Has anyone seen any dyno sheets where 1 5/8 headers were swapped to 1 3/4 or vice versa? Im curious what the trade-offs were.

 

Smaller ones will have better low end and mid-range power, bigger ones will improve top end power.

 

That...it would seem is just an old wives tale.
Once you hook them to the street exhaust system it doesn't make much difference.

Running uncorked....well that's different.

Remember one thing....if you can't get it out ....you can't get it in
(air and fuel that is)

 

does anyone think that my hedman longtubes (1 5/8" primaries with 3" collectors) with 2 1/2" dual exhaust with moroso spirals and h-pipe will be a restriction on the new combo?

holley 650 double pumper (will change to the 750cfm+ flowing proform main body when funds permit)
holley street dominator intake manifold
comp XE274H cam
crane energizer aluminum roller rockers.
083 heads with 3 angle valve job, gasket match and bowl blend
9.8:1 c/r

 

Matters what your looking for, but possibly yeah they are.

 

Im trying to build a motor that has 550-600hp with a supercharger, without revving to 6500 rpms. I want really good streetability and I dont want to lose any significant low end torque to build top end, and Im wondering if 1 3/4" headers would do that. I want real good torque in the 3-4k rpm area.
Has anyone upgraded to 1 3/4" from 5/8 with 500+ hp and have anything bad to say about low rpm torque?

 

I don't think you will see any loss being that your exhaust dumps before the axel.I have a friend with an 85 camaro with an AFR headed 406 using hooker LT's and a full over the axel 2.5 exhaust and he only picks up about .2 when he opens the cut outs.He has the XE288HR cam and runs deep into the 11's @ almost 120MPH.

 

i'm going to go all the way over the axle with my new setup though.

 

I did that exact test while my motor was on the dyno. I'll have to scan them for you. Basically the smaller headers brought the torque peak down about 300 RPM and made the tq. curve much more broad. The HP was shifted down a little too but the tq. showed a bigger difference. The 1 3/4 showed "peakier" curves.

 

Then just do what my friend did and use some cut outs if you have to have the .2 of a second.

 

Full length 21/2" system is plenty for your new 360.

It's not as "wild" a combo as you think. It'll be around the 350hp mark, which dual 21/2" are plenty.

 

That would be awesome if you could do that. Thanks.

 

i imagine it will be making more than 350hp. the 305 propelled my 3300lb. butt to 103mph, which is like 283rwhp...that would be roughly 325hp at the flywheel.

i picked up a set of 113 casting aluminum vette heads for next to nothing and that bumps the c/r to 10.3:1 instead of the 9.8:1 it was at. and they are better flowing heads too than the 083's. i hope to be making more in the 375hp+range.

 

With a blower your going to want a large primary to help evacuate the huge volume of gases you will produce, so sticking with a 1 5/8" pri is going to hurt you.

mw66nova: I ran the heddman 1 5/8" LT and full exhaust behind my 355 and never picked up anything when I dropped the exhaust- 2.5" exhuast and H pipe, going into flowmaster 40 series.

same with this combo, Hooker super comps 1 3/4" into 3" exhaust-NO H or X pipe and flowmaster 40 series...never gained anything dropping the exhaust..did gain a headache with that loud azzed noise though.

If you gain a hundredth or so I'd chaulk it up more to dropping the exhaust system weight off the car more than I would the restriction.