AirBusPilot

My 71' has a cnc motorsports 415ci engine that came with a dyno sheet showing 551 hp.

551hp - 6000 rpm
550hp - 6100
551hp - 6200
545hp - 6300
533hp - 6400
526hp - 6500
532hp - 6600

10.25 compression
Edelbrock RPM gap
Tremec 600

I haven't weighed the car, but it's an LT-1 with Van Steel coil overs up front, and the rear is a composite spring. Weight is likely below 3000 lbs.

So, which header size? BTW, it was dynoed with a 1 3/4" stepped to 1 7/8" dyno headers. Right now, it has wimpy 1 5/8" Tri Y's. Not sure how much power is being killed with those.

Thanks!

Les

At your CI and power level I'd go with 1 7/8" without hesitation. Hell, I run 1 3/4" on the 355 in my avatar car, which has a fairly pumped 355. My throttle response is strong right above idle. You'll have gobs of torque with your engine, so even if you lose a small bit down low you'll have more than you can use- and you should have some nice gains from mid-range on up. If you don't already have it, adding a 3" mandrel system after the headers would add even more power. It should be a beast!

AirBusPilot

The engine builder recommended the 1 3/4", but I didn't get a chance to tell him what car it was going into and it's low weight. I was going to go with what he said, but I'm now more inclined to do as you recommend.

Looks like Maryland Speed is having a Black Friday sale..I'll have to call and see how much of a savings.

Les

I'd be curious as to why he recommended 1 3/4"- and I'm certainly not saying he's wrong. He may change his mind after knowing more about your car, or maybe he won't. Did he say you'd lose too much low end torque for a street application?

AirBusPilot

It was a quick phone call, I didn't want to bother them. But I suspect his rec was based on your typical near 4000 lb muscle car.

Here's a must watch video on header comparisons. It's a BBC, though 427ci. I'm going with the 1 7/8.

Les

Interesting video. The torque gained w/ the header extensions was noteworthy, but the real eye opener was the huge amount of HP & torque lost with shorty headers. FYI, I'm running 2" Hedmans on my 427 and that thing has silly torque down low. The 2.5" mandrel system I added definitely picked up some and, overall, the pipes probably act somewhat like those header extensions do.

I'm sure you'll be happy with 1 7/8" primaries.

ajrothm

1 7/8" if you can find some... A good mandrel bent 3" system like Les said is very important over 500hp.

That car should be nasty!

hugie82

When you are going for every last HP, 1-7/8 would be best but drivability on the street, the 1-5/8 may make your engine respond better at low rpm and make it feel tamer than it really is.

Les

I must respectfully disagree. On a mild 350 or even a mild 383 that might be true, but on a high HP/torque engine with 415 CI those headers would gag the engine and result in a big hit on both HP and torque. Take a few minutes and watch the video he posted.

AirBusPilot

Kooks makes 1 7/8. Pauldana has those on his car.

It's pretty quick now..I raced a couple of LS3 camaro's and just destroyed them.

The original owner was all about trying to turn this engine into a low rpm tractor. He had a 6k rev chip, which I replaced with a 6.8k rev chip. That helped wake it up tremendously.

That being said, I'm thinking these small tube Tri Y's that are installed are costing me about 30-40hp up top, and adding nothing to the bottom end torque. The dyno sheet I have shows 456 lb/ft at only 2500 rpm, then it climbs to 532 lb/ft at 4800 rpm (with the dyno headers).

63mako

1 7/8"
1971 corvette curb weight 3,593 LBS + you and fuel = heavier than you think

AirBusPilot

GM Heritage Center shows 3200lbs for a stock 71' convertible. Considering how this car has been modified, I'm confident it's below 3000 lbs.

https://www.gmheritagecenter.com/doc...t-Corvette.pdf

63mako

Yes it does. These are heavier than you would think, Nova, Camaro are within 100 LBS plus or minus

Another source showed 3,593.

Car and Driver 1971
"Then there is car weight, which varied in this test from 3370 to 3675 lbs."
http://media.caranddriver.com/files/...corvette-1.pdf

AirBusPilot

No way does this car weigh anywhere close to 3500 lbs.

I'm confident in the number published by the manufacture. Throw in the weight reduction my particular car has, and it's no stretch to believe it's below 3000lbs. But, with all due respect, so what if it isn't.

1 7/8" headers is the choice. Thanks for your input on that, I was hoping you'd chime in.

lars

As you guys know, I do quite a bit of dyno testing: I dyno run and tune every engine I build to assure a documented level of performance and to optimize every combination I put together.

Before I got heavily involved in the testing, I believed the articles published by the popular magazines: "For the street, you gotta' keep that exhaust gas velocity up, and big tubes will kill bottom-end torque..."

In spite of what I believed to be true, testing has proven me wrong: In every single test I've done on the dyno, on small block engines from 350 through 427 cubic inches, the bigger headers produce more power and torque throughout the rpm range. Even at low rpm. Not by a small margin, but by a significant difference - we're talking differences of 15 horsepower across the rpm range. The larger tubes (1-7/8) will outperform the 1-3/4 small-tube headers every time, even on a little 383, and there is no sacrifice in power or torque from 2500 rpm-up (I don't start my dyno pulls below 2500, so I don't have numbers or facts below that engine speed).

The bigger headers can be a bit more painful to install, but if you're looking to maximize power, run the big tubes.

Lars

Caveat: The engines I build are not usually stock engines. The statements above apply to engines above 450 horsepower. I do not have any data about big-tube headers on near-stock engines or on stock iron cylinder heads. I doubt the big-tube headers would have any gain on modest-rpm engines with near-stock cams running stock iron heads. On a near-stock engine, any header, such as a set of 1-5/8" tube headers, would produce a nice performance gain over the stock cast manifolds, and there is likely not much to gain by going with the big primary tube size.

AirBusPilot

Thanks Lars! Great info.

63mako

bluedawg

You won't really know until you weigh it.

DO MA NEU!

vstol

I have a LS3 427 and the builder as all the major builders say 1 7/8's. Mine are actually stepped 1 7/8" to 2"

waljr

there are header length/diameter calculators on the net that you can put your details into.

http://www.wallaceracing.com/header_length.php

let us know how you get on