Z28 vs Evo 8 vs single turbo supra

[evokevin]

actually your both kind of right.

the volumetric flow rate is the same with a big turbo as it is with a small turbo at the same boost level. the volumetric flow rate is dictated by the engine displacement, not the turbo size. granted the volumetric efficiency changes with a smaller turbo to a bigger turbo, so it will be slightly different because of the turbine backpressure. an example of volumetric flow rate would be cubic feet per minute, CFM.

the mass flow rate of the two turbos will be very different though. the large turbo which generally is more efficient will have a cooler discharge, and thus a higher density. its still the same volume, but there is more air packed into the same area because there is less temperature. an example of mass flow rate would be lb/hr.

[2000T/A Guru]

Yes but he said, 18psi on a engine would flow the same using a 67trim compared to a 76trim, now it may flow the same, but when you talk efficiency well the trim size and being efficient go hand in hand. All im saying is if you put a 67trim on a ls1 and ran 10psi then put a 101mm and run 10psi there is gonna be a big difference.

[Chris Arnold]

Ok so you are telling me, if you run 18psi on a supra with a 67trim you are flowing the same amount of air if you ran 18psi on a 76trim.

Other than the fact that you seem to also have a misunderstanding of the meaning of "trim," that is exactly what I'm telling you.

I dont think so, the whole point of being efficient is getting more air flow.

Well, not quite. The point of being more efficient is getting more for your money... or in this case, getting more power from the same amount of air.

A bigger whole will flow more air, not to mention you saying a bigger turbo takes more power away from the motor, either im misunderstanding you or you are just silly.

I'm not just silly, and relax. Understanding will come in short order. Turbos are complicated, but the more you read the more you'll understand. Initially, I didn't grasp all of it intuitively. I still don't know it all. A lot of people think that they do. Bigger turbos do take more power to spin that's precisely why they take longer to spool up..... Even once they're spun up, they still may not be more efficient at a given boost level than another smaller turbo. That's why a 67mm and a 101mm will not work on the same displacement engine. Let me see if I can dig up a couple of turbos and a single displacement where I can make my point. You'll have to bare with me. I don't think many people other than the secretive class racers switch turbo sizes by only a little on the same otherwise untouched engine combination. But hopefully, I can at least show you a couple of compressor maps that illustrate my point.

Thats why when they say bigger trim that pretty much goes hand in hand with a more efficient turbo

hmmm.... that saying may have some merit to it, but again I question your understanding of the word trim. Earlier when you mentioned 67trim and 76 trim..... well those are very weird trim numbers, but are very common wheel sizes. Though they use the word "Trim size." Trim really isn't a size at all.

The saying bigger trim is more efficient.... Possible, but not necessarily. On the same size turbo a higher trim will be capable of moving more air, and that in turn would change the dynamics of the wheel - shifting the islands on it's efficiency map. I think that it would likely make higher volumetric flow at the same pressure more efficient i.e. using a larger displacement motor, but it would probably also shift efficiency away from moving lower volumetric flow. Whatever, that's really a tangent to our discussion, and I'm just speculating in any case.

Last, evoken made exactly the same points I did in his own words.

Chris

[Chris Arnold]

Take a look at the above compressor maps. Take for instance a 180 cid motor (roughly what supra's have, right?) at 2 bar at say...5300 rpm. That's equivalent to moving ~35 lb/min of air. So on the graphs above look where 35 lbs/min at the bottom intersect 2 bar from the side. As you can see, the 76mm graph indicates only 70% efficiency while the 61 indicates 76%. That tells me that for these specific compressors, the T61 will cause a greater amount of power to be produced on a 180c.i. engine at ~5300 rpm and ~14.7 psi boost. Yet, the T61 is the smaller compressor. Sorry I don't have the tests to back it up.... Like I said earlier, we'd be very hard pressed to locate any back to back testing done with turbos like this.

Also, keep in mind, that just because it is more efficient at this point, does not mean that this disparity in efficiency exists throughout the rpm range. Helping you to determine the efficiency of your turbo on your specific combination is exactly why you should pay close attention to the compressor map for your turbo. This will go to show why you don't just slap on the biggest turbo you can afford, or go and slap on a smaller turbo than will meet your goals.

Chris