Twin Turbos - do you really need twin waste gates?

[RarlyL8]

In a rear-mount application would it not be better to have one waste gate mounted centrally on a crossover pipe that connects both banks of cylinders?
This would reduce weight, increase system balance, and reduce complexity. I understand the reasoning behind dual gates in a side mount configuration, but a rear mount should lend itself to one wastegate. Thoughts?

[BURN-BROS]

Its been suggested that 2 seperate ones as close to the turbo as possible yield the best performance. All the things that you have listed are the upside to one larger one but the down may be a poor control of boost. If you decide to run the single let us know how it goes.

[beepbeep]

Well yes, you can use only one as long as WG tubing isn't too long. With other words, you want to avoid long closed cavities that those pipes essentially are.

But theoretically, there is nothing wrong with your approach. Thing is if you go TT your turbos will be pretty small and will probably have integral wastegates. Almost nobody uses external nowadays...996TT has integral ones. New turbos have even integrated diverter valve.

[RarlyL8]

Turbo selection is another issue. Most modern units are water cooled leaving few choices for small displacement applications. I also like having a dump pipe on the wastegate and quiet muffler on the turbo. It's only loud when I want it to be.

Here is a picture close to what I may do. The wastegate could be mounted right between the turbos with a fairly short run of pipe from each collector.

[beepbeep]

Originally posted by RarlyL8
Turbo selection is another issue. Most modern units are water cooled leaving few choices for small displacement applications. I also like having a dump pipe on the wastegate and quiet muffler on the turbo. It's only loud when I want it to be.

Here is a picture close to what I may do. The wastegate could be mounted right between the turbos with a fairly short run of pipe from each collector.

Yeah, that would work. There are even some benefits of using only one wastegate, as you don't have to synchronize and adjust two wastegates to same boost.

BTW, what I see on the picture is OEM 3.3L 930 engine with original intercooler and CIS sporting what seems like twoKKK K27. That thing must be laggy as hell...nothing I would do. One of those is enough for original IC and FI.

[RarlyL8]

Yes that picture is wrong in a lot of ways.
What I needed to convey was the approximate location of the turbos. I have not decided on a header design just yet but they would be tighter to the motor and closer together than the above configuration.

Any thoughts on turbos?

[ohecht]

Rarly,

I ordered two of the $200 T-25s to start the long process of sorting out possible locations of everything.

This will be a long, slow project that will also require EFI, WB O2, back dating heat, and probably a crank-fire ignition of some sort (either Electromotive, Megasquirt with Ford EDIS, or Ultra-Megasquirt if it is ready before I am). With that level of control over fuel and ignition, and the feedback of WBO2, I should be able to safely run decent levels of low-RPM, intercooled boost even with 9.3 compression. I will have to put the controls in place first so I don't put the engine at risk.

I figured one of the places to start the project is to see how the turbos could fit and still accomodate exhaust piping, intake piping, mufflers, etc.

I am considering intercoolers in the wheel wells if I can fit them, or possibly an air-to-water setup so I can mount the "radiator" in the tail with flexible lines and still keep my AC setup.

The T-25s should be here any day, and they are water-cooled with integral wastegates. I will probably try to work a coolant system for the bearings, too, rather than just running without cooling them (which I also think is an option). Especially if I have the water IC anyway.

I think the biggest advantage will be tbitz's EFI kit, which will open up a ton of room in the engine compartment behind the fan after I cut down the airbox to almost nothing. Maybe enough for a big enough air-to-air IC even if it is not located in the perfect position in the airflow.

Still, with all the challenges of getting an IC in the airflow in a 911 with AC, etc., I am leaning towards a water IC, even though it has drawbacks like the potential of heat-soaking the coolant. Probably manageable as long as I am not constantly on boost, and the low-boost of the system should not soak it too fast.

I look forward to seeing more of your project as I work on this one. Hopefully most of the hard work after making all these decisions will fall over the winter when I can drive my Jeep instead without regretting driving the 911 every day!

By the way, I saw a picture of a Ferrari model in Bell's book on turbocharging that did use a single gate for a TT setup, so it is certainly possible.

Olivier

[ohecht]

PS...in that picture, would the turbo outlets to the engine run through the engine sheet metal and behind the engine crossmember?

I can't remember now without looking at my car if the sheetmetal is in front of or behind the crossmember, but I thought it was in front of it, which would make those outlets seem to be blocked by the car itself.

Olivier

[ohecht]

Now for a lot of trial-and-error fitting options!



Olivier

[turbo6bar]

Olivier, are you going to mount the T25s to a pair of SSIs? jurgen

[ohecht]

Yes, I have SSIs and will not be going the custom header route, even though I know the exhaust length to the turbo will be long. I am hoping that the two small turbos will make them very responsive despite this length.

Olivier

[RarlyL8]

Where do you get $200 turbos?
How do they fit functionally with the 930?
I plan to get started on this in the fall. Picks will follow.
The left turbo will go in the stock 930 possition, the right one tucked in to match.

[ohecht]

I got them from someone whose father bought 500 T-25s for a kit that his professional turbo shop was putting together, but then went with a different turbo.

This person has been selling hundreds of them at this price, and the same ones show up with $250 "buy it now" listings on ebay.

I am not sure if they are the best fit, since this is the one turbo for which I could not find a compressor map anywhere, but they were too good a deal at that price and I think they are similar in characteristics to the GT-25s, which you can find maps for.

Olivier

[ohecht]

Speaking of wastegates, is there a way to adjust an internal wastegate "down" in terms of opening psi without shimming or messing with the actuator internals?

I understand how the external boost controllers can bleed off the actuator signal to increase the boost level, but it seems impossible to go the other way without somehow amplifying the boost signal, which I'm not sure how you could do.

I am told my turbos are set at 7 psi, which is right where I probably want to get eventually with good fuel management, ignition management, and intercooling on my 9.3:1 engine, but I would like to be able to start out a little lower just to make sure everything is working as expected before pushing it.

Olivier

[beepbeep]

Originally posted by ohecht
Speaking of wastegates, is there a way to adjust an internal wastegate "down" in terms of opening psi without shimming or messing with the actuator internals?

No. You have to adjust it manually by lengthening the rod on internally wastegated turbos. I don't recommend adjusting base boost more than just a fraction on pre-set internal wastegates with rods. Their behaviour isn't consistent.

Just go with 7 psi...it's pretty hard to synchronize two wastegates to same boost (w/o one working harder) once you change them, especially if they blow into common plenum.