I know there's a few rebuilt 2.8's with an increased compression ratio out there... just wondering how you guys did it? Did you shave the block? Use a thinner head gasket? Any problems aligning the intake manifold?

Anyone remember the compression of a Gen II 2.8l (FWD, aluminum heads)?

 

I know on these 3.4l engines, they use the dohc piston which is a doamed top.

I belive shaving the bottom of the head would be easiest correct?

 

Mill the head.

Boring your black will also up cr.

Domed pistons.

Thinner head gasket.

 

if you do raise the compression, don't go up a lot. my car spark knocks quite a bit on pump gas and i've got about 10.75:1 compression. although...a couple of gallons of 114 with a tank of 93 seems to work pretty good.

 

Hey, another central florida-er! What's your setup?

 

Only mill the heads just enough to straighten the deck surface. They should not be milled for CR. The use of pistons and head gaskets should determine CR only. Milling the head too much will produce a mismatch on intake alignment AND the power manual claims that it can cause "reduced head gasket clamping and lead to premature gasket failure"

Very minimum valve-to-piston clearance is .045" / But need to be increased in higher revving engines. General accepted minimum clearance is .100".

The best head gasket to use on a cast iron block for endurance racing is GM part# 14090561. .035" thick and block should be machined for .041" stainless steel o-rings used in conjunction with the gasket.

 

hey Doward, i had never noticed you were near here. i'm in tampa. you ever go to the dragstrip at lakeland? i got a 91 RS with a built 3.1. let me know if you want to try to meet up sometime.

 

Ocala here... I run at G'ville... but maybe sometime before Fall semester, I'll run to Lakeland!

 

That's why I wanted to know how people have raised this up "in the real world"! I don't want any intake manifold problems, or blown head gaskets. I'd like to see a small gain; 9.5 would be nice, but I don't want to rebuild an engine and have it only last 5,000 miles after my original has lasted 249,555.5 miles. (Saw that one turn last night... simple things for simple minds, ya know?) Thanks for the info on that head gasket; the "I want to turbo" guys might like that one. I won't spend that much coin to o-ring my block; it's just a v6, after all.

Boring the block raises the compression ratio? I didn't know that would happen...

So, AM91Camaro_RS, you told me your compression ratio, but how was it done?

 

Think about it, TomP... you're now pulling that extra .030" (or whatever bore you do) of air/fuel, yet compressing it into the same cylinder head. Increases cr!

 

when my motor was rebuilt i had the heads milled to gain compression. i have 9:20-1 comp with my set up. its not a big gain but better than the 8 somethin it was runnin stock. my block is bored .040 over but i never knew that increased the comp ratio?. i also have the HO size valves in my heads. that just increases air flow.

 

TomP:

My FWD 2.8l V6 was rebuilt 2 years ago:

Compression was from 125 PSI to 242 PSI per cylinder.

The block was machined and the dished stock pistons were replaced for simple flat pistons (no milled). Now, valve adjustment is only possible until 0 lash + 3/8 turn. With 0 + 1/2 turn + heat valves knocks pistons. The other problem was audible knock sound. The stupid mechanic said me: The car (DIS) needs retard ignition timing modifying the PROM, but, sorry, your car is not programmable, then I called to GM Chile, and an engineer said me the same: "You car is not programmable". Finnaly, I looked at internet and I arrived to the Traxion Tech. article in this forum and my car WAS programmable.

The knocks for too many time turned the valve nuts and it produced bad valve adjustment.

 

v6camaroman, did they write down on your receipt how much they milled off the heads? I wonder if they had to mill the intake manifold down, too.

Denis, 242 PSI? Was that an error? Wow! I didn't think you could use flat top pistons at all in the FWD Gen II 2.8's; cool.

Doward, I never thought about it that way! I dug up this message from ws6transam- but I don't know all the figures to be able to plug the #'s in. https://www.thirdgen.org/techbb2/sho...threadid=20216

 

Yes Tom, you read O.K.: 242 !!! PSI. I did the tests and I saw the compression tester and the flat 0.020 pistons with my own eyes!!! (the same compression tester read 155 in a 2.8 Corsica with 100.000 miles no rebuilt. I think with original dished piston).
I used FELPRO head gaskets without sealant.

