I have a ZZ4 (355hp stock was a 208/221 dur -.480/.510 lift cam & SMALL 168cc heads and crappy intake).

Not great parts: " The aluminum heads are basically the same as late-'80s L98 Corvette heads; the dual-pattern hydraulic roller cam is lazy; and the low-profile intake manifold, although it offers ample hood clearance, just doesn't flow the air." ---Hot Rod Magazine

I replaced the heads with 195cc Trick Flow heads, an RPM manifold, and a 230/235 dur. - .530 lift cam. I'm assuming 400-420HP (+/-)

So... if I'm 85% street/freeway...and 15% strip... which will suit me better? 1 5/8" or 1 3/4"?
I will (when this needs rebuilt....could be years from now) be turning this into a 383 but the use will be the same (85% street 15% strip).

1 5/8 should be fine for you. WIth both motors

Mike

Yep 1 5/8 should be fine and by the time you get ready to step on that engine, you should be ready for a new set anyway.

I'd like a little more info on 1 5/8 versus 1 3/4. Is there a big difference in top end performance? Torque? Is there a horsepower rating that the 1 5/8 can't handle?

Ran 11.90's@114 n/a with the 1 5/8 $25 cheapo headers (used) will try the new hooker 1 3/4 this spring. I bet there won't be that much difference.

Low end torque is better with 1 5/8, and based on your 85/15 split, these would work better for you.

I'd like a little more info on 1 5/8 versus 1 3/4. Is there a big difference in top end performance? Torque? Is there a horsepower rating that the 1 5/8 can't handle?
That's a good question, I'd also like to know at what point you should go to larger size headers.

On my last '78 I built a 406 with 10.5:1 compression, Dart Sportsman II heads, Crane solid cam 244/252 dur. @ .050 .518"/.536" lift 106 LSA, Weiand Team G intake, Holley 750 DP, 10" 3800 stall converter, 4.10 gears.

My machinist at the time recommended I use 1 3/4" headers with 3" exhaust. So I took my 1 5/8" headers and my very nice fitting and sounding full length 2 1/2" Flowmaster exhaust system off and switched to 1 3/4" headers and dual 3" two chamber Flowmaster mufflers that ended under my rear seat. It was loud as h_ll and honestly, I didn't like driving it on the street!

This '78 is going to be a nice daily driver/mild street performance car. I'm building a 385, but I'm not going to turn it more then 5800 RPM's. When I bought it, I just went ahead and bought 1 5/8" Hedman headers and the same full length 2 1/2" Flowmaster exhaust system I took off my last Camaro.

I'm sure the 1 5/8" headers will work just fine on my current set up. I don't know if 1 3/4" headers would be any better or any worse, I don't know.