Looking for some information about sizing twins on a BBF engine, 540cid. Drag only, looking for about 1,200hp. My initial thought are a pair of T4 based S475's or S480's. Using the 74/83mm turbine wheel. Want some room to grow when the new engine is done, 1,500ish HP.

 

How much na hp? There are a few here that have run twin T6 475/1.32's on similar cid. I would consider those or maybe the same turbo with the T6 1.10 ar on the 96mm turbine wheel.

 

N/A HP is around 800. Made 1,130hp on a 325 shot .

 

That is with an N/A cam, not a dedicated nitrous cam.

 

Yeah, T6 S475, 96mm 1.32. Assuming you have a brake.

 

A turbo shop recommended twin 80's, in a T4 platform, with the 88/96mm wheel. Is this primarily for space savings? I'll admit, twins on a BBF is a little out of my realm as far as sizing goes.

 

The T4 housing is slightly smaller and is setup for a 4" dp. Other than that i see no real advantage.

 

Think of it as 2 350-400 hp engines.... a S475 or 80 is quite common on something like that.

 

That's kinda what I figured about a T4. I was thinking about it on a cubic inch basis, not a HP basis. Makes 100% sense though, 75's and 80's are common place on engines in that 350-400 hp range. Thanks again.

 

Ive run a T6 S465 on a stockish 5.0 ho, and it worked great. I think the T4 vs T6 thing is much less of a factor than many make it out to be. I personally wouldn't run a T4 on a 475 sized turbo unless it was strictly a space thing.

 

And it may come down to space being a factor. It's a BBF in an unmolested foxbody engine bay. Now add twins, downpipes, and class rules mandate exhaust dumps no farther forward than the back of the door line.

 

For twins I just divide by two. 540/2=270. So what size turbo would you use on a 270" motor making 400hp? I think a 7675 or S475?? But ideally, i'd call Jose or someone really turbo smart and go with what they say.

Your goals seem really soft. Any reason for that? 800N/A would be 1600 at 14.7#. You're thinking just like 9-10# max?

 

Not my car, just a friends. the hp goal is a somewhat safe zone for the current parts. Future setup will be higher power. Jose recommended 80's with a 88/96 turbine wheel, but in a T4. Seems like lots of 500"+ BBC guys get along rather well with a pair of T6 S475's.

 

Under no circumstances would I deviate from what Jose tells me to do.

 

So as not to question Jose, what benefit is there to running an 88/96 wheel in a T4 frame 1.25 a/r vs a T6 with a 1.32? I only ask so I might learn something. Thanks.

 

Drive force on the compressor. Back pressure will change. The smaller A/R should spool faster. T4 reduces volume and therefore should spool faster. Jose is smart so that will make it spool faster.

 

Got it. Not sure why my brain is temporarily stuck on the notion that flange size is the deciding factor, and not scroll volume. A T4 1.0 a/r moving across a 96mm wheel will have more exhaust velocity than a T6 1.0 a/r moving across the same 96mm wheel, correct?

 

That's the general idea, yes. "They" say to always fit the biggest flange you can, but "they" aint always right.

 

Jose definitely knows his stuff. I would be very interested in knowing why the 1.25 T4 over the 1.10, or 1.32 T6.