I talked to metal jim on the phone the other night and doing some thinking about this subject. If the rods and pistons are forged why would you want to run a lower compression. First you will push out more power, and second you wouldn't be losing all your low end torque. Just some thoughts...

The only reason people go down in the compression is that its easier to tune for, lower compression allows for more mess up room
When you up the compression your more likely to detonate and cause motor damage

So most drop the compression to give them some mess up room

But your correct you will not loose any low end power when your not boostin but

If I finally save up the money for turbo, im going with the 10.5:1 compression pistons and tune correctly and safely

You will have approx and 8% increase in power vs. the same setup/same boost levels if the other guy is 8.6:1 compresion

I am a proponant for high compression and turbo. I would not turbo on less than 10.5:1 with a rebuild.

The reason it is done it it is easier, the margin for error with lower compression pistons is greater. More room to be off on your fuel and not mess your engine up. But we have good fuel systems and modern technology is great, Howell has some Neons running 12.5:1 compression with turbo. I think that's a bit much for a street/strip car, but it can be done. It's all in the tuning. There should be a link on the main page to a thread with some good info from Howell and the board on this.

Una salus victus.
He will triumph, who knows when to fight, and when not to fight. - Sun Tzu

Michael J. Kulaga
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http://highlander.dsmpower.com/

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I know the reason why I wouldn't do it, b/c Kansas has 91 oct(pee pee water) for pump gas. Sure the detonation can be fixed by retardation, but then your back to making less power. Not to mention I like driving my car daily, and anybody who has owned a high comp engine knows how hard it is to start in the winter.

Yeah I thought one of the neons (phatridez??)(Cox??)is running 14 points of compression and 30lbs of boost
Its all in the tuning

there are several people around here that have destroyed higher compression pistons while making serious turbocharged power. Kiku broke several 10.5:1 pistons at 9psi, Glacius has broken at least one recently running the same. Greg (992gnt) broke a few 9.5:1 pistons.. the only people who have broken/melted 8.8:1 pistons are Armond, Dave_FL and MWC (PureRage, Ray).. Armond makes 400wHP, and Dave.. well.. doesn't but broke them anyway ;-) MWC melted a few running 20psi on pumpgas while on the dyno with no timing retard.

You didn't mention Stan who ran on 10.5:1 for quite some time before other problems arose in the car.

Una salus victus.
He will triumph, who knows when to fight, and when not to fight. - Sun Tzu

Michael J. Kulaga
e-mail: Michael_97RS@2gntDSM.zzn.com aol IM: MJKulga
http://highlander.dsmpower.com/

All motor in the making.

Injen CAI, P&P Intake Manifold, Crower Stage 2 Cams, Hooker Header, GS-T Muffler, AFX UDP, S-AFC, 8.5mm Magnecor Wires, Crane Fireball HI-6DI2, FU/RU/RL STBs, TSW Blade 17x7.5, Bridgestone Potenza RE730, Front KVR Cross-Drilled Rotors & Carbon Fiber Pads

To Be Installed: P&P Milled Head, Rear Disc Brakes, Rear KVR Cross Drilled Rotors & Carbon Fiber Kevlar Pads, SS Brake Lines, AFX 55mm TB (re-installed once exhaust is done, intake was outflowing exhaust)

One of the main reasons I went with 8.6:1 is because I've heard from a few reliable people that with these pistons and stock cams, it makes the motor a non-interferance motor. Meaning that if the timing belt breaks, no bent valves for me :7

Your all talking like tuning is easy. I have a hard enough time tuning for 8.6:1. There's no way in hell I would want to tune for any higher.


Unless you have hella cash to go spend days at a time on the dyno tuning isn’t really that easy, especially when you are tuning for maximum power but trying to keep daily drivability.

Thats basically why people go lower.

Just my .02cents

Tuning sucks! x( I do miss the 9.5:1 power though. If I put another motor together (likely), I'll go with 9.5:1's again.

jamesman had the 10.5:1 compression till he had a boost spike to 18psi .. not enough tuning etc done... but yah little mistakes make your car go bommm...



Michael 99gs
HRC Stage 2
Stage 3 process begins anyone got a 420a block thats not spun/head for sale?

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wow...but the people who blew the 10.5:1 blew it because they didnt tune correctly right? maybe it would be safe to go with the 9.5s im still considering the 8.5s because of reliablity

A good example is a night like tonight for me. It was like a high temp of 19 deg today and I have my MBC set at 18psi. Well w/ all the cold ass air coming into my CAI and then through the FMIC I was boosting 22-23psi. I didn’t realize it for a second though and if I was running a higher compression that would be the kind of thing that could really hurt.

It's kind of weird when that stuff happens your like wow I seem to be faster and you look at the boost gauge and it’s at 23.

ill be running 10.5:1 i decided and if your boost is electonicly controled and tuned properly via a ems nothing to worry about, ill be running minimal boost on the streets but beable to up it to higher boost if i want, u dont break a piston(forged) for just putting 20psi to it it breaks because of detonation IE improper tuning... if u know your motors read enough elarn enough and play with it tuning becomes easier ive been racing methonal atvs and avgas burning atvs for years(that u have to tune the old fashion way by feel and plug readings) so a car isn't much different other then u get fancy tools to make it perfect...

if u want to u can get 10.5:1 pistons run the tti head gasket(drops compression 1 full point so 9.5:1) till u get a handle on tuning then switch it out witha mls gasket and get your 10.5:1 back....

