[KJ223]

Hi everyone, I’ve been reading several different forums about increasing compression ratio, but they are mostly all about Pontiac 400s. I have a 75 Pontiac 350 and as far as i know it’s the original engine and never rebuilt. It runs great and I put a summit 2800 in it a couple months ago. I’ve been looking into how to get more power out of this set up and I’ve heard about increasing compression. From what I understand my motor has around 7.5:1 compression ratio. What can I do with these 6X-4 heads to increase power? Do they need milled, and if so how much can be taken off? I also want to put headers on, right now I have a pypes dual exhaust and stock manifolds since I can’t remove the manifold bolts without them snapping off in the head, so if I have to pull the head for milling and headers I might as well do them at the same time. I also don’t want to have to pull the engine out of the car and change anything else as I don’t have an engine stand or crane, and I also don’t want to rebuild the motor if it’s not necessary. Thanks

[JLMounce]

I don't think I would spend much time or money trying to get the 6x-4's to get you where you want to be. I'd look for a set of #48 heads which would have been found on a 69 350HO. They are a small chamber head with the 1.94 intake valve. On a stock displacement 350, it my opinion that you don't really need the 2.11 valve. It's shrouded by the small bore anyhow.

Put a good spring for your 2800 on the head and add a quality set of 1.5 rockers and pushrods. You should end up with around 9:1 compression ratio with these heads and with a bit of bowl work and a quality header, you should see a decent bit of performance gain over what you currently have.

I know you don't want to pull the engine, but one thing to keep in mind is that you've probably got a 40psi oil pump in that engine. If you're after more performance and are going to be driving the car to take advantage of it, upgrading that oil pump should be high on your priority list before you're forced to rebuild the engine. Food for thought.

[b-man]

Plenty of compression increasing/head swapping advice in your previous thread:

https://forums.maxperformanceinc.com...d.php?t=868214

[Stan Weiss]

Originally Posted by KJ223

Hi everyone, I’ve been reading several different forums about increasing compression ratio, but they are mostly all about Pontiac 400s. I have a 75 Pontiac 350 and as far as i know it’s the original engine and never rebuilt. It runs great and I put a summit 2800 in it a couple months ago. I’ve been looking into how to get more power out of this set up and I’ve heard about increasing compression. From what I understand my motor has around 7.5:1 compression ratio. What can I do with these 6X-4 heads to increase power? Do they need milled, and if so how much can be taken off? I also want to put headers on, right now I have a pypes dual exhaust and stock manifolds since I can’t remove the manifold bolts without them snapping off in the head, so if I have to pull the head for milling and headers I might as well do them at the same time. I also don’t want to have to pull the engine out of the car and change anything else as I don’t have an engine stand or crane, and I also don’t want to rebuild the motor if it’s not necessary. Thanks

What ICL did you degree the Summit 2800 in at? Have you done a cranking compression test? If so what numbers are you seeing?

Stan

[KJ223]

Originally Posted by Stan Weiss

What ICL did you degree the Summit 2800 in at? Have you done a cranking compression test? If so what numbers are you seeing?

Stan

I’m not sure what ICL stands for, I used a new stock style timing set, so I didn’t degree the cam other than what stock was. I’ll have to do a compression test to see what it says.

[KJ223]

I’ll read through my previous thread again, but I do have the 60psi pump, I know this because when the engines cold the psi will hit 60-62 according to the stock gauge, and not climb any higher. Unless that’s not how you determine what psi your oil pump is, I don’t know how else I would check that

[steve25]

Was your motor originally a 2 bbl 350?

1967 and up small valve heads are 1.96” intake valves, not 1.94”

[grivera]

Originally Posted by KJ223

I’ll read through my previous thread again, but I do have the 60psi pump, I know this because when the engines cold the psi will hit 60-62 according to the stock gauge, and not climb any higher. Unless that’s not how you determine what psi your oil pump is, I don’t know how else I would check that

When the engine is hot, what psi are you seeing at ~3000 rpm? Assuming the engine isn't worn out, should be close to 60

[KJ223]

Originally Posted by grivera

When the engine is hot, what psi are you seeing at ~3000 rpm? Assuming the engine isn't worn out, should be close to 60

When hot it’s right below 60 probably in the 50-60 range, hot idle psi is ~25. I use valvoline vr1 10w-30

[JLMounce]

Originally Posted by steve25

Was your motor originally a 2 bbl 350?

