I am building a 383 TPI for my car right now, and was wondering how much of a gain would I see if I went up in size from my current Hooker 2460's which are 1 5/8" primaries, to a set of Dyno Don's headers which are 1 3/4" primaries? I have heard that 1 5/8" primaries cover a little bit of the exhaust ports on AFR heads, which this motor has. So would the swap be worth it?

 

I went from my edelbrock 1 5/8 headers along with there y pipe, to dyno don's headers and y pipe and made a big difference. On the edelbrock primaries when they get to the collector they looked like they were crushed a little and were smaller than 1 5/8. When I looked at the y pipe, it was about the same situation where both of the pipe met. And that was how the edelbrocks came from the factory. I would highly recommend a set of dyno dons stuff. Really nice and no problems with the install.

 

You should find a difference above about 4500 rpm, but with long-runner TPI, your average power will suffer. However, if you're heavy with the nitrous, then I'd definitely like the larger tubes.

 

So your saying that i will lose power down low? Will it be a big difference? I heard that you should get the Dyno Don headers with the AFR heads because the 1 5/8 in primaries block part of the exhaust port. Won't the 1 5/8's be too small for the 383?

 

If 99% of your driving is road trips under 2000 rpm, then stay 1.625". Below 2000, 175s would cost you enough to really feel. If you'll check out my best builds thread, which is a sticky in the engine swap sub-forum, you'll see that I found 1.625" primaries adequate for nearly 460 horses. On the 450 horse carb'd Vortec 383, 1.75s actually cost over 10 horses.
You just aren't moving enough air to need huge tubes.

 

Well it is a street driven car but I would like to squeeze out as much power as possible. I was still thinking I needed the 1 3/4" primaries because of the AFR port issues with smaller primary headers. I was also thinking that the 1 5/8" headers would be holding back some power with a 383. Has anyone tested these two sizes next to each other?

 

1989fastgta built a strong 350 with a heavily modified TPI intake to flow more air. I believe he saw improvements stepping up header size. I know that when he switched to long tubes, power did not really increase at all since Dyno Don's headers are that good. Try pm'ing him. Many high performance cars use 1-5/8" headers. My John Lingenfelter book says to use 1-3/4" primaries for a 383 that will produce 400+ hp. Any less hp on a 383 and 1-5/8" are recommended. We know from the "ten times torque article" that stock TPI intake on 383 will produce 400 hp and full big mouth system yields 450 hp. I see that you have big mouth runners. Will you be using those on your 383? If so I think larger headers will help you. May not be a huge power increase but having exhaust ports not partially blocked has to be good.

 

I will be putting my TPIS Bigmouth intake and Large Tube runners on along with a 52mm TB and a ported plenum.