Johny GTO

I see posts about guys changing your pistons. You all are talking about changing your compression ratio by doing this. What is the point of doing this? How does changing the compression ratio affect the power being put out by the motor?
Does it change the amount of air you can jam into the cylinder with FI safely?

Correct me if i am wrong, but I would imagine changing the pistons would allow the super\turbo to push more air into the cylinder at the same boost pressure, thereby making more power safely. Like if at standard 11.1 CR you are getting like 100cfm into the cylinder at 10psi of boost, changing the piston to 9.1 CR would allow lets say 160cfm of air into the cylinder at the same 10psi of boost.

I am making up numbers here, with the emphasis on what is happening. Can someone explain this? i tried searching, and i dont comprendo.

Jake@EPP

Generally speaking, dropping the compression makes less power. The more compression, the more cylinder pressure.

With boost its nice to reduce the static compression so you can run a lower grade fuel for the boost. If you tried 20psi on 11.1cr with 93 octane, you will have a problem. Now for a high octane race-car enviorment then you can use that compression to your advantage.

Josh@MASPORT

iTrader: (3)

haha, i dont think he will completely get that

stevieturbo

Without being cheeky, you need to learn how engines work first, then it will start to make more sense.

Perhaps people can reccomend decent books on the subject ?

One I like, and it is quite general, but covers a lot of topics well, is Julian Edgars 21st Century Performance.

Johny GTO

No, i understood what he said.

My point is, why would you lower the compression on your motor? If you are worrying about detonation, why not change pulleys on your blower and decrease the amount of air you are stuffing into the cylinder?

Josh@MASPORT

iTrader: (3)

cause then u couldnt make any power!

more air and fuel, more power

Project Dc2

iTrader: (4)

because you can run more boost at lower compression. overall itll make more power . but HP per psi wont be as significant.

Jake@EPP

Think of it as cylinder pressure, a motor making high pressure with a supercharger putting low boost will make x amount of pressure then. While a motor with low compression with high boost will make around the same.

For a street car, ultra low compression is less then desired, the lower compression means less power down low ( part throttle), its like driving a 400ci honda, but still with no power .

ChrisUlrich

Actually... if I heard correctly. Don't you make more horsepower per pound of boost in comparison to per point of compression?

Little off topic but figured i'd throw that in there.

Higher the compression the more turbulence going on in the cylinders. You add FI aka Forced Induction you are basically doing just that. Forcing air into the cylinders. So now the turbulence in the cylinders are much greater and the gas is much more prone to detonating before the spark plug goes off due to the extra cylinder pressure. Lower compression to 6.7:1 and running 20psi on pump. **** would be mint!

Johny GTO

ok, well explain this to me. This car here on Speed Inc. Here is the stats...

Engine- 370c.i. LS1 motor, iron block, stock crank, eagle connectiing rods, and diamond pistons. Out of the box AFR 225 heads. 236/242 112lsa camshaft.
Driveline-TH400, TCT 4000 stall converter, 9" rear end w/ 3.42 gear.
eed Inc Harness for factory gauge cluster, Innovative Boost Controller, 96lb injectors, custom fuel system using twin Bosch 420L pumps.
Turbo system- Custom System using a midframe Innovative 88mm Turbo.
Misc- GM Truck Ignition Coils, Radiator relocation
Performance/driveability- 995 RWHP / 8.93@153mph (with 850RWHP). Driven to many cruise nights and around town.

So he came in at 8-900 hp. His engine is boosted, but i dont see any crazy work done to it. So i am assuming he ran a **** load of boost on the motor, and the thing didnt explode. So what allowed him to run all that boost? was it these parts right here...eagle connectiing rods, and diamond pistons. Out of the box AFR 225 heads. 236/242 112lsa camshaft. Did that keep his piston from flying out of the hood?? If so, how come he isnt getting any detonation? It doesnt seem a pair of heads, pistons, and connecting rods is a huge $$ hit. Am i missing something here? If i did these mods to my motor, could i crank my boost way up, gain loads of HP, and keep my motor in one piece?

2002/Black/SS

iTrader: (3)

theres alot to it, i think theres alot of research u need to do.

he had good comp, prob race gas, amazing tune, perfect sized turbo, the right amount of boost. and great engine builders

im 100% sure if u gave that car to someone who wasnt a great tuner, theyd blow a rod out of the engine on the first day trying to tune it

Inspector12

iTrader: (29)

Well you didn't list the SCR!, but that is there shop car and it is usually run on race gas. They can run low boost and run pump gas I am pretty sure, but don't assume that they are running it on pump gas just cause it is driven to street meets LOL! It is running BS3 and is real close to being a race car no A/C and you can make that kind of power on pump gas all my numbers are on 93 so dropping the CR helps to do that. I think you might need to read a little on a few of the different princibles of combustion engines so that you might have a better grasp of what the different ways people try to make more power etc... Not trying to **** you off but tyring to help you before some know it all start flaming you etc...