1967 and up small valve heads are 1.96” intake valves, not 1.94”

Thank you for the clarification

Originally Posted by KJ223

When hot it’s right below 60 probably in the 50-60 range, hot idle psi is ~25. I use valvoline vr1 10w-30

definitely sounds like a 60psi pump. Also does sound like the engine is in decent shape.

[bdk1976]

Why do you want to put headers on a fairly stock P350? Are you a masochist?

[steve25]

Maybe he’s looking for the extra 15 HP he will have at full throttle and the extra fuel mileage and no cost power at times when his foot is not on the floor.

[gtospieg]

I've never heard of a set of 48's having 1.94 or 1.96 valves(unless custom). I was under the impression they all have 2.11/1.77 stock.

[steve25]

Yes, all of them where born with 2.11” intake valves.

[gtospieg]

Thanks Steve I was 99.9% sure that was the case....

[grivera]

Originally Posted by KJ223

When hot it’s right below 60 probably in the 50-60 range, hot idle psi is ~25. I use valvoline vr1 10w-30

Thanks for the info!

[78w72]

Originally Posted by KJ223

Hi everyone, I’ve been reading several different forums about increasing compression ratio, but they are mostly all about Pontiac 400s. I have a 75 Pontiac 350 and as far as i know it’s the original engine and never rebuilt. It runs great and I put a summit 2800 in it a couple months ago. I’ve been looking into how to get more power out of this set up and I’ve heard about increasing compression. From what I understand my motor has around 7.5:1 compression ratio. What can I do with these 6X-4 heads to increase power? Do they need milled, and if so how much can be taken off? I also want to put headers on, right now I have a pypes dual exhaust and stock manifolds since I can’t remove the manifold bolts without them snapping off in the head, so if I have to pull the head for milling and headers I might as well do them at the same time. I also don’t want to have to pull the engine out of the car and change anything else as I don’t have an engine stand or crane, and I also don’t want to rebuild the motor if it’s not necessary. Thanks

You can definately increase the comp some by milling the heads, 6x-4 heads will have about 6cc reduction with a .030 milling, ive went close to .060 on 6x4 heads before with no problems bolting to an intake, but you can also take the same amount off the intake surface if thats a problem, most I had to do was elongate the bolt holes on the intake a little if they didnt start easily. This chart shows how much milling is needed to reduce CC's. http://www.wallaceracing.com/headmill.htm

You can add headers to a mild built street engine for a little extra power & exhaust sound... they are usually easy to install if you follow the instructions most headers come with, jack the car up as high as possible & install from underneath with a helper up top to guide them in. The key to leak free headers is using bolts that will lock, lock washers or header bolts with serrations under the heads to help keep them tight... & most of all re-torque after a few heat cycles & check again once or twice a season. I have headers in use for 15+ years using the supplied paper gaskets that came with the hedman headers that are still leak free.

[KJ223]

Originally Posted by 78w72

You can definately increase the comp some by milling the heads, 6x-4 heads will have about 6cc reduction with a .030 milling, ive went close to .060 on 6x4 heads before with no problems bolting to an intake, but you can also take the same amount off the intake surface if thats a problem, most I had to do was elongate the bolt holes on the intake a little if they didnt start easily. This chart shows how much milling is needed to reduce CC's. http://www.wallaceracing.com/headmill.htm

You can add headers to a mild built street engine for a little extra power & exhaust sound... they are usually easy to install if you follow the instructions most headers come with, jack the car up as high as possible & install from underneath with a helper up top to guide them in. The key to leak free headers is using bolts that will lock, lock washers or header bolts with serrations under the heads to help keep them tight... & most of all re-torque after a few heat cycles & check again once or twice a season. I have headers in use for 15+ years using the supplied paper gaskets that came with the hedman headers that are still leak free.

What would having them milled .030 do? I looked at the chart and I see it’ll reduce the combustion chamber but what does that do to the compression ratio, like how much does it increase by? I’m still trying to learn all this. I’m at 7.6:1 right now based on other posts and previous replies, what would it change to 8:1, 9:1?

[b-man]

Milling .030 might get you up to 8:1 at most.

For any real increase in compression ratio a head swap is in order, no way around that.

[steve25]

With 6X castings for every CC you want to reduce the chamber size by to increase compression you will need to mill .006” off of the deck surface of the head.

For every 5 CCs you mill off the head you will gain a 1/4 point in compression.