Cam terminology

[88 coupe]

My response to a PM I got, earlier today.

http://www.aperaceparts.com/tech/camterminology.html

Norm

[Jamesbo]

Thanks Norm

What about this "cam bleeding off compression"

[Jamesbo]

Here's another one not on the link

"long-overlap" but delayed intake closing.
__________________

[88 coupe]

Originally Posted by Jamesbo

........ What about this "cam bleeding off compression"

I don't know.

If I did, I would not have asked the question.

Paraphrasing: "Your compression is too high for that cam. You need a bigger one to "bleed off" the excess cylinder pressure". Similar statements on OP and ROP, by him, and others.

My thinking is: How can a cam, that is designed to make more pressure (power) at a given CR, make less pressure (power) than a "smaller" cam.

Norm

[captjim]

Originally Posted by Jamesbo

Thanks Norm

What about this "cam bleeding off compression"

It bleeds off cylinder pressure. not compression.

"My thinking is: How can a cam, that is designed to make more pressure (power) at a given CR, make less pressure (power) than a "smaller" cam."

The difference is the RPM that the engine will be operating in. Norm, that statement ignores cylinder head flow, exhaust flow, plus other factors. A bigger cam will make more power, but at a higher RPM, as long as the other components are matched to it. IMO, most engines have an effective power range of 3500 rpm or so. A mild street engine that operates from 2500-4000 rpm with have completely different requirements than a nasty engine that makes power from 4000-7000 rpm. The latter engine does not give a hoot about idle quality or vacuum, where on a street engine, throttle response and driveability are factors to consider. As always, IMHO.

[88 coupe]

Originally Posted by captjim

It bleeds off cylinder pressure. not compression ….....

Not exactly. Without compression, there would be no cylinder pressure. But that would be a different topic.

Originally Posted by captjim

The difference is the RPM that the engine will be operating in ….....

That would be the difference between a street engine and a race engine, which has nothing to do with this topic.

Originally Posted by captjim

…..... that statement ignores cylinder head flow, exhaust flow, plus other factors ….....

So did the statements that prompted this thread.

Originally Posted by 88 coupe

…..... Paraphrasing: "Your compression is too high for that cam. You need a bigger one to "bleed off" the excess cylinder pressure". Similar statements on OP and ROP, by him, and others ….....

The question was: How?

Norm

[tonycpe]

how ? the larger cam holds the valves open at the same time longer than a smaller cam. and don't forget there is actual compression ratio and static compression.

[88 coupe]

With apologies for the improper code. The quoted thread is locked.

Originally Posted by 88 Coupe

Overlap occurs between exhaust and intake. It is increased, for better scavenging, in order to promote increased cylinder filling, which would increase cylinder pressure.

http://classicoldsmobile.com/forums/50090-post29.html

Originally Posted by 88 coupe

........ How can a cam, that is designed to make more pressure (power) at a given CR, make less pressure (power) than a "smaller" cam.

Increased duration and increased scavenging = increased cylinder filling = Higher pressure.

Norm

[MN71W30]

I didn't see some other technical terms like...3/4 race, and full race.

[cutlassefi]

You forgot one. How about "RV" cam? That'll be exactly the same for every engine right? Have a good day fellas!

[captjim]

Originally Posted by cutlassefi

You forgot one. How about "RV" cam? That'll be exactly the same for every engine right? Have a good day fellas!

Mark brings up a point that has always annoyed me. The cam descriptions go something like this,
"Good power and mid range torque for a 9-10.5 to 1 engine with some gear and converter"
The thing is, the same cams fit 307s-455s. The same cam in a 9.5 to 1 455 will act quite a bit differently than the same cam in an 8.7 to 1 350. My Engle "RV" cam that Norm likes to make fun of would indeed be a good choice for a 455 that has low compression and is used in a towing application. The specs are .472/.488 210/216 @ .050 on a 112. It certainly is no monster, but compare it to the stock 350 cam which was in the neighborhood of .400 lift and 194 on the intake duration, the the "RV" cam is a significant improvement. I'll take my high 13s in a 9 to 1 355 in a 3950 lb car that is dead reliable. Speed demon? Nope. Fun car to drive" You bet'cha.
Every professional cam grinder that I spoke with stated that using the factory iron SBO heads (slow burning, big chambers) they would recommend a compression ratio between 9 to 1 and 9.5 to 1 and a cam with an intake duration between 210 and 218 @ .050. So, the Pros like the "RV" cams in a mild street SBO application, too.
I guess what I am saying is that it is all about the complete combination. Match your components and you will have a fun car to drive. Oldsmobile engines are not noted for being fast revving, high rpm engines. Build to their strong suit, low end torque, and that usually means being conservative in camshaft selection.
The above is just my persona opinion, as always.

