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.
single rear mount turbo c5 with boost issues (lengthy post)
Good morning tech,
I have been getting the bugs out of the my Single rear mount turbo setup on my c5 corvette. I looking for some advice to possibly an idea where to check on resolving my boost issues. First a little info about the car.
2001 C5 corvette:
Stock 4l60e with 3600 stall.
Stock bottom end.
Comp lsr cam 231/239 113
stock 243 heads
melling oil pump
ls7 lifters
Head studs
BTR springs with supporting mods.
Now with my cam only setup mentioned above the car did 400 + hp on the dyno and i was running in the 7's in the 1/8.
After my rear mount setup I am currently only able to make 5lbs on a 10 lb waste gate spring.
Turbo setup:
stock manifolds
3 inch exhaust feeding a t4 flange with a on3 turbo 78/75 with a .96 ar exhaust housing.
55 mm or 60mm waste gate (sorry i cant remember at the moment)
2.5 aluminum cold side
A&a early gen intercooler
4inch tube that feeds from intercooler to throttle body
2 bar map SD tune.
Manual boost controller runing directly off the wastegate.
down pipe is 3inch pipe with vband that is approximately 15" long.
oil feed is 4an and drain is 10an both with one way check valves pre turbo and post turbo, along with remote oil pump.
Yes i have checked for exhaust leaks and cold side leaks and there are none currently present.
After testing the car at the track last night it wouldnt make more than 5lbs on a 10 lb spring. I have not ran the car without the boost controller that will be my next step, but i am also looking for other things to cross of my list in hunting this boost issue. Sorry for the lengthy post, but I didnt want to leave anything out. Any help would be greatly appreciated.
What exhaust manifold gaskets and collector gaskets are you using? How did you check for leaks? Plumb that boost reference line coming out of the turbo to the side of the wastegate without a controller hooked up.
Edit. I would by a new elbow and tap the compressor cover instead of having that fitting in the rubber hose. I would also check your boost pressure at the rear of the car.
What exhaust manifold gaskets and collector gaskets are you using? How did you check for leaks? Plumb that boost reference line coming out of the turbo to the side of the wastegate without a controller hooked up.
Edit. I would by a new elbow and tap the compressor cover instead of having that fitting in the rubber hose. I would also check your boost pressure at the rear of the car.
the manifold gaskets are the metal ones i replaced when i took the longtubes off when i started this turbo build. (cant remember the brand maybe feel pro?) There are no collector gaskets as the later c5 manifolds use the doughnut style flange with the 3 bolt studs and nuts. Yea my next thought was to bypass the boost controller all together and hook up a vacuum gauge to the back and see what it does. The reason i did not tap the compressor housing was to not void the on 3 performance warranty. As far and checking for leaks i made a pressure testing system out of pvc and used the soapy water method and checked for bubbles as well as watching a gauge that was on the pvc for pressure loss.
Here is a picture of the cold side when i checked for leaks. i checked the hotside while it was off the car.
thank you for the advice as i am open for any suggestions at the moment.
Well if you are leak free and using good gaskets the next thing I would check is that fitting in the 90* elbow. Then I would the make sure the valve in the wastegate is fully closed/fire ring installed. I would also check how much boost you have at the rear of the car.
Where are you getting your boost reference from? I'm not sure what you mean by "directly off the wastegate" as that's the only way to hook up a manual boost controller. Boost reference should be on the intake manifold (since manifold pressure is what you want to control). From there to the wastegate with the boost controller in between.
It should be worth noting that running 3" downpipes into a 3" single pipe into the turbo is about the worse possible way to do it. You're losing a massive amount of kinetic energy an pressure between the engine and the turbo. If your setup is fully sealed it's possible that recompressing the air to get into the turbo is jacking exhaust pressure so far up that the engine and turbo can't breath. Smaller pipes retain velocity and KE that spool the turbo as well as allowing exhaust to flow freely out the system instead of requiring mechanical work from the engine to push air through the bottle neck that is the turbine.
