I have a 400 cube mopar big block with a pretty mild combo. I have the S400sx 75 mm turbo with the small 83/75mm turbine and I was on the dyno with it at 8 psi. It made 525 RWHP at a low 5300 RPM so I turned it up a little to 12 psi and it made 581 at 5000 RPM. So I suspected either valve springs or to much backpressure. I test drove it today and found that the back pressure is the culprit. It makes 36 PSI back pressure at 12 psi boost and it is climbing I only took it to around 4800 on the street.

Now for the questions
#1 Will this backpressure hurt the motor?
#2 What turbo would be a better choice without sacrificing spool?

 

Wow, you'd think the fact that its still a large frame t6 would keep the bp down more than that. Blown385 has a post on yb where he was at 2:1 with his 1.10 87mm hotside. If I were you id just get the hot side upgraded.

 

How big is your DP and what's the rest of the exhaust ofter that look like? Are you dumping the WG gasses back into the DP as well?

 

It is a 4" down pipe with a single magnaflow muffler staight through design cut at the rear axle. wastegate is dumped into the exhaust. Suppose I could rig up a gauge into the exhaust and see backpressure there. Should be pretty good though. Have a feeling it is the turbine wheel killing me but I thought I could take it further than this.

 

What A/r turbine housing?

 

Yes that much backpressure can hurt the engine if you were to stay in it for too long. At the very least you should upgrade to the 96/87 1.32 A/R turbine on that engine. I don't think it would hurt spool much but should really wake it up on top especially if you keep your 75mm compressor. How is the spool now?

 

s400sx are t4 exhaust housing, you can step up to the t6 if you want. i have a 1.10 t6 housing for the 83/74 wheel that i just took off mine if your interested? i just put the t4 .90a/r on mine. what size exhaust housing are you currently running, i might trade ya even up for my t6? give me a call or text 7178163880 my name is Ryan

 

The sx series are either t4 or t6. I have the t6 1.0 A/R. That is why I was surprised with the readings. I would feel better if it were at least 2:1 but that seems very high as well. The spool is very good but not looking to loose that. I don't have a trans brake so I still can't get tue truck to leave with crap but at least this tells me why my top end seems limited.

 

CW,

Are you checking right at the turbine inles, or closer to the head? Have you checked pressure post turbine yet?

Dave

 

It has been raining and now sleet is coming down so I havn't been able to test anything. I am checking 2" before the turbine.

 

That much backpressure is killing everything. When do you see full boost? I'm willing to bet that even sacrificing a little spool will help the truck leave better. I know if it comes on too quick it can be worse than too slow on these tanks. Having a brake is a whole different world.

On my truck with twin t4's I never had a transbrake because I didn't want to blow my two piece driveshaft all over and I could keep the 60's around 1.53-1.55. Switching from .68 to .96 turbine housings lowered the backpressure to 1:1 but didn't really hurt spool much. Sure made the mph jump though.

 

CW,

Interesting. I can't offer advice, but it's a good subject for me. I have logs, and I hope that they don't kill my setup. I have my recently built TT setup running/driving easy, but I can't boost until I swap my 11.7-1 engine out for a low CR engine. I'll be checking pre-turbine pressure then. I don't know what a good post turbine pressure is [how close to zero], but please post your findings.

BADGMC,

Darned good sixty there. Great, really, at least compared to what I read other guys here doing for sixty. My 3960 lb lump went 1.59 in the sixty N/A'd with 4.30s and a 3200 converter [11.66 at almost 115]. I hope it's not a toad with the turbo setup with same converter and 3.55s.

Dave

 

I can tell you from experience that a 3:1 backpressure should not hurt anything (didnt on mine), just wont make the power that you want.

 

I not sure when it makes full boost. As boost comes up the converter starts stalling higher. Seems things really start happening at 3000 RPM.

What size motor? When do you shift? Ptrim? I have often thought of switching to twins to keep the spool and backpressure down. Guess I will get ahold of a vendor and see what they might recomend for a exhaust wheel/hsg or different turbo.

 

I have a 383 sbc. I shift @ 6200. Starts to spool about 3500, full boost by 4500 with the .96 housings and 9psi backpressure with 8psi boost. With .68 housings my backpressure was 13psi with 8psi boost and they are full boost by 4000 but the start of boost isn't any earlier, it just comes on quicker. Also these backpressure readings were measured just before the turbine housing with my exhaust on the truck which dumped at the rear axle. Truck went 10.80's with 8psi.

 

Thanks for the info. Just to be sure I measured backpressure in the exhaust system am it was right on 2 psi. It is certainly the turbine on the turbo. Have to save money and prep the truck for more power. Trans brake, boost controller, turbo, roll bar, diper.... Lol I just can't leave things alone.