cam rpm range and cubic inch

[bigdozer]

building 388 sbc and have a question about cams in general, cam has rpm range of 3700-7000, how does the cubic inch of a engine change the rpm range of a cam, iam not talking about idle quality or drive ability but where the cam makes power. what is a cam grinder basing the rpm range on. will the rpm range change in a 327 vs 434 for example

[A Atwood]

The port size and stroke of the engine will have a very large influence on the rpm where tq and hp will peak at. The cam needs to work with the rest of the combination.

ARN

[bigdozer]

well with a 350 and 383 is the rpm range going to be the same if you had the same top end, with the stroke being differnt or would the 383 make peak power at a lower rpm

[Orr89rocz]

383 with longer stroke will peak at a lower rpm. In general it seems larger inches will peak at a lower rpm when using same heads/cam/intake. Power may be about the same HP wise, but where it peaks changes. Torque may change in favor of the larger inch motor or larger stroke motor but it depends on the combination.

[bigjoe1]

A quarter inch of stroke will move the HP peak about 400-500 RPM with the same cam and heads.


JOE SHERMAN RACING

[bigdozer]

so in a perfect world is that going to bring the whole rpm range down 400 to 500 rpm, and does a cam manufacture base rpm range on a 350 sbc with a certain head, iam just wanting base line, some cams with the same specs have totally differnt operating ranges

[bigjoe1]

What you said is pretty much correct. Almost cam catalogs use the 350 Chevy for their examples.


JOE SHERMAN RACING ENGINES

[Orr89rocz]

Some rpm ranges can be misleading tho... like Comp Cams XFI hydraulic rollers for SBC. In a 350, the biggest cam in the line up says 2200-6200. The 292xfi. 242/248 at .050. .584/.579. I can tell you right now that may be true if you dont have enough valve spring for it. Or if that was in a 406-427 sbc... It may float at or around 6200 with a weak spring but if you control the lifter on those lobes with enough spring, that cam will go to 7K on a 350 with the right heads and supporting intake. If you dont have the heads/intake, it wont go nearly that high. So the range really depends on the total application. I usually ignore the advertised rpm ranges unless it was based on a very similar motor to what I wanted to build.

[bigdozer]

so with the 292xfi cam, if i can keep the lifter on the cam it would have more cylinder pressure with the smaller duration and with the higher lift have more rpm, would that be a perfect sbc cam. maybe not that set up but small duration and big lift for a custom grind

[bigjoe1]

The duration is what makes a differance when it comes to max RPM. The lift is just along for the ride. This is where the rocker ratio might come into play.,







JOE SHERMAN RACING

[Strange Magic]

I have found, that if the port volume from the back side of the valve to the carb throttle blades does not change, then where the peak torque number comes in doesn't change. The numbers under that rpm figure that the peak torque comes in does change becoming broader and bigger when you increase the stroke and less broader and less in the amount when going smaller in stroke.

[rewguy]

In my opinion.....cam guys like to take credit for where an engine makes peak power. But sometimes you will find that a great flowing cylinder head with "adequate" cross section will make peak power at say 7500 at 400 cubic inches, with a cam that is 260 @ .050. This would imply that the mcsa of the cylinder head had alot more to do with where power was made than the camshaft's duration did. We had a large runner (252cc) brodix 10x head make peak power at 7800rpm on a 409 cube sbc, with a camshaft with 269@.050. When the camshaft was swapped for a 278@.050 grind with similiar lift and lsa, power moved only 100rpm. (yes.....valve springs were more than adequate in both cases...) If you were to look up both of these cams in a catalog, it would more than likely tell you that they would operate in entirely different ranges.......say..5000-7000, and 5500-7500. To prove the same point in an entirely different way.......years ago we had a 421 sbc on the dyno, with some decent flowing, but small (msca) ports. 2.38" at the pushrod to be exact. The camshaft had a good deal of lift and a ton of duration. The engine was a general turd and made peak power at a dismal 6200rpm. (also had adequate valve spring) But if you were to ask most cam guys where this would peak at, they would likely tell you "upwards of 7500". Cross section was AGAIN key in where it made power. Matching cross section with valve timing is where I think power is found. I think that camshaft guys should be asking "how large in area is your intake runner at point A, and points B and C?" instead of asking what the head flows. If the sizes and shapes are correct for what you are doing, the flow will usually correspond.

