what size primary tubes would i need to use to make turbo headers for a t6 turbo 91.5? 370ci motor. I was thinking 1 5/8 would that be too small that it will choke the engine? thanks

 

I used 1 3/4 for my 370 t6 88mm. 1 5/8 seems to small.

 

1 3/4", No need for anything bigger unless you are shooting for 2000+rwhp. 1 5/8" is to small in my opinion.

 

1 3/4 into 2 1/2 crossovers

 

+2 Josh knows his stuff. (obviously)

 

Considering we had our best luck with 1 3/4" to 2.5" crossover in a 438", I'd almost imagine that 1 3/4" stepped to 1 5/8" to a 2.25" crossover would be **** on that size motor. Bigger in a turbo hotside is not always better. If a 1 5/8" choked a 370, what does the 1 3/4" do to a 438"? Its all about velocity the bigger the tube the slower the air moves.

 

It's not always about motor size...

 

I'm still new at this...So assuming you have 2 similar motors, just the size difference, both heads have a 1.690-1.720 cross section on the exhaust port, why wouldn't you size the exhaust tube to yeild a similar velocity yet make the tube fit the cross section of the cylinder head?

 

ok so phil seeing what you have done and how fast you been, no dissrespect to everyone advice i to think the 1 5/8 are too small but not sure. What Phil says makes sense on paper though. Im gonna measure the exhaust ports on my heads and see what they are. im shooting for 1000 hp at the rear wheels. I have the dart pro 1 cnc 250cc heads on my 370, as i say 91.5mm turbo. I just want to make sure i can use these headers as i start mocking up my turbo kit. Would i be ok with a 1 5/8 runner. my exhaust housing a/r is 1.0 if that matters none. Again thanks for all of your advice. I did see a guy make 1000 hp on stock 317 heads with very little port work. hummmm

 

I just measured my exhaust opening and the cross section opening is 1 3/4. I never get to take the easy way out. lol

 

x2000 gotta let the heads breathe alil

 

Just my experience....

My 438" motor has 1 3/4" headers with a 2.5" crossover, LS7 heads and more compression than 90% of the turbo LS setups out there. With the 1 3/4" headers on my LS7 head/carb style manifold and the marcella, the combo spooled very quickly and was fast went 160+ to the 1/8 mile. We never really leaned on it hard with the 1.75" headers.

We switched to 2" headers(what you would run on a turbo BBC) and now its harder to spool and doesn't seem to make any more power.

I'm still learning at this but I dont understand why you would run a 1 3/4" header on a 370 and on a 438. A 438 is going to make more power than a 370 considering they have the roughly the same volumetric efficiency. The amount of power you make will give you a certain air speed number in each header tube which at some point will cause problems if the airspeed is too high.

A 370" motor with 90# VE will make X amount of power and yield X amout of CFM in each header tube.
A 438" motor with 90# VE will make X amount of power and yield X amout of CFM in each header tube.

I spoke with Corky bell about a year ago and he said that there are some key air speeds that should be used when sizing header and crossover tube diameter, too big and airspeed falls, too small and it gets turbulent. When I appiled the forumula to my setup it said that the 2" header was too big for my 438" and the track results/spooling said the same thing.

The difference between a 1 3/4" header and a 1 5/8" header is airspeed. I'd be willing to bet that there is very little power difference between the true 1 3/4" header vs the 1 3/4" header stepped to 1 5/8". The difference is going to be in the velocity or airspeed and that will show up in spooling, not TEC®.

I'd still use a 2.25" crossover into a megs 2 x 2.25" to 3" collector/tube to the turbo. We use a 2.5" to 3.5" and we're making enough power to go 165 to the 1/8 mile. The key is get as big of an exhaust after the turbo as possible, run the wastgates to the atmosphere. Just my .02

What is the cross section of a truck manifold at its smallest point?

 

I let my heads breathe and my turbo doesnt spool like it used to. This isn't a N/A combo.

 

I think it's best to run the smallest primary tubing that can be matched to the exhaust port on the head. I don't see any horsepower gained by a large primary tube as the turbine is the restriction.

As for the op's question, if it was my own car I would run truck manifolds into a 2.25 crossover. If headers were necessary due to an obstruction than I would use 1 3/4 into 2.25. (I like phils idea of 1 3/4 stepped to 1 5/8 as well but it would have to be done with some nice cone reducers)

 

Man great info. The truck manafold on the driver side will not fit cause my car has motor plates. I'll figure out something. Auss I want to run like the orange ls1 turbo mustang from speed inc. Lol lol lol

 

There is A LOT of experience in this thread. I will use ALL of this when I decide to tackle my own headers

 

Well we have quite a few different stock exhaust manifolds at the shop. You're welcome to borrow them and test fit them if you like.

 

I have always heard that for maximum efficiency, you should try and make the volume of each primary equal to the volume of each cylinder. Anyone with more real world experience than I care to comment or elaborate on that and if it relates to primary diameter (within reason)?

 

A good exhaust shop should be able to put a 15-20 degree angle on a 1 5/8" tube to 1 3/4".

I still want to hear why you need to run the same size header on a 1900hp motor and a 1000hp motor.

 

Well I wouldn't say you "need" to, but it is convenient to fit an 1 3/4 tube to the average LS exhaust port.

I still like the idea of using manifolds as the flange can be ported to match the head but the overall internal volume will be lower than any tube header.

I've seen 1000hp twin turbo SBC combinations that use 1 1/2 headers so I would say you could use smaller tubing as long as it doesn't shroud the port. (or just step it down as you said).