Factory Exhaust Manifold Flow Improvements - Chevelle Tech
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post #1 of 16 (permalink) Old Oct 10th, 19, 4:31 PM Thread Starter
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Factory Exhaust Manifold Flow Improvements

Last year I installed headers on my 'Bu. I like the power increase, but The Wife is not wild about the header noise or the heat in the passenger compartment. I could better insulate the firewall/floor for heat and noise, but I'm also considering going to a modified "factory appearing" exhaust system. What am I missing? As always constructive suggestions are appreciated. This is what I'm thinkin':

FACTORY APPEARING
BBC EXHAUST SYSTEMS
For Street Performance Applications

An exhaust system flows only as well as its smallest diameter or restriction. In the case of BBC Exhaust manifolds this smallest diameter will be located in the manifolds themselves.

Extrude Honed Exhaust Manifolds can increase low end performance, but won’t do much for horsepower. Exhaust Manifold Extrude Honing is expensive. An alternative is to use a flexible hone and do-it-yourself. Flexible hones are available in many sizes and grits. The manifolds must be ported and the inside diameter opened up as much as possible by honing.

BBC Exhaust Manifolds of this era incorporate small inside-the-manifold protrusions for the AIR Pump System’s injectors (this is true whether the manifolds are drilled and tapped for the system or not) and the nubs must be ground away. The manifolds are then ported and honed (an inside diameter of 1 inch would be ideal for the inlet ports and 2.5 inches at the outlet). This process is one of the simplest and most important and can add up to 5 to 10 percent of flow.

A non-operational AIR System can be mounted, provided the AIR tubes do not intrude into the interior of the manifold. Whereas, it would be simpler to start with non-AIR manifolds and simulate the AIR Tubs’ being attached, there are zero performance gains to be had, therefore use the manifolds you already have.

If you don’t already have a pair of exhaust manifolds, consider these:
3989310 1971 - 1972 Right Hand
3989343 1971 - 1972 Left Hand

The 3989343 Left Hand Exhaust Manifold has less of a kick out angle at the outlet end than the 1969/1970 manifolds (something to keep in mind when purchasing head pipes) and a less restrictive casting internally at the number one cylinder (making for a slightly better performing manifold when extruded).

The factory Heat Riser Valve must be eliminated. If you want the look of a factory exhaust system a “Heat Riser Eliminator Block” could be used to give the external appearance of the valve. The block itself must be bored out to be circular and then honed to a 2.5 inch inside diameter.

Use 2.5 inch inside diameter mandrel bent exhaust tubing for the head and tail pipes with a 2.5 inch inside diameter H Crossover (the “H” pipe will provide better low end Torque with Exhaust Manifolds than a “X” crossover will). Use high flow mufflers with 2.5 inch inside diameter offset inlet and 2.5 inch inside diameter offset outlet. Use 2.5 inch inside diameter Exhaust Tips. Consider heat wrapping the head pipes to keep heat in the system which provides for better flow.

Use a Camshaft which is designed to produce low end torque, not high end horsepower (i.e. less overlap to lessen exhaust gas reversion).

Use an Intake Manifold which is designed to produce low end torque, not high end horsepower.
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post #2 of 16 (permalink) Old Oct 10th, 19, 9:40 PM
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Jim
 
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Re: Factory Exhaust Manifold Flow Improvements

I remember when I first saw "extrude honing" advertised in the magazines...long time ago. As I recall, the reviews weren't so great and the price was pretty high.
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post #3 of 16 (permalink) Old Oct 10th, 19, 10:24 PM
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Re: Factory Exhaust Manifold Flow Improvements

Wouldn't it be easier just to get your headers coated???
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post #4 of 16 (permalink) Old Oct 10th, 19, 10:31 PM
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Re: Factory Exhaust Manifold Flow Improvements

Bruce, is this a copy/paste from some exhaust vendor ad? Looks like it.

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post #5 of 16 (permalink) Old Oct 10th, 19, 11:40 PM Thread Starter
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Re: Factory Exhaust Manifold Flow Improvements

Quote:
Originally Posted by Tom Mobley View Post
Bruce, is this a copy/paste from some exhaust vendor ad? Looks like it.
Nope, I wrote it, I like to do research and plan out what I want to accomplish (my memory isn't, wait what are we talking about?).

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post #6 of 16 (permalink) Old Oct 10th, 19, 11:42 PM Thread Starter
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Re: Factory Exhaust Manifold Flow Improvements

Quote:
Originally Posted by JJ'65 View Post
I remember when I first saw "extrude honing" advertised in the magazines...long time ago. As I recall, the reviews weren't so great and the price was pretty high.
Correct. It's about 800 bucks (for a pair, I hope) and mediocre results at that.

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post #7 of 16 (permalink) Old Oct 10th, 19, 11:56 PM Thread Starter
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Re: Factory Exhaust Manifold Flow Improvements

Quote:
Originally Posted by blm View Post
Wouldn't it be easier just to get your headers coated???
My headers are the Hedman "Black Maxx" Ceramic Coated. Noob question: Is there a "better" coating to contain heat and noise?

