Zilla's rebuild (exhaust mod revealed, gauges going in.....) - Page 8 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 03-09-2007, 05:37 PM   #71
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Please note that my exhaust manifolds are already ported......did that a year and a half ago. I did a good measurement today before I left for work and created it to scale for this topic.

The full circle is the inside diameter of my exhaust manifold ports. 1.435". Area of 1.6"
The inside profile is my head port. The head port has a calculated area of about 1.2 square inches.

By matching the head port BUT KEEPING THE PROFILE (flats have a purpose), I can squeak another easy 15-20% in exhaust port flow from the heads. If I made it a full matching round (which I won't), I could gain 33%.

It'* better to take half what you WANT if you don't like creating scrap. The D-shaped exhaust port is designed to cancel out the shockwave that returns back to the cylinder from inside the exhaust manifolds. This actually helps performance.

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So I'll be enlarging the rounded sections in WIDTH, but leaving the flats where they are.
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Old 03-10-2007, 09:27 AM   #72
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Not even a proportional increase in size, eh?

I'm not privvy to the equations that went into designing those flats, so I'm just going to go ahead and ask: is it the true position of the flats, or the presence of the flats that work to cancel the shockwave?

If you had a spare set of heads, it would be interesting to see the effects of a proportional increase to their size...
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Old 03-10-2007, 11:04 AM   #73
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It'* both the position and surface area of the flats that are important on a D-shaped exhaust port.

I've found some interesting work by Edelbrock on Harley Davidson heads that are very similar to my exhaust ports. I actually spoke to one of the Edelbrock guys about them, which is what I'm basing my porting on.



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The "D" shaped exhaust port is designed to provide a larger area for increased flow and velocity as well as to create a mismatch between the exhaust pipe and the exhaust exit. This purpose-built mismatch reduces "power-robbing" reversion by keeping the majority of the exhaust gasses from going back into the engine.
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Old 03-14-2007, 07:28 PM   #74
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It has begun. I'm porting the rear head right now. The intake runners are roughed in, very little material removal, just smoothing things, as I will in the valve bowls.

The exhaust ports are getting a little more attention than that, when my compressor catches it'* breath.
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Old 03-14-2007, 09:22 PM   #75
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These are progress pics, not finished pics.

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Old 03-14-2007, 09:35 PM   #76
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Alot of good clean work is what I see right there.
Zilla will thank you
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Old 03-14-2007, 09:38 PM   #77
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He'* talking too fast to get done tonight. Expect finished pics tomorrow.
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Old 03-14-2007, 11:58 PM   #78
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Thats looking very good!

hurry up and get then done!
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Old 03-15-2007, 12:52 AM   #79
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Looking very good. And clean!! Wow, that'* a clean engine for the miles and tear that'* been on it. :P

With the increased flow, what'* you're thoughts on flat pistons with relief cuts for the valves?

(Haven't really seen any topics on flat pistons.)
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Old 03-15-2007, 09:09 AM   #80
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Pistons stay the same.

All I'm doing here is what I'd call a 'Stage 1' job. I'm SLIGHTLY increasing the intake ports in order to smooth them. The LIM will be further ported to match them.

Increased the width of my exhaust ports to match my ported exhaust manifolds, and left the upper and lower flats relatively untouched, other than smoothing them down.

I still have to enlarge my valve bowls and blend back to the intake and exhaust ports. Then do the other head.

Typical warning applies......this isn't your typical garage job. This is difficult work, and I take great pains to do all the ports exactly the same. I move from port to port, and measure several times.
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