Why is it that the twin turbo kits(like GReddy) can be set to 8/8.5psi and pull 420+whp but it takes an ST kit (like Turbonetics) 9.5/10psi to make the same. Yet everyone feels that the twin kits are safe at those boost levels...and ST kits get cautioned on how high they push it. Why the difference in the numbers?

Help me out here...I'm curious...been searching...but still dont get it.

 

two small turbos vs. one bigger turbo, that's basically what it comes down to.

psi doesn't really mean squat, btw...so i'd be wary of throwing around boost numbers when you don't mention turbo differences or lb-min flow per turbo, or efficiency ranges of the compressors and turbines.

 

If you haven't read the book Maximum Boost by Corkey Bell yet you should purchase that and read. When you are done read it again. After the second time you should have a good fundamental understanding of turbochargers and how they operate as well as the answer to this question. I'm not being a d!ck just that its not an easy thing to explain and have you fully understand.

 

+1 .... that book was very helpfull!!!

 

To talk about specific kits - excluding psi - The GReddy TT kit would have a higher flow or effeciency than a larger kit like the Turbonetics.

I guess what I'm really wondering is why the TT kits seem to come out of the box making more whp than the ST kits.

 

Because the total Mass Flow of the two turbos at the given pressure ratio is greater then the Mass flow of the single turbo at the same pressure ratio. Again not being a d!ck whatsoever but that book would should some light on all of this. I and several others consider it the "bible" of turbocharging.
Edit: The TT kits don't all come out of box making more then the ST kits. The Greddy kit at 5.6 PSI makes roughly 350-370 RWHP, ditto for the PE kit. The Turbonetics kit makes 365-380 RWHP at 8 PSI for the reason I listed above. Of course I'm talking out of box no tuning or modifying of pressure ratio.

 

Thanks bro...that makes sense now. I was looking at it as if the VQ could handle a certain whp number on a twin, why couldnt I up the boost on a single and get the same results (somehow thinking that the pressure had to be less on the single for it to yeild less results). I wasnt looking at it as a higher volume at at a lower pressure on the dual. So the simple answer is that the ST would have to put a lot more pressure on the engine and components to match the flow of the dual.

Thanks again,

-Blaise

 

See thats not exactly the answer either. Depending on the single turbo it could achieve the same Mass Flow Rate as a twin setup at the same pressure ratio if the compressor produced enough air at that pressure ratio. Thats why I'm saying there is no cut and paste answer here. It involves several factors which you are neglecting to add into the model. For instance (everything in this example is theoretical and just for explanation purposes. These are not actualy flow numbers)

A single lets say T70 turbo produces 500 CFM of air at 9 PSI
A pair of GT28RS turbos produce 475 CFM at 9 PSI of air.

For the same given pressure ratio the single is producing MORE air and thus more power.
Conversly lets look at this:

A single T62 produces 420 CFM of air at 9 PSI
A pair of GT28RS turbos produce 475 CFM of air at 9 PSI

For the same given pressure ratio in this scenario the smaller twins will produce more air and thus more power. Again these are examples and not actual numbers of any kind.

I would go into the single vs twin argument but it would take a lot of typing that quite frankly I just don't have time for right now. Best bet grab a copy of Maximum Boost. Theres no way I could break this down into the kind of detail available in that book.

 

Very good explaination MIA. +1 on getting the book. Learn to read a compressor map and you will understand exactly the point he is trying to get across.

 

Its an easy answer..........volume

 

No its not that easy...thats an easy answer but does not include factors such as turbine wheel trim and effciency, A/R, compressor trim and efficiency, ambient pressure, pressure ratio, exhaust backpressure, and about a thousand other factors.

 

Looking at the smaller picture, different companies will use different dynos, and henceforth, get different numbers. Not saying that is the answer, just another variable.

 

I couldnt have said it better myself. PSI does not determine power potential...only the MASS of air. (lb/min)

 

very informative thread

 

I ordered the book. Just waiting for it.

"PSI does not determine power potential...only the MASS of air. (lb/min)".
So the PSI only counts for how much pressure is spooling the turbo, not how much volume it is pushing into your manifold...got it.
So if a certain volume is deemed safe (with appropriate tune of course) on a twin, then the single 'should' be able to have the pressure increased to produce the same volume?

 

short and simple 2 power sources make the same power with less effort than 1 power source

 

Not necessarily. Each turbocharger has a peak efficiency zone. Think of efficiency as the region where the turbocharger is flowing a high volume of air, at its lowest temperature (all things being equal). Beyond this point, the turbo will produce less and less air, for each psi increase in pressure, and the result is also a hotter intake charge.

So you still need to match the twin or single turbocharger to the intended applications. Of course, if we are talking the difference in 8psi on the Greddy kit, vs. 10psi on the Turbonetics kit, then all is good.

 

exactly what I was talking about...

Which is why you would need to invest in cooling mods like water/alc/meth injection, oil coolers, etc...to dissapate the heat caused by increasing the load on the TC. Right?

-Blaise

 

Actually no not at all. If your turbo is properlly sized for your application it will be in its peak efficiency range all the time and thus you are getting the best airflow possible with the coolest outlet air temps. Again, there is a TON of stuff that goes into all this. I wish it was a simple... "This is the answer" question but its not.

 

Oops...Sorry...Misread Sharif's post. Push the turbo out of its efficiency zone, get LESS airflow at higher temps...