I suppose I should chime in. I really do appreciate this thread. Its good to see people on the fence about the compression issue. First I'll say - I know that tuning has everything to do with it. I'd talk about my plans, but I think I'll just say - there are a ton of things to consider. The pee pee water in Kansas for instance - can't run to hot on that. Motivation or ability to tune - that's up to the owner. I don't want a slugmobile. A turbo car is N/A until it hits boost right? There are way too many factors to say that it is good or bad to boost with high comp. All I know is that it is possible. With knowledge there is power.

....also, I own Kiku's cams - the ones he used when he blew his pistons. God bless you Vinnie.


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It is all simply about that margin of error. It depends on what you want. For an amateur, and we are ALL amateurs, it is far easier to have a larger window to work with. If you are not looking to make extreme power it is easier to get away with the high compression and the car would feel much livelier around town. The more power you make, the less room for error. That simple.

Honestly, I would love to build a car that runs 20psi on a S20G with 10.5:1 compression on pump gas. That would kick ass. No arguments here. It can be done as well, but do not pretend that anyone can do it. I respect those who can.

For a daily driven street car, I prefer the 8.8:1 compression that allows me to be a bonehead from time to time. If you guys knew some of the things my car has endured it is amazing the engine has over 30k miles on it after my rebuild.

Oh, and the idea of runnind a thick head gasket with forged pistons. I would drop that idea. Even if you did tune it for the lower compression, you would have to start all over again when you changed it.

The change from 9.5:1 to 8.8:1 pistons equaled almost a 10% in my base fuel map. They are quite different. Put the motor together with the compression you want, skip the thick gasket, and tune slowly and in small steps. Don't be surprised if you blow a motor or 2 in the process as you learn. And from a guy who has tuned all kinds of performance/race vehicles from muscle cars to superbikes, all the fancy tools with an electronically controlled car don't make it as simple to tune as a carburetor. If anything, it's harder due to the large number of 'other' things affecting how the engine runs. Sometimes I wish my car had a carb. I'd already be in the 12's.

My pistons are fine (I guess), they were the last time I drove the car anyway. although I had 10.5 compression my pistons were only about 9.8 compression (stock profile with the domes raised .020". keep in mind only about half the stock piston top surface area is an elevated dome and its only elevated .02" as well. the rest is mostly valve relief cut out and in my case over size cut outs for future mods that will never happen). I only bring that up because my pistons are still relatively flat top in profile while actual 10.5:1 and up pistons larger domes could contribute to detonation. Plus I only ran 10psi, not really a challenging task, I think I could have ran that forever that way (hell I planned on up to 15psi). Had the car not developed the mysterious "cut off while driving for no good reason" disease so many have caught I'd probably still be driving it today (too many tunnels and bridges around here to be cutting off even if it does only happen about twice a month).

hehe I'm going 10.5:1 after having crap ass compression and mega turbo lag I ain't goin low, then again i got the pistons and rods cheap used so I coudln't complain either way haha!

I heard you save more gas with higher compression too. I'm not tuning for monster horsepower like everyone else though, I just want a little something to play with like 250HP. Also if your running higher compression the more power your boost gets, like for 9psi@8.6:1 you have X amount of HP and 9psi@10.5:1 you'll have X+more haha. I didn't feel like doing math.

Same here Hades. I think 250-300hp would be great. Here, I'll do some math.

Lets take a 420a making 180hp N/A. One of the calculations flying around here is the gain/loss from a point in compression - it seems to be the opinion of most that this is at 4%. Our 180hp N/A is running on 9.5:1 pistons, so when we change them out to:
8.5:1 - the power would drop 4% to 172hp
10.5:1 - the power would rise 4% to 187hp

Ok, enter the turbo. The power calcs for pressure say that if you run one full atmosphere of pressure(14.7 psi) then your power doubles. 9psi is approximately 60% of one atmosphere, so if you're running:
8.5:1 at 9psi - you get 60% increase in power = 275hp (172 * 1.6)
10.5:1 at 9psi - you get 60% increase in power = 299hp (187 * 1.6)

The actual power numbers(275 and 299) may be higher or lower - the'll be something like the example....BUT the difference in power between 8.5:1 and 10.5:1 should be pretty close to the numbers in this example. It'll be about a 25hp difference boosting at 9psi, and about 15hp difference when your not boosting(or your N/A alltogether).


ride__________95 Eclipse RS : 5 speed
power________Gude cams : Jeep TB : AFX UDP : ram air : test pipe : GST catback
suspention___BOMZ F/R upper strut bars : ES inserts
sound________Kenwood KDC 5000 : Audiovox 8" sub toob - no sub amp(damnit!)
shizzle_______AutoCommand remote start : dual air horns : Caspers o2 simulator(rear)
incomming___Hurricane F/R lower strut ties : Field SFC Hyper-R

You gotta look at it both ways there is negitives and positives one + would be more power and when u run outta stuff to do 25 hp is alot. But if there is a chanse of more probloms? Some one should make a pole on this or sometihng i would really like to no from the top dogs what they think is better.


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