Can a N/A tC can running with 8.5:1 Pistons instead of 11:1?
It can run with them for a while?
What could happen?

 

It will be the slowest tc in the world for one. Mostly because you have no compression, and no turbo or s/c to add volume or compression so your going to have a turd of a car pretty much. I may be wrong though so see what others have to say.

Jay

 

im not going to do that, it was only for curiosity.
Thanks!!

 

Yep, slow as crap. You run lower compression when you are adding a lot of boost to the vehicle. This is to keep the actual compression to a level that is manageable. Although you want higher compression overall, the huge benefit in boost (And an intercooler as well) is to not only add pressurized air, but a more dense charge to allow for more fuel at one time. You dont want to start with too high of a compression ratio because the added boost will cause detonation as well as a lot of stress on other components.

So, I wouldnt run them unless you are going FI. And even then, if you are running 10 - 12 psi you should be ok without them on our engine given its lower CR of 9.6:1.

 

so the 11:1 pistons for a N/A tC would be ok?

 

If you are not planning on going FI then they would be great. They sell the higher compression ones for that purpose.

 

what could be the benefits of the 11:1 pistons in N/A in whp? (estimate)

 

Think of it this way...The higher the compression, the bigger/faster 'explosion' in the combustion chamber. The bigger/faster the explosion, the more power. The smaller/slower explosion, the less power it will produce. The trade off is that gas ignites under extreme pressure, so in turn, the higher compression ratio, the more likely a chance of it igniting sooner than the spark wants it to. This is known as 'engine knock' or 'pre-ignition'. There are ways around this by using higher octane and removing 'hot spots' from the combustion chamber/piston top, but those take some time and by an experienced mechanic. They require the 'massaging' or lightly sanding/grinding of rough edges from the piston top as well as casting lines and rough/sharp edges from the combustion chamber.
If you run too high, and ignore the 'knock', you could blow a hole in your piston, bend a rod, crank, valves, etc. In other words...BE CAREFUL and let a GOOD mechanic/engine builder do it.

 

nobody knows... not even ZPI knows...

i've been waiting for there reply for forever...

i want to be teh guinea pig but... $$ is always the issue

 

Higher horsepower and maybe a little more torque!

 

Well... since higher compression increases MEP, he's gauranteed to see big torque increases.

His HP increases will be more determined by the SHAPE of the torque curve, which is more affected by flow through the head. If the shape doesn't change, then he'll see an equal % gain in Hp as torque.

Expect about 10-15% HP/TQ gain going from 9.5 -> 11.1 --- this is what the hondas typically see.

 

ALso if you are changing the piston heads, you might as well do the P&P and valve work to save you time and money.

 

I love seeing people saying "you can't run higher compression pistons with FI" when that is in fact completely false.

Adding boost to an engine increases dynamic compression. The end result is all the same. You can run higher compression and lower boost and make the same, sometimes more power then you can with a low compression high boost car. It all depends on the application. S2000 owners dont seem to be to worried about throwing 6 psi at their engine. They have high compression. 11:1 if I am not mistaken (might have changed in different years) but its still high none the less. With higher compression you just can't run as much boost safely.

A street car, with a good engine would be best complimented by a moderate compression ratio and a smaller amount of boost. Low compression cars have much less off boost power for daily driving needs. It's a balancing act that many people do not yet understand.

Charles

 

^^ I fail to see where anyone said that you cant run FI with higher compression.

 

I wasn't referring to any particular example in this thread, but there have been many threads on this board before with people saying pretty much that exact thing.

To make a point about your specific comments from above, adding boost to a higher compression ratio car will not cause detonation. NA cars can detonate just as FI cars can. The boost has nothing to do with it directly. Detonation occurs when the cylinder temps in the quench exceed the point at which the fuel you are using can maintain stability. Race gas is more stable in nature and can therefore withstand higher cylinder temps(leaner fuel mixtures) and added timing. Which is where the benifit of running race gas comes from.

Charles

 

Very true, but for detonation you need 2 things. Heat and compression.

FI set ups produce a lot of heat and compression. If you are running too hot and compression too High. You'll be needing some trick racing fuel to run your car @ optimal performance and safely.

 

^^ Absolutely T. Compression DOES play a BIG role in this area. That is one of the reasons high compression engines require higher octane fuel. The more compression you run, the less heat it takes to combust. When you compress a gas you create heat as well. So running higher compression also contributes to higher temps. A very basic example of this outside of the engine is the intercooler. You are not simply creating a hotter charge due to the heat transferred from the compressor housing and exhaust, the simple act of compressing air heats it up. So, with a higher compression engine you will run into this issue at a much lower boost level than you will with a lower compression engine.

And yes, you can run higher compression and a smaller turbo and make a streetable quick car. But it seems that you (t.customs) are saying that compression before ignition is the only purpose served by a turbo. FI works best by creating a denser charge of air. The denser the charge, the more fuel you can cram in. The more fuel and air crammed in per intake stroke, the larger the boom on the power stroke, hence more power. Simply raising the compression will yeild results, but I would rather start with a lower compression and add a large amount of intercooled air and fuel, thus raising the amount of charge per ignition cycle AND the compression.

Your point about having performance before boost begins is a by product of a turbo or centrifugal supercharger to start with. For a street driven tC I would rather keep the stock CR and run a good amount of boost without over doing it. This will keep the stock performance outside of boost rather than stifle it by lowering compression. The other alternative would be to run a roots style SC (which isnt in existance for our car) which provides a more linear performance response.

 

I couldnt agree more. Hence the reason why I have stock compression ratio pistons in my built tC. I want the car to act as if it is stock when Im not in boost.

 

It always tends to be a common misconception.

Let me tell you it plagues the rotary world... but high-comp+boost in a rotary can be a tricky wicket.

 

Her cool points icreased exponentially.