hey guys, currently running a 331sbf with twin 57mm turbos. i am currently running 18psi, not sure on the hp since i street/strip tune it with megasquirt. i am currently running duel 2.5" exhaust form turbo to tips and i was wondering at this boost level would i gain anything going to dual 3" exhaust. it seems like at this hp level 2.5" exhaust would be a restriction. i plan to increase the boost up to 24psi if these turbos will let me. running e-85 for fuel if that matters.

 

I've got a 355 SBC, 58mm T3/T4 turbos, on E85, and I went to 3" and got rid of the internal wastegates pretty quickly after I got the car running, so I couldn't tell you for sure how much improvement there was after the switch, but I went from low 11s~high tens the first few passes, on down to a best so far of 9.66, and hope to be in the 8s soon. I would definitely make the switch if I were you....

 

Are the turbos a 57 trim? or actually 57mm wheels? What brand turbo? What size is the turbos exhaust wheel minor diameter? 120% of that is the rule of thumb for choosing exhaust size. If you meet that criteria, going to larger piping isn't going to help. (assuming you don't have 50 foot of piping, 100 bends, or a restrictive muffler setup.

 

It would be a good idea to go to dual 3". You could always take a backpressure reading so you at least know what u have.

 

Purely from a flow and motor size standpoint I would say your post turbine piping is too small. I'm running twin 6262 turbos, t3/3"vband turbine housings on a 9:1 compression 4.6L motor with 3" downpipes and 3" cutouts on the down pipes. Even with the 3" downpipes, opening the cutouts picked up 50rwhp at 20psi during testing. So going through a full 2.5" exhaust is definetly leaving power on the table at your boost level and power goals imho.


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If he has the typical dinky 56mm exducer found on many ebay turbos, 3" is overkill.

Do the math. Don't just throw 3" piping on because someone else did. 120% of 56 is 67mm. Or 2.6". So 2.5" exh would be fine IMO. 64-65mm wheels are where 3" should be used.

 

I understand the math completely and I agree, its a good starting point but I am much fonder of theory to practice. It doesn't take into account pre turbine pressure (largely due to motor specs, cams, intake manifold etc) as that's whats going to hurt power production, slowing air flow through the motor. A motor of that size with the ops desired boost and flow level will generally see a drop in pre turbine pressure by going to larger post turbine piping as the exhaust gas has more room to expand. Also what is the rpm range of the setup that's a good data point as well.

Easy test before spending any money, changing mufflers, making dumps, making bigger pipes etc. is run open downpipes and see if you don't pick up a good amount of power.
That will give you a snapshot of how the current exhaust is doing and how well it is currently working on your motor setup.

I've seen a 100rwhp gain on a 75mm turbine wheel on a 4v 5.0 motor going from a 3.5" downpipe to 5" downpipe. The 5" downpipe was only kept for 2-3ft and then went back down to 3.5" piping again. The large pipe diameter at the turbine outlet allowed the exhaust to expand and greatly reduced pre turbine back pressure which was excessive at the time (4:1).

 

The way I see it... (and I may be incorrect) The major restriction isn't the exducer exit, it's the scroll exit. This is the smallest point in the system and what will mostly determine pre-turbine back pressure readings. If you stick to the "120% rule", I don't think you would add to the existing pre-turbine back pressure levels.

 

the turbos are the cheap ebay t3/t4 turbos. i know that most are not fans of the cheap stuff but ive been running the same stuff for years without issues so i cant complain.

the other reason for wanting to redo my exhaust in 3" is because right now, the down pipes run all the way from the front of the engine bay along the motor and then down behind in. changing spark plugs is impossible.....not to mention all that heat. i want to run the exhaust from the turbos, straight into the fenders and then dump it. here is a current pic
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i am currently running bullet style mufflers so there is no restriction there.

based on the simple exhaust i want to do and the cheap prices for pipe i found while searching this forum, i know i can do the exhaust in 3" the way i want it for about $100 so not a huge investment


and im shifting at 7000rpm

 

If you have the space, bigger sure won't hurt anything. And you'll have room to grow when you max out those turbos. I doubt they will put out much more boost. Similar setup in a bone stock 5.3 would spike to 21 and settle to 18 by 6k. I like the idea of 3" 90* bends facing the fender with an an electric cutout/dump. Then you can snake a much smaller and easier to work with 2" piping around or under the plugs and back to mufflers.

 

Install a pressure gauge post turbine and post some numbers... I also like the 3" to a cutout, and then to the remainder of your exhaust.

 

well if 21psi is all im going to get it might just be time for bigger turbos. i was looking at the on3 70mm turbos. im thinking my motor has enough cubes to spin two of them. they have one that has a .68 AR turbine, should be good to 1200rwhp. and the only reason for picking those turbos is the price, i know there are better options

 

I would look at the 67's before the 70's. That's a good ar to go with. Dump them downpipes I say!

 

Figure out how much power you want first, then size the turbos. Twin 67's or 70's on a 331 sounds like a lag monster to me. I'd go with 61-66 range.

These would be about perfect for an 800-900hp street/strip IMO. (61.4mm compressor 63.56 /73.60 exh wheel 1.0AR T4 )

http://www.ebay.com/itm/151288186055?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

And these if you really want to move some air. They make the same turbo in a with a 66.7mm compressor.

http://www.ebay.com/itm/God-speed-G...Parts_Accessories&hash=item2ec9bc64ca&vxp=mtr

 

id like to see 1100rwhp safely. according to aeromotive, their pro pump is only good to 1400bhp on e85. i suppose i could go back to race gas but i like buying fuel at the pump

are then any inexpensive singles that will get me to my goal? i dont mind switching.

 

That pump is also rated at 45psi. Depending on your base pressure and boost pressure, I seriously doubt it will make 1400 crank hp on E85. Flow drops a ton with pressure. A BW S476 with the big 92 exh wheel would do it. Not going to be cheap though.

 

well guys i appreciate all the responses. i think switching to a 3" exhaust is my best bet. itll look good (imo), itll get heat out of the engine bay, it make changing plugs easy and it should free up some hp......all for about $100. with the money i save over buying new turbos i think i might invest in new suspension. currently running mega bite jr's and stock replacement tubular uppers. that double adjustable upper and lower kit that team z makes with the relocation upper bracket looks promising.

 

Fitted twin3" on my 330 and gained 100 rwkw and better spool..
after new tune ..380 to 482 rwkw..
Prev exh was 2.5 to 3" single over axle..

 

What size turbine wheels?

 

forcedfed, you think that first turbo you posted is only good to 900hp? what about the second one?