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Discussion Starter #1
Why does an SP63 make more power at less boost, than say an SP57? Is it because of the backpressure caused by the more restrictive exhaust side of the turbo?

Does an SP57 heat the intake air more then an SP63 at the same power levels, like the stock Supra turbos do when they're running near their limit? For example, would an SP57 equipped car, putting out max power, (approx 580rwhp) strain the engine more than an SP63 equipped car at the same 580rwhp level?
 

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Yes, that's one reason. Also, a smaller wheel flows a lower volume of air than a larger wheel at the same pressure differential(assuming the same wheel design.)

compare a T60-1 http://www.turboneticsinc.com/comp_maps/fig9.html
to a T66
http://www.turboneticsinc.com/comp_maps/t66.html

A T60-1 might make 580rwhp at ~24psi ~= 2.6 PR (this is a stretch for the T60-1)
A T66 will make 580rwhp at ~20psi ~= 2.4 PR

The T60-1's peak efficiency island of 76 percent? is from a PR of 1.5 - 2.0. So if one runs at 2.6 they miss the peak efficiency completely and therefore heat the air more causing less mass of air to be forced into the engine.

The T66's peak efficiency island of 76 percent is from a PR of 1.7 - 3.4 and is nice and wide at 2.4. Obviously it will heat the air less causing more air mass to be forced into the engine.

Steve
 

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Yes but most FMICs do a great job of removing heat from the intake charge, so charge air temps are probably pretty consistant between turbos. Even the stock IC does a good job (within 30F of ambient) of this during most street pulls @ BPU (although I admit I haven't monitored air temps @ 2 bar of boost).

But the energy used to heat up the air more (by an inefficient compressor) has to come from somewhere. In this case, its the exhaust. An ineffecient turbo will require more exhaust energy to make the same amount of power as an effecient turbo. In other words, more exhaust backpressure, which also means the engine is less resistant to knock, so less timing advance, etc.

Of course, this isn't to say an SP57 isn't effient (I'd imagine it is). Its just a smallish compressor wheel (effecient at lower flow rates) mated to a smallish turbine wheel (spools at low RPMs where the engine can't flow enough). But a smaller turbine does = more backpressure at higher RPMs, where other, larger, turbos would be more ideal.

IMO YMMV.
 

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Interesting point regarding IC's and consistent charge temps, I thought the IC's would remove a certain percentage of the heat which could lead to noticeable differences in charge temps.

Hmmmm.

Steve
 
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I think the main issue is the pumping action the engine does to get that air out of the combustion chamber, along the lines of what Grant said. The more the exhaust restriction, the harder the engine has to work. High pressure differential ratio between intake and exhaust manifolds just robs power that would otherwise end up at the rear wheels. This lost power ends up as heat, from an energy transformation standpoint. The intercooler sizing certainly plays a role, but to a lesser extent; especially in short runs where it acts as a heat sink, and doesn't reach saturation.
 

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Grant said:
Yes but most FMICs do a great job of removing heat from the intake charge, so charge air temps are probably pretty consistant between turbos. Even the stock IC does a good job (within 30F of ambient) of this during most street pulls @ BPU (although I admit I haven't monitored air temps @ 2 bar of boost).

The I/C does make a good heat sink to lower the air temps, but they can only do so much. Once a turbo moves out of it's efficiency range the temps go up really fast. That is why you might be able to turn up the boost, but not get any more power.
The difference in temps between a t-60 making 580 HP and the T-66 at the same level will be significant. Not sure what the actual #'s will be but the 66 will be lower. That said, the I/C can only remove heat based on the difference in temp (outlet of turbo-temp of I/C). The lower the temp in the lower out.
At lower HP they will be close, BUT when trying to push the limit, the 66 will heat the air less at the same pressure, which gives more hp.
ERIC.
 

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I think the power difference at same boost levels is more due to the exhaust backpressure difference as opposed to the intake temperature differences. Obviously a smaller turbine wheel will create much more back pressure between the wheel and the combustion chambers. The piston has to work much harder to exhaust the spent gases. With high back pressure, not all of the spent gas will be exhausted out of the cylinder. This will also decrease the power output on the next stroke as the residual exhaust left in the cylinder will mix with the new boosted air. The residual exhaust obviously has no O2 left....... = less power.


Lar
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See, Lar agrees with me, :p

My reasoning comes from using a thermocouple with the stock IC @ BPU, and by Eric Typpo's and Steve Haye's testing of the GReddy 3 row. For me, the stock IC held temps close to ambient on a run of 1st through 4th, only at the very end of 4th did it start to get heat soaked. Eric and Steve have said that their ICs hold temps close to ambient during all conditions and boost levels. That tells me intake temp is mostly constant, its the exhaust backpressure that makes the difference.

Steve, remember the heat trasnfer between the boost and the IC is proportional to the delta T, so the hotter the charge, the more heat trasnfered.
 

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This leads into a question I posted earlier. If we are talking about backpressure and how the amount of it effects horsepower (which is something I think we all agree about), how do we figure in terms of exhaust piping how large piping must be for a certain amount of power generated?

In other words, how can we mathematically calculate at what point a 3" downpipe and 3" exhaust is becoming a restriction and creating a certain amount of power loss? Through research of various MKIV Supra tuners worldwide there seems to be a varience in opinion as to at what point a Supra requires more than a 3" downpipe and 3" exhaust.

Thoughts on this? Do you think exhaust design (as in how many bends) can have a lot of bearing on restriction and is also a key variable when determining exhaust diameter necesities?

-m
 

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Discussion Starter #12
Marcus Frost said:
This leads into a question I posted earlier. If we are talking about backpressure and how the amount of it effects horsepower (which is something I think we all agree about), how do we figure in terms of exhaust piping how large piping must be for a certain amount of power generated?

In other words, how can we mathematically calculate at what point a 3" downpipe and 3" exhaust is becoming a restriction and creating a certain amount of power loss? Through research of various MKIV Supra tuners worldwide there seems to be a varience in opinion as to at what point a Supra requires more than a 3" downpipe and 3" exhaust.

Thoughts on this? Do you think exhaust design (as in how many bends) can have a lot of bearing on restriction and is also a key variable when determining exhaust diameter necesities?

-m
I wonder by having an exhaust at all, how much power you're losing? Someone should try a dyno run with only a downpipe and see. I wouldn't imagine that you'd gain much.
 

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Of course, the majority of the backpressure that an engine sees is between the turbo and the engine.

I think Andi B. told me that Peter's old black car was dynoed with and without the cat-back exhaust (just running out of the down pipe) and there was about 15 RWHP difference up in the 700 range versus running with the cat-back (can't recall if he was running the Power Extreme or the Super Dragger...).

Steve

Kevin Hoare said:


I wonder by having an exhaust at all, how much power you're losing? Someone should try a dyno run with only a downpipe and see. I wouldn't imagine that you'd gain much.
 
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