[charlie66]

Is 10.5 to 1 a safe doable compression ratio for the street on 91 octane with a 461ci with aluminum heads? I used to think so, but I have guys telling me that it is pushing it. Where are you guys at with your street cars that are pushing 500+hp? the cam I'm using is 521/540 lift .050 is 236/242 on a 110. Please share

Thanks

[JLMounce]

My 465 with KRE D ports is right at 10.3:1 and uses a 240/248 on 112 hydraulic roller. 580hp, 600tq. I use 91 octane pump gas in it, but realistically I could probably run 87 mid-grade. If I knew I was going to be traveling long distances and not pushing the engine, I would probably do that.

[jerry455]

My 468 is 10.5:1 and on the dyno it made no difference between 30-36* timing. I run 93 octane pump gas. I have Edelbrock Performer RPM heads, ported with bigger valves and a 241/251 at .050 hyd. roller cam with .618 lift with 1.65 rockers. It made 580 hp and 590 tq.

[mgarblik]

I am at 10.1:1 with D-Port Edelbrocks and a solid flat tappet cam. A little under 500 HP, (487) 540 Torque. My engine runs just fine without a hint of detonation on 89-91 Octane non-alcohol fuel. If I had to do it all over again, I would bump the compression to 10.5:1. I feel I am giving up an honest 10-12 HP with the lower compression.

[Jay S]

Looks like a 433 XR from Comp, it has a lot of mid range, 9.5 to 10.5 is a good place to be. With that cam in a pump gas combo generally a bigger port will like the higher end of that range, the smaller the port will like the lower end of the range.

[P@blo]

11:1 is the maximum

[ta man]

I'm at 10.8 to 1 with SD's 310cfm KRE heads and SD's OF cam 235/245. Cranking compression is 190. I have zero issues with 91 octane..at times it is a mix of 87 and 91 since I have a pump at home and no issues. Compression is your friend..engine is smoother more power more efficient. I've seen 18mpg with my car a friend of mine has been at 20mpg with a 474 with the same head cam package.

[charlie66]

Thank you guys!

Looks like I'll be good then! I was going to try and knock it down some with a thicker gasket with a big bore but looks like I won't have to now.

[Skippy597]

I errored on the side of caution and went with 10.2:1 after talking with butler. Honestly the performance increase for half a point on a 500hp build is not substantial and isn't worth the chance to me. But mine is a street build.

[charlie66]

Originally Posted by Jay S

Looks like a 433 XR from Comp, it has a lot of mid range, 9.5 to 10.5 is a good place to be. With that cam in a pump gas combo generally a bigger port will like the higher end of that range, the smaller the port will like the lower end of the range.

Here is the cam card

[Jay S]

Originally Posted by charlie66

Here is the cam card

I see it has a Bulter part number and it is ground on B lobes. It is the same cam I mentioned, the off the shelf 433, except the 433 is advertised with S lobes. It is generally ground on B lobes though and advertised as an S in literature.


https://www.summitracing.com/parts/c...1/make/pontiac

[grivera]

What’s the difference between the S and B lobes?

[charlie66]

Originally Posted by Jay S

I see it has a Bulter part number and it is ground on B lobes. It is the same cam I mentioned, the off the shelf 433, except the 433 is advertised with S lobes. It is generally ground on B lobes though and advertised as an S in literature.


https://www.summitracing.com/parts/c...1/make/pontiac

Haha I have no clue what that means. Hopefully it will work well with 10.5 to 1. It seems that everyone here is doing it, so I feel better now about it.

[charlie66]

Originally Posted by Jay S

I see it has a Bulter part number and it is ground on B lobes. It is the same cam I mentioned, the off the shelf 433, except the 433 is advertised with S lobes. It is generally ground on B lobes though and advertised as an S in literature.


https://www.summitracing.com/parts/c...1/make/pontiac

Wow that is the same cam! interesting . Butler told me that it was there won custom design. I guess they made it there's by changing the 520 lift to a 521 . lol

[Jay S]

Originally Posted by grivera

What’s the difference between the S and B lobes?

The B and S are just suffix’s for the grinders to adapt the lobe to different cam tunnels. Th B is for a little bigger base circle and cam tunnel up to 50 mm. It more or less is adding a slight amount of intensity and lift (.001”) without increasing the lifter acceleration. It won’t make any meaningful difference on this.

[72LuxuryLeMansLa.]

The biggest caveat is your tuning ability. If you push it too far and don't have it tuned correctly you will regret it.

[65 Lamnas]

As usual, "IT DEPENDS".

I'm running a 400-based 462 with ported O-port 72cc E-heads with the "fast-burn" chambers. My block is zero-decked, and I'm running 14cc dished pistons, Fel-Pro 1016s. After the valves/seats were all done, the chambers were cc'd and the actual SCR is right at 11.0:1. Cam is a CC HR with 238/248 on 112 and about .650" (QNI lobes) with 1.65 rockers.

During break in on the dyno, and subsequent intake testing, we ran 92 octane and now that the engine's in the car, I run 91 octane non-ethanol. 34 degrees of total timing with about 16 degrees of vacuum advance.

I'd still be OK at 11.3-11.4.

[ta man]

Originally Posted by 72LuxuryLeMansLa.

The biggest caveat is your tuning ability. If you push it too far and don't have it tuned correctly you will regret it.

Anyone pushing it, degree your cam when installing.

[mgarblik]

Originally Posted by 72LuxuryLeMansLa.

The biggest caveat is your tuning ability. If you push it too far and don't have it tuned correctly you will regret it.

That is certainly the MAJOR factor when determining the maximum compression ratio. Not based on my personal experience with Pontiac engines. But with other brands, all things being equal, you can get away with a full point of extra compression or even more with something like a Progression ignition system, port fuel injection, and a knock sensor system.

Going full "old school", like my Pontiac engine, mechanical/vacuum advance system, 2X4 Carter carburetors, no electronics, you have to play on the conservative side to be safe.

[charlie66]

Originally Posted by mgarblik

That is certainly the MAJOR factor when determining the maximum compression ratio. Not based on my personal experience with Pontiac engines. But with other brands, all things being equal, you can get away with a full point of extra compression or even more with something like a Progression ignition system, port fuel injection, and a knock sensor system.

Going full "old school", like my Pontiac engine, mechanical/vacuum advance system, 2X4 Carter carburetors, no electronics, you have to play on the conservative side to be safe.

So, with a carburetor and a MSD distributor with vacuum advance and a 6al box is 10.5 to 1 still a save zone for 91 octane for street driving?