Dbluegoat

and do not lower your compression much with that magnacharger (I wouldn't lower it at all or build a motor).......you wont be able to add that much more boost to make up for the lost HP

you need to figure out your goals for the car

A maggie, cam, headers, meth inj. will do pretty damn good for a street car on a GTO

Been there done that.....

EPP

iTrader: (22)

On an engine running a centrifical supercharger, we shoot for a compression ratio of 9.5/1. This will allow us to run a lot of boost without having to worry about detonation problems on pump gas. Bob

PREDATOR-Z

Again the majic word "fuel", so yeah you can run higher compression provided you have the "right" fuel for the job.
Also AFR heads have a dual quench area, running a tight quench helps as well as the right plug heat range for the application.

stevieturbo

You are most definately missing something. An understanding of how to make power.

A build, and subsequent tune, are only as good as the person doing it.

So you could build the strongest motor in the world, and an idiot could destroy it in seconds.
or you could use a totally stock motor, with weak parts, and an expert tuner could have it make good power, with pretty good reliability.

There is more to makjng power, than just throwing a lot of parts together, and then trying to tune it yourself, with a limited understanding of whats going on. That is a recipe for disaster in anyones book.

GR33N GoblinM6

iTrader: (14)

ahhahahah my mom tells me this all the time..she's english, funny to hear you say it..

Johny GTO

ok. First, i would like to thank most of you for your help. I know that i am no engine guru. I never claimed to be. I am just a kid who has loved american muscle since i rode in my dad's nova in a car seat.
However, i do know more then the average shmuck, and i can understand and absorb whatever you throw at me. So telling me i need to go read a book, or throwing things together wont make power, while true, isnt exactly the kinda of response i was looking for. I am pretty sure that i will understand whatever reasoning people throw out there at me.
So again, why change the pistons? I thought that putting a dished or reverse domes piston in would increase the overall size of the combustion chamber allowing for more fuel\air to be stuffed in. But i didnt know there were reasons other then this, such as intentionally lowering the compression ratio. If one was worried about detonation, why not lower the amount of air pushed into the cylinder (boost) as opposed to changing the piston (lowering compression)??
The only thing i can come up with (with my limited understanding) is this....
By changing to a piston that lowers compression, you have more room in the cylinder to stuff more air and fuel into the cylinder with less resistance (not as much pressure in the cylinder due to more room), resulting in a larger bang and more power.
is this correct?
Also, a question regarding boost in this application. Say you have 2 engines, 1 with standards LS2 heads\pistons and 1 with lower compression pistons. Both engines have a super\turbo spinning the same speed thereby moving the same amount of air into the motor. Since the lowered compression engine has more room (cc's) in the cylinder due to the dished piston, the super\turbo will be moving the same amount of air per second, but at a lower pressure (PSI).
Is this correct?

Josh@MASPORT

iTrader: (3)

no, its more complicated

let me try to give u 2 good examples:

1) a heads and cam 11.5 to 1 compression LS1 with a turbo. at 10 psi that motor will need race gas to stay out of detonation. and race gas is expensive as u know

2) or that same motor with the same heads and cam, but with dished pistons lowering the compression to 8.6 to 1, will be able to run that 10psi with no detonation at all with only 93octane. granted it will make less power, but much more streeatable cheaper gas etc.

also u may ask why not give it tons of compression and tons of boost, well the answer is race gas can only accomplish so much, then u get into the realm of C16 (powerful race gas) and alchohol etc

ddnspider

iTrader: (26)

the higher the CR the higher octane you need at a given psi to fight off detonation.its been said that for every point you drop your CR you give up roughly 3-4% of your hp on motor.....so you lose 20rwhp,big woop if it lets you throw another 5psi on there.a high CR and then introducing FI will give u rediculous cylinder pressures and if the proper octane isnt there you have problems.if you have 2 motors same setups,1 running a lower CR everything else being equal including boost,the higher CR motor will make more power....BUT is this at a level where both can manage to do this safely on pumpgas is the question?if your so high that the higher CR motor needs racegas then the comparision isnt fair anymore,run the low compression motor on racegas and u can double the boost,now whose making more power