When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Quick rundown on my setup now. gen 3 6.0 lq4,forged dss pistons/rods - .030 ring gaps,stock crank,ljms stg 2 turbo cam,stock 317 heads,btr dual valve .660 springs,turnion bushed stock rockers,seimens 80lbs injectors,ls6 stock intake. cx racing 4' intercooler,stock flipped truck manifolds to 2.5 merge to my on3 84/75mm billet turbo,2.5 cold side piping.full 3' exhaust to a qtp 3' e-cutout just before my gmmg muffler due to floor pan/sfc's is the best place for it fitment wise. Also running aem progressive water/meth kit.Timing is 14* and Im hitting roughly 17psi and tapers down around 6100rpm. My car is a t56 and gets compared to everything being stalled autos making way more jam with less boost.Albeit most are using e85 which I dont have here.I run 94 octane.
The car just dynoed its best of 692whp/656tq 2 weeks ago.Previous best was 650whp last year on stock gen3 pistons/rods which i bent twice and went forged.My tuner and myself both think the car should be making far more power.i cracked 630whp with ease at 14psi.Turning up boost was gaining minimal results.At the end of it we put a backpressure gauge in the afr bung just slightly after the turbo and made a pull at 17psi boost. I measured 8psi backpressure!!! On what is a very free flowing exhaust to my cutout that is open during all dyno runs. Video shows the gauge at the end here:
So Im curious to hear what everything thinks.Ive already asked my tuner on his thoughts.Before everyone jumps on blaming the tune i will say it had 0 knock ..afr was clean 11.2-11.0. But something is fighting this setup from doing better..way better. I dont have the realestate to go up in exhaust size to 4' off the turbo,some say the cutout while open is just not enough flow and the gmmg muffler is creating the backpressure.others think its timing/tune related,other think the on3 turbo is just too small.Curious what you guys suggest i test out to make this thing breathe/exhale more efficiently to make the power..i think the combo should be mid 750whp/680tq.8psi of backpressure is not going to make this motor live long under high boost.
Generally speaking, turbos don't like chambered mufflers. You may have better luck with something that is more of a "path style" or even straight through muffler since the pulses are not as intense after the turbine and you really just want constant flow.
Also, does your boost taper off only on the Dynojet (assuming it's an inertia only run), or also on the street/track? Turbos respond differently to increases in load (wind drag) and you may see a difference in the real world vs the inertial dyno at the top end, which may also affect backpressure.
Def sounds like you need to free up the exhaust, but here are a couple points to consider...
1.) What kind of power do you think a cam only 6.0 is making? What are you revving it out to? What compression ratio are you at?
Figure the absolute best you can do is come close to doubling the NA HP at the crank per 15-16lbs... So 692whp sounds pretty normal to me. 15% is used alot for guessing drivetrain losses with a manual trans. That would be 104HP. So add 104 to 692 and you’re at 796 crank HP. Basically 800 crank. Half of that is around 400 NA. All checks out IMO.
Also you seem to be making round 20hp per pound if you went from 630 at 14 to 692 at 17. Which is normalish gain wise for a mild 6.0 and single turbo setup like yours. Esp. once you get up into the higher boost ranges. By addressing the things below I think it could do a bit better, but it doesn’t sound that far off to me. Figure you are also losing a little bit with mild timing and pump gas. E85 guys typically see 7-9%+ gain in WHP just switching fuels (same timing and boost) So don’t compare yourself to them.
2.) Back pressure is measured in the exhaust manifolds pre-turbo generally, not at the exhaust pipe. This would also be a good number to have. That said you should have zero psi in the exhaust post turbo. Any pictures of how your setup is laid out? Is your 3” dump positioning now done correctly? Can you go to 4” for a short run with a 4” dump, then neck it down to 3” after?
3.)Have you checked the pre and post IC boost levels? The cheap china IC’s are pretty bad. I’ve seen as much as 11 psi lost post IC. Treadstone is good middle ground. Garrett and Bell are some of the best. Treadstone makes a drop in replacement fot that same CX unit you have. Made a BIG difference on my setup. I ran the same cheapie china 32x12x4 IC all those vendors sell.
4.) How much “meth” are you using? And what type of fluid ratio? Might try some race gas and leave the water/meth off. I’ve seen alot of people drown out power with too much water doing much more harm than good. (myself included). I actually picked up 3mph at the track turning my water/meth off. I was just spraying too much with too high of a water content. Not saying it’s not a great product, but you need to ensure you are using it correctly. Personally with pump gas I’d run like 98% meth and 2% water at big volumes.
Last edited by Forcefed86; 08-07-2018 at 11:06 AM.
I have a 87 TA too and your not going to fit a 4" dump anywhere withou alot of mods. I too run a 3" DP into a 3" cutout in the same location and a 3" catback (dynomax ultra flow SS straight through perf core). With the cutout open Ive gained 2psi with no changes, 500rpm increased spool time and this is with a 78/75 VS cast turbo. The backpressure for sure is killing you a bit but unless you dump out the fender I dont see how your going to do it easily. I also feel that your turbo is a bit missmatched 84/75 seems a tad on the small side (exhaust) even if its a billet on the 6.0 and the pressures your pushing. But once you go over a 3" DP exit your going to have to redo the exhaust and DP which you dont have room for it. With that being said as forcefed86 explained your power doesn't seem that "off". But a decent sized borg S480 and a short 4" fender dump would work well but be loud!
