frustrating situation, can't make more than 14psi
 

This is driving me nuts. I had a .70 divided housing on my T76 and could make boost easily to 16psi, but anything over that and the car would slow down at the track. If I took the rpms any higher than 6000 rpms the power would fall off drastically and detonation would happen, even on 111 octane and methanol.

Swapped to my .81 divided housing, car spools up just about the same, but i'm unable to turn the boost any higher than 13-14psi, the same boost level I was limited to when I had headers and the small housing. I've fixed all my known boost leaks but i'm going to check it again tomorrow. Waste gate never opens now. With the t76 and .70 housing with stock manifolds I would hear the waste gate open at 13psi but would never hear it at 16psi.

So here is the history of my car at the track and the mph it traps.

370 cu in 8.7:1 motor, stock 317 heads, z06 cam

T76, .70 housing, slp long tubes, 13-14 psi, best trap of 126
T76, .70 housing, stock manifolds, 15-16psi, best trap of 126

When I had the long tubes I set the waste gate to stay shut under boost and would still only make 13-14psi. I'm only making a max of 12psi in 2nd gear. The turbo spools up fast, but it will only make so much boost.

With the .81 housing on it now and the boost controller maxxed out to keep the waste gate shut it only makes 13-14psi :(

I'm guessing that my trap speed will be the same since the car feels the same as before. I'm becoming very frustrated with this. I know of two other cars with T76's, a stock motored ls1 camaro that has gone 11.1 @ 124 on 14psi (rear mounted .81 p-trim).

Anyone have any ideas?

A stock motored cobra was at the track the same night i went 10.8. He had a t76 on his stock motor and was running 9.6 @ 140mph on 20psi.

I've gone 143 mph at full weight with the T76 at 16 psi. There's definitely more there. You need to measure your backpressure.

Mike

Just got back from measuring backpressure. With 10psi at 4000 rpms I'm at 20psi on the trans brake. As the rpms go up so does the back pressure. By 6000 rpms i'm at 35psi back pressure on 12-13psi! 3:1 is no good. The back pressure seems to be getting out of control over 5000 rpms.

I can hear a boost leak when i'm under boost because it will wistle so I will be fixing that later today when I can pressure check the system.

I'm never hearing the waste gate open on the car now either.

If I can get the exhaust hotter and cause the wastegate to open the system should become more effecient, right?

Yikes! With my old exhaust, I used to see 34 psi backpressure at 12 psi boost. However, with the new exhaust, I'm seeing 34 psi backpressure at 18-20 psi boost.

Ok, got an update after getting back from the dyno. We pressure checked the system and found a HUGE leak. Fixed the leak and I am able to get way more boost.

Was only able to hit 15psi on the dyno but boost was going 16-17 on the highway aftwards. Back pressure was 40-45psi (fluctuated) at 16psi over 5500 rpms. At 13 psi the back pressure would to 35 over 5500 rpms.

The dyno stats. Temp 85 in the dyno room. 91 octane gas, 14 degrees timing at before 4800 ramping up to 19 by 6000. All pulls done in 2nd gear because I was worried about the length of the pull on 91 octane. Keep in mind these numbers are from a th400 with an unlocked torque converter.

On 10psi the car made:
415 uncorrected HP @ 5500 rpms 452 SAE
492 uncorrected TQ @ 4300 rpms 537 SAE
AVG HP from 4500-6500 is 445 SAE

On 15psi the car made:
476 uncorrected HP @ 5500 rpms 519 SAE
517 uncorrected HP @ 4500 rpms 563 SAE
AVG HP from 5000-6500 is 510 SAE


Hp was a flat line on the dyno but on the road it actually feels like it pulls harder over 6000 rpms which it would not do on the .70 housing.

It looks like i need a larger a/r housing to help with the back pressure when I rev over 5500. The back pressure comes on fast once it gets to 5500, up until that point it's more like 2:1 and then goes 3:1. Holds the same back pressure out to 6500 rpms.

Witht he .70 a/r housing in august I managed 424 uncorrected hp (470 sae) at 5500 rpms. With this power I was able to run 11.0 @ 124.9

sorry to hijack your thread but how do you check back pressure?

I was wondering the same damn thing.

Pressure gage in the exhaust before turbo, duh!

Ok, simple how to:

drilled and tapped a 1/8 NPT hole right before the turbo inlet, used 5' 1/4" copper tube to 1/8 NPT adapter. Ran 1/4" fuel line to el cheap $8 air compressor gage with 1/4" NPT fitting and used an 1/4" NPT fitting adapter. Total cost about $20. Time to install about 15 mins, about 45 mins faster than I anticipated LOL

Ran copper pipe into trunk (rear mount turbo) along with 12' of hose and gage so I could watch what was going on under boost. Dyno operator watched gage on the dyno since i had 10 other to watch.

Great information to have when trouble shooting.

i figured something like that but thought heat would mess the gauge up after a while. cool gonna watch mine soon.

The gauge never gets hot. Without flow, the exhaust never actually reaches it. I use a 1/8 pipe 6" long adapted to rubber hose. The 6" long pipe isolates the rubber hose from the hot header.

When I finally measured my backpressure, I found alot of power.

BTW, we checke backpressure on a T76/377 combo. It only had about 26 psi backpressure at 18 psi boost. The downpipe was 3.5" and very short with no muffler. Just an example of what is possible with a good enough exhaust.

Mike

Good to hear man! Where was the leak?

Ok me and Jermey were talking about this some last night... and Mike you have been in these conversations with us before....

How much backpressure do you think will drop when he wraps the exhaust and is able to maintain higher exhaust temps? I am thinking 5-10PSI.... thoughts?

I can figure it out, but won't have time until later on this week. Send me a reminder Wednesday?

Mike

good post!! This is a rear mount setup? Where was that leak anyways?

which leak grr.

For some my car decided to leak out of the majority of my couplings in the last week. I had 4 different leaks i found in the last 3 days. Things just got loose, I wonder how long some of them have been this way.