Where are you getting your boost reference from? I'm not sure what you mean by "directly off the wastegate" as that's the only way to hook up a manual boost controller. Boost reference should be on the intake manifold (since manifold pressure is what you want to control). From there to the wastegate with the boost controller in between.
It should be worth noting that running 3" downpipes into a 3" single pipe into the turbo is about the worse possible way to do it. You're losing a massive amount of kinetic energy an pressure between the engine and the turbo. If your setup is fully sealed it's possible that recompressing the air to get into the turbo is jacking exhaust pressure so far up that the engine and turbo can't breath. Smaller pipes retain velocity and KE that spool the turbo as well as allowing exhaust to flow freely out the system instead of requiring mechanical work from the engine to push air through the bottle neck that is the turbine.
Currently i have a vacuum block at the front of the car that is fed from the brake booster, I have a mechanical boost gauge that is fed from the vacuum block and that's what i am using to check boost. I was thinking of running the gauge from a source closer to the turbo to check there. I also must add (and i forgot) prior to this hot side i had originally designed this system with 2.5" exhaust and had such a huge back pressure problem it was actually losing horse power on the dyno however it wouldn't make boost with the 2.5 hot side as well. i have checked shaft play at the compressor wheel and the exhaust wheel have zero shaft play. I Am going to eliminate that 90 degree coupler and tap the compressor side of the turbo and by pass the boost controller to see what that results me. Again thank you guys for the suggestions.
Also i have added a video to my youtube channel of me bench testing the wastegate. The hellfire ring was installed when it was on the car...
Link to video:
What psi is the wastegate opening at when you are checking it? If it is opening at 10psi then it will open sooner with the exhaust pressure pushing on it.
Rear mounts won't work like front mounts. Running your boost control from the cover results in low and erratic manifold boost pressure. You must run your boost reference from the intake manifold. Even then, running that much rubber line causes problems. My old solution was to plumb steel brake line tubing from the intake to the boost controller to the wastegate. My new solution is to ditch the bullshit rearmount like I should have done in the first place. Rearmounts are a simple starter solution for someone who wants to learn about turbocharging, but the ease of install is absolutely their only positive attribute.
Thank you guys for the advice. Since I rarely get to drive the car on the weekends. I had planned on running the vacuum source from the vacuum block at the front of the car instead of that add a port coupler. And also bypassing the boost controller and running it off the 10lb spring before I change the spring in the wastegate. One of the ideas I have been throwing around is, and again Ifor im way off just let me know, my my wastegate (with the 10best spring) is fully open by 6 lbs would that be a reason the car isn't making over 5 or 6 lbs ? And by switching to a 15 lb or 12 lb spring would that help me make 10lbs ? I know the rear mount is a lot more complicated than a front mount but with my limited fabricating skills and race car funds, it would have to be awhile before I did a custom front mount turbo. Thanks again for the advice. As soon as I test some things out I'm going to report back with my findings.
Yeah low boost has nothing to do with any boost source, vacuum hoses or boost controllers. You probably want a smaller pipe running to the rear, say 2.5" at a guess, and just as important to heat wrap it the whole way. May also need a smaller rear housing.
There was a reasonably detailed thread on here that was almost identical to this, and he went through and resolved it with heat wrapping and getting rid of long tube headers for standard manifolds. Can't remember enough detail to find it, but someone else on here might.
Yeah low boost has nothing to do with any boost source, vacuum hoses or boost controllers. You probably want a smaller pipe running to the rear, say 2.5" at a guess, and just as important to heat wrap it the whole way. May also need a smaller rear housing.
There was a reasonably detailed thread on here that was almost identical to this, and he went through and resolved it with heat wrapping and getting rid of long tube headers for standard manifolds. Can't remember enough detail to find it, but someone else on here might.
Yeah I remember that thread I belive his name was zombie, really good thread. However I was getting way too much backpressure with the 2.5 piping, when we had it on the dyno with the 2.5 the curve looked good (making power) until late 6psi then it started declining in power. I was spending some time yesterday looking for a picture of the graph but couldn't find it. most of my exhaust is wrapped except the part that goes over the axel. I have been thinking about the exhuast house sizing. But I'm going to try to test all the stuff I have before spending more money at this point.
