I thought I was the last one you would need to tell this. Anyone into turbochargers knows that it is very important to have an unrestricted exhaust discharge from the turbos if you want good performance.

Well, after driving my Turbo Z24 all winter, I finally got the TT IROC out again last weekend. I haven't driven it much with the new, larger turbochargers (TO4E 57 trim compressors, T3 Stage III, .63 A/R turbines), but it really wasn't feeling as strong as it should. I was seeing 6psi on the boost gauge, but it just didn't seem snappy (honestly, it felt like my Z24 would roast it!). I don't know why I didn't try it before, but I decided to open the electric cut-out to see how it felt. What a world of difference! The boost increased by 3psi to 9psi, and it revved right up to 5500rpm (it would labor above 4500 before).

The old 45trim T3s really didn't feel that much better with an open exhaust, so I didn't think my exhaust was actaully that restrictive. I guess the larger turbos really opened the thing up to the point were the exhaust was a problem. After the cut-out, the exhaust is a 2 3/4" pipe back to the muffler, and then a cross-flow Flowmaster muffler with two 2" outlets. It just choked my engine!

Here is the cut-out and y-pipe:
(When I race it, I'm going to run open down-pipes this time!)

 

you should put a hobbs switch on the intake to open the cutout as soon as boost reaches like 5 psi. you could also use it to activate the alcohol injection

 

Yeah, I thought of that too. The only prblem is that it takes 4 seconds to open the valve.

Yes, I should add water/alky injection, or get a more serious EFI controller and learn to program it (and use larger injectors- I only have 24lb SVO's in now).

 

time for a 3 or 4inch catback.

BTW how are the manifolds looking?

 

what are you using for a cat back?

 

The manifolds are doing great. I'm so glad I got it done. The cheap header paint would be probably be showing some rust speckles by now.

 

After the cut-out, the exhaut reduces to a 2-3/4" pipe back to the muffler, which is a flowmaster cross-flow (no-idea what series or anything). The muffler has two 2" or so (maybe 2.25") outlets. (think maybe there is a rats nest stuck in there yet too?!)

Yeah, either upgrade the cat-back or run the cut-out open. The problem with leaving the cut-out open is that it sounds like a stock engine with open exhaust- you all know how bad that sounds (it really sounds sick except for th turbine whistle).

 

Nice set-up you ought to see some nice numbers on that set-up. Just a suggestion. In my limited experience, turbo's do not like flowmasters or chambered mufflers in general. If you upgrade to a larger system you may want to consider magnaflow or bullet. Or you could run one of these

I like the purple one

 

good point on that chambered muffler thing. the way they are designed to work doesnt help turdos much

 

That makes me want to get a cutout for my setup. I have 3" downpipes merged to a single 3" (really the biggest i managed to fit with my limited fabrication skills) and then out through a hooker 3" catback (crossflow muffler, 3" in, dual 2.5" out).

Ive been thinking about the idea of welding in some flanges where the cat would be, and making a segment of pipe w/ a (electric?) cutout, and another one of the same size/shape with a cat so i can make a laughable attempt to pass emissions (visual cat check and sniffer).

 

Where have I heard the chambered muffler deal before? (now that you brought it up, I think I did hear this previousely). Anyone know the science behind why they hurt turbo performance so much? I mean 3psi pressure loss in the intake manifold due to exhaust restriction seems like a serious problem. I really would like to see the difference in the exhaust manifold pressure! (I should test it I guess)

Not only was the peak boost higher, but the response was much nicer- with the exhaust choked, the powerband was more linear. Opened up, it gives that 'slingshot' feel like a proper turbo system should. My old turbos worked great through the muffler. I can't believe that the new turbos flow that much more air at the same boost pressure.

 

I know that most chambered mufflers advertise that they use pressure waves inside the chamber to cancel out sound. I don't know the science behind it but this has a negative effect on the turbo. Turbos just like to breath, they are a big enough restriction on their own. I guess the design philosophy behind a turbo exhaust system could just be described as, "get the hell out of the way."

