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 Intake / Carb and Fuel Delivery Tech
 Heat Crossover?
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52 Indian Delivery
Tribal Scout

USA
38 Posts

Posted - 17 Mar 2016 :  10:58:50 AM  Show Profile  Reply with Quote
I am a little confused about whether to block off the heat crossover or not. I have read different opinions on this so advice would be appreciated.
I have a 1968 Pontiac 400 with a "068" Cam and hopefully the long branch cast iron headers (providing they will fit). I will be running a '68-'69 cast iron intake #9794234 with a '76 Olds Quadrajet #10756252.
This will be a street engine in my 1952 sedan delivery.

Steve 1952 Sedan Delivery with 1968 Grand Prix 400/400

Phil
The Great White Buffalo

USA
7216 Posts

Posted - 17 Mar 2016 :  11:41:06 AM  Show Profile  Reply with Quote
Are you in a cold climate? What method are you planning to use? Filling the crossover passages in the heads or using the block off plates that come with some gaskets?

I do NOT recommend the block off plates in the gasket kits as they are junk steel and blow through after a little while. Stainless would be a better choice in that area but the best way is to fill the exhaust crossover passages in the heads.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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52 Indian Delivery
Tribal Scout

USA
38 Posts

Posted - 17 Mar 2016 :  12:47:47 PM  Show Profile  Reply with Quote
Thanks Phil,
The first thing is if I should the second would be to decide how to do it. I would follow your advice and make my own stainless steel plates if I decide to.
It usually is cooler up here so maybe I should not block off or only partly.

Steve 1952 Sedan Delivery with 1968 Grand Prix 400/400
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Tub
Sitting Bull

172 Posts

Posted - 17 Mar 2016 :  7:41:51 PM  Show Profile  Reply with Quote
"...It usually is cooler up here so maybe I should not block off or only partly."


For cold weather driving, I'd definitely leave 'em open. It helps get the carb up to operating temp much quicker.
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52 Indian Delivery
Tribal Scout

USA
38 Posts

Posted - 18 Mar 2016 :  11:20:00 AM  Show Profile  Reply with Quote
Thanks for the advice.
The car will normally be started at 6,500' elevation but the driven down the mountain and in the desert areas.


Steve 1952 Sedan Delivery with 1968 Grand Prix 400/400
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Blued and Painted
Chief PONTIAC

USA
3406 Posts

Posted - 18 Mar 2016 :  12:30:13 PM  Show Profile  Reply with Quote
Blocked heat crossovers will raise intake heat soak time from 5 minutes up to 15 or 20 minutes while driving.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
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52 Indian Delivery
Tribal Scout

USA
38 Posts

Posted - 18 Mar 2016 :  8:56:57 PM  Show Profile  Reply with Quote
Thanks Blued that helps me understand better. I just got the Eldelbrock Performer off this afternoon and noticed that the previous block off plates did burn through. I understand why others state they made their our stainless steel plates because I don't have very many miles on the car after trying that mod.

Steve 1952 Sedan Delivery with 1968 Grand Prix 400/400
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Phil
The Great White Buffalo

USA
7216 Posts

Posted - 19 Mar 2016 :  10:24:14 AM  Show Profile  Reply with Quote
Yeah that's the typical problem with those. Fwiw, I've been running without exhaust crossovers for decades and have no issues. However, I run an electric choke (a manual choke would be fine too) and I don't drive my cars in the winter (once salt is on the road which is typically Nov-April here in PA.). There have been a few times however when I needed to start the cars (to move around the garage etc) in colder weather and it can take an extra couple of minutes to stabilize the idle. But most of that is attributed to the cars setting for the winter.

My new engine has KRE heads which have no exhaust crossover at all. I run an HO intake and like the wide open area under the intake. I have a Qjet w/electric choke on that too and it starts right up when it's cold, idles and runs fine. I don't beat on my cars anyway when they aren't fully warmed, none of them-new or old.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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DanM
Buffalo

80 Posts

Posted - 03 Jan 2017 :  10:01:07 AM  Show Profile  Reply with Quote
That whole exhaust crossover design is just bizarre. I really wish I could talk to the engineers who originally designed it to hear what the true considerations were.

It improves drivability in cold weather for the first 15 minutes at the expense of possibly increasing the risk of engine knock in hot weather. Over the course of the car's lifetime I am guessing that crossover serves to reduce emissions. If you assume that the emissions of a few minutes of a cold car are as much as a hours of a warm car than it's worth it. (Don't quote me. I just made up this statistic for the sake of argument.)

It all stems from a need to adjust the carb choke so that it tracks the engine temperature and to help fuel atomization during cold weather. These issues are solved with fuel injection.

I am still waffling on the subject but will most likely leave the crossover intact when I swap my heads in a few weeks. I guess I'm too chicken to try to make things work up here in New Hampshire with the crossover blocked.

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
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cortcomp
Coyote

USA
5335 Posts

Posted - 03 Jan 2017 :  11:44:10 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
I am daily driving a 56 chevy with a 327 and an air gap manifold that totally eliminated the crossover. Edelbrock carb with electric choke, automatic car. Hit the key while pumping once or twice first thing in the am, put in gear, and go. If you're not using it to control the choke via heat riser tube or spring and you have the choke manual or electric, you won't notice it even when cold out.
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DanM
Buffalo

80 Posts

Posted - 05 Jan 2017 :  4:56:41 PM  Show Profile  Reply with Quote
Hi cortcomp.

I'm using the heat stove with bimetal coil/spring. I fashioned a custom rod using a coat hanger cut and bent just so. I have a service loop bent into it so I could adjust the rate of choke opening. I'm so proud myself and the precision of my pliers-bends that I hate to swap to an electric choke.

So though I still feel the whole heat crossover/choke routine is a nutty Rube Goldberg design I went with it for the sake of originality. Plus some of my car's soul comes from that elaborate yet effective choke mechanism.

My car has soul!

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
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Blued and Painted
Chief PONTIAC

USA
3406 Posts

Posted - 06 Jan 2017 :  11:17:34 AM  Show Profile  Reply with Quote
Car guys must have a little engineer built in from birth. Nice work.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.

Edited by - Blued and Painted on 06 Jan 2017 11:22:30 AM
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