I'm in the process of planning my headers, equal length, 36" primaries.

I know the engine will respond better than with the stock exhaust manifolds, but what is the range of improvement that is generally to be expected if I do the job right? I'm hoping for at least 8-10%. Is this reasonable?

Anybody?

Thanks,

Arn

There is no real 'typical' gain for headers. Most headers flow enough to support an engine producing WAY more power than most customers need.

Any gains in the form of power comes down to how restrictive the factory setup is.

As for percentage, if there was a typical gain, Id say less than 5%. 10% is HUGE for a simple bolt on. Thats saying a 300hp motor would gain 30hp just with headers. Not likely

Actual gains will most likely come in the form of smoother revs and torque curve, as well as "cool" factor under the hood. And depending upon the material used, headers normally make the motor a good deal noisier.

5% as a max increase is ok with me. I am hoping to have a nice spread of increased torque and response particularly from 2000 - 3500 rpm.

The smoothness of the power curve is really good to get along with some more torque where you can use it.

My understanding is that longer primaries lead to lower end torque. If that is true again are we talking 5% difference in the midrange at the max, or something less?

there is next to no gain at 2500-3000 with headers.... stock manifolds are designed work at their best at 2500-3000....the pulses are to slow and spread apart to far to create scavaging with equal length headers at low rpm's

just a typical example:
3,000rpms, log manifolds=298ft/lbs
3,000rpms tube headers, 34" collectors=296ft/lbs...not a typo, it is lower

Gains happen across the board. Long tube hedders help down low, unless you get to large a diameter tube for your installation.

Good rule of thumb is 20/20 for a 350 cube.

Hedders are a lot like carbs, cams. Stay smaller (diameter) for low rpm power.

Thanks guys,

What I've been researching indicates exactly that. Too fat, no bottom and longer is better.

What I've been able to find out is that 1.5" pipe at 34-36" is the best primary size for my street engine. Maybe 1&3/8's but I'm going with 1.5"OD because the stock is 1&1/8" ID and 1.5" OD is not too fatter but is just a bit. So that is where I am aiming. What I don't know is what to expect. The car will be going on a dyno when it is done, so I'll eventually know exactly, but in the meanwhile, I'm not seeing anything in print so far that shows an average improvement or an expected improvement. Thanks for the input.

Arn

attached dyno chart is from the fifth in a series of articles done by Chevy Performance magazine It's on their website, under "complete builds" good articles

article #1 was they added headers and performer intake to the stock 239hp 350=line 2

if you "just gotta have" headers then small diameter Tri-Y's (exoug Thorley) can boost low end hp and tq by a modest amount

Attached Images

untitled.bmp (62.0 KB, 21 views)

here's a more clear view of the chart

Attached Thumbnails

 

I went to
http://www.chevyhiperformance.com/t...power_increase/

and it was very informative. Not engine specific, but overall a real good read.

It kind of looks like an approximate 10% inprovement for a street engine. Thanks for the reference. I've bookmarked it.

Arn

arn,
when they added headers and a manifold, they did gain 20hp "at the hp peak", probably around 5,000rpms....

look at the left bottem side of the graph I posted.....at 2500rpms it added nothing compared to the test#1 bone stock 350 with stock manifolds

Got it.

Arn

arn,
your link is a different magazine article....

here's the graph from your link-article

note that they are doing a "flywheel dyno" not rear wheel dyno in a car...

add 5 fwhp @2600 = nothing at the rear wheels
add 11 fwhp @3000 = "not much" at the rear wheels in a 3400lb car

the extra 20 ftlbs @ 3,000 will make it a little more fun to drive car but you can gain up to 40ft lbs with a different cam and stock manifolds

Attached Thumbnails

 

I'm not looking at the header dyno info completely on its own. The more examples the better to understand it, IMHO. (and thanks BTW)

A whole lot depends on just how good the stock manifolds were in the first place. Also, a lot depends on how much more efficiently the carb can flow with a little help in the extraction department. As you know different engines respond differently to headers, and different header design (length, volume, bends) is a huge factor for what you end up with in performance. I'm going to 36" primaries to help out before the peak hp is reached.

In my case the engine has a case of constipation. It has log exhaust manifolds and unequal length primaries. REALLY unequal. I'm realistically thinking 10 hp at the wheels but the headers giving more oomph around 3000 rpm. I'm advancing my cam 4* to also bring up the bottom end.

My power band is currently 4200 - 5800 rpm and I want to bring it down some for some more grunt in the working rpms.

So I really appreciate all the info I've been given. I've got my mandrell bends and I just set up a jig to form the headers today. This is a custom application and nobody makes what I want, so I'm going at it myself. Wish me luck.

Arn

Arn, I conclude you have an 85 Mustang with the cast iron logs.

Using the Mustang tubular stainless factory pipes you can expect about 8 across the board, 15 with equal length shorties, about 20 with long tubes. 1 1/2 x 2 1/2 is a good torquer and if you use a 18" collector with a 10" divider it will be even better.

Most longies are 1 5/8 tubes and 3" collectors, do the same extensions/dividers. Step tubes are better.

I'd research what the header mfgrs are recommending for your combo.

A good general rule ...... any cheapo header is significantly better than the logs.

arns,
got a picture in my head of you huddling close to the just welded pipe to stay warm.....good time of year for that project...."ah"

so what is your "constipated" beast car/motor combo