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post #8 of 16 (permalink) Old Oct 11th, 19, 12:14 AM
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Re: Factory Exhaust Manifold Flow Improvements

Quote:
Originally Posted by NickeyChevelle View Post
My headers are the Hedman "Black Maxx" Ceramic Coated. Noob question: Is there a "better" coating to contain heat and noise?
MAYBE jEThOT as they are coated both inside and out,
some thermal .cool it. insulation and sound deadner mat worked on my ride

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post #9 of 16 (permalink) Old Oct 11th, 19, 12:18 AM
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Re: Factory Exhaust Manifold Flow Improvements

After 10+ years of dealing with leaking collectors, rattling on the frame, always in the way headers, I swapped out for exhaust manifolds. No regrets. I like the stock look and I really don't notice any performance loss. It is noticeably cooler in the interior and no more fumes. Also went with 2.5" plumbing (was 3") and straight thru Magnaflow mufflers (no manifold gaskets req'd). My wife enjoys not smelling like exhaust fumes after a cruise in the ol' heap...
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post #10 of 16 (permalink) Old Oct 11th, 19, 7:53 AM
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Re: Factory Exhaust Manifold Flow Improvements

Quote:
Originally Posted by Stickman View Post
After 10+ years of dealing with leaking collectors, rattling on the frame, always in the way headers, I swapped out for exhaust manifolds. No regrets. I like the stock look and I really don't notice any performance loss. It is noticeably cooler in the interior and no more fumes. Also went with 2.5" plumbing (was 3") and straight thru Magnaflow mufflers (no manifold gaskets req'd). My wife enjoys not smelling like exhaust fumes after a cruise in the ol' heap...
Hey that's like trading in Ellie May for Miss Hathaway.
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post #11 of 16 (permalink) Old Oct 11th, 19, 8:33 AM
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Re: Factory Exhaust Manifold Flow Improvements

Best improvement for the BBC exhaust manifolds is to remove aforementioned "hump" from the AIR boss and this is key, get them hi temp coated inside and out,from someone like this:
Central CT Coatings | Home

They have coated a few exhaust manifolds for our SuperCar guys. The coating is more beneficial HP wise than the extrude hone process. This is according to a few friends that I race with that own 10/11 second stock appearing,stock tire cars.

And I will also tell you this: years ago I went from exhaust manifolds to headers and I had a 4.5-5 MPH gain in my trap speed,so stock manifolds are horrible and don't let anyone convince you otherwise.

A 114 LSA cam also works well with manifolds. And so does ALOT of compression. Like north of 11:1

And FWIW,I have untouched,out of the box repro AIR pump exhaust manifolds on my 11 second heap.
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post #12 of 16 (permalink) Old Oct 11th, 19, 9:02 AM
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Re: Factory Exhaust Manifold Flow Improvements

Quote:
Originally Posted by NickeyChevelle View Post
Correct. It's about 800 bucks (for a pair, I hope) and mediocre results at that.
FWIW, that place is a couple minutes from me (North Huntingdon/Irwin PA) They changed their name to something with Extrude in it. I have always wanted to stop in and see what all they do. I think back in the day, doing intakes was a good benefit, something you can't get to with hand tools.

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post #13 of 16 (permalink) Old Oct 11th, 19, 9:27 AM
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Re: Factory Exhaust Manifold Flow Improvements

There’s a lot of improvement to be had by removing the restrictive 2.5” outlets and welding on some 3” turbocharger dumps. That is something that I have long dreamed of doing and adding a resonance chamber, which would simulate a header collector.

The resonance chamber is like a large diameter crossover tube. But instead of it being tied directly to the smaller pipes, it’s like two pipes run side by side run in to a larger tube which allows for the pulse effect from both side. Functionally, it is an empty vessel that is designed to suck in the gas and create a slight negative pressure on the cylinder so as to pull in unburnt gas (without pulling it straight through) and pull as much burnt gas as possible out. It is like a muffler that uses dead air space instead of packing. The exhaust note gets into the resonating chamber through one or more small bleed holes. The resonating chamber creates an interference in exhaust flow and uses the positive and negative sound waves to cancel each other out. A chamber also gives the tuner more options for minimizing back pressure at the crucial moment of valve overlap.
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post #14 of 16 (permalink) Old Oct 11th, 19, 12:28 PM
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Re: Factory Exhaust Manifold Flow Improvements

No exhaust manifold for a SBC or BBC will ever out do a header performance wise. Even the 427 Corvette ones that I thought were the bomb were way down compared to headers. As for heat the manifolds were always hotter for me the last set of manifold I used were on my Son's 409. We cleaned and silver POR coated them but there is still a lot of heat form them. You are right the headers are a little tingier than manifolds but the way an engine runs with headers keeps me using them. Seems though the LS exhaust manifolds are pretty good IMO.
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post #15 of 16 (permalink) Old Oct 11th, 19, 2:16 PM
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Re: Factory Exhaust Manifold Flow Improvements

This is something I have wanted to do with my exhaust manifolds. I have a numbers matching motor and have kept the engine bay original, but I would love to get a little better flow from my manifolds.

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