I decided to do a little back to back test on my truck today, swapping the old Masterpower 76 q-trim 1.00 a/r divided housing for a PT7675 .96 a/r non-divided housing. I have felt that the little turbine wheel on the MP turbo left a lot to be desired especially on my LQ9 with the 6 liters of displacement. My motor consists of the following

-LQ9 stock bottom end
-LSA heads with the "wing" removed from the intake port so it flows a bit more than what an L92 head does, chambers machined a bit to open it up and drop compression
-Small turbo grind from Ed Curtis (223/227)
-KB Racing turbo kit
-stock L92 truck intake manifold with my original 2006 75mm TB using a 4-bolt to 3-bolt flange adapter
-4L80 with stock converter and 3.73s in the rear on a 20" wheel w/31.5" tire

Long story short I got a screaming deal on this new PT unit so I figured I could swap em and sell the MP to make up for some of it and not be out too much money and at the same time it would make me feel better about using a larger turbine to better match my cubes... well I guess the added power should be a nice benefit as well.

This is about as back to back as I could get it with an hour and a half in between to make some small changes to the cold side and downpipe. The correction factors were 1.02 on the dynojet for all pulls so it's about as non-corrected as you can get and accurate.

First up the comparison between the two turbos. I made no wastegate changes and the boost level between them was pretty close however the MP turbo hit ~7 psi and dropped to ~6 psi up top while the PT turbo hit ~7 psi and stayed pretty much at the same level the whole way up. I really wanted the boost levels to be identical but seeing as how the MP turbo would drop off approximately 1 psi up top no matter what and the PT doesn't it's about as accurate as can be just as it is on the street in the real world. The MP turbo comes in a bit faster/stronger down low due to the smaller turbine and divided housing but the PT takes over quickly and feels just very slightly laggier on the street.



Here is another graph comparing 2nd gear vs 3rd gear. The dyno reads differently in different gears but what I'm really trying to see is how much the tires wadding up at higher speeds affects the power curve. 2nd gear's graph is lower overall because of this but what I'm really looking at is that it doesn't drop off as much in the higher RPMs due to the lower tire speed and less resistance since the tire isn't trying to wrap itself around the roller nearly as much as it is in 3rd. This tells me that it's still making power up top farther out in the higher RPMs than the 3rd gear pull shows so I should be shifting higher since I know 3rd gear's upper RPM area is artificially lower than what it's really putting out due to tire wrap on the dyno roller.


Overall I was happy because it picked up 57 rwhp and 32 rwtq at such a mediocre boost level. Obviously the higher I would have gone with the boost it would have continued to make more of a gain as the little Masterpower turbo wouldn't be able to keep up with the larger higher flowing Precision turbo.

On a side note with these heads it puts my compression right around 9.6:1 which is stretching our crap 91 octane gas here in AZ on all of 7 psi in this heavy of a truck. My timing is pretty low right now and even putting in another couple of degrees or bumping it up another psi or two puts it into detonation so I'm just octane limited at this point with this combo. Maybe at some point I will put on a meth kit or step it up to E85 but seeing as how this thing doesn't get all that great of gas mileage already I really wouldn't want to see it on E85. Maybe I should also start thinking about the bottom end some more seeing as how I have 130k on this motor and at some point I better start shopping for some good rods/pistons.

 

Nice writeup as always, Nic. I wonder how much further behind the MP would be if it could maintain boost level...

 

Thanks! On my truck combo it's right around 20-25 rwhp per pound of boost so we could probably just subtract/add that number to either. The MP always just drops off a bit up top though and there is nothing I could do to stop that from happening so I just see it as part of the gains on the new turbo. Hell in the past I tried race gas and putting in a stiffer spring in the gate to combat the presumably bad backpressure on the hotside and it would always still just drop off ~1 psi in the higher revs. I just chalk it up to a restrictive turbine on the MP.

 

Is that the 65mm turbine exducer?

 

I didn't measure with the micrometer but I am pretty sure it's either a 65mm or 68mm on the MP q-trim. I should have taken a picture of both side by side because the PT7675 is noticeably larger than the MP even just looking at it.

 

Q-trim is 67mm exducer, P-trim is 64.5mm on the MP's.

 

Nice comparison. Good to see that kind of a gain.

 

Good write up.

 

Nice write-up, confirms what I had been thinking, let that puppy breathe