 

Real world, it has little to do with chambers… the fact is that they just don’t flow well. WRT to flow, in raw cfm #’s from lowest to highest typically you’ll start with the flowmaster style chambered muffler, then go to the typical “turbo” muffler (originally actually designed for a turbocharged corvair), and then finally the straight through designs. There is some variation with them (ex, seemingly identical glasspacks can flow dramatically differently depending on the perforation design). Even when flow is in excess of what you need to prevent a restriction with all the designs, they still act differently.

WRT to why this effects a turbo so much is that it draws power from the turbine, essentially the amount of power that is available is proportionate to the ratio of the pressure before vs the pressure after the turbine. The power level you’re running at results in a pressure ratio of 2:1 across the turbine, with no restriction after the turbine that means that you’ve got 14.7psi pressure before the turbine. If you have 3psi restriction after the turbine, you need 20.7psi before it to get the same pressure ratio. That means that the cylinder is seeing 6psi more at the exhaust port if you increase the restriction after the turbo by 3psi. In both cases you’ll be seeing roughly the same boost on the intake side (in most setups about 6-8psi) but you’ll make less power due to less efficiency proportionate to how your cam allows that pressure difference to effect the engine during overlap.

You’ll also see slower spooling because you have to reach a higher pressure before the turbine before you’re spooled to the same point, which is probably the reason why you were seeing less boost (this also implies that your boost control isn’t great, but that’s sorta irrelevant to the rest of this discussion).

OTOH, if you had a cam with an actual, advertised negative overlap then this wouldn’t have much of an effect on power, just slow spooling somewhat.

 

how does all this effect cars with the turbos at the rear? the idea initially to me sounded pretty hinky, how well do "expanded" exhaust gasses drive a turbo?

 

I've been thinking about this too. The RMT usually have nothing after the turbo, so its just like running an open down-pipe. The greater pressure difference acreoss the turbine may help recover some of the lost response due to lower EGTs.

Yes Mark, my boost control is'nt that great. I seriousely modified my stock T3 Volvo wastegate acuators and swing-valves to work on the new turbos. I guess I should have gone all the way and installed external gates, but I wasn't really looking for more fabrication work since the turbo system was already up and running (plus Chris had already coated the manifolds before I decided on the turbo upgrade).

 

How loud is the cut-out system with the turbos, compared to a cutout without turbos. Does the turbo really quieten it as much as they say?

Want to impliment something similar soon on my car.

 

Well, here is the thing… typically, there is about as much energy dumped out the exhaust as the engine puts to the crank, and turbos use about the same amount of energy as a similar supercharger, so you can build a setup that is very inefficient at getting that energy to the turbine and you’d still have enough to drive it.

It’s almost never better to do things in a less efficient way, but if you don’t have any other choice… In this case you either have to live with lesser response and/or more exhaust restriction then if you mounted it closer to the engine.

 

With the old set-up (small T3 45 trim turbos), the exhaust was very quiet, even with the cut-out open. It is significantly louder now with the larger turbines, but still streetable. In the car, it probably sounds even louder than it actually is because the cut-out dumps the exhasut right under the passenger seat, instead of out the tailpipes. I have actually considered building a new exhaust with no muffler- I think it would be very tolerable with an open exhaust, especially with a rear exit.

Its very hard to explain how loud it something is with words- I do have a video camera, so I may try to get a video of my car with open and closed exhaust so you can actually hear it. I will have to wait for a nice weekend though....

 

89YJTurbo,
Did you make your own flanges for the downpipes to mount to the turbo wastegates? I have a couple of Saab turbos that I think have the same wastegates as the Volvo turbines you used on your setup. I was searching for some flanges for it that I can weld downpipes on to.
I could problably have an exhaust shop bell some 2 1/4" pipe to fit the wastegate but I would still need the 3 bolt plates to hold them on. Did you find a place on the web that sells the bell & plate?

