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Intake Manifolds [message #274898]

Fri, 03 April 2015 15:04

thorndike

Messages: 406
Registered: January 2011
Location: Conifer, Colorado

Karma: 0

Senior Member

I have a stupid question.

For what reason do folks replace the intake manifolds?

Do they provide better performance?

Are they required for FI systems?

Robert Peesel 1976 Royale 26' Side Dry Bath Conifer, Colorado

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Re: Intake Manifolds [message #274902 is a reply to message #274898]

Fri, 03 April 2015 15:18

Bob de Kruyff

Messages: 4260
Registered: January 2004
Location: Chandler, AZ

Karma: 1

Senior Member

thorndike wrote on Fri, 03 April 2015 14:04

I have a stupid question.

For what reason do folks replace the intake manifolds?

Do they provide better performance?

Are they required for FI systems?

There appear to be all of those reasons but I think the major one is that they (mostly 455) crack in the area below the carburetor or through the web below the carburetor. Depending on the crack it can cause a major debilitating vacuum leak. Some people run throttle body FI and the intake isn't an issue there, but if you go port FI you need to modify the existing intake or get one with the bungs already in place like the aluminum replacement.

Bob de Kruyff
78 Eleganza
Chandler, AZ

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Re: [GMCnet] Intake Manifolds [message #274903 is a reply to message #274898]

Fri, 03 April 2015 15:12

Gary Worobec

Messages: 867
Registered: May 2005

Karma: -1

Senior Member

Stock Olds 455 manifolds will more than likely have a crack in the casting
right under the carb on the floor of the manifold and that will cause all
kinds of trouble. Also they have an exhaust manifold crossover between
cylinder banks that is also a cause of issues. There is a choke tube for
carb heat that comes out of the crossover. Make sure you do not have an
exhaust leak at that gasket or you could set the adjacent wiring loom on
fire. The aluminum "Rockwell" replacement manifold does away with crossover
and does not have the hot spots as in the cast iron manifold. However, the
Rockwell manifold is a bitch to get fitted correctly without air leaks
unless it have been modified in the last 4 years or so. I had to hand cut
intake manifold gaskets for mine.

Thanks,

Gary and Joanne Worobec
1973 GMC Glacier
Anza, CA

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Robert
Peesel
Sent: Friday, April 03, 2015 1:04 PM
To: gmclist@temp.gmcnet.org
Subject: [GMCnet] Intake Manifolds

I have a stupid question.

For what reason do folks replace the intake manifolds?

Do they provide better performance?

Are they required for FI systems?

--
Robert Peesel

1976 Royale 26'

Side Dry Bath

Sterling, Va
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Re: Intake Manifolds [message #274905 is a reply to message #274898]

Fri, 03 April 2015 15:29

thorndike

Messages: 406
Registered: January 2011
Location: Conifer, Colorado

Karma: 0

Senior Member

what are the tell-tale signs of a crack in the manifold?

Is there a way to test for a crack? We are considering the Atomic FI system and if I need a new manifold I'd like to find out BEFORE I install the system.

Bob

Robert Peesel 1976 Royale 26' Side Dry Bath Conifer, Colorado

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Re: [GMCnet] Intake Manifolds [message #274909 is a reply to message #274905]

Fri, 03 April 2015 15:48

Gary Worobec

Messages: 867
Registered: May 2005

Karma: -1

Senior Member

Once you take the carb off if there is not too much debris in the bottom you
should be able to see any cracks. But, if it is not cracked now there is a
good chance it will. Others may have more insights than I do on this. When I
added my EFI I went with the new aluminum manifold. Yes, it was an extra
$600 or so but I only like to cry once!

Thanks,

Gary and Joanne Worobec
1973 GMC Glacier
Anza, CA

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Robert
Peesel
Sent: Friday, April 03, 2015 1:30 PM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Intake Manifolds

what are the tell-tale signs of a crack in the manifold?

Is there a way to test for a crack? We are considering the Atomic FI system
and if I need a new manifold I'd like to find out BEFORE I install the
system.

