Which are best? Stock, High or Low compression pistons?

[inspector-111]

O.K., I'm about to order an engine kit (still in the very slow rebuild process), and I need to know if I should stick with stock, use higher or use lower compresion pistons. Is there any advantage to changing this if you are going to run about 15psi? (for now, hopefully more later when I have the means to tune way down the road. . .)
This is my DD and will need to be reliable. I have polished the rods (too poor to buy upgrades) and so far have SS valves with stock springs and cams.

Thanks for any help!

Chris.

 

[Locke]

The stock 1g pistons CR is 7.8:1, that is fairly low to begin with, especially if you are only going to be seeing 15psi. Higher CR will yield more power, but you will be more prone to knock. 2g CR is 8.5:1, they run 15psi all day with few issues.

For safety and reliability, I would personally stay with somewhere near 7.8:1CR, this would also allow easier tuning in the future if you wanted much more boost.

In the end, it really comes down to you goals and plans with the vehicle.

 

[JayRolla]

I agree with Locke. 7.8:1 is great for the street and allows you to run a good amount of boost on pump gas. I wouldnt go above 8.8:1. I run 8.5:1 and I like it.

 

[inspector-111]

I agree with Locke. 7.8:1 is great for the street and allows you to run a good amount of boost on pump gas. I wouldnt go above 8.8:1. I run 8.5:1 and I like it.

How much boost are you running? Can I get away with 91 octane or will I need to run 94? That being with 8.5:1CR @ 15psi.
Is there that much of a gain to go to 8.5:1 from stock CR? If there is, I will definitely get those pistons instead!
If not, I may just have to put the boost to it down the road. . . . way down the road . . . .$$$$$ . . .

Just a note, I also still have the stock injectors, hope that this won't cause any problems.

 

[inspector-111]

Has anyone else out there run 8.5:1CR on a stock engine? (Up to 15psi)

Can you run 20psi (possible future plans) with an 8.5:1CR?

 

[LeroyBKR]

yea you can run 20psi with 8.5:1 with a good tune of course.. Might want to invest on some arp headstuds if you want to up the boost some more..

 

[Locutus78]

I have 10:1 2.3 stroker and run 26psi on pump. Tuning window is a bit smaller, but not as small as people told me when I first started out. Definate a lot of fun!

 

[JayRolla]

Yea you will be fine with 8.5:1. Its the sweet spot on our motors. I cant run shit for boost but thats because I have fuel delivery problems and I think my stock SMIC likes to heat soak even at 14psi.

 

[brute]

The higher compression ratio will produce results so minute that there is no way you will notice them. It will make tuning different. The 7.8:1 stock 1g compression provides a lot of, how should I say, slop room. Your tune does not have to be dead on. With higher compression, like 9:1, or even Locutus's 10:1, you are going to need to do a lot of tuning to get it perfect.

Higher compression pistons decrease the amount of boost you can on stock timing maps. The 1g's have a pretty aggressive timing map, and with higher compression pistons, you have to back down the boost to stay away from detonation. Basically, any compression can work, but it just depends on what tuning methods you are going to use, how much money you want to spend, how many times you want to redo everything, and how much time you have.

 

[nightspeed87]

The higher compression ratio will produce results so minute that there is no way you will notice them. It will make tuning different. The 7.8:1 stock 1g compression provides a lot of, how should I say, slop room. Your tune does not have to be dead on. With higher compression, like 9:1, or even Locutus's 10:1, you are going to need to do a lot of tuning to get it perfect.

Higher compression pistons decrease the amount of boost you can on stock timing maps. The 1g's have a pretty aggressive timing map, and with higher compression pistons, you have to back down the boost to stay away from detonation. Basically, any compression can work, but it just depends on what tuning methods you are going to use, how much money you want to spend, how many times you want to redo everything, and how much time you have.

The guy above me has the most accurate post about this so far.
In my opinion stay with the stock compression ratio or find something in between the 2g 8:5.1 and the 1g pistons,
like the 8:3 pistons for example. The issue is your stock timing maps will give you knock just for running the higher compression. If you dont tune your timing maps, adding compression may only hurt you overall.
Id stay with the stock ratio personally because ive played with 2g pistons before. learning that compression yields the least power in what your trying to do. Id rather lower compression with more aggressive timing and more boost, than higher compression with less aggressive timing and boost.
And you cant run high boost, compression think of it like you had to pick two and lower the other one on a pump gas setup.
The only time this exception doesnt work is like on race gas or meth injection or when nitrous is added it changes the variables outcome a little. On a race gas only motor id run 10:1 compression with 20 something degs timing and 30psi etc.