Thanks,
J

 

This really isn't worth the effort, but you could install an electric motor that moves much faster, such as one from an RC vehicle. It would open almost instantly.

 

isnt it a nice stepper motor with a gear box? Im pretty sure it is to make sure it is reliable and closes tightly.

 

That is all it looks like to me. Makes me wonder. Has anyone made their own external wastegate with a stepper motor or solenoid with a standard GM heat riser?

J

 

large GM heat risers are in the 2.25" range, the heat involved would require a good amount of research to produce a productive combination. Also I dont believe they are designed to close 100% failure to fully close it what has plagued the reputation of manually operated dumps for years until these new electrics came along (borrowed from expensive marine applications).

I would like to see a selenoid operated "slide" type unit. That would be much faster.

 

I got the parts from a local auto parts store. I know the flared 2.25" pipe is from an S10 truck (the intermediate pipe has the flare where it connects to the cat). I don't know what the application is for the 3-bolt flange, but a good auto parts store should be able to help you. I checked back in my records, and was not able to come up with any part #s. If I find them, I'll let you know.

I like the flare connection because it gives some adjustability in the connection to make assembling the exhaust piping easier.

 

This weekend I checked a stock iroc-z exhaust Y-pipe I had lying around to see how it fit the saab/volvo wastegate flange and it was a perfect fit in terms of the bell size. The flange is two bolt though. I think an older 1970-early 80's three bolt manifold flange might fit. I am going to check it out this weekend. Haven't had much time with trying to tune the ECM in the Camaro right now. The weather is still kind of nice so I would rather be making steel parts than messing with an ECM, but I would like to have it tuned somewhat decent before I store it away.

Having the flare at the wastegates is nice in terms of making downpipes. I think I am going with 2.5" downpipes on this setup. Only looking for 400-450hp out of a 350ci TPI w/street (218 duration) cam and stock GM 882 heads. Rev no higher than 5200-5500rpm.


Thanks,
J

 

Did you custom make your turndown that's connected to the cutout?

 

I wouldnt blame all the restriction on the muffler. I think it has alot to do with the smaller exhaust pipe. Dont get me wrong, I know chambered mufflers do restrict exhaust flow, but not that much to make 9lbs of boost differance. I had a flowmaster 3in cat back on my car before and i switch to a 3in bullet with it dumping right before the rear axle, and it did make a noticleable differance. I'd go with a bullet muffler, if i were you. Good Luck

 

Thanks for the input. However, there is not a 9psi difference in boost, it was just 2-3psi (goes from 6 to about 9psi manifold pressure by opening the cut-out).

My plan is to build a new y-pipe using larger plumbing, then work the whole way back with at least 3" tubing. A have the welding skills now to make a much nicer exhuast than when I originally built my turbo system.

 

Yes.

 

I saw these the other day and thought how prophetic my statement was.

Pertronix Ignitions just bought Doug's Headers. Dougs headers and Pertronix just relesed this new slide style cutout at SEMA.

 

How well would a glasspack muffler work? I was thinking of having a 3" downpipe from the turbo and ending right before the rear axle. Or should I just go no muffler?

 

I like the idea, and it's the first of it's kind. It seems like they could make the design a bit less bulky. I can't seem to find more information on it.

 

probably not for another month or so

 

yea, really, it's like a pocket door... it takes 2x as much space as the opening that it covers... Does the thing actually open any faster? Those dayton gear drive motors can be very slow.

there is no good reason that a butterfly type assembly wouldn't work, you just have to cut the blade at the right angle (about 2.5* works well) and make the whole thing out of similar materials so everything expands/contracts at roughly the same rate (yes, different shapes expand/contract at different rates but this would be close enough to work) and you should be able to get some thing that you can open and close quickly and seals well (at least as well as the typical, clamped slip joint in exhaust tubing tends to seal, which is enough that you can’t really hear it)