Bob
--
Robert Peesel

1976 Royale 26'

Side Dry Bath

Sterling, Va
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Re: [GMCnet] Intake Manifolds [message #274910 is a reply to message #274905]

Fri, 03 April 2015 15:43

James Hupy

Messages: 6806
Registered: May 2010

Karma: -62

Senior Member

Take the air cleaner off. Remove fuel line, vacuum lines, throttle and
cruise linkage and the carb. Shine a light down into the intake and look
forward cracks in the floor of the intake. If you see anything that even
appears to be a crack, pull the manifold and inspect the bottom of it. Some
here have gone so far as to melt aluminum and pour the crossover full of
it, thus sealing the cracks and blocking the crossover. Don't know if I
would go to that much trouble for a cracked manifold, but the Rockwell
aluminum manifold has to be very carefully installed to prevent coolant,
vacuum, and oil leaks. Trouble stems from the bottom of the intake ports
on the Rockwell not having any forgiveness for misalignment. I have pulled
two of them installed by other people and reinstalled them without issues,
but due diligence must be applied. Just how they are.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Apr 3, 2015 1:30 PM, "Robert Peesel" wrote:

> what are the tell-tale signs of a crack in the manifold?
>
> Is there a way to test for a crack? We are considering the Atomic FI
> system and if I need a new manifold I'd like to find out BEFORE I install
> the
> system.
>
> Bob
> --
> Robert Peesel
>
> 1976 Royale 26'
>
> Side Dry Bath
>
> Sterling, Va
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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Re: [GMCnet] Intake Manifolds [message #274926 is a reply to message #274898]

Fri, 03 April 2015 17:02

USAussie

Messages: 15912
Registered: July 2007
Location: Sydney, Australia

Karma: 6

Senior Member

Robert,

The reason they get replaced is because they crack. There are several areas where this occurs:

1) the bottom of the large exhaust crossover that interfaces with the lifter valley
2) the bottom of the plenum chamber under the carb
3) the vertical section between the two secondary barrels

I have discussed this with Dick Patterson and agree with his assessment; if you block the crossover ports it becomes a non issue.

Here's why:

The temperature of the exhaust gas in the crossover between the lifter valley and manifold carb plenum is 1250°F+. That is what
bakes the oil into carbon on the bottom of the turkey tray and manifold exhaust crossover. The temperature of the air in the plenum
directly under the carb will run around ambient minus a bit because of the gas changing phase from liquid to gas. The differential
temperature between the surface of the plenum that "faces" the carb and the surface that "faces" the exhaust crossover will be over
1000°F.

If the exhaust crossover is blocked the temperature in the crossover drops from 1250°F+ to the temp of the oil vapors in the lifter
valley. The temperature differential of over 1000°F as noted above drops to less than 200°F. Dick sent me a photo of an infrared
thermometer aimed down through the choke exhaust heat port on an engine that was running. It was reading the temperature of the
bottom of the exhaust crossover plenum. The engine had block off plates installed; it read 179°F. A crack in the bottom of the
exhaust crossover that opened a mile wide at over 1250°F+ might not even open at 179°F. A crack in the bottom of the plenum where
the differential dropped from over 1000°F down to less than 100°F (179°F- 85°F = 94°F) might not even open either. Of course there
would be a vacuum leak; however, if it would be large enough to create a mixture problem is another question.

In earlier emails to the GMCnet I had noted that I had two manifolds in Houston I was going to repair by filling the crossover with
Hard Blok and a product that Moroso sells. After what I learned from Dick I decided not to bother. A much easier way to repair
cracks in the bottom of the crossover and carb plenum would be to make up a couple of plates to fit over the crossover ports in the
manifold and evacuate the crossover. While it was evacuated smear some red hi-temp RTV (O2 sensor compatible) over the crack and see
if it gets sucked in. No wait a minute, I forgot I was the Pedantic Mechanic for a moment. It would be better if the manifold were
heated in an oven to 250°F to simulate operating conditions.

I have a third cracked manifold here in Sydney and I'll put a GMC round toit chip on the manifold! :-)

Regards,
Rob M.
The Pedantic Mechanic
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426

-----Original Message-----
From: Robert Peesel

I have a stupid question.

For what reason do folks replace the intake manifolds?

Do they provide better performance?

Are they required for FI systems?

Robert

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Regards, Rob M. (USAussie) The Pedantic Mechanic Sydney, Australia '75 Avion - AUS - The Blue Streak TZE365V100428 '75 Avion - USA - Double Trouble TZE365V100426

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Re: [GMCnet] Intake Manifolds [message #274929 is a reply to message #274903]

Fri, 03 April 2015 17:29

Loffen

Messages: 1087
Registered: August 2013
Location: Norway

Karma: 1

Senior Member

Good question Gary, has it been updated ? Have to ask since I am thinking of buying one soon

Gary Worobec wrote on Fri, 03 April 2015 22:12

However, the
Rockwell manifold is a bitch to get fitted correctly without air leaks
unless it have been modified in the last 4 years or so.

Thanks,

Gary and Joanne Worobec
1973 GMC Glacier
Anza, CA

1973 23' # 1848 Sky Blue Glacier called Baby Blue and a 1973 26'-3 # 1460 Parrot green Seqouia Known as the Big Green, And sold my 1973 26'-2 # 581 White Canyon lands under the name Dobbelt trøbbel in Norway

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