425 compression question

[Bugzaper]

What is the best way to up the compression ratio on a 77 425.


1. Pistons
2. Shaved head
3. Both

Thanks!

[curtis73]

Not sure how far you can go without piston-valve clearance, but shaving the head is probably the simplest way. New pistons means a complete teardown, honing, and rebuilding.

You'll have to upgrade from the stock valvetrain. Not only are they wimpy rockers, shaving the head changes the geometry. You'll need shorter pushrods too.

[PeteR]

I calculated the compression ratio on my stock
77' 425, and the result was 9.34:1 :shock:
I assumed heads were 96cc, pistons were flat and
head gasket was 1mm thick.

Weird, because GM specs says 8.5:1

[cadillac512]

I wonder if the 425 heads are really 96cc....I have heard they may be closer to 106cc. Has anyone here ever cc'd one? It would be good info to have. Thanks for bringing this subject up-there are a lot of 425's out there and no reason they can't be put to use!
Terry

[Bugzaper]

I calculated the compression ratio on my stock
77' 425, and the result was 9.34:1 :shock:
I assumed heads were 96cc, pistons were flat and
head gasket was 1mm thick.

Weird, because GM specs says 8.5:1

You know thats interesting as I am only able to buy around 89 octane where I live and it pings on occassion. When I add octane booster the pinging goes away. Maybe it's a timing issue but I think the compression is higher than advertised.

I also added a 1" birchwood spacer under the carb and went to a higher lift hanger in the carb and it "Really" woke the old beast up! I think these 425's have more potentional than some people think.
Thanks!

[STDog]

I calculated the compression ratio on my stock
77' 425, and the result was 9.34:1 :shock:
I assumed heads were 96cc, pistons were flat and
head gasket was 1mm thick.

You assumptiions are probably wrong.
For example I doubt the pistons are flat.

[cadillac512]

STD..
The pistons in a 425 are indeed flat with no reliefs and the head gasket is close enough to 1mm thick (approx .040")to give accurate calculations.
Do you have accurate information that will help us here? :?:
Terry

[cadzu]

as cad 512 said the pistons are def flat.as i also verified this on a 78 425 engine we have.

[Bugzaper]

I pulled the pan off this afternoon to replace the rear main seal and replace the stock timing gears with a set of Cloys timing gears. I pulled number one piston to check the rings and sure enough the pistons are "Flat Tops".
If I was to have .10ths shaved off the head (I think that would be a safe amount) or maybe went to a steel .30ths head gasket I think this would put me where I want to be without problems.
As to the valve train, I think there may be enough adjustment to allow for this change.


That Cloys timing set is sure a step in the right direction, I had at least a 1/2 play in my old timing set and this engine only has 78,000 miles!

[cadillac512]

Bugzaper-
My opinion only here,so just take it as that...... I think you would be better served by replacing the cam/lifters with a mild performance grind than milling the heads and trying a different gasket.Power comes from filling the cylinders with a greater amount of air/fuel mix and the stock cam is not a good choice,even in a stock application. A good cam with carb and distributor tuning will get you far more power than a bit more compression will,and won't increase the octane requirements of your engine. I'd consider swapping on an Edelbrock intake or a stock dual plane from a 472/500 as well.I think the results may be surprising. :wink:
Terry

[dave_brode]

Bugzaper,

The valve length on 425 valves is same as 120cc heads', so the valve heads are .250" or so further away from the piston, compared to engines with 76cc heads and flat tops. So, valve to piston clearance will not be an issue, even if you chop the heads way down. There have been several threads on the subject of 425 heads at CB7 in the past. Iirc, experts claim that they are 106-107cc, not 96cc, as published elsewhere. Same expertsd claim that the 425 heads' deck isn't as thick as 472/500 heads, and should not be chopped more than .050" or so. The subject of 76cc heads on a flat top 425 has also been discussed. Iirc, with moderate cam TDC lift, it is possible that it could be done w/o piston to valve clearance issues.


"]What is the best way to up the compression ratio on a 77 425.


1. Pistons
2. Shaved head
3. Both

Thanks![/quote]

[Al from MTS]

Had to comment on the head cc's. To me the cc sizes we have quoted for heads has always been an average number.
All Cad heads will cc at higher numbers than they are refered to. The numbers MTS has always quoted are what our finished heads come out to. Give or take a couple cc.
Been my experience that stock stuff is 6-8cc larger than the head is commonly refered to. After a new set of valves and a resurface, 76, 96, 120 are close finished numbers for chamber sizes of a fully reconditioned head.