I've repeat the test a lot of times: max reading was 248 in one cylinder. (engine cold at normal environment temperature).

 

Like Doward was saying, compression is a RATIO. It is the bottom dead center volume of the cylinder divided by the top dead center volume of the cylinder. The only two ways to increase the compression ratio are to either decrease the denominator (make the chamber smaller, domed pistons, thinner gaskets, etc...) or to make the numerator larger (increase the total volume of the cylinder, by boring and stroking).

Don't be fooled, though, a .030 inch overbore alone won't send your compression very much higher at all. That is why most people never notice. By my rough calculations, a .030" overbore will increase the static compression on your 2.8 liter engine from 8.9:1 compression to 9.0:1 compression. It will be slightly more on the 3.1 liter, since it has a longer stroke, but still not too noticeable.

 

Right; that's why I wanted to know from our little group how the few that increased the compression ratio did it! Like I said, I'd like 9.5; enough to make a difference, but not enough to require extensive engine tuning. The only bore I wanted done to the block would be to clean up the holes; I'm not going to ask the guy to try and go 60 over!

Or is 8.9:1 enough? From looking at the tech data on the site, all the later TPI models are 9.3:1. The 3.1 is actually 8.5:1! Is it even worth it to increase the CR? I don't think I'd have any problems lighting the a/f mix up with my MSD setup...

 

I have a 3.1 liter, and the documentation I got with my car said it was 8.9:1 compression. The only motor listed for 1992 that was 8.5:1 compression was the 305 TBI, if I remember correctly. All the Tuned Port engines were over 9:1.

I bet that with our iron heads and old style combustion chambers, the thing would need at least 89 octane fuel if you went much over 9:1 comp. I would think that taking about .030 inch off the deck would give you the compression that you are looking for. Actually, have them take about .010 off the heads, just to clean them up and make sure they are straight. Then have them take about .020 inch off the block to clean it up too. After that, use a thinner head gasket, and you should be right where you want to be. I wouldn't add domed pistons unless you want to be mixing your fuel from now on.

 

I guess they must've typed the numbers in wrong at https://www.thirdgen.org/newdesign/tech/techdb.shtml ... I thought the 3.1 compression was higher then 8.5:1, also. They show the LO3 TBI as having 9.3:1 compression. Actually, that chart shows ALL v8's starting in 87 up to 92 as having 9.3:1 compression. That doesn't make sense...

Running 89 octane's no problem; I run 93. (Yes I know we just need 87. ) It'd be interesting to scavenge a set of the 2.8 Gen II heads & pistons, and test-fit them into a Gen I block. Think a junkyard would let me do that?

 

I'll confirm that all L03s ran 9.3:1, but that figure for the 3.1 does sound a tad off..

 

the way that i have about 10.75:1 compression is... I was building a new motor and was going to use flat top pistons. we went to put the rods and pistons in the motor and noticed that we had about .100" pop up. we knew that would not work so, we did some custom work. we did alot of measuring on the pistons and decided we would put the pistons in the lathe and cut a ring off the outter edge of the pistons (to clear the heads). so, we ended up with, in a sense, domed pistons. the block is also bored +.030". here's a picture...

 

if you do end up milling the heads/block for cr, also mill the intake manifold the same amount. that's what keeps everything aligned.

fwd pistons, & the 3.1 pistons were dished to clear the valves.

interesting technique, milling the outside of the pistons. there have been engines w/positive deck clearance. increases the CR, but the pistons did have valve releafs in them.

 

Mill too much, tho, and you WILL NEED NEW PUSHRODS.

 

My best thoughts would be to have your cobustion chamber in the head welded and shrunk to a smaller cc. This could be done quite easliy with welds in the right places and some minor grinding and polishing- then simply re-cc the heads all to match the same size.
Or if you deside you want to drop the pistons out, then buy 12.5:1 forged pistons and have them milled down to desired CR.

I wouldn't mess with milling the head deck- everything I have read says to stay away from that route with these motors- I tend to trust the GM engineers on this one.

Boring a block will not always effect CR- it is determined by the size of the head CC volume and shape. If the cumbstion chamber of the head is just about as large as the cylinder bore, then thae added width of bore will only compress the same at the top of the stroke.

 

What if you installed the High compression pistons (part# 14044833) then went with thicker head gaskets to bring the compression ratio down to 9.5?