Boost vs C/R

miafori · #1 (**permalink**) 12-26-2003, 11:55 AM

If we change the compression ratio, does the boost change? For example two engines with all same components and specs, one of them has 7.0:1 and other has 8.5:1, do both of them have same boost?

Dan951Man · #2 (**permalink**) 12-26-2003, 12:22 PM

The one with the higher c/r will spool faster but you won't be able to boost as much because of detonation. The lower c/r will have more lag, but you will be able to turn the boost up higher in the end getting more top end, because pinging is less of an issue

miafori · #3 (**permalink**) 12-26-2003, 12:34 PM

Mmm, then if we have a non-adjustable boost, can we use thicker gasket in order to decrease the boost level (even a little) ?

revo86 · #4 (**permalink**) 12-26-2003, 02:41 PM

you cant control the amount of boost you run buy lowering or raising a compression ratio. You are changing the level of boost that you can run safely.

you need a boost controller to adjust boost levels, hense its called a "boost controller"

HyperBaja · #5 (**permalink**) 12-27-2003, 02:42 PM

FYI- 1 CR Point is about 4psi of Boost.

KolobM3 · #6 (**permalink**) 12-27-2003, 06:59 PM

Quote:

Originally posted by miafori
Mmm, then if we have a non-adjustable boost, can we use thicker gasket in order to decrease the boost level (even a little) ?

FYI - Boost pressure is measured at the manifold, and it has nothing to do with CR.

Also, lowering CR does not necessarely = more lag. You can tune the lag char. with the turbine a/r.

stimpee · #7 (**permalink**) 12-28-2003, 06:12 AM

Lower compression ratio will actuall REDUCE lag, not increase it. We have been having this discussion on Bimmerforums too. By reducing the compression ratio, you are REDUCING combustion efficiency. What this means is that more heat is LOST out the exhaust. That lost heat is used to turn the turbine = less lag.

Contrary to intuition, but true...

Steve

RAiMA · #8 (**permalink**) 12-31-2003, 07:09 PM

Quote:

Originally posted by stimpee
Lower compression ratio will actuall REDUCE lag, not increase it. We have been having this discussion on Bimmerforums too. By reducing the compression ratio, you are REDUCING combustion efficiency. What this means is that more heat is LOST out the exhaust. That lost heat is used to turn the turbine = less lag.

Contrary to intuition, but true...

Steve

It's only a half truth. Reducing the compression ration does reduce combustion efficiency. IT DOES NOT MEAN MORE HEAT IS LOST THROUGH THE EXHAUST. All it means is with less compression, there is less heat generated with each combustion. This in turn allows the engine to run higher boost within safer limits. More compression allows the engine to produce more power, therefore enabling to have a slightly quicker rise rate, which in turn would reduce lag.

Keep in mind that higher compression ratio engines are more likely to detonate. When running boost, it all comes down to heat and how to keep it as low as possible to keep things working as efficent as possible.

stimpee · #9 (**permalink**) 12-31-2003, 07:15 PM

I disagree with you, but that is nothing new.

A certain amount of heat is generated when combusting a certain mass of air/fuel. This mass will be much more closely related to displacement of the engine, and some other factors which affect VE (of which CR is one) than it is DIRECTLY to CR.

In the case of a high CR engine, more of this heat goes into "pushing" the piston down, since the CR also increases the EXPANSION ratio of the engine. If this heat is NOT used to "push" the piston down, it goes out the exhaust. Combustion efficiency IS reduced.

But, feel free to disagree with me, as I am sure you will...

Steve

mertdastan · #10 (**permalink**) 01-01-2004, 08:55 AM

Miafori,

Selam. When you keep the supercharger or turbocharger's set up the same and lower cr you will have less boost. Assuming that intake piping, intercooler piping, manifold stay untouched, boost will be lower with a lowered cr.

In order to have the same boost which you had prior to decreasing cr, you need to spool the sc or tc faster and more.

MERT

stimpee · #11 (**permalink**) 01-01-2004, 09:06 AM

Quote:

Originally posted by mertdastan
Miafori,

Selam. When you keep the supercharger or turbocharger's set up the same and lower cr you will have less boost. Assuming that intake piping, intercooler piping, manifold stay untouched, boost will be lower with a lowered cr.

In order to have the same boost which you had prior to decreasing cr, you need to spool the sc or tc faster and more.

MERT

Mert,

I am not talking about absolute boost in my comments, I am talking purely about available exhaust energy to spool the turbo...

I stand by my statements, and can back it up with references if need be...

Steve