[CamKing]

In my opinion.....cam guys like to take credit for where an engine makes peak power. But sometimes you will find that a great flowing cylinder head with "adequate" cross section will make peak power at say 7500 at 400 cubic inches, with a cam that is 260 @ .050. This would imply that the mcsa of the cylinder head had alot more to do with where power was made than the camshaft's duration did. We had a large runner (252cc) brodix 10x head make peak power at 7800rpm on a 409 cube sbc, with a camshaft with 269@.050. When the camshaft was swapped for a 278@.050 grind with similiar lift and lsa, power moved only 100rpm. (yes.....valve springs were more than adequate in both cases...) If you were to look up both of these cams in a catalog, it would more than likely tell you that they would operate in entirely different ranges.......say..5000-7000, and 5500-7500. To prove the same point in an entirely different way.......years ago we had a 421 sbc on the dyno, with some decent flowing, but small (msca) ports. 2.38" at the pushrod to be exact. The camshaft had a good deal of lift and a ton of duration. The engine was a general turd and made peak power at a dismal 6200rpm. (also had adequate valve spring) But if you were to ask most cam guys where this would peak at, they would likely tell you "upwards of 7500". Cross section was AGAIN key in where it made power. Matching cross section with valve timing is where I think power is found. I think that camshaft guys should be asking "how large in area is your intake runner at point A, and points B and C?" instead of asking what the head flows. If the sizes and shapes are correct for what you are doing, the flow will usually correspond.

You need to find some new cam guys. The cam will only dictate peak HP rpm, if something else isn't choking the engine.
We use the flow numbers at multiple lifts to calculate what the minimum area would be to reach those numbers at a certain fpm, and use that area to calculate the required cam lift and duration. It's more accurate then using a mechanical CSA measurement, because there's too many variables that can change the choke point of a given CSA on a port.

[Rick360]

In my opinion.....cam guys like to take credit for where an engine makes peak power. But sometimes you will find that a great flowing cylinder head with "adequate" cross section will make peak power at say 7500 at 400 cubic inches, with a cam that is 260 @ .050. This would imply that the mcsa of the cylinder head had alot more to do with where power was made than the camshaft's duration did. We had a large runner (252cc) brodix 10x head make peak power at 7800rpm on a 409 cube sbc, with a camshaft with 269@.050. When the camshaft was swapped for a 278@.050 grind with similiar lift and lsa, power moved only 100rpm. (yes.....valve springs were more than adequate in both cases...) If you were to look up both of these cams in a catalog, it would more than likely tell you that they would operate in entirely different ranges.......say..5000-7000, and 5500-7500. To prove the same point in an entirely different way.......years ago we had a 421 sbc on the dyno, with some decent flowing, but small (msca) ports. 2.38" at the pushrod to be exact. The camshaft had a good deal of lift and a ton of duration. The engine was a general turd and made peak power at a dismal 6200rpm. (also had adequate valve spring) But if you were to ask most cam guys where this would peak at, they would likely tell you "upwards of 7500". Cross section was AGAIN key in where it made power. Matching cross section with valve timing is where I think power is found. I think that camshaft guys should be asking "how large in area is your intake runner at point A, and points B and C?" instead of asking what the head flows. If the sizes and shapes are correct for what you are doing, the flow will usually correspond.

I've seen the same thing. It takes a large change in duration to make much change in peak TQ rpm or peak HP rpm.

The same heads and cam will peak lower (TQ and HP peaks) on a larger ci engine.

Rick

[rewguy]

right on rick.......