BBC header size - Chevelle Tech
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post #1 of 22 (permalink) Old Jul 24th, 18, 9:29 AM Thread Starter
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BBC header size

I recently discovered a thread on this forum by Chris Straub, which outlined header primary size based on bore and stroke. It was very eye opening! I always knew that bigger primaries did not always equate to better performance but I thought that 2Ē primaries were standard for a 500 hp 427 bbc.

After reading the posts, I realized my 4.251 bore/3.75 stroke engine would perform better with 1 7/8 primaries.

I had intended to put 2 inch Lemons on it, until I read this thread.

I donít plan on spinning my engine much past 6000 rpm. 😁 I contacted Dan Lemons last night and we spoke about primary size. He is making me a pair of his street headers in 1 7/8. He feels that itís best to use 2Ē flanges because of my AFR heads. Great guy to deal with and his products are works of art! Iíll post them here when they are done, in 8 weeks.

Joe
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post #2 of 22 (permalink) Old Jul 24th, 18, 9:32 AM
 
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Re: BBC header size

A solid engine combination starts with math.

Chris Straub
Mfg Performance Parts

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Why is it we never have time or money to do it the right way in the beginning but we always have time and money to do it over again?
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post #3 of 22 (permalink) Old Jul 24th, 18, 9:35 AM
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Re: BBC header size

The trouble with many 1.875" is you may have to open up the flanges to the port, so good deal on the stepped headers. Primary tube size sets peak HEADER torque, and with my 440" and 2" its near 5000 rpms, so a 427 with a peak at 6000 dont need 'em. 427s that spin to 7500 do. I base it simply on peak header torque. If I recall 1.75" was near 3500 rpms, and 1.875 just over 4000 rpms. You may have been able to use really small 1.75" tubes with good effect!

You would notice the 2" pipes coming off a stoplight slowly. Smaller pipes work slow best. What I noticed with the 2" were on, low speed got a bit rattier, yet over 6000 they were decidedly better. Other than that, the math here has a pretty marginal effect for the overall system, but you done right with smaller.
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post #4 of 22 (permalink) Old Jul 24th, 18, 9:47 AM
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Re: BBC header size

Yes, smaller headers are better. And like Lemons is doing for you, the biggest issue is that the exhaust ports are usually too big for the headers you want to use. So most times you wind up with shelf headers that are too big, just so that you can get a gasket to seal on the port. I went through this with my Nova with the 355 in it. I used AFR 195 heads and wanted a 1 5/8" header because I felt that is all it needed. However, the hooker competition headers hung inside the port and would not seal well to the size of the port as a 1 3/4" header would.
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post #5 of 22 (permalink) Old Jul 24th, 18, 10:10 AM
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Re: BBC header size

Quote:
Originally Posted by bracketchev1221 View Post
Yes, smaller headers are better. And like Lemons is doing for you, the biggest issue is that the exhaust ports are usually too big for the headers you want to use. So most times you wind up with shelf headers that are too big, just so that you can get a gasket to seal on the port. I went through this with my Nova with the 355 in it. I used AFR 195 heads and wanted a 1 5/8" header because I felt that is all it needed. However, the hooker competition headers hung inside the port and would not seal well to the size of the port as a 1 3/4" header would.
I had the same issue with my AFR's. It hung in on all 4 corners of the exhaust port on my AFR 195 Comp ptd heads. I went to 1-3/4 supercomps from hedmans 1-5/8".
My car picked up with the larger header BUT it wasn't the only change in the exhaust. Went from 2.5" with an H to 3" to a 3" in X pipe 2.5" out of it to the existing 2.5" ultraflo mufflers and tailpipes at the same time of header change.
Kept that exhaust for the 406.
Worked ok when I ran the times in my sig.
I guess that's why Lemons is starting with a 2" flange then the 1-3/4" primary.

73 nova,3515#(all steel)Pump gas!(91octane)406ci N/A, best 1/4,1.45 '60,[email protected],10.6 @ 122.38, short shifting,NO tuning!


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post #6 of 22 (permalink) Old Jul 24th, 18, 11:07 AM
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Re: BBC header size

Here is the gasket fitment on my BBC with AFR 305's and 2 inch Headman headers. The tube that protrudes through the flange hangs in the port so there is nothing for the gasket to seal on. I filled the top of the tube with weld and smoothed it out to help with gasket seal. Then I used Remflex gaskets.


