I've a 60mm HKS wastegate that my engine builder is saying will be too big for my power requirements.
He believes we'll have problems with control of our low boost requirements. (I suspect he means some sort of boost fluctuation given the closed loop boost control we use)
We're aiming for a conservative 400-450whp using a BW 300sx. The only caveat being throttle response is paramount.

 

how low is low boost ?

lower the boost the bigger the wastegate.

 

There's a lot of things to consider but for example on my full bridge 2-rotor with a Japanese HPI T4 manifold and a Borg Warner S475 with 4 inch exhaust we are running a Precision 66mm waste gate. No problem holding 14 pounds at low boost making 570ish RWHP.

The lower the boost you want to run the bigger the waste gate you will need or you will have to run two smaller ones especially if you have a big turbo.

 

Small turbo in a BW 67.5/76 with a .91AR exhaust housing. Effectively a 300sx.
Trying to get turbo response as quick as possible, hence the small turbo.
Running enough boost to get approx. 400-450whp, about 18psi from what I've found here.
Exhaust is very short, approximately 2 foot of tube then a muffler.

 

Forgot to mention I also need to switch the boost down to a "limp home" mode, spring pressure only. Somewhere around the 10lb mark. It's simply a conserve fuel, reduce temp, limp back to pits setting.

 

I used a HKS GT 60mm wastegate on a stock hybrid turbo (380rwhp @ 14psi boost) for about a decade.

There is no issue with a large wastegate on a rotary.

The lightest spring available is 10psi but even after a bunch of work to give WG priority on exhaust flow my set up ran 12psi boost off this spring with a 3" exhaust and crept to 17psi with 3.5" exhaust.

You may not be able to run low boost on a small turbo unless you have another restriction in the system.

 

You got boost creep with a 60mm? Bad manifold design?
I also thought that a free flowing exhaust dropped the pressure in the exhaust. As such the wastegate would be more efficient (as it's got a greater pressure differential). As such shouldn't you see lower boost with a free flowing exhaust?

I suspect this HKS already has a 10psi spring in it as it was mated to a stupidly small hotside, p trim turbotechnics. However will check.

 

^No, a free flowing exhaust will allow more exhaust to flow through the turbine raising boost.

FD's don't have boost creep problems (stock twins) until people start messing with the exhaust, then they have to port the wastegate.

 

That applies only to the N/A

The larger the exhaust on a turbo car. The larger the flow.

 

I believe boost creep from larger turbo back exhaust has to do with flow and pressure differential across the the exhaust turbine.

You got boost creep with a 60mm? Bad manifold design?

I think too small an exhaust housing with too big of an exhaust.

I ported the runners up to the WG to twice the volume of the turbo inlet runners and with flow heavily favoring the wastegate.

I could run 3.5" downpipe/midpipe to 3" RB exhaust and hold 12psi on the "10psi" spring, but with full 3.5" exhaust it was minimum 17psi.

I had this problem with the hybrid turbo since it had the stock internal WG and only knocked a few psi off the boost creep each time I raised wastegate flow with many different mods- but the trend was the same.

 

I don't think I've seen a scenario of a wastegate being too large for a rotary, I've seen many where they are too small. If it were me, I'd say go for it especially if you plan on running a sustained low boost setting. 400-450 rwhp will probably be in the high teens or low 20's pressure wise so assuming you have a good manifold and no serious restrictions in the exhaust flow I'd guess you have a little more than just enough wastegate for your goals.

 

Tial 60 mm dump to atmosphere 10lb spring,
Pte6766cea .96ar undivided,
Short manifold t4 undivided,
4" down pipe, 4" to 3" megaphone around 12" long, 3" rb muffler,
Pressure source UIM,
Pulls hard for 10.5 to 11 psi solid boost to red line,

Working on ac and boost controler now will test with higher boost hope soon, will get pressure from turbo cover

 

Excellent Gentlemen, looks as if I'll retain the HKS 60mm then. Thankyou.