Compression Ratio question

[Spraynlog]

I'm running E85 now, so I could use a bit higher compression.

If I put a 782 'swirl' head on an '85 block, with the stock '85 pistons, what would the compression be? I have the parts on hand to do this, and the beefier '85 press pin rods would give me some peace of mind over the LW rods.

[turbovanman²]

Probably 9:1, the G-head generally lowers compression 1/2 a point when using swirl pistons so its safe to say, that you'd be around 8.8-9:1.

[Shadow]

Did that on a Shelby Charger about 10 years ago. REALLY woke that little 2.2 up!

[Spraynlog]

Did that on a Shelby Charger about 10 years ago. REALLY woke that little 2.2 up!

Very interesting! Thanks!

[Vigo]

hmm id like to know what kind of boost you were running at the time and any downsides you saw, shadow?? ive got an old 85 ghead motor and a bunch of 782s laying around i could use...

[Shadow]

hmm id like to know what kind of boost you were running at the time and any downsides you saw, shadow?? ive got an old 85 ghead motor and a bunch of 782s laying around i could use...

Everything I used was stock, untouched (as in NO porting or mods of any kind). I took a good running SC that I had and (at that time) I loved to swap the turbo roller cam in and get rid of the slider. I can tell you a lot of stories about how everyone I talked to said I would go slower!lol The roller mod always worked well. The car would instantly run better and at 14 psi with stock Garrett turbo I would gain a little better power everywhere! This perticular time I needed to change the HG and I had a low milliage 782 from an 89 T2 Daytona. At first I was just going to rob the cam, but when I got the head off I started thinking of the higher compression and how it might work. Long story short, I bolted it all together with an MP HG and the car was MAD! (at the time anyways) I was a little worried about the T1 bottom end at the time, but the car never had a problem in the 2 or so years I drove it like that. I remember seeing it still driving 4-5 years later but there's no way to tell if it was the same mtr. I ran 14-15psi on premium fuel, but I may have had an MP upgrade PCM as well (not sure) As that was also one of the first things I would mod on a T1 5-spd. So, no down side as long as you run enough octain!

[Spraynlog]

Everything I used was stock, untouched (as in NO porting or mods of any kind). I took a good running SC that I had and (at that time) I loved to swap the turbo roller cam in and get rid of the slider. I can tell you a lot of stories about how everyone I talked to said I would go slower!lol The roller mod always worked well. The car would instantly run better and at 14 psi with stock Garrett turbo I would gain a little better power everywhere! This perticular time I needed to change the HG and I had a low milliage 782 from an 89 T2 Daytona. At first I was just going to rob the cam, but when I got the head off I started thinking of the higher compression and how it might work. Long story short, I bolted it all together with an MP HG and the car was MAD! (at the time anyways) I was a little worried about the T1 bottom end at the time, but the car never had a problem in the 2 or so years I drove it like that. I remember seeing it still driving 4-5 years later but there's no way to tell if it was the same mtr. I ran 14-15psi on premium fuel, but I may have had an MP upgrade PCM as well (not sure) As that was also one of the first things I would mod on a T1 5-spd. So, no down side as long as you run enough octain!

Thanks for the description, I was going to ask if there were any valve to piston worries, but it sounds like it was OK.

[Shadow]

Thanks for the description, I was going to ask if there were any valve to piston worries, but it sounds like it was OK.

I did this one other time, with an 88 T2 Daytona. Replaced the bottom end with a good running 85 T1. Same effect, 14psi and everything else stock. I remember driving that Daytona around and telling ppl that I was sure it would go 14.7 no problem. A friend of mine had one of the first gtec's around our area and I decided to give it a try. He figured 15.3 on street tires at best. So with him (200lb) in the pass seat I let'r rip! 14.98@99mph, he couldn't believe it, cause we both knew, he gets out of the car and that's 2 tenths for sure! That was the begining of accuracy for the'ol but dyno, been deadly accurate ever since! (o ya, I was even more concerned for that bottom end, because it was in a heavier car making more power. But no problems with it either!)

[Pat]

Not the same swap, but we ran a Shadow with an N/A bottom end (which is what, 9.0 to 1?) and a T-I mitsu swirl head top end for about 100k miles. Granted it was only running about 10 psi, but it was non intercooled, ran on 92 octane and it never gave us any trouble at all. Intercooled and timed right, i'm sure it would have been ok at 13-14 psi.

Particularly if you're going to run E85, I would think the added compression would be no problem.

[Spraynlog]

Not the same swap, but we ran a Shadow with an N/A bottom end (which is what, 9.0 to 1?) and a T-I mitsu swirl head top end for about 100k miles. Granted it was only running about 10 psi, but it was non intercooled, ran on 92 octane and it never gave us any trouble at all. Intercooled and timed right, i'm sure it would have been ok at 13-14 psi.

Particularly if you're going to run E85, I would think the added compression would be no problem.

Good info! Thanks Pat.

[jl93sundance]

Not the same swap, but we ran a Shadow with an N/A bottom end (which is what, 9.0 to 1?) and a T-I mitsu swirl head top end for about 100k miles. Granted it was only running about 10 psi, but it was non intercooled, ran on 92 octane and it never gave us any trouble at all. Intercooled and timed right, i'm sure it would have been ok at 13-14 psi.

Particularly if you're going to run E85, I would think the added compression would be no problem.

Thats crazy!!! How did it last that long?

[Vigo]

10 psi is practically stock. the difference in compression really isnt all that big, and 10 psi leaves some wiggle room.

im actually planning on doing it with my van.. but it will be garrett so intake charge should be even cooler.

[Pat]

Thats crazy!!! How did it last that long?

When we finally parted the car out, it was actually still running fine. It had about 200k on the body, rust, brake issues, had taken two hits in the front end and the trans was acting a bit funny. It's last ride up the driveway was a monster burnout before I took it apart!

[4 l-bodies]

I come up with 8.66:1 assuming standard bore, std. deck height, uncut head (doubtful) and .068 gasket thickness. Probably closer to 8.8-9.0 if the heads been resurfaced a couple times. You should be fine.
Todd