C6C6CH3vo

iTrader: (9)

Did a quick test on turbine pressure relative to charge pressure for those curious about backpressure on small turbo's.

Setup: VIII turbo with 10.5 HS, stock WGA with about 5psi preload over stock, stock cam gear settings, methanol injection, MBC, stock exhaust manifold and O2 housing, stock intake manifold, IC and charge assembly, temperature about 75 degrees.

compressor psi / turbine psiRpm data is crude relative to pressure differences. Pressure will peak at 48 psi I believe due to pressure induced WG relief.

Pressure was obtained by tapping into egr valve hotside and readings were taken from a pressure guage.

Eventually I will do the same test with delayed intake valve openening

JKav

iTrader: (8)

Good info. FYI if you were to do a pull from ~2krpm or so and plot backpressure vs rpm, you will see a increase in slope as the wastegate starts opening.

Assuming VE doesn't fall off a cliff beyond 7krpm, backpressure will keep rising beyond 48 psi.

CandCPerformanc

at those back preasures I would start looking for a new turbo, granted we run a v8 in our race car but none the less we had a very simalar looking chart in the start of our quest for a championship. We had tried 2 turbos both same GT4708 compressors 2 different exhaust wheels and 11 exhaust housings before we got the setup we wanted. our original peak preasures were 19psi boost @ 49lbs backpreasure making 849 hp at the wheels on a Dynocom dyno. Our final turbo and exhaust housing combined with a HKS 60mm wastegate neted peak power at 17psi boost at 21lbs back preasure making peak power of 978hp all gained by getting rid of the backpreasure. Downfall is lack of spool up time in a street car, but I am trying other ways of getting rid of the backpreasure by going overjill on the wastegates.

C6C6CH3vo

iTrader: (9)

Here's the vid

http://s49.photobucket.com/albums/f2...t=102_1124.flv

JKav

iTrader: (8)

There are definitely a lot of gains to be had by reducing backpressure. What's amazing is that so few shops/tuners/racers even measure it. So you're one up on the others right there.

With that said, bigger wastegates won't do a thing for backpressure. All a bigger gate will let you do is run less boost.

jid2

iTrader: (62)

Good info for us nerds. Thanks.

CandCPerformanc

I could understand that theory of not making boost if you were using a small turbo but when using a large turbo like what we are using I just can't see it

crcain

So the assumption is the stock gate will be forced open due to exhaust backpressure at around 48 psi?

Ted B

iTrader: (6)

Good info. The same type of info was used by DB and Robert of FP to expose the shortcomings of the ill-fated 6-blade '20G' unit, and why it did not deliver the expected power.

Agreed. I'm putting together a little rig to monitor intake manifold press, turbine press, and EGT. This should provide insight as to turbine housing sizing (a fuzzy area with respect to twinscroll applications), as well as finding the limits of the boost pressure / ignition map picture.

Sizing turbo compressors for an application is the easy part. It's the sizing of the turbine side that is the challenge to getting the best compromise with respect to power vs. response.

JKav

iTrader: (8)

Very cool. If possible, also include turbine discharge pressure.

Will you be using the Ford EBP sensor?

Agreed. I can possibly help here, just let me know

Aby@MIL.SPEC

iTrader: (161)

nice info there...

what's really neat about this too, is most people have the ability to measure their Pr @ the same location

Ted B

iTrader: (6)

Will do JKav. I'm presently using a smallish .70 A/R TS T4, but that's going to be too small for what I want ultimately. I've already secured a 1.00, so we'll plug it in soon and see where that takes us.

As for the EBP, the Ford sensor might be a good idea. I'll have to look into that. I hadn't really considered the discharge press. I may end up needing too many fittings to squeeze that one in, although I'd like to run the EPB and EGT from the same manifold tap in order to make all as practical and easy to retrofit as possible.

C6C6CH3vo

iTrader: (9)

Yes, but just assuming this. The only way to tell for sure would be for me to permanently fix the WG closed and take a reading for comparison. Even though the boost may be higher as well, I'm assumming the turbine/compressor ratio will also be greater.

The 48 psi backpressure itself does not scare me with at least 26 psi on the charge at 7K, what scares me is what it would be beyond 7K when boost tapers even more.

I'm anxious to see how decreasing overlap slightly will effect this, like about 2 degrees delay on intake.

C6C6CH3vo

iTrader: (9)

I'm wondering if there in any profit by just increasing the turbine housing if there is one available larger than a 10.5. Hate to go through the trouble and expensive of a new turbo when mass flow is the primary problem. A test with a 9.8 would give a good indication.

Ted B

iTrader: (6)

There's no telling until/unless someone tries it. The compressor will still be limited at high rpm, but relieving some of the exhaust pressure should pay off. The real question is to what degree will spool be affected, and will the tradeoff be worth it.