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#1
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Increase compression, Pontiac 350 6X-4 heads
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
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#2
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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.
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-Jason 1969 Pontiac Firebird |
The Following User Says Thank You to JLMounce For This Useful Post: | ||
#3
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Plenty of compression increasing/head swapping advice in your previous thread:
https://forums.maxperformanceinc.com...d.php?t=868214
__________________
1964 Tempest Coupe LS3/4L70E/3.42 1964 Le Mans Convertible 421 HO/TH350/2.56 2002 WS6 Convertible LS1/4L60E/3.23 |
#4
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Quote:
Stan
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Stan Weiss/World Wide Enterprises Offering Performance Software Since 1987 http://www.magneticlynx.com/carfor/carfor.htm David Vizard & Stan Weiss' IOP / Flow / Induction Optimization - Cam Selection Software http://www.magneticlynx.com/DV Download FREE 14 Trial IOP / Flow Software http://www.magneticlynx.com/DV/Flow_..._Day_Trial.php Pontiac Pump Gas List http://www.magneticlynx.com/carfor/pont_gas.htm Using PMD Block and Heads List http://www.magneticlynx.com/carfor/pont_pmd.htm |
#5
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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.
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#6
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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
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#7
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Was your motor originally a 2 bbl 350?
1967 and up small valve heads are 1.96” intake valves, not 1.94”
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#8
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Quote:
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Will Rivera '69 Firebird 400/461, 290+ E D-Ports, HR 230/236, 4l80E, 8.5 Rear, 3.55 gears ‘66 Lemans, 455, KRE D-Ports, TH350, 12 bolt 3.90 gears '69 LeMans Vert, 350, #47 heads: work in progress |
#9
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When hot it’s right below 60 probably in the 50-60 range, hot idle psi is ~25. I use valvoline vr1 10w-30
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The Following User Says Thank You to KJ223 For This Useful Post: | ||
#10
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Quote:
definitely sounds like a 60psi pump. Also does sound like the engine is in decent shape.
__________________
-Jason 1969 Pontiac Firebird |
#11
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Why do you want to put headers on a fairly stock P350? Are you a masochist?
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#12
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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.
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#13
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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.
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#14
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Yes, all of them where born with 2.11” intake valves.
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#15
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Thanks Steve I was 99.9% sure that was the case....
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#16
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Thanks for the info!
__________________
Will Rivera '69 Firebird 400/461, 290+ E D-Ports, HR 230/236, 4l80E, 8.5 Rear, 3.55 gears ‘66 Lemans, 455, KRE D-Ports, TH350, 12 bolt 3.90 gears '69 LeMans Vert, 350, #47 heads: work in progress |
#17
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Quote:
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. |
#18
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Quote:
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#19
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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.
__________________
1964 Tempest Coupe LS3/4L70E/3.42 1964 Le Mans Convertible 421 HO/TH350/2.56 2002 WS6 Convertible LS1/4L60E/3.23 |
The Following User Says Thank You to b-man For This Useful Post: | ||
#20
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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.
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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