View Full Version : Sizing issue that confuses me
Say you've got two engines.
Engine A is 2.0L & capable of 200kW@14psi
Engine B is 2.3L & capable of 200kW@14psi
Surely, if both are tuned correctly, the same turbo would suit both engines?
I'm a little confused over why you'd put a larger exhaust housing on the larger engine ... or is that only because it's pumping more exhaust-gas at idle?
Sorry, trying to match disparate info from different sources. I'd like to believe the source that's 30% cheaper, if possible ...
I'm not an engineer, but how does this sound:
Engine B is 15% larger swept volume.
Therefore, it inhales 15% more air and 15% more fuel.
Therefore it has to exhale 15% more exhaust gas.
Therefore, it needs a bigger exhaust housing to cope.
That is not quite right.
If they make the same power, assuming one or two other things remaini the same (like no significant differences in internal friction etc etc) then the 2.0 has higher volumetric efficiency than the 2.3. And they swallow the same amount of air and fuel each rev coz they are making the same power.
Of course there are other differences like, it is possible to make more power from the same amount of fuel and air if your ignition timing is more correct on one engine than on the other....which might happen if one of the engines was a bit prone to pinging and so had to be retarded a bit.
I reckon that Forg is right and that you could/would use the same turbo for both.
But if the 2.3 had a different off boost torque characteristic (ie made more torque off boost) then it woul dbe consuming more air at that point than the 2.0 would, and so the exhaust flow would be higher and it would come on boost a bit earlier.....so you could spec teh turbo with a larger exhaust housing to swap some of that earlier boost for less restriction at high revs and gas flow.
Does that help?
Just a thought (and it's 4am so i could be off track) but would it not be important to know what the redline/max power RPM of each engine is?
the 2.3 could be a long stroke torquey thing only pulling 5000rpm and the 2.0 could be a screamer with no torque pulling 9000 RPM (hmm sounds like a S2000 :) )
Surely then the 2.0 would be pumping more air... ?
Just a thought
So it depends, if the 2.0 and 2.3 are the same family of engines, i guess it could be ok, but if the 2 engines are a different philosophy, then you might be comparing apples to BBQ's
If the revs are totally different, yet they're both tuned accurately, then air/fuel ratio should be roughly similar; and so when making X kW they should surely be pumping the same amount of fuel+air through. In the 2.3-doing-5000rpm vs 2.0-doing-9000rpm, if they're both pushing out the same amount of power (and are both tuned properly), then I'm fairly sure that theory says they're both basically pumping out roughly the same amount of gas.
Yeah, GTSBoy, I think you're thinking along the same lines that I think I'm thinking along. I think. :) That means the bigger the engine, at a given kW-rating & with the same turbo, the smaller the lag; I think (hope)(and obviously with provisos/exceptions).
Same power = same volumetric flow rate
Different capacity engines but same power = different volumetric efficiencies
Flow rate and efficiency…same kind of wording 2 different things :)
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