Piston Swap

CDeutsch · 03-23-2005

Here's the plan I've been kicking around to lower the compression ratio on my 2.4L. I want to know how bad of an idea this is (desperation is good at blurring my logic).

I have the exact specs of the pistons in my car from Ross. If I keep everything else the same and increase the dish from 13cc to around 32cc it should lower my compression to where I want it.

The sketchy part is doing the swap. How cheap can I get? Since the pistons will be the same diameter can I just transfer the rings and spacers from the old pistons to the new? If I do that would it be better or worse to hone the block?

Is doing this just asking for trouble? The rotating assembly was originally balanced but this will probably throw that off.

**JET** · 03-23-2005

How far are you going to lower your compression? Do you think it will be worth it? It should help HP, but lose low end TQ and spool up.

CDeutsch · 03-24-2005

Quote:

Originally Posted by JET

How far are you going to lower your compression? Do you think it will be worth it? It should help HP, but lose low end TQ and spool up.

I'll probably try lower it to 8:1 to 8.5:1. The crank case pressure makes the car undrivable with stock cams and tuning has always been really difficult, so it's either this or part the car out.

Jakey · 03-23-2005

What kind of a weight difference are you looking at between the different piston models?

Raptor · 03-24-2005

As long as the pistons are matched to each other, it shouldn't effect the balance drastically. They basically will cancel each other out for the most part. You can do what you are suggesting with the rings as long as you keep them in exactly the same bore they came out of. You shouldn't need to hone anything. Not something I would normally tell people, but honestly there should be no reason you will encounter trouble as long as care is taken just as in original assembly.

**1ViciousGSX** · 03-24-2005

Where is it at now and where are trying to get it down to?

CDeutsch · 03-24-2005

Quote:

Originally Posted by 1ViciousGSX

Where is it at now and where are trying to get it down to?

It's between 9.5:1 and 10:1.

A compression test with stock cams came out with 225-230 across the board.

http://www.rosspistons.com/calculator.php

Here's the numbers I used.
Bore: 3.426
Stoke: 3.937
Head: 46
Gasket: .045
*Deck Height: .0001
Dish: -13
Num Cyl: 4

CR=9.9:1

*Deck height needs to be negative since the pistons slightly come out of the bore, but the ROSS calculator doesn't support that.

I'd like to get the static CR down to around 8:1 (8.5:1 max) so I can run more boost on pump gas.

CDeutsch · 03-24-2005

Quote:

Originally Posted by Raptor

As long as the pistons are matched to each other, it shouldn't effect the balance drastically. They basically will cancel each other out for the most part. You can do what you are suggesting with the rings as long as you keep them in exactly the same bore they came out of. You shouldn't need to hone anything. Not something I would normally tell people, but honestly there should be no reason you will encounter trouble as long as care is taken just as in original assembly.

Thanks for the advise Raptor! I appreciate it.

I think I may give it a shot. The heads are already off. The next big question is if I can get the spiral locks removed so I can reuse the rods. I don't think my pistons have the indent for easy removal.

If anyone want a cheap set of high compression pistons for a 4G64 let me know.

**Goat Blower** · 03-25-2005

Depending on what HG you have, a cheap and easy solution would be to switch to the thickest HG you can find, it'd probably only lower your CR to about 9:1 though. And the options for the 2.4 are much more limited than the 2.0.

CDeutsch · 05-10-2005

Well it's done. I started putting things back together Saturday morning and I turned it over for the first time last night. Tonight I'll finish changing the oil and then take it for a test drive. This thing better make me happy or blow up because I'm getting sick of putting money into it.

http://christopher.implex.net/pics/pistonswap/

Keep an eye out for that sweet downpipe modified by QPR.

Raptor · 05-10-2005

Looks good Chris! Should work out just fine. I won't give you shit for numbering you pistons backwards, although it is tempting

Steve, just noticed your other post. Trying to compensate for compression issues with thicker head gaskets is a good way to push your quench area out of acceptable range. You only really have a .020" window for it.

CDeutsch · 05-10-2005

Quote:

Originally Posted by Raptor

Looks good Chris! Should work out just fine. I won't give you shit for numbering you pistons backwards, although it is tempting

That's they uhmm, "European" way of numbering. Yeahh..that's it.

Actually what happened is I labeled the valve cover with the cylinder numbers wrong a while ago. And I keep using those marks when changing spark plugs (I always label and keep my old ones for some reason), etc. Now I have some good incentive to fix those labels; humiliation.

Raptor · 05-10-2005

Hahaha, don't be humiliated, I knew there was a silly reason and that you know which side is really #1. I just don't have the self control to prevent me from not having fun with it anyway. Character flaw on my part. But I can live with it.