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Block or leave open?
#1431335 Thu Nov 25 2021 03:43 PM
Joined: Sep 2011
Posts: 2,344
L
Lugnutz Offline OP
'Bolter
I’m gonna redo my intake gasket on this 283ci.
The circled ports circulate exhaust gases through the manifold to warm the carb. When I removed the manifold from my other engine you could see that oil was cooked on the underside of the shield beneath the carb. Seems like it gets too hot. Warming the area is good, but is it possible to restrict the heat just a bit? Is there any good reason to block off or restrict these passages?

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Re: Block or leave open?
Lugnutz #1431340 Thu Nov 25 2021 04:23 PM
Joined: Feb 2004
Posts: 23,214
H
'Bolter
That carbon buildup is normal. Check it in about another 50 years, and I'm sure you'll find similar deposits. Blocking a heat riser passage is never a good idea, especially on a street engine. On later model engines with an EGR valve, small stainless steel restrictors were used to control the gas flow to the intake manifold, but the 283 was never equipped with them.

Try drilling a few 1/4" drain holes in the bottom of the heat shield to prevent oil from puddling and carbonizing if you wish.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Block or leave open?
Lugnutz #1431378 Thu Nov 25 2021 11:50 PM
Joined: Nov 2002
Posts: 2,256
C
Carburetion specialist
In a different lifetime (well about 35 years ago) I started on a performance Pontiac 350.

About 20 years ago was still working on the car, and the internet stated block the crossover for maximum power.

Unfortunately, I listened (later found this mod is good for maybe 1.5, that is one and one half percent power at WOT). Since my 350 puts out maybe 430 HP, I gained a maximum of 6 HP.

Having some experience with inline performance engines some 50 years ago, I was aware of the increase from using an aluminum flywheel, so I did.

Approximately 8 years ago (yes, I was working on customer stuff, and slow on my own), the engine came together. ONCE up to operating temperature, engine ran great from idle (about 1000 to WOT). This, in town, was at least 30 minutes. Until the 30 or so minutes was up, the engine (with a stick transmission) died at every stop sign until I relearned the heel and toe technique I had mastered 50 years ago, and forgot. It is embarrassing to have to start a collector car at every stop sign.

The solution, an 850 CFM Carter TQ with a MANUAL CHOKE! Pull the choke about half way at each stop sign for the first 30 minutes.

I realize the vehicle is off-topic, but possibly a real-life experience by someone who has worked on more than a few carbs will reinforce what Jerry said about blocking the crossover.

Do I need the maximum 6 HP??? NO! Terrible trade for driveability. I am so glad the transmission is manual; not sure these mods could be made to work with a dogmatic.

But yes, the car runs like a scalded dog!

Would I do it again? ONLY if competing in a dyno HP test with an engine that would NEVER see the street!

Jon.


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
[image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
Re: Block or leave open?
Lugnutz #1431380 Fri Nov 26 2021 12:10 AM
Joined: Feb 2004
Posts: 23,214
H
'Bolter
It's also extremely difficult to block a heat riser passageway effectively. Since we were required to run "stock-appearing" parts in the circle track racing class my friends chose to run, they tried various methods of keeping the exhaust heat out of the manifold. Blocking plates at the manifold gaskets, even ones made of stainless steel, burned out quickly, so we tried melting a bunch of scrap pistons and pouring the heat riser passages on both sides of the manifold full. At the end of the race season we tore the engine down- - - -and found the passages open again, with no sign of the cast plugs. I guess it would be possible to fabricate a thick, tight-fitting steel plug, drive it into place, and secure it with a bolt through the manifold runner, but why bother?
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Block or leave open?
Lugnutz #1431383 Fri Nov 26 2021 01:31 AM
Joined: Sep 2011
Posts: 2,344
L
Lugnutz Offline OP
'Bolter
To my thinking, a plate with a small hole in it would merely DELAY the heating of the intake, but wouldn’t keep it any cooler after reaching operating temperature. It’s not like there’s a one way flow beneath the intake plenum. Blocking it off would lower the temperature quite a bit, but would make it run poorly, especially when cold. Switching to an aluminum intake wouldn’t help unless it had that area blocked off already. I’m surprised nobody has made a tube of some sort that crosses THROUGH the intake. It would warm the intake more slowly but transfer less heat. Or is that just dumb?

Re: Block or leave open?
Lugnutz #1431388 Fri Nov 26 2021 02:15 AM
Joined: Feb 2004
Posts: 23,214
H
'Bolter
Ford did that on their 3.8 liter V6 in the mid-1980's. It was a dismal failure. Three stainless steel tubes about 1/2" in diameter passed through the fuel-air stream about an inch below the carburetor mounting flange. That worked fine until the tubes rusted out on the bottom and started mixing lots of exhaust gas directly into the intake manifold. It took me half a day of diagnosis to find the problem on the first one I worked on because it was impossible to see the holes from the top, even with the carburetor removed. The engines refused to idle, and performance at highway speed was atrocious. Another one of Ford's "Better Ideas!"

The restrictor inserts on the EGR equipped Chevy engines were not intended to control the temperature- - - -they were there to slow down the flow and prevent too much exhaust gas being introduced into the intake by the EGR valve for part throttle emission control. Thanks, tree huggers!

Most of the time, that coked oil residue in the heat shield is caused by a heat riser valve stuck in the "hot" position so exhaust gas is diverted through the manifold all the time instead of just during warmup. Keep the heat riser freed up and the oil won't coke, or at least it will take several years to do it.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Block or leave open?
Lugnutz #1431400 Fri Nov 26 2021 06:41 AM
Joined: Apr 2005
Posts: 2,885
C
'Bolter
The after market heads we ran on our sprints/supers did not have the heat riser ports and also ran on alcohol. On cool nights the fans in the stand would get impatient with the flagman for giving so many warm up laps but the cars were dogs until warm. Alcohol fouled plugs will clean themselves so sometimes a "tail end Charlie" would come alive 10 laps in and start kicking butt. People would tie the air flapper doors open on Corvairs and make them very cold blooded. In short, engines run much better on fuel vapor than on liquid fuel.


Evan
Re: Block or leave open?
Lugnutz #1431409 Fri Nov 26 2021 03:45 PM
Joined: Feb 2004
Posts: 23,214
H
'Bolter
Dad's flathead Ford powered dirt trackers ran alcohol, but he used Hydrogen Peroxide as an oxidizer instead of Nitromethane. Gettin downwind from one of those engines would clear one's sinuses in an instant!
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Block or leave open?
Hotrod Lincoln #1431424 Fri Nov 26 2021 05:10 PM
Joined: Dec 2018
Posts: 921
F
'Bolter
Hotrod Lincoln I guess those circle engines ate that aluminum in the form of liquid that didn't hurt em !! ??

Re: Block or leave open?
Lugnutz #1431428 Fri Nov 26 2021 05:35 PM
Joined: Feb 2004
Posts: 23,214
H
'Bolter
We were pretty sure it just melted and coated the inside of the exhaust manifolds with aluminum. It never got near the intake passages. I've measured exhaust port temps at full load on the dyno approaching 2K degrees F.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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