This may not be true in all scenarios generally, you gain the most power from raising boost, then 2nd most power from air fuel ratios and timing, and then the least power gain comes from compression ( In a turbo car that is )...

 

[inspector-111]

^^Thank you guys, this has cleared things up for me quite a bit. I am going to stick to the stock CR on this build, I have no means of tuning and do not want to go too far without the proper tools.

 

[DSMReviver]

8:5 is the best and all ,but what about someting in between 10.1 and 8:5 like say 9:1. how much power and boost can you run. Just carious.

 

[silver bullit]

8:5 is the best and all ,but what about someting in between 10.1 and 8:5 like say 9:1. how much power and boost can you run. Just carious.

Spelling?

There are to many factors to factor in.

7.8-8.1-8.3-8.5 Is what i would recommend, as it allows room for error in tuning.

9.1-9.5-10.1 is for huge turbos, Great tuning/stand alone management. Lots of money on tuning...

If i was to build a motor i would use 8.5 or 9.1, on a 2.4 block. Maybe destroke it to a 2.3

 

[b00sted]

Bump it up to 11.5:1, bolt an 88mm turbo on it, and run it on Methanol.

 

[silver bullit]

Bump it up to 11.5:1, bolt an 88mm turbo on it, and run it on Methanol.

Yep...

Live like Vin diesel off the fast and furious, (I live my life a quarter of a mile at a time, for that 10 seconds or less i am free)

Great, just in time for the next fast and the furious.

Its only good if you run your car drag only

 

[b00sted]

Yep...

Live like Vin diesel off the fast and furious, (I live my life a quarter of a mile at a time, for that 10 seconds or less i am free)

Great, just in time for the next fast and the furious.

Its only good if you run your car drag only

It was obviously a joke.

 

[haioku-rocket]

I run 9:1. Couldn't run above 20-21 psi on pump without severe knock. Got a methanol injection kit now tuned car runs 26 psi with 4-7 counts of knock. ;-)

 

[rarson]

If i was to build a motor i would use 8.5 or 9.1, on a 2.4 block. Maybe destroke it to a 2.3

Huh? How do you destroke a 2.4 to a 2.3? Offset grind the crank to 98mm instead of 100mm?

I would probably run somewhere around 8.7-9:1. With meth injection. 7.8:1 is too low in my opinion, and if you're having issues with knock at something simple like 8.5:1, then you're probably running too much timing, or too much boost, or both, for the gas that is in the tank. I tend to not be able to run more than about 20 psi on pump, on the stock 8.5:1 pistons, but that's what I expect because the small 16g is heating the air up a lot just trying to keep up with the engine.

I should just buy a new turbo already.

 

[meliketoball]

I learned something new today.

 

[Turbo Talon DL]

9:1 compression is fine. You will get about 31 mpg on the freeway with mods also. I run 13psi with cams and evoIII 16g right now knock free. Lets put it this way, i get more part throttle knock than under boost.

 

[TunaTalon]

In this performance forum it took 20 posts for someone to mention gas mileage, even with gas at $4.00/gal.

Yes Turbo_Talon_DL is right, higher compression ratio brings better gas mileage along with better off boost torque.

Thermal efficiency is a strong function of compression ratio. The higher the compression ratio, the more energy is extracted from the combustion cycle. That's what makes the diesel engine more efficient that the gasoline engine.

 

[rodent]

I'm thinking of going with 10:1 with a built 6 bolt and large turbo. I also live at 5800ft and we can usually get away with it up here. Few other guys are running 10:1 with great success.

 

[eski]

9:1 compression is fine. You will get about 31 mpg on the freeway with mods also. I run 13psi with cams and evoIII 16g right now knock free. Lets put it this way, i get more part throttle knock than under boost.

now that really interests me. say you were to go with a stock car and raise the compression to 9.1, how much of a performance gain would there be?

 

[Turbo Talon DL]

now that really interests me. say you were to go with a stock car and raise the compression to 9.1, how much of a performance gain would there be?

I couldnt tell you. I jumped straight from the 4g37 to the 4g63 turbo with most of my current mods now. I figure it wouldnt be as sluggish off boost and you would get better gas mileage. Im not sure about knock because these things run rich from the factory anyhow. I have no EGR or evap canister either.

 

[TunaTalon]

now that really interests me. say you were to go with a stock car and raise the compression to 9.1, how much of a performance gain would there be?

The math and a good write up of the effect of static compression ratio is available at Engine Compression Ratio - Tech - Popular Hot Rodding Magazine.

The article is for big iron hot rods but the physics is the same for the DSM.