I just started reading David Vizard's book "How To Build Max Performance SBC On A Budget", and was startled to learn that he does NOT recommend headers with 1 3/4" primaries and 3" secondaries/collectors, UNLESS the engine is producing MORE than 375 hp. I almost blew my own head gasket when I read this because this is exactly what I'm doing, and I see alot of us here are going with headers that he would deem too big, especially us L98 guys.

Why would this guy claim this, apparently he's quite accomplished and since I'm an idiot I would have to take his advice. Are many of us just dead wrong and screwing up? Are we LOSING any performance vs. 1 5/8" primaries with 2 1/2" secondaries by going with the larger diameter?? :confused:

Many of us spend big $$ getting the 1 3/4", are we flat on our faces :cuss

 

It's possible that the 1 5/8 are better for us, but that 375 HP he is talking about is at the flywheel.

Also, I like to plan for the future. :D

Vic

 

:iagree:

 

I dont know about that but Im shooting for 500HP NA and then maybe a 100shot of N20 ontop of that later on. I already have hooker supercomps and a 3inch exhaust would go perfectly with that. :yesnod:

 

I've seen many build-ups with 500+HP (flywheel) using 1 5/8" headers. Bigger will give up some bottom end; Joe Sherman has used them on many engines he's built. One problem is that the only company I'm aware of that builds them for our cars is Hedman.

:chevy

 

The larger primaries on an L98 would hurt the L98's low-end power some in theory, because the exhaust gases would have a lower velocity going into a tube thats very big. Smaller header pipe increases the gas velocity, resulting in efficient removal of exhaust gases at low rpm.

1 5/8" would be slightly better in theory than 1.75" for the L98, but I personally dont believe it matters, some with 1.75" have commented that they didnt believe they lost any low-end on the L98. Its not like you wont get that power back at a higher rpm anyway.

If you were sticking with the TPI intake, 350ci, 1 5/8" might be best in the long run. If you're ever going bigger, get the 1.75"

 

Dave Vizard is basing his estimate of "best" header tube length on a table that shows exhaust port velocity (CFM at 28" of H2O) vs header pipe dia.

For example, from his table we have the approximate numbers:
PipeDia.....Flow Rate
1.750"......190cfm
1.625"......160cfm

Further, TPIS shows some exhaust port flow numbers for #113 Al heads:
Flow at .500"....Head Status
159cfm............Stock, with 1.500" exh valve
177cfm............Pocket Port, with 1.500" exh valve

IMHO, this data tells me that 1 5/8" headers would be fine for a stock Vette SBC, but with heads & cam 1 3/4" would be a better choice. I mean don't you want the header tubes to flow a little better than the exhaust ports, though not too much so that gas velocity will be lost.

And if you limit the amount of cam overlap the issue of charge blow through at low RPM will be minimized. Also, the step caused by our "D" ports tends to minimize inversion, even with larger header pipes.

 

Thanks everyone for your input. Jim, I see exactly what you're saying.

Ever since I joined this forum 15 months ago the conventional wisdom for L98s when it came to doing any mods the FIRST THING was to upgrade the exhaust completely - which obviously inlcudes getting rid of the exhaust manifolds in favor of headers.....and many have said Hooker headers as one of the best. But they're 1.75" and now I'm going to lose bottom end. Thats the reason I got the L98 to begin with. 375chp from what I'm seeing here isn't going to happen with a majority of modded L98's. So here many of us are running with larger than optimal size headers for the application. We need a serious 125 more chp - which is big time serious - and I just don't see that happening except in very limited cases.

If so many people advise on full exhaust being the FIRST thing, it seems interesting to find out now that its actually the LAST thing. Us L98 guys have to have do cam, head porting, etc FIRST before anyhting else, which then knocks out a majority of us because thats obviously big $$.

However.....then again if the LT headers simply move up the TQ/HP curve a bit and those gains are realized there, a good thing happens. I have no problem with this, then its seems the L98 would behave more like an LT1.
Am I about right on this one?

 

Thats the controversy, youre not assured of losing any noticable bottom end with the larger headers. It wont quite be like an LT1, but the midrange will be much stronger.

 

That sounds real good then, would be very satisfied with that ;)

 

Well, when running a full exhaust system it might not make a lot of difference between 1 3/4 and 1 5/8; I think Jessie showed that a while back. But, LT headers will make an improvement over your stock exhaust at any rate, and needed to fully utilize any engine mods, so thats why most people start there. It's the combination that counts. The thing with Hooker and some others, is a good fit and finish. I agree they're expensive, I bought a set (Cyclone) for a friends Nova in 1985 and they cost $35.00.
:chevy

 

Brad, you can use those "simple" mods to add quite a bit of power and an AFPR to tune to new air flow.

Indeed you will push the power curve up a little but you won't lose much if any low end torque.

I might add that I have an '86 IROC with the LB-9 motor, to which I added full exhaust mods including the Edelbrock TES. Those consist of a mix of 1 1/2" & 1 5/8" header tubes. I felt now loss of loss of low end though the motor developes only 245chp.

Also, when I my Desktop Dyno setup from small-tube headers with mufflers to large-tube headers there is a slight gain in chp at 2,000rpm and gains above that rpm too.

 

Thanks for the latest round of posts :cheers: I've now been able to take off my straight jacket and throw away the Thorazine pills :lol:

Oh yes......if there happens to be any loss of low-end, I'll be sure thats taken care of with the Accel AFPR unit I have, by bumping up fuel pressure a few psi from stock, as well as adjusting the base timing up 2-4 degrees; these two tuning tool should go far in solving the whole thing.

:cheers: To everyone :cheers: