346 FAST 102, LS7 TB, GT35Rs, 8.5 to 1, MAST 225s. Got the dyno results after visiting another dyno. The car was shutting down the previous dyno. At 10 lbs of boost at 5900 car made about 600 then laid down. I do not have the dyno to post. At 15lbs car laid down again this time 4000rpm. Backpressure was 4.4 to 1 at 10lbs though and this is the reason for the car losing power. There are no cats.

Downpipe 3" and 18" long. The 2.5" S pipe to primary exhaust which is 2.5" all the way back. Two air to air intercoolers up front on the smallish side. One wet to dry intercooler full of antifreeze.

Turbo housing has A/R .5 cold side and .86 hot side with ??model T712. Turbos are supposedly one offs with the smallest rectangular exhaust flanges.

Car ran on pump gas 10.1s at 138-140 traps with 1.8 60s prior to this build. I never noticed shift points in car as is a handful and car is A4. Car then had 317s, LS1 intake, and stock TB. Cam is roughly 600, 230/234, 116+2 now was smaller. No dyno previous my ownership but track numbers suggest roughly the same HP as now.

Car was used as a street drag car and was successful at it prior to my ownership.

I've been told the .86 A/R is good enough but if so where is the restriction coming from? Can't be the heads IMO. Springs upgraded again with the new heads. The 2.5 connector and 2.5" exhaust isn't helping but I'm not sure going to 3" all the way back will relieve backpressure enough. What do you guys think?

No cats so that's not it. SS mufflers probably not a problem with backpressure as no cat material to stop them up. The car has sat for some time so I suppose something weird like wasp nests and dirt dobbers(sic) could be a problem but that would likely keep the car from ever getting to 5900 rpm at 600 HP.

I figure car should make 800-900RWHP at 15-20lbs of boost maybe less with normal backpressure.

I'm guessing the 2.5" pipes aren't helping but that's not it all. I think the turbos may have too large a compressor in them. I know the internal housing has been hogged out by hand. I suspect they wanted to spool quickly and didn't care about power over 5900-6000rpm as they were shifting before and using the car to drag.

What do you guys think? Also I'm planning on talking to the owner of P1 in MD tomorrow about the problem. So I may be getting recommendations from him. I'm thinking the turbos need to come out and be inspected for the compressor size and to determine for sure what A/Rs are. And I'm thinking I'll have to consider going to a 1.06 A/R with a 'normal' compressor. If they come out I will rebuild them.

 

Who and what is p1 ...I'm in md, never heard of them

 

P1 Performance. They are a Garrett dealer for one thing. That's all I know.

 

Talked to James at P1 Performance. He reiterated another Garrett tech would said turbos were likely not the problem. Apparently with new heads, intake and TB I am critically restricting the GT35s with a 2.5 exhaust. I knew they weren't helping matters being so small but he attributes most if not all the backpressure problem to the exhaust.

He suggested 4" exhaust all the way back from the downpipe.

Anyone with experience with 4" pipes? Is this overkill? Would 3" pipes be enough?

 

While normally I would steer clear of these type of posts, I want to step in an make a point to save you a lot of aggravation. Every TTi Stage X kit (And LPE and others) has 2.5" exhaust installed on them and virtually all have 2.5" all the way out the back. That being said based on their performance I think your chasing you tail with a pipe change at these power levels.

Phil

 

Hey buddy-do you have any pics you could post of the setup? Uptop and underneath?

 

Did I read correctly you have 2 air to air intercoolers and one air/liquid for a total of three?

 

Hey, Arun. Someone is supposed to have emailed you. I'm not near the car but I can have them emailed to you tomorrow.

We're going to drop the exhaust back of the downpipes and dyno the car again. That should isolate the problem to the turbos or the exhaust for the most part. At least we hope.

I'd really like your input though.

We've never heard of 4" pipes but I can envision a way to do them. I can't see how to easily do a X pipe though with a 4" exhaust. I'm not sold on that being the answer either. There are a lot of cars out there with 800-1000HP with 3" exhausts.

There is a H pipe which is causing some added restriction.

We're scratching our heads around here. We really don't want to have to drop the engine again and dismount the turbos unless we have to.

There is a 3" downpipe of 18". Then a 2.5" flange and S shaped pipe towards the tunnel of 16". Then 2.5" to the cat back. There are no cats. Then you have the H pipe crossing over.

Thanks for your interest and help.

 

Yes. The air to air are on the smallish size.

 

The car is making roughly 600 HP at 10lbs at 5900. We think we should be getting 800+hp at realistic boost levels easily. The car is falling off for a reason. Now whether it's the exhaust or the turbos or both is the question.

 

Phil, first I'd like to say I am not questioning your expertise because I know it far outweighs my own, however, I have the APS kit that has 3" pipes all the way until it mates with the cat-back exhaust and I have been told (verified by dyno data) that cutouts just before the bottleneck into the 2.5" cat-back will net 35-50rwhp. I would assume this is a backpressure problem as well.

What I do not have it dyno sheets to prove it. A well known shop owner gave me this information.

In a way, I'd like your thoughts on this claim.

 

Hey Mike, couple of things that caught my attention. You say at 15 psi the car fell off faster than it did at 10 psi.

You mentioned the you have those trick Mast 225 heads? What springs/valves/seat pressure are you exactly running?

As for exhaust-I really don't know what to tell other than just drop the catback first and see what happens-then the mid-section-do this before spending a dime.

Also-is the car shutting down from any airflow codes? I noticed you have a 102 TB? Depending on what you punched in for the TB scaler you may trigger some airflow codes. Sometimes the math doesn't work like it should-especially with 2 or 3 bar on the drive by wire TB's.

And lastley the obvious-stuff like plugs and wires?

 

150ish on the seat pressure closed and I think 450 or so open. Springs are proprietary by Mast and are double of course. Reaonably stiff. I wanted stiffer springs but the guys at Mast said 150ish is good enough for my application. I think these heads are PI and not the new Mast heads.

I'll relay the info about the airflow codes. Didn't know they existed. Wires new. Plugs regapped a couple of times. Even coils changed out to ones we knew were good for sure. Fuel is good although we did get a pinhead leak in the passenger side FAST fuel rail (which is new). They're drop shipping a new one for us even before a return BTW.

The car just dies like you shut it down. Don't think this is valve related. We've already addressed that. God I hope so. I'll try and post a dyno if possible. I just don't have it yet. Died at 4000 at 15lbs of boost BTW.

Again thanks.

 

Does the turbos have the t2 or t3 flange? If it has the t2 flange its causing your problems. The smaller exhaust housing is creating the back pressure and the only real fix is to replace the housings. Do you have cast or tubular manifolds?

Carl

 

If you're running a total of 3 intercoolers, I would begin to wonder about pressure drop on that side. Just because you're seeing 10-15psi at the intake doesn't mean the compressor isn't working much harder. Your 4.4:1 ratio may not be what the compressor and turbine are really seeing. Regardless, a single/twin efficient intercooler setup with low pressure drop goes a long way toward making decent power.

Also, why are you running such a huge, exhaust biased cam on a turbo car? A big part of your potential efficiency in a turbo setup comes from exhaust velocity.

Aside from the already mentioned check of plugs/gap/wires, what kind of timing will it tolerate without knocking? Do know you know where either the knock limit or MBT is? (Are you running pump or race fuel?) Timing and AFR can have a significant impact upon exhaust energy, backpressure, and power.

I seriously doubt you're going to find 200rwhp in pipes and a catback without changing anything else.

 

Plus you've now had 2 of the best turbo tuners I have ever seen calibrations of weigh in. Greg and Phil are both legends in their own respect.

EDIT: No disrespect meant to Arun, I just haven't seen any of his work, he's certainly more than helpful and from his posts is no stranger to making big power with snails.

 

 

I don't expect to get 200 HP from the fact of changing the exhausts. The problem is the car is running into a brick wall. Remove the problem or restriction and the car should make some decent power. There is something wrong with a car that stops making power at 4000rpm at 15lbs of boost. The fact we're getting to roughly 600HP (it's slightly less) at 5900 on 10 lbs of boost is just somewhat of a break. The car should make more power at 10 lbs as it should be able to reach 7000 easily with the parts on the car.

It's the backpressure that's killing the power. It's not a lack of parts on the car to reach a goal of roughly 800 HP. GT35Rs should support 500-600 HP each. A 346 with the parts that car has should make 800+HP pretty easily.

 

We're measuring back pressure right at the turbo.

The cam is mediocre IMO. The lift is just less than 600. Duration is 230/234 which isn't pushing it that much. Or am I wrong? We've talked with Comp Cams as it's a custom grind and they see nothing wrong with it. In fact they suggested we go to a 236 on the exhaust side to try to relieve the back pressure. Are we missing something here?

I'll have to ask about the timing and knock. To my knowledge we're getting no knock at all. But we're running half race gas to keep it safe until the final tune on pump gas and meth. I'm afraid this is all at the limits of my knowledge. In fact while I understand the basic concept of back pressure I don't fully conceptualize it in my mind. I do understand the banana in the tailpipe concept though.

I didn't build the car. I would have used larger air to air intercoolers and would not have a wet to dry on the car. I do understand why they did it. Apparently the car was run to race on 116 and they put dry ice in the well to lower the IATs significantly.

 

I was going to use Mast double valve springs on a set of their large bore LS3 heads. The machine shop (that my tuner used to assembly my heads) tested them and said that even if they shimmed the crap out of them they would barley work on my motor N/A. I ended up going with PRC springs and ate the $375 I paid for the Mast springs.