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post #7 of 22 (permalink) Old Jul 24th, 18, 12:43 PM
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Re: BBC header size

Joe, I hope you are using the GM Tahoe Turquoise and not the Ford.
Headers, gaskets and ports are not always true and do not have to be to get good performance. Unless you are trying to get absolutely everything from your setup you can drive yourself nuts with details.


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post #8 of 22 (permalink) Old Jul 24th, 18, 12:56 PM
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Re: BBC header size

Quote:
Originally Posted by Kopfjaeger View Post
I recently discovered a thread on this forum by Chris Straub, which outlined header primary size based on bore and stroke. It was very eye opening! Joe
Can someone post the link to that thread?
Thanks to the OP for starting this.

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original paint, M20, & 3:31 posi.
Unrestored, but dreaming of doing so
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post #9 of 22 (permalink) Old Jul 24th, 18, 2:51 PM Thread Starter
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Re: BBC header size

https://www.chevelles.com/forums/13-...wers-here.html

Here is the original thread.

Mrpaticular, yes, 67 GM Tahoe Turquoise, of course! 😁. Most likely adding a bit of silver flake to add some bling!!

Is going an 1/8 smaller in the primaries going to make a huge difference over 2 inch? I donít know but everyone chimed in that it would indeed matter. Itís no extra cost and Dan didnít even flinch when I asked. He takes into account the raised exhaust port so The header certainly better match the exhaust port better than in the photo that was posted in this thread! 😳

I know the engine certainly capable of more power than Iím requesting from it. Could have gone with much larger AFR rectangle port heads but I wasnít looking to have the engine scream to take advantage of the extra ponies. Iíll be going with a relatively mild hydraulic roller cam, probably, so as to retain suitable vacuum for power brakes. As I stated in prior threads, I went overboard on building an untamed 440 six pack motor back in my early 20ís. Breaking stuff got very old very fast. I never want to go there again.

Joe
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post #10 of 22 (permalink) Old Jul 24th, 18, 5:07 PM
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Wonder what a 4.03 bore & 3.400 required at 6500?
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post #11 of 22 (permalink) Old Jul 24th, 18, 8:04 PM
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Re: BBC header size

Quote:
Originally Posted by bracketchev1221 View Post
Yes, smaller headers are better. And like Lemons is doing for you, the biggest issue is that the exhaust ports are usually too big for the headers you want to use. So most times you wind up with shelf headers that are too big, just so that you can get a gasket to seal on the port. I went through this with my Nova with the 355 in it. I used AFR 195 heads and wanted a 1 5/8" header because I felt that is all it needed. However, the hooker competition headers hung inside the port and would not seal well to the size of the port as a 1 3/4" header would.
thats because they are hookers, they dont fit anything i have ever put them on.
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post #12 of 22 (permalink) Old Jul 24th, 18, 8:37 PM
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Re: BBC header size

Quote:
Originally Posted by Mike 70SS View Post
thats because they are hookers, they dont fit anything i have ever put them on.
Best brand, in your experience?
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post #13 of 22 (permalink) Old Jul 24th, 18, 9:33 PM
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Re: BBC header size

Quote:
Originally Posted by Mike 70SS View Post
thats because they are hookers, they dont fit anything i have ever put them on.
All I had to do was put a small dent where the pitman arm is.
Other then that, 10 years later, they're fine.
This is a Small block tho..

73 nova,3515#(all steel)Pump gas!(91octane)406ci N/A, best 1/4,1.45 '60,[email protected],10.6 @ 122.38, short shifting,NO tuning!


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post #14 of 22 (permalink) Old Jul 25th, 18, 1:48 PM Thread Starter
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Re: BBC header size

Quote:
Originally Posted by any4xx View Post
Best brand, in your experience?
For a big block Chevy, Lemons. The guy is an artist with Chevelle and Camaroís. Iíve seen his headers fit in vehicles with big blocks, that were not designed to have big blocks in them, fit right out of the box. Expensive, $1,500. Uncoated, but they will be the last header I put on this vehicle and he tweaks them to fit the AFR heads Iím using.

Joe
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post #15 of 22 (permalink) Old Jul 25th, 18, 4:49 PM
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Re: BBC header size

Headman should have a set of coated headers for your car. They fit nice under the hood and do not hang down like Hookers. If you go 2" be aware of how far the collector will move back. It might get close to your trans cross member causing an issue there. 2' primaries will require a larger collector and exhaust. But getting a 3" system tucked up nice is not hard for a good installer. Cost for 3" pipes, mufflers and install could be $600-800.
What state are you located in? I know a good shop in Michigan.


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