Compression ratio w/turbo blowing ?

If I understand correctly (fat chance) the quoted compression ratio for a Saab motor would be the "mechanical" ratio. If this is correct, what is the effective compression ratio when you have

11 or more pounds of boost blowing ? (If I'm incorrect, please correct me :)
Reply to
-Bob-
Loading thread data ...

The compression ratio does not depend on the boost pressure, it is the ratio of the smallest and largest volume in one cylinder (piston at TDC and at BDC). see

formatting link
Max boost pressure in a FPT 900 is 0,7 bar (over atmospheric pressure = 1 bar), at that pressure the waste gate will (or should) open.

------- MH '72 97 '77 96 '78 95 '79 96 '87 900T8

Reply to
MH

But wouldn't the "effective" compression ratio be higher since the charge is denser than a normal charge ? Or does the turbo simply make the pump run faster ?

Reply to
-Bob-

But such an "effective" compression ratio wouldn't be constant, since the cylinders don't fill so readily at higher rpm. But the geometric compression ratio is lowered to compensate for the higher degree of filling of the cylinders due to the turbo pressure, effectively increasing the volume of the engine compared to an NA engine. (Positive displacement engine). Hence the compressed space must also be larger.

Reply to
Johannes H Andersen

So a turbo motor runs faster because the effective _volume_ of the engine is increased not because the effective flow is increased ? (noting that as the motor runs faster, flow will go up too).

In other words, cramming a denser charge in a cyl that would normally only hold an atmospheric charge effectively makes the motor run as if it was larger ?

Does anyone (engineer) attempt to measure this as an equivalency ? What I mean is... if you have 14lbs of boost, the charge will be "X" times more dense, can we predict that the motor will generate "Xn" times more power (ignoring the obvious layman's 'standard' that we usually see 100hp/liter in most production turbo's).

Reply to
-Bob-

It doesn't necessarily run faster, it produces more power at same rpm. Since the cylinder filling is less restrictive at low rpm, the turbo pressure moves the torque curve towards lower rpm. That is provided that there is enough gas flow to spin the turbo wheel. You typically see high performance NA engines have peak torque around 4000-5000 rpm, while turbo engines have peak torque at 2000-3000 rpm. The positive displacement effect works particularly well at low rpm, the engine feels larger than the bhp number suggests, even taking into account the peak power figure of the turbo machinery.

Well I know some fluid dynamics, but I'm not an mechanical engineer. The relationship is complicated since it is not a direct relationship between boost and effective volume. This is because of the inevitable flow resistances through the many nooks and crannies before the gas mixture ends up in the cylinders. And these flow resistances grows non-linearly with flow speed.

Reply to
Johannes H Andersen

-------------------- The turbo boost fills the cylinder (at TDC) with more air (fuel air mixture), or if you will is pre compressed ("boosted"). The

**RATIO** that volume (as explained above TDC [Top Dead Center/piston head at top] to BDC [Bottom Dead Center/piston head at bottom] is compressed remains the same as if the cylinder had 'only' a normal atmosphere's worth of fuel/air mix.

To restate: the turbo stuffs more air/fuel mixture into an uncompressed (by its piston) cylinder, but the *ratio* of the compression from largest to smallest cylinder volume - by the piston - remains the same.

Reply to
Andrew F. Dicker

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.