Hi guys, after your thoughts on an ideal cam for a high back pressure scenario, on a rear mounted turbo.

The back story is I've LS1 swapped an RX8 Mazda. Originally the set up was NA, with a 224/228 110LSA cam. Car made 292rwkw/390rwhp on a rolling dyno, with peak power achieved at around 6500rpm. It's worth mentioning also I'm running a T56 manual gearbox.

Like many similar stories, not satisfied with that power level I decided I'd try my hand at boosting the engine. No room in the engine bay, but plenty of room out the back where the factory rear muffler lived, I decided to set up a rear mount turbo system. The turbo of choice was a BW369-SXE, with a 1.0AR turbine housing. Given the LS1 still has a SBE, my goal was 410rwkw/550rwhp. Borg warner themselves rate this turbo at over 900hp capable, and when I reviewed the compressor map it seemed a reasonable choice of turbo for the power I was chasing, even though in the world of boosted LS engines, it's deemed on the smaller side. I've also read and seen plenty of examples around the place of people using the BW366 on LS1's (front mounted though), that have supposedly made 550rwhp or more. So I thought my expectations were realistic....

So after having the block honed and rings gapped, I fitted the engine, put some miles on it, then took it to the dyno for tune. Tuner made over 30 pulls, and the best he could achieve was 387rwkw/520rwhp, and 800nm/590ftlbs of torque at just over 11psi of boost. This is on 98 octane pump gas. Peak power was just before 5000rpm, and peak torque roughly at 4500rpm. At 5000rpm power drops off dramatically - refer to the attached dyno chart.

Tuner felt the 1.0AR turbine housing is choking the engine and not letting it make power in higher RPM. Although I agree the turbine size is probably not helping the top end horsepower, I'm also suspicious that the cam, with 6 degrees of overlap, isn't helping my situation either and the reason why power drops off so hard, In NA form the same engine and cam made power much higher in the rev range.

With 4.11 gears in the rear and a quickly spooling turbo, it's a hoot to drive. However, I still think there's more on the table to take, and I'd like to optimize the set up where I can. I will try to look for a bigger turbine housing as part of the solution, but I think also I should consider a different and more appropriate cam, given the system is likely running higher than usual (when compared to a front mount turbo) back pressure. Does anyone have any direct experience of changing their cam in their boosted LS from a tight LSA, to a wider LSA, and what was the outcome? Did power improve or fall away? What sort of cam specs do people suggest?

Probably worth mentioning I've asked this same question elsewhere, and the typical response has been "Richard Holdner says every cam is a turbo cam"...... which I get, but not really helpful. My car makes power now with the NA cam, but my question really is around finding a cam that will optimize my set up, which I think is a little unique given my chosen turbo and one that's bolted to the back of the car.

Looking forward to your thoughts and feedback, and thanks in advance for any help and guidance. I'll try and answer any questions you may have, but I'm no tuner, so probably can't shed much more light on any the tuning specs or parameters.

 

If I were to spec a cam for you with that little of info, it would be no different than you guessing yourself. You need to do two things: 1) Post a boost curve and 2) Install a fitting into your exhaust to log exhaust pressure. Without having those two pieces of information, there's no way to know what cam events will be beneficial. However, judging by your power curve, it's probably 90% likely it's the turbo size that's killing power, not the cam. I'm going to be running a 78/75 with a .96 A/R on a 5.3, and that's a little turbo for my application. You have more cubes and less turbo, so your exhaust flow is simply getting choked off. I'm still just guessing here, of course, but this seems the most likely scenario. Log your backpressure and go from there. A cam swap is probably not going to do what you need it to here.

 

@ddnspider is running a VS billet 7875 in a rear mount configuration and has had excellent results with it.
Hopefully he'll chime in.

 

There's NO reason you should have power peaking that early and fall off like that. What's the entire hotside? Manifolds? Cats? Merge? Etc. If you're not running stock manifolds and a decatted stock diameter style y and I, you're hurting yourself. Heat wrap everything.

