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I recall instructions supplied with "universal sbc" intake gasket sets packaged to fit all model years with carb.
Some gasket sets installed with a partial block plate that has a hole in it, some years take fully blocked one bank and partial other bank, some partial both banks etc etc etc many combinations for many years sbc; installer would pick & choose from a common set to fit his model.. or if he didn't RTFM and didn't choose correctly, the plenum might very well sustain too much heat. Of those sbc with proper intake & carb