Step one-send pressure to the top-this should be all of it. See if it spikes past 10psi.
Since you have already boost leak tested it, there is literally no point to doing anything else. Until you determine if your system can actually make the boost.
IMO-you will have to kick down to a 2.5 hot side to get some exhaust velocity back.
So does the spool look the same with the larger piping or does that make it slower?
I would imagine that a single 2.5 should be plenty to feed the turbo. What makes you say it had too much backpressure? Just the boost dropping off? Or were you actually measuring it? If anything I'd use the smaller piping to help maintain heat and velocity and put a bigger housing on the turbo to reduce backpressure.
So does the spool look the same with the larger piping or does that make it slower?
I would imagine that a single 2.5 should be plenty to feed the turbo. What makes you say it had too much backpressure? Just the boost dropping off? Or were you actually measuring it? If anything I'd use the smaller piping to help maintain heat and velocity and put a bigger housing on the turbo to reduce backpressure.
Yeah the dyno looked the same.I did measure backpressure and under boost it was around 60psi. i thought i had a picture of the 3 inch piping dyno but i only have the video. that was another theory i have of maybe the exhaust housing needs to be bigger. I plan on testing the car today and ill let everyone know how it goes.
60psi of backpressure is huge for that boost. That, to me, is a clear indication of what needs to be fixed, not the boost issue. Where were you measuring it from? I'm not overly familiar with that turbo but it's a reasonable size, should not be causing such a restriction.
Have you got any of those flexible bellows in the exhaust before the turbo? And what is your current back pressure? It sounds like you have some sort of restriction in your exhaust. Reading your first post again, it sounds as though it made 400whp NA, and 465whp at 9psi with your old turbo setup in the graph above? You should have picked a damn site more than that.
60psi of backpressure is huge for that boost. That, to me, is a clear indication of what needs to be fixed, not the boost issue. Where were you measuring it from? I'm not overly familiar with that turbo but it's a reasonable size, should not be causing such a restriction.
Have you got any of those flexible bellows in the exhaust before the turbo? And what is your current back pressure? It sounds like you have some sort of restriction in your exhaust. Reading your first post again, it sounds as though it made 400whp NA, and 465whp at 9psi with your old turbo setup in the graph above? You should have picked a damn site more than that.
i was measuring backpressure about a few inches from the flange of the hot side going into the turbo with a small brake line that runs into a mechanical gauge up front. no bellows, no flex pipes, just stock manifolds that have 2.5 Y's into a 3 inch single that runs all the way into the turbo.
I did test the car a little bit in mexico. Both these test bypassed the boost controller. First i ran the car with the supplied vacuum hose from the front of the car to the wastegate, ditched the add a port silicone coupler and it made no difference. Then i took the vacuum off the wastegate and ran "spring only" and although it felt a bit stronger it only resulted in 5 lbs on the top end. My next test will to be trying a high psi spring and see where that gets me.
Gas Monkey Built a 6-Wheel Ferrari Testarossa With a Corvette LT4 Engine
Slideshow: The controversial Ferrari F6 swaps its original flat-12 for a Corvette Z06-derived LT4 V8 and sends power to four rear wheels through a custom-built drivetrain.
7 Most Reliable High-Performance Engines GM Has Ever Built
Slideshow:These GM engines didn't just make huge power, they survived abuse, boost, track days, and six-digit mileage with a reputation for refusing to quit.
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Slideshow: From wobbling harmonic balancers to failed EBCMs, these are the issues that define long-term C5 ownership and what repairs typically involve.
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Slideshow: A modern Camaro transformed into a retro icon, this limited-run "Bandit" build blends nostalgia with brute force in a way few revivals manage.
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Slideshow: Cadillac didn't just crash the high-performance luxury vehicle party, it showed up loud, supercharged, and occasionally a little unhinged...
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Slideshow: A one-off sports car that looks like a vintage Italian exotic-but hides a C6 Corvette underneath-just sold for the price of a new mid-engine Corvette.
single rear mount turbo c5 with boost issues (lengthy post) 