What does the post turbo exhaust look like? Also very important. Any restriction in the post turbo hot side will choke it. This would cause a fall off like you're talking about.

That cam isn't ideal but 6 degrees of overlap isnt massive issue. Overlap is a concern on a rear mount because of back pressure for sure. Have you measured your back pressure? My Titan 2 is about 0 degrees of overlap and I've revd over 7k on my 5.7.

I wouldn't be surprised if your gears were too steep for the setup.

 

my billet 7875 is an ebay knockoff...$280 and going strong like 3 years later lol. Thnx for the tag.

 

That turbo may be rated at 900hp, but not on a 5.7 liter. Its just too small for what you are doing. Measure back pressure and go from there to confirm.

You always have overlap. But if you want to run 2:1 or higher drive pressure ratios you want a healthy amount of negative overlap @ .050. Something like the LS9 cam would be the cheapest solution. It would also limit power before 5k which would prob help keep things alive if it's a gen3 rod LS1. Esp. since you are making boost so early. Would be cheaper and easier to swap out a larger turbo than to fart with cams if you can live with the response change. Also I believe the SXE line was designed for smaller engines with higher pressure ratio's. Think DBRODS found that the S366 actually flowed more and was better suited for the larger engines w lower pressure ratios than the S369 SXE. He made less power with a pair than he did with the S366's. Ended up with precisions w around the same wheel specs and it blew the S366's outa the water. Has a post with pics and all here if you search

 

Thanks for the feedback. Firstly, I haven’t measured back pressure.

The hot side use factory cast manifolds, and off each manifold I run a short length of schedule 10 steam pipe with an internal dia of approx 2.25in, to a flex joint on both sides. From each flex joint pipe size drops to 2in , where both 2in pipes meet and merge at the back of the gearbox, to a single 2.5in all the way to the turbo. Whole hot side is heavily heat wrapped, and in fact from the flex joints to the turbo it’s double wrapped. I also run a turbo beanie on the turbine.

There are no cats or mufflers from the engine to the turbo. Post turbo it has a small (14in long) , straight through 3.5 inch muffler, which exits out the back of the bumper through 3,5 pipe. I think the post turbo set up is as good as it could be, without running virtually any muffler.

 

Engine is coming out for another reason, hence why I just thought whilst it’s out, why not try another cam if there’s a good chance it will help me achieve my goal.

And yes, it is a gen 3 bottom end LS1 and the reason why I have a self imposed hp limit of 550rwhp. I guess as I’m only 30hp away from target is the reason why I’m looking at making some changes to achieve what I set out to do. Given the set up is a manual, rear mount, I wanted to select a turbo that was small enough to meet my goals and still be responsive. I thought I could do it with this turbo.

What about going to a larger turbine? Surely that would help?

 

Thanks for confirming your hotside. Sounds like a combo of turbo sizing and cam overlap then.

 

The system seems pretty well thought out, I'd only question the 2" sections, but those could be ok IDK. Do you have the cam card or any of that? Is it set up 110 or 110+2 or 4 etc? What valve springs are you running and if you know what setup height? What intercooler if any? More data will help us all a bunch so that is probably the next thing to do, but the curve definitely looks like other high backpressure rear mount ones I've seen. What happens at lower boost levels does the curve look "better"?

 

Thanks for the feedback.

What do you see as the issue with the 2 inch pipe? Too small? It’s a fairly stock standard set up in the rearmount turbo world.

I’ve attached the cam card. I installed the cam myself and it was simply done “dot to dot”. No advance or anything like that. Before I installed the cam I called the supplier and that’s all they recommended to do. Valve springs are PAC 1218. Manley push roads. Don’t know the height. It runs 241 heads, but just prior to fitting the turbo I ported the heads myself, and had the valves all seated and angle cut by a professional. The porting of the heads worked because I picked up 20kw of power on everage throughout the rev range, and also a whole heap more torque, particularly off the bottom end.

intercooler is an eBay job, advertised to suit a GTR Nissan. Tube and fin, 75mm thick, 280mm x 600mm wide. 2.75in inlet and outlets. On cruise inlet temps were only 5 or 6 degrees above ambient. When it was on the dyno I don’t know what they were exactly, but tuner commented they were fine.

