Just wondering what everyone thinks about header primaries when constructing turbo headers. I see a couple people use 1 3/4", but then in HotRod last month there was a 1000 hp Mustang using 1 5/8" primaries. Does the smaller size help increase exhaust gas velocity, and not hurt top end, or is it pretty much a cubic inch thing. I believe the mustang was only 306 cu.in, would 1 5/8" primaries be too small for a 350 or 383

 

Guess it would depend on your goals and your choice of parts. For a 350, you could probably get by with 1 5/8" primaries, but for 383+ I would be looking at 1 3/4". I'm planning to use 1 7/8" primaries for my twin turbo project, after I finish paying for Christmas. So, header contruction should begin in June, maybe.

 

See if you use Mild steel you could be done and driving your car by June

 

and if I bring you the early xmas gifts of car parts and $$ tomorrow, then light a fire under your *** for the next 4 months, we can be driving both of our cars by may

 

I wish!! I've projected to have this project completed in like 3 years (maybe). I'm doing the whole car including new engine (427 small block chevy), TH400 with all the goodies, a new 9 inch, new rear suspension, new k member, and new interior including a cage. Not to mention the cost of fabbing the headers and getting the turbos/intercooler. I'm in no hurry, till I win the lottery anyway. Plus I have to keep my 97 running so I can get back and forth to work (and I really want a new SQ trim, my V1 is too noisy).

All the above mentioned work is for my 87 GTA. I'll make a new sig then with stock parts listed.

 

I'll have 1-5/8" primaries into a 2-1/2" MAX log. This is on a 383 though. Depending on the size of turbos you're running, I'd bump the size up a little on that 427.

Are you getting the headers coated afterwards?

 

No, I'm planning to use 321 stainless, so they will be put into a burnishing machine to make them shine. As far as turbos go, I'm probably going to use two PT68s or something like that, haven't completely decided yet.

 

427 cu.in WOW... I have been thinking lately you are better off spending money on fuel system, turbos, and intercoolers, then spending money on huge cubic inches. I mean look at Buick GN's most are under 4.0 liters in displacement and some are running 9 second quarters. I think CID matters less with a turbo, than with a supercharger

 

keep the cubes down, (dutweiller style) and run 1-5/8".. velocity is everything, back pressure is caused by the turbine wheel/exhaust housing.

I've currently got a 427" small block but I'm knocking the stroke back to 3.5" and increasing the bore from 4.125 to 4.130.

You don't need cubes, remember, there is a replacement for displacement.

good luck!

 

Sorry to hijack, but I've got one quick question for Brady:

What distributor setup did you use on that big block? Looks like it fits under the cowl just fine. Can something like that be used on a SBC?

 

it's the MSD crab-cap crank trigger distributor.. it's cheap and fits with plenty of clearance... you can lift the distributor and turn it 180* if you need to (don't ask how I know)..

laterz
...Brady

 

FWIW This guy is a friend of mine
www.hartlineperformance.com ...... His car is a 4.5L V6 making 1000HP (That's more displacement per cylinder than a 5.7L V8) and he runs 1.5 inch primaries and a 2.25 inch crossover.

Velocity makes a big difference ...6 second Mustangs at the Orlando Streetfinals were in the 377-408 CI with 1 5/8 primaries, 2.5 inch crossover pipes, and 101-104 MM turbos.

So run 1 7/8 headers if you think you know something those guys don't, but your car will probably be faster with 1 5/8 or 1 3/4 MAX

 

run a primary that will handle your motor "NA".

that 4.5L v6 probably runs very high boost pressure, which means higher exhaust pressures, which means he can get away with smaller tubing.

I would say 1000rwhp v8 is where you get close to the 1 5/8th or 1 3/4 gap. I would say you are still aways away from 1 7/8th territory


I plan on making 900+ rwhp with 1 5/8ths.

 

I can see using the smaller diameter pipe with a huge turbo, 100+ mm turbos take a hellacious amount of exhaust velocity to make them spool. However, with a large displacement and two smaller turbos (like 60 something mm), I don't forsee a problem with a larger exhaust primary (as always, I could be wrong). And, if there is a problem, I can always make another set of headers in a smaller diameter. Besides, a little lag on take off will probably be a good thing, don't want to completely overpower the tires now.

So Jordon, what do you consider to be 1 7/8" territory?

 

maybe as you approach 1500hpish?