Something has been bugging me for a while.
Here it goes:

[what follows is non-turbo]

the s4 engine has a 9.4:1 compression ratio

the s5 engine has a 9.7:1 compression ratio

the s4 FSM says that 85 psi is acceptable compression numbers

the s5 FSM Says that 85 psi is acceptable compression numbers

How is it possible that the s5 fsm says that 85 psi is acceptable when the s5 engine is a higher compression engine than the s4 engine ? Weird. I would expect to see a higher number but I don't know how much higher.

On this very forum, most pple seem to agree that 110 psi is the norm for an healthy motor. It would seem to me that the numbers for an healthy motor should depend on the engine (s4 or s5), and not be some unique number.

How does the compression ratio translate to acceptable compression pressure numbers ?

Just wondering,
Hugues -

 

That's what I thought too but I'm stupid. Some one needs to chime in with why it is like this.

 

Well I think it is acceptable to have 85psi compression because it is still hight enough to combust the fuel and air mixture so the engine wouldn't run ruff.

In theory The S5 should have high compression.

But 85psi is enough to keep the engine running smooth.. so that why it is said to be the minimum.

Don't know if that makes sence to you.

 

That makes sense to me. For identical sealing levels, the higher compression ratio engine will read a higher PSI when compression tested, but to combust the fuel adequately, 85 psi is 85 psi . . . in other words, the air / fuel mixture does not care what compression ratio engine it is run in, but rather _how much it gets compressed_, and the engineers determined that 85 psi of real world compressing ability, at whatever rpm the compression test is done at, was enough for the engine to run properly.

 

Im going to agree that is is probally the minimum to have the engine run, obviously it isnt going to be running at its full potential but it runs decent nevertheless.

 

I dont think that compression ratio and compression numbers are directly related.

 

Of course they are!!! I don't even have to argue that.
Anyways, at 85 PSI, the engines will both run acceptably, keep the ECU hapy with sensor inputs, run smoothly and have the exact same power (VDI and other vaiables aside)
In optimum form (like a BRAND NEW engine with 1k miles in it) the S5 will have higher compression reaidngs than the S4, same for NA compared to TII. The higher compression ratio engines just have a higher potential for generating higher compression numbers when they are in perfect form.

 

Actually, given all the variables, the compression PSI ratings will all be within +/- 10 PSI of each other. Other variables include wear, etc.. So in yes, they are related, but the difference in them isnt as pronounced as some people seem to believe they are.

 

Muahahahaha. Everybody knows that the S5's have more power because they have VDI and a higher redline! I would guess that the 0.3 increase in compression ratio adds a few HP out of the 15 extra.

 

So does a VDI bolt onto an S4?? probly have to change a few electrical things with it to...