I’ve spoke yesterday to VCM, the guys who made and supply the cam (I’m in Australia). They agreed my current cam isn’t the best for boost, particularly at higher rpm, and suggested another one of their off the shelf cams they recommend for boosted applications that has zero overlap. Specs on that one are 224/232 with 114.5LSA, 580/590 lift respectively.

 

I’d test exhaust backpressure and pre intercooler pressure and report back. That intercooler might cool ok for now, but be a huge flow restriction for this size of an engine causing you to push the turbo way harder than you think you are.

Me personally going into a higher backpressure type of build like a smaller rear mount I’d sort of lean towards a single pattern type cam, nothing too large like a 220 220 or 226 226 etc. that is one area I’d love to see more dyno testing on a high backpressure setup and different cams used. My old setup was a 383 tt up front and 226 226 cam they made 1000 relatively easy so you don’t need a giant cam to do what you want.

 

If the motor won't pull past 5k, its corked up. Housing changes help a bit. what is the largest available housing? Larger exh wheel would help more. I wouldn't just want 0 overlap at .050... I'd want a healthy amount of negative overlap at .050. But since you don't know back pressure its hard to spec anything out. The issue is when you widen the LSA a ton it shifts the power band up... and if you are corked up at 5k... you never take advantage of that. What would that new cam cost VS a larger turbo would be my question. I think you'd get more out of different turbo.

 

No matter what he does if it’s turbine related it will take decent rework since he is in a s300 chassis. If they had a good turbo shop nearby they might fit a larger turbine wheel and mod his housing, but the most I’ve seen is going from a 73ish exducer wheel to a 75mm so not sure that would cut it. I’ve seen another rear mount gto make 700+whp with a 5.7-6.0 and sxe372 so same hot side and is why I also suggested double checking the intercooler flow. Just bypass the intercooler, run low boost and see how it does. Either way need some testing done.

 

Off the shelf the largest turbine housing is the 1.0 AR, which is what I have. I have seen online a couple of places selling a1.1AR, and I’m in the process of confirming whether this would fit my turbo.

Interestingly when I was putting this build together, I posted my turbo specs to a couple of rear mount turbo FB groups, to get feedback from others who had similar builds. Often I was told the 1.0AR turbine would be too large, laggy and the suggestion was to go down to a 0.91AR! But this was only other people’s opinion of course, based on their on experiences.

Coincidentally I did end up having an online chat with the guy that was supposedly involved with the design of the s300-SXE line of turbos. Can only assume a guy that works at Borg Warner who designs turbos for a living would have a reasonable idea, but he also said the s369 turbo was more than capable of achieving my set power goals on the LS1. So it’s a bit of a mystery why right now it isn’t.

 

Thanks, will consider what more efficient intercoolers are available.

 

I do remember what forcefed said about the 366 vs the 369 , someone here had pictures of the 2 turbine housings while disassembled and the 369 was very restrictive. You mention flex joints after the manifolds - the type with mesh or weave on the inside ? Have been many cases of them closing up and choking the engine, not likely when very new but might be worth checking

 

Thanks, interestingly you say that as they're the mesh type. They are only small ones, like 3 inch long, and doubt they are the issue as they are new. However I'm going to replace them anyways as they have a slight leak around the bead.

 

Hmm, a 224/228 110LSA cam should pull past 5000 rpms even with an undersized turbo. Also I hate dyno graphs that don't have matching axis for power and torque and have it plotted over speed, it makes the curve almost unreadable.

At a glance I would say you have some valve float due to the single beehives and extremely high exhaust backpressure.

 

I'm curious, what makes you suspect I have valve float from the dyno chart? Not disagreeing, just trying to learn myself.