Compression Ratio w Vortec

[rcazwillis]

Need thoughts and experience.

1974 block with flat tops bored 60 over. Standard stroke. Was going to put the heads (883s I think) that came off it back on it. However, upon inspection valves are stuck in the guides. Don't know the extent yet, but they are 75cc chamber heads. Compression would be in the 8-8.5 based on calculators on line.

I have new vortec heads that will come off another engine that has dished pistons and an oil burning issue and will be replaced by this one. Vortec has 64cc chambers and will give 9.8 compression best I can tell. Research indicates the vortec has a better quench area and can handle 9.8 on 87 octane with correct timing.

I don't want to back timing off to nothing and I don't want to run 91 octane as it is Zane's daily driver. Am I going to get away with this or should I do replace pistons with dish to drop the compression?

Thoughts, experiences?

[KJSR]

I have done this and think the compression ratio is actually 9.5 to 1. Regardless I run 85 in my truck and it has no issues. I can't remember what my total timing is set at but it's certainly not low. Carb is tuned right as well. It's no real issue if you tune the package....

[rcazwillis]

I was guessing on some of the numbers as the engine is torn down. Thanks for the feedback. Hoping others have done the same and will give the same feedback.

[nightrider388]

Also there are thicker head gaskets out there that can bump compression down .5 point or so, if you feel like you are borderline.

[GASoline71]

Make sure the oil burning issue wasn't because of bad guides or seals... or your new engine will be an oil burner too.

Gary

[rcazwillis]

Been down the path. Brand new out of the box vortec heads. Burning about a quart per tank. Thought it was intake as this is a common problem, but replaced intake gaskets with the $50 version that is not supposed to collapse. Existing gaskets were seated properly. Still burning oil. Thinking we have a piston issue.

In addition, tired of listening to that (&*$ electric fuel pump. One should not hear the pump before the truck arrives.

[Hammond68]

My 383 is 10.6 to 1. Heads planed to 63 cc. Pistons .025 in the hole. + 5 cc for valve reliefs. .041 head gaskets.

[BCOWANWHEELS]

new heads on old bottom end isn't a good idea.............

[rcazwillis]

The bottom end was reported to have about 12k miles on it from a relative. Checking bearings indicated it was probably accurate as the plastigage showed .002 gap. So it was not an "old" bottom end. The new heads were due to a mistake I made on one of the original vortec heads (different story, lessons learned).

[HO455]

You truly won't know the compression ratio until you confirm the deck height and cc the heads. Lots of room for variables to put your combo over the line. What are your camshaft specifications? How was the cam installed?
Static compression won't be the problem but dynamic compression will. That is the reason one person will be able to run 10.2:1 and the next one has problems with 8.8:1.
Unfortunately there is no easy way to be sure. Either take the time to measure all the components or assemble the engine and take your chances.

I would suggest reposting this in the engine section as you will have a larger audience.