Phish806

iTrader: (5)

So, i am on my third stock shortblock in my project car and i think i have finally found my issue. Backpressure. Long story short, after each motor ate a piston, i did small upgrades trying to fix the issue. This last time, i went with a new spec'd cam, some straight through mufflers instead of chambered (expected a backpressure issue) and I also went with the MS3-Pro to monitor and control the engine. When installing hte MS3, i added a sensor in the exaust manifold to monitor pre turbo pressure - thus comparing it to boost and seeing my BP ratio. What i found, well, not good

Long story short, i have a BP ratio of around 3:1. Obviously this is a problem. So here are a coupel questions:

1. I found a quite large boost leak around the BOV. When this is fixed, will that help lower the BP ratio any, or am i just hoping for a mericle?




2. If i go from a 1.15 AR housing to a 1.32 housing, how much will that reduce my back pressure? Would that be great enough to fix the issue?


3. Any other suggestions about how i can reduce the back pressure? I am up for ideas, but can not really change the turbo thats on the car. Physically, it barely fits and would require a ton of modification to change.



Some info about the motor/turbo: LQ9 shortblock with an LS3 top end. The turbo is the - notorious for backpressure - Comp Turbo CT43 8079 with a cast wheel, 1.15 AR and a 3.5" DP.


Thanks for the replies guys.

1999Black_Z28

iTrader: (1)

I don't know the answer for sure but I would think that fixing a boost leak should help at least a little. Keeping drive pressure the same, you would gain boost pressure thus lowering your BP ratio. How much will depend on how much boost you are losing through the blown seal.

I realize that BP is bad for making power but I'm not sure I understand the link to eating pistons. What is the theory behind that? I'm still new to turbos...

Phish806

iTrader: (5)

Excessive back pressure can cause extreme heat among other things and will cause detonation. The head will ignite the fuel instead of the spark plug doing so.

THADD

iTrader: (12)

What are your cam specs

Phish806

iTrader: (5)

223 231 595 610 115+5 LQ9, LS3 topend

asubennett

iTrader: (6)

Yes, boost leak fix will help a lot. Your spinning the compressor insanely fast to achieved,your target boost because your pissing all of it out your charge pipe where that leak was.

Your back pressure ratio probably doesn't add up because of your leak. Probably measuring pressure before leaking point and therefore it looks way out of whack.

My guess, is back pressure is just a ghost you are chasing. Tune up is probably breaking your motors.

yenkomike

iTrader: (2)

what are you running for boost? timing? and on what fuel. just trying to figure this out.

back pressure will be an issue but you still should be able to keep it out of spark knock.

TurboS10

iTrader: (12)

No experience, but from reading people seem to break stuff with the smaller turbines and around 16-18 psi boost where people making more power on stock parts don't with lower BPR.

What are you doing to the rings to prep for boost?

Need a high mileage motor to do nothing to them.

SPRAYED 01

iTrader: (22)

Search my name and read all about my ct80/79 issues. I knew from the beginning you were going to say you have that turbo. Get rid of the turbo, its a back pressure monster. Lost my *** trying to figure out my issues.

I still run a comp but its a ct43 comp cover ct5 turbine housing hybrid. 88mm billet comp wheel and 92 or 94mm turbine wheel with 4inch dp. Im now about 1.5:1 bp and have been running strong for over 2 years now.

SPRAYED 01

iTrader: (22)

And yes the leaking bov will cause a high bp reading. But get rid of the turbo like now. I mean read this and put it up for sale on a supra or honda forum, it has no business in the ls world.

Alex

LJMSJohn

Get that turbo off the car and get something different. All your issues Lie in that turbine wheel.

Forcefed86

iTrader: (4)

PT76/75 is small and would be a much better match.

rotary1307cc

iTrader: (5)

Alex has been here, listen to him

Phish806

iTrader: (5)

Yeah i have definately ruled out everything but the turbo. So swapping it out, i am thinking about going with an S475 with the 92 turbine. The issue with fitting that, is i would have to neck the 5" dp off the turbo down to 4" to get it out of the engine bay. I still think for my low boost, it would be ok with the 4" dp. I would also have to modify the radiator a bit as well to get it all to work and for it to fit

TurboS10

iTrader: (12)

Cast 476/92
not much diff in price & still cheaper than the 8079.
Neither will spool like the 8079 though.

Phish806

iTrader: (5)

Thats something that i am worried about. Spool time. I know it will not be as responsive, but i also dont want it to be crazy slow to get spooled.

rotary1307cc

iTrader: (5)

Spool is overrated

Phish806

iTrader: (5)

^^ thats not encouraging lol. I am sure at the track it will be fine. I mean auto, trans brake, two step, antilag if needed. But just roaming around town and rolling into it, is it going to take me three city blocks to get up to 5 psi?

rotary1307cc

iTrader: (5)

You like smoking the tires a light switch tq delivery will do it

Forcefed86

iTrader: (4)

No, don't worry about it. Jump right into the biggest wheel you can fit into a T4 1.25 housing. My 6.0 with a S476 1.25 housing and the 77.5/87 wheel lights off too quickly for my taste... (very mild cam and tight converter as well) One of the vendors S476... 92mm T4's would be ideal IMO if you can fit a BW unit. If not, I'd still look at the PT 76/75 units. MUCH smaller, still capable of great power and will light off easily on a 6.0.