[88 coupe]

Originally Posted by MN71W30

........ ........ ........

Originally Posted by cutlassefi

........ ........ ........

Originally Posted by captjim

........ ........ ........

At the risk of repeating myself:

Originally Posted by 88 coupe

........ The question was: How?

The question was: How?

Norm

[MN71W30]

Originally Posted by captjim

Mark brings up a point that has always annoyed me. The cam descriptions go something like this,
"Good power and mid range torque for a 9-10.5 to 1 engine with some gear and converter"
The thing is, the same cams fit 307s-455s. The same cam in a 9.5 to 1 455 will act quite a bit differently than the same cam in an 8.7 to 1 350. My Engle "RV" cam that Norm likes to make fun of would indeed be a good choice for a 455 that has low compression and is used in a towing application. The specs are .472/.488 210/216 @ .050 on a 112. It certainly is no monster, but compare it to the stock 350 cam which was in the neighborhood of .400 lift and 194 on the intake duration, the the "RV" cam is a significant improvement. I'll take my high 13s in a 9 to 1 355 in a 3950 lb car that is dead reliable. Speed demon? Nope. Fun car to drive" You bet'cha.
Every professional cam grinder that I spoke with stated that using the factory iron SBO heads (slow burning, big chambers) they would recommend a compression ratio between 9 to 1 and 9.5 to 1 and a cam with an intake duration between 210 and 218 @ .050. So, the Pros like the "RV" cams in a mild street SBO application, too.
I guess what I am saying is that it is all about the complete combination. Match your components and you will have a fun car to drive. Oldsmobile engines are not noted for being fast revving, high rpm engines. Build to their strong suit, low end torque, and that usually means being conservative in camshaft selection.
The above is just my persona opinion, as always.

Years ago we were racing a 67 442 with a 70 455 motor and 390 gears. It would run low 14's. I remember riding in it and it felt like the engine couldn't rev up as fast as the 390 gears would alow. If you can understand what I just wrote. So we changed the gears to 355 and it ran high 13's.
We then put in an Engle cam real close to what you listed and it made a real firebreather out of the 455. I would say we picked up at least a half of a second or more et. We never got the car back to the track to know forsure but the difference was huge. It would idle perfectly and ran strong.

[captjim]

Many guys over gear Olds engines. They don't rev particularly fast, and don't need to. I did a bunch of passes one day, and the car ran within a tenth shifting at 4400 as it did 5200. Why? Flat torque curve. IMO

Norm, I really don't understand what you are getting at. If you keep the valve open longer, open it slowly, and have both valves open longer at the same time, physics will dictate that the cylinder pressure will be lower. Scavenging can be a factor that will increase the pressure. Yes, the larger cam will allow more air and fuel in, but you need more compression to make it work. If I am wrong, please explain how going from the 214/224 to a 210/216 raised cranking compression from 145 to 185 and increased the vacuum at idle?

[gmrocket]

"bleed off" is a term used to describe i situation where you have excessive cranking compression because your cam isnt suited to your static comp ratio&whats needed is a later intake closing point. the chart in the link that Norm posted explains it, "decreased" can be substituted for "bleed off". in jims case it seems like he was in the opposite situation where his bigger cam was bleeding off what little dynamic he had..going to a smaller, earlier closing intake point cam, increased his cranking psi.

there is no hard&fast rule on pump gas. as an example a 9:1 455 could rattle itself to death on good 93 octane yet another properly built &set up 455 with 10.5:1 will run like a champ

[380 Racer]

Well Jim I'm back and I see not much has changed around here huh?