L78 intake plenum divider - Chevelle Tech
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post #1 of 43 (permalink) Old Dec 19th, 18, 9:09 AM Thread Starter
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L78 intake plenum divider

I'm about ready to reinstall my bbc in my 65 after a rebuild. The motor is a .30 over 454 , 2.19 valve 781's with a summit 1303 cam, Holley 850dp. I have an L78 163 intake with rec port gaskets I want to install for the stock appearance. Is there any downside to modifying the divider? I've seen them cut down an inch or so and sometimes completely removed. Suggestions?
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post #2 of 43 (permalink) Old Dec 19th, 18, 9:26 AM
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Re: L78 intake plenum divider

Usually runs better at rpm with a .500"-1.00" gap. You can mimick this with a open spacer/heat shield. Ps UltraSeal 5828 is a good gasket to do this with.
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post #3 of 43 (permalink) Old Dec 19th, 18, 9:27 AM
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Re: L78 intake plenum divider

Well, the Chevrolet Performance intake manifold which you could say is a modern day interpretation of the L78 manifold has a cut down plenum divider so I'd assume those in the know believe there is an advantage to be had in doing so.

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post #4 of 43 (permalink) Old Dec 19th, 18, 9:36 AM Thread Starter
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Re: L78 intake plenum divider

Thanks Gene, 5828 is what I intended to get but I ordered the Mr gasket 108 by mistake. Also, thanks for that pic Rich.
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post #5 of 43 (permalink) Old Dec 19th, 18, 9:51 AM
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Re: L78 intake plenum divider

Quote:
Originally Posted by aimhard View Post
I'm about ready to reinstall my bbc in my 65 after a rebuild. The motor is a .30 over 454 , 2.19 valve 781's with a summit 1303 cam, Holley 850dp. I have an L78 163 intake with rec port gaskets I want to install for the stock appearance. Is there any downside to modifying the divider? I've seen them cut down an inch or so and sometimes completely removed. Suggestions?
Removing the divider and more or less leaving a .090-.100 lip on a 163 intake is worth 35 HP on 4.25" stroke 291 casting untouched square port heads.
tested by Eric @ Vintage Musclecar
You can find the info doing a search in this forum.


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post #6 of 43 (permalink) Old Dec 19th, 18, 11:47 AM
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Re: L78 intake plenum divider

Quote:
Originally Posted by mr 4 speed View Post
Removing the divider and more or less leaving a .090-.100 lip on a 163 intake is worth 35 HP on 4.25" stroke 291 casting untouched square port heads.
tested by Eric @ Vintage Musclecar
You can find the info doing a search in this forum.
What do you mean by "leaving a .090-.100 lip" on the intake?

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post #7 of 43 (permalink) Old Dec 19th, 18, 11:56 AM
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Re: L78 intake plenum divider

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Originally Posted by Rich-L79 View Post
What do you mean by "leaving a .090-.100 lip" on the intake?
the .090-.100 lip refers to the height of the plenum divider.

This is my intake in this pic
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post #8 of 43 (permalink) Old Dec 19th, 18, 12:45 PM Thread Starter
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Re: L78 intake plenum divider

I just read that test. Those are some big gains for a simple change. Mr. 4 speed thanks for your photo.
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post #9 of 43 (permalink) Old Dec 19th, 18, 1:07 PM
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Re: L78 intake plenum divider

Quote:
Originally Posted by aimhard View Post
I'm about ready to reinstall my bbc in my 65 after a rebuild. The motor is a .30 over 454 , 2.19 valve 781's with a summit 1303 cam, Holley 850dp. I have an L78 163 intake with rec port gaskets I want to install for the stock appearance. Is there any downside to modifying the divider? I've seen them cut down an inch or so and sometimes completely removed. Suggestions?
What is your intended use? If low rpm carb response, low rpm torque and street driving efficiency is important to you, then you want the divided plenum in place.

WOT romp for 1/8th or 1/4 mile? Then sure.

Try the spacer idea first, experementation is fun, eh?
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post #10 of 43 (permalink) Old Dec 19th, 18, 1:29 PM
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Re: L78 intake plenum divider

Quote:
Originally Posted by NickeyChevelle View Post
What is your intended use? If low rpm carb response, low rpm torque and street driving efficiency is important to you, then you want the divided plenum in place.

WOT romp for 1/8th or 1/4 mile? Then sure.

Try the spacer idea first, experementation is fun, eh?
I can assure you that removing the plenum divider will NOT cause any of issues that you mentioned. In fact,it increases throttle response,torque/HP
Eric's dyno tests indicate that plain as day. FWIW,my engine has ridiculous,instant throttle response and outrageous low end torque.


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post #11 of 43 (permalink) Old Dec 19th, 18, 2:08 PM Thread Starter
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Re: L78 intake plenum divider

It's a street car, but will go to the track some. I'd love to see some 12's.
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post #12 of 43 (permalink) Old Dec 19th, 18, 2:08 PM
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Re: L78 intake plenum divider

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Originally Posted by aimhard View Post
I just read that test.

I must be having a search brain cramp Could you post a link to it, I would like to see it.

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post #13 of 43 (permalink) Old Dec 19th, 18, 2:14 PM
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Re: L78 intake plenum divider

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Originally Posted by BillK View Post
I must be having a search brain cramp Could you post a link to it, I would like to see it.
Hi Bill,read post #39:

https://www.chevelles.com/forums/13-...intakes-3.html
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post #14 of 43 (permalink) Old Dec 19th, 18, 2:35 PM
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Re: L78 intake plenum divider

Quote:
Originally Posted by aimhard View Post
I'm about ready to reinstall my bbc in my 65 after a rebuild. The motor is a .30 over 454 , 2.19 valve 781's with a summit 1303 cam, Holley 850dp. I have an L78 163 intake with rec port gaskets I want to install for the stock appearance. Is there any downside to modifying the divider? I've seen them cut down an inch or so and sometimes completely removed. Suggestions?

Quote:
Originally Posted by aimhard View Post
It's a street car, but will go to the track some. I'd love to see some 12's.
A good friend of mine runs [email protected] 107-108 MPH with a flat top 454 with 215 heads w/2.19 intake/1.72 exhaust, the same cam,RPM Air Gap/850 DP,small tube headers. 3.42 gear/TH400/3200 rpm or so 10" convertor. 4000 lbs. 69 SS 396 full street trim. He drives it everywhere,family goes out for ice cream in it. And it burns 87 octane since it has about 8.9:1 compression.
Your 65 should have no problem getting into the 12's with your combo

(FYI-the engine in his pic is NOT his engine,someone uploaded the wrong pic )
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post #15 of 43 (permalink) Old Dec 19th, 18, 3:23 PM
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Re: L78 intake plenum divider

Quote:
Originally Posted by mr 4 speed View Post
the .090-.100 lip refers to the height of the plenum divider.

This is my intake in this pic
Gotcha. Thanks for the clarification.
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