Your Turbine is wayyyy to small.. a 75mm on a 6.0 is going to cause backpressure issues.
I agree bigger is generally better to a point... But to say he is going to have “back pressure problems” and a 75mm exducer is Waaaay to small is BS. He isn’t even talking about turbo back pressure here. He is talking about post turbo exhaust pressure. Thats not going to change with a larger turbine wheel.
Tons of guys run a 75mm wheel and it’s very capable. The small S475 has a 74mm exhaust wheel and it’s been deep 8s at 900+whp on 6.0’s.
I agree bigger is generally better to a point... But to say he is going to have “back pressure problems” and a 75mm exducer is Waaaay to small is BS. He isn’t even talking about turbo back pressure here. He is talking about post turbo exhaust pressure. Thats not going to change with a larger turbine wheel.
Tons of guys run a 75mm wheel and it’s very capable. The small S475 has a 74mm exhaust wheel and it’s been deep 8s at 900+whp on 6.0’s.
this seems like such a common misconception and i dont know where it came from. ill admit i dont know much. but everyone seems to recommend the 96 wheel 1.32 ar for people wanting 650 hp.... i dont understand it?
There is no “talk” when it comes to back pressure. You want absolutely nothing after the turbo. The shortest and largest diameter pipe you can possibly fit.
There is no “talk” when it comes to back pressure. You want absolutely nothing after the turbo. The shortest and largest diameter pipe you can possibly fit.
Exactly. Pre-tubine back pressure hinders power because of reversion. POST turbine pressure does multiple bad things. One to note is that it ADDS to the pre turbine BP. so if you had 25psi with a good flowing downpipe that now will become 33psi pre turbine because of the restriction after the turbo. Get it to zero after the turbo, plain and simple.
Yes thats correct...8psi AFTER the turbo in my 3" downpipe with the cutout being 2 feet away maybe on a fairly straight piece of pipe. So reading the comments,its clear something needs a change so lets start with the simple things to try.Im going to upgrade to a larger cutout.4" badlanz,i really enjoy the sound of the gmmg muffler but if its going to risk damaging the motor then it will have to go also
Its clear that for the setup right now..i can gain some power and longevity making the exhaust flow better.that is easier said than done for my car and very limited space.All I can do is approach things 1 mod at a time and see what makes the best improvements. I dont think is as much a turbo issue as opposed to exhaust flow overall and that it wouldnt matter what turbo i had on..8pis of backpressure needs to be addressed.
Boost pressure from the turbo to the intake on my 2 seperate gauges show about 1-2psi difference fyi.Nothing I think requires to change coolers yet anyways.
There is a lot of confusion with Borgs wheel specs too. They advertise the large side of their exh. wheel, not the small side like everyone else. Their “Small S475” Lists the Exh. wheel as 83mm, but the minor is 74mm. Borgs advertised “96mm” exh. wheel is about 87mm.
Personally for a max effort SBE auto. drag car I would jump straight to a T6. They plain make more power per pound. With a manual trans “street” car with moderate goals like 650whp... A t4 would work great with less lag.
To the OP –
1-2 psi is fine. Move on.
Look into the Hooker MAXFLOW muffler. It’s one of the quietest non-chambered mufflers out there.
Also make sure the cutout gets priority, not the piping to the muffler.
You want a straight shot out to the cutout like this.
what are common numbers for POST turbine back pressure on cars with full exhausts? if measured right after the turbo, wouldnt you expect to see some pressure there? theres alot of air moving there
Never have tried to prove it,, but I was told that on a NA motor,, between the engine and the cat.. .25 lbs back pressure meant the cat was on its way bad..
On the healthy street cars I've played with ,, I've never seen a BP number at the O2 above 1/2 lb with a cat that wasn't fried and without a cat and any
decent muffler you can't even hardly measure it with a vacuum gauge..
700 to the tire seems healthy for your setup to me. could the 8psi be a false reading somehow? just throwing that out there
definitely possible it could be a faulty gauge. I really dont want to change out the gmmg muffler as I do like its tone.Im going to step up in size for the cutout as that might be an improvement going with a 4". We shall see.Im chasing minor things now for the setup.
I have 4' of 3.5" down pipe with a kooks bullet to my dump and I see less than 1 psi of post turbine pressure with the dump open.
even that costs power. when I did just a 18" hater pipe thru the hood, it made the truck so much stronger and faster spooling. probably 50 wheel or more.
definitely possible it could be a faulty gauge. I really dont want to change out the gmmg muffler as I do like its tone.Im going to step up in size for the cutout as that might be an improvement going with a 4". We shall see.Im chasing minor things now for the setup.
How is enlarging the cutout going to make a difference if the piping leading to it is still 3". Doesnt make sense... the bottle neck isn't the cutout... its the pipe coming out of the turbo to the cutout. Going to 3.5-4" from the turbo to the cutout with an equally sized 3.5-4" cutout and then reduce down to the original 3" catback would make a huge difference. But if you can't increase the pipe size then the cutout size doesnt matter. unless you place the cutout right outside the turbo DP at an angle that provides a direct shot out.
Might be some improvement based on the cutout positioning. Think of it as similar to poor WG positioning causing boost creep. Which can sometimes be cured with a larger waste gate. OP still hasn’t posted his cutout positioning so this might not be the case. I do agree though. Run 3.5 or 4” to the cutout. Where there’s a will there’s a way... oval pipe...big hammer... whatever it takes.
08-07-2018, 01:12 AM
Let's talk backpressure vs boost 
