Header size?

[jeff5347]

Im back, I have a question for all. I have a 68 cam that sees mostly street duty but like to get it going to Hits 6 g once in a few moons.
Im not sure on the header size- 1 5/8, 1 3/4, 1 7/8,.
I have a 350 bored 40 = 357
kb hyper piston with about 10-1 comp.
a comp cams roller cam 280hr .525 lift
tfs twisted wedge heads g1
vic jr intake and 750 vac sec holley
th 350 wit h3.55 gears.
electric fan
Over the winter may move to AFRs but thats a big maybe
if not ill polish and clean the TF's
also polish and clean the vic jr. (also have the rpm manifold if engine needs call for it)
and will get a thinner HG to up the compression
Also want to move to a bigger cam such as a 550-570 lift.
Will prob re ring and re bearing the engine also.
So my question is this which may be hard.
I dont know what size headers or brand to get
I have hooker 1 5/8 super comps but they got twisted by a rock so i need new ones.
If i dont do all that work over the winter what is the size i should shoot for as well as if i do the mods planned what size then. Would 1 3/4 be to big either way? Also ive been looking at the hedman headers to and they seem nice and looking to get them with the HTC coating. Any suggs?

[parsonsj]

You might want to give the guys at Stainless Works a call. They've got some software that may help, and some nice offerings in stainless steel that might be of interest.

Off the cuff, I don't know why you'd use anything less than 1.75.

jp

[jeff5347]

really,
Right now i have 1 5/8. Is that to small? I thought the 1 3/4 would be overkill but if not ill try them

[68sixspeed]

1-3/4 are pretty good for most applicaions with improved heads- you shouldn't outgrow them and it still keeps some bottom end in it.

[BA.]

You can throw out the thought of 1 7/8", those would be just too big for your bored-out 350. (even with your mods and future mods)

That leaves you with the 1 5/8 and 1 3/4 options.
I agree that the 1 3/4" will work and work relatively well, but I dont' see any great argument against the 1 5/8" either.

You do realize that the bigger the pipe diameter, that this will move your torque peak into a slightly higher RPM band right? This is usually at the loss of a little low-end torque.


So, if that is a willing trade-off, you could go with the 1 3/4", but if you put a lot of street miles on the car and only race occasionally, I'd get the 1 5/8". They are the normal size for your cubic inches and rpm range. 1 5/8" is not too small.


JP, if you jump back in here, what are your thoughts concerning exhaust velocity and torque peaks for the 1 5/8"? Seems a better match to me...


PS. If you had a 1st gen F-body, I'd sell ya my 1 3/4" ones.

[jeff5347]

thats what im looking to do. Clean the junk in the vic jr and also the trick flows if i dont get new heads. Up the rockers to 1.6 and not sure on the 1 5/8 or 1 3/4 . It is a 1st gen camaro. I would like the torque down lower. Something that will boil the hides at 20.

[parsonsj]

I'd make the trade-off and go with the 1.75. Low end torque is usually wasted energy from a performance POV: you just spin the tires anyway. So you move the torque peak up a couple of hundred RPM, but it may be worth 10-30 hp. And ... I think there are lots more offerings for 1.75 than 1.63.

Ya know, I'm looking over the engine specs again, and it's pretty mild. What kind of rear gear are you running? 1.63 is probably just fine if your rear is 3.5 or less.

jp

[Norwoodx55]

I would go with the 1 5/8" personally. You may spin the tires off in 1st. gear, but what about the rest of the gears? Probably not. A little less throttle could cure the 1st gear issues if needed, or a lower RPM launch. Is this an auto or manual trans. car? If auto, what RPM do you expect the converter to stall at? What tail gears? Street driving? Dragstrip hero or track time? All things I would take into consideration. I hate to give up low end torque unless it is a serious application. I have tried too many HP priority combinations that required RPM to have fun. I enjoy the grunt these days. Just my .02, take it or leave it. Brett

[jeff5347]

its a auto w/ 3.55. stall speed is 2800. This is a mostley street driven car but want something that will turn good timeslips to in the mid to low 12s. Has anyone had experience with dougs tri y headers. I read some articles on the int and forums they like them alot then i read david vizard in his how to build a small block chevy and he says that the 4-1 is better. so im still in a rock and hard place.

[parsonsj]

Well, given all this, short of doing back to back chassis dyno tests, I doubt we'll ever know which is the better choice.

Got a quarter? Give it a flip.

jp

[FabUThis]

don't know if this helps you

Primary tube dia = valve lift x valve dia divided by 295 = A
then get the square root of A

example= 525 lift x 1.60 dia divided by 295 = 2.847
square root is 1.687

1 3/4 would be fine.

[jeff5347]

sweet, thats all i needed to know. Ordered a pair of Super comp hookers from Jet hot in the sterling coating. Anyone use this coating. How do they look. Im so excited to see.

[jeff5347]

well i got the headers yesterday. Man, what a surpirse. They have the sterling finish and look sweeeet. Question. Im gonna rebuild the mule this winter. As always a new engine gets a little hotter the first couple times. will this affect or run the coating of the headers?

[gearbanger]

Unless you are using a flat tappet cam, I don't see any reason to do the extended 2000 rpm run for intial break-in. That is usually when you can ruin your headers. I have heard of several people hurting their coated headers when they are breaking in a cam. If you are using a flat tappet cam or are planning to run the engine like that on break-in, I would use other headers for sure.