think Marlin Davis wrote it; but do not quote me on that.

It made sense though. Have a cam that is right for the application and play with cam timing + or - a few degrees and see what gave the best cranking compression.
I think Panic is right on this. Intake valve closing at BDC +/-0 will always produce the highest cranking compression. Advancing the cam from recommended setting will always increase cranking pressure and retarding it will always do the opposite. That is assuming, of course, that the cam is a "normal" cam where IVC is considerably after BDC.
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Re: How much does moving the cam affect peak rpm?

Post by 70GS455 »

I'm in a quandry over 2 deg. I read the thread about degreeing an assymmetrical cam til my eyes bled. I have a Bullet hyd roller that is non-symmetric. I am up against a hard structural limit at 6000 rpm, and want to make sure the hp peak occurs below and near that. I asked for 236/244 on 113, + 5 deg advance (108). Well I have a bunch of 108s lol, depending on the method. I hesistate to use the term centerline

in position A:
The point of maximum lift is 107 (+6 deg adv)
The mid-point between 0.050" events is 108 (+5 deg adv)
The mid-point between 0.006" events is 110 (+3 deg adv)

This puts my ICL at 77 deg ABDC and DCR at 7.8. Moving this 2 deg to get ICL at 75 and DCR=8 I get:

Position B:
The point of maximum lift is 105 (+8 deg adv)
The mid-point between 0.050" events is 106 (+7 deg adv)
The mid-point between 0.006" events is 108 (+5 deg adv)

I left it at A, the overlap events seemed more balanced. Is 2 deg even enough to worry about? Seems as B is too advanced, every where but the advertised events. B gets my DCR closer to 8, and maybe keeps my peak a few hundred below 6k. This is on a 482 ci, 93 octane, somewhat weak block. Will dyno tomoro so I will know something
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Re: How much does moving the cam affect peak rpm?

Post by Nut124 »

Scott, in your post, is there a typo? IVC vs ICL? "ICL at 77 deg ABDC" makes no sense.

What is the displacement of your engine?

If you are saying IVC at 77ABDC, That would seem like a lot of cam for 6000 redline to me. Are your cam specs at 0.050? What does the mfg suggest for the powerband for that cam?

244@0.050 seems a bit hot to me for 6000rpm structural limit based on my very limited experience.
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Re: How much does moving the cam affect peak rpm?

Post by 70GS455 »

Nut124 wrote: Thu Nov 19, 2020 8:59 pm Scott, in your post, is there a typo? IVC vs ICL? "ICL at 77 deg ABDC" makes no sense.

What is the displacement of your engine?

If you are saying IVC at 77ABDC, That would seem like a lot of cam for 6000 redline to me. Are your cam specs at 0.050? What does the mfg suggest for the powerband for that cam?

244@0.050 seems a bit hot to me for 6000rpm structural limit based on my very limited experience.
Yes, I was thinking Intake CLosing, should've abbreviated IVC. It's a 482 ci. Cam is 236/244 @ 0.050".

Bullet didn't specify rpm range. With it being non-symmetric, intake closing @ 0.006" is later than one would expect for a 291 adv cam (i measured adv int. duration at 294 and 295 on some cylinders)
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Re: How much does moving the cam affect peak rpm?

Post by treyrags »

For that many cubes I think you are splitting hairs on the icl. I would put it in the more advanced position to allow for chain stretch if its a street car. If anything that will favor keeping the peak below your target 6000 rpm and might boost your low range torque slightly.
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