Hello,

I am wondering if say an engine that is 2.0L 9:0 CR on 10psi will spin a turbo with more pressure than the same displacement engine with a lower CR of 8:0 & 10psi boost? In other words, will both turbo setups have the turbine revolutions when measured at the same RPM or will one spin faster than the other when measured. Will the 9:0CR engine put more stress on the turbo?

 

CR at 9.0 will get the turbo to come up on boost easier vs the 8.0 CR.You can also play with the AR ratio to get the turbo to spool quicker but will hurt flow at high rpm.
There are some choices.
Tony

 

Tony,

are you certain that the 9:1 will spool quicker? I would have thought the opposite, but there may be other factors involved I'm not taking into account...

My reasoning is that a 9:1 compression engine should, according to thermodynamics, have a greater thermal efficiency than an 8:1 engine. With the same amount of air and fuel going in (ie the same energy input) and a greater thermal efficiency, you'de get more power at the crank, with a lower EGT as a result (just like more advance increases appearance AFR and lowers EGT). With less energy in the exhaust I'de also expect the turbo to spool slower (all other things being equal). But I guess that the blow-down pressure could be slightly higher if your exhaust valves open a LONG time BBDC... food for thought and dyno testing

Cheers,

pat.

 

volumetric efficiency goes up and the 9:1 will spool up faster than the 8:1 - the reason why Cobb's project cone basher is a 9:1 500 hp crank motor with gobs of torque for auto cross

 

The effect on the turbo would be nil, the difference would be fuel quantity.

 

The 9:1 comp motor will spin the turbo at low speed quicker.It will also make the car less laggy and throttle responce quicker.
Now for egts.Timing will effect egt temp.The only way to correctly tune the car is by o2 voltage or the use of a wideband o2 setup.
We run our 2.5l block with 2.2l heads with a lot of compression.
We don`t know what it is but a comp test reveiled pressure at 190 psi.The turbo is run at 9psi and hp should be around 300 at the flywheel.Our egt temps are 1475-1575.
Timing is factory settings.
Tony

 

I'm not entirely sure why a higher compression ratio should improve volumetric efficiency, other than perhaps because there is less clearance volume, hence less combusted gas left at the end of the exhaust stroke, and hence a slightly better induction stroke, but I would have thought this is marginal ?

With regard to high compression ratio, and torque, the higher thermal efficiency of the higher C/R engine will result in more torque in the crank and so the motor will feel more responsive, but then there are many other ways of achieving the same effect (stroke, conrod ratio, cam profile & timing, AFR, timing etc).

With regard to achieving the correct AFR, I run an NTK UEGO wide band sensor permanently in the exhaust system, closed loop lambda on cruise; I could use the four dimensional fuel mapping capability of my ECU to aid spool up, but half the problem with Subaru EJ series is the length of the exhaust manifold (compare to say Toyota 3S-GTE, 2JZ-GTE, Nissan RB26-DETT, Mazda 13B etc etc), running a shorter manifold would improve spool up... I'm gonna be trying a pair of really short exhaust manifolds on my next engine (with two NTK UEGO wide bands, closed loop control per bank).

With regard to EGTs, my previous setup (wire ringed EJ22, STi crank, billet rods, forged pistons, copper head gaskets, tubular manifold, TD05H-18G, external wastegate, about 8.5:1 C/R, thermocouple *IN* the turbo) ran very cold, barely getting over 600 degrees C on cruise, lucky to see 800 degrees on a charge. The original TD05H-16G would get up to 830 on a charge, 950 with the antilag out on track, then I had the ECU switch to the ALS "recovery" map (less agressive ALS) to save the blower.... nice blue flames

Still trying to figure out my eventual compression ratio, starting with a high compression motor and then gonna drop it in the full build, don't think a 10:1 motor will be too impressed with getting 2 bar stuffed down its throat! Current theory is to set it as high as I can without getting detonation all over the place, don't really want to be running too far away from MBT just to save the motor; then on the other hand higher CR would be great on the road, just run it with less boost, and for fun I could sling in some 102 RON race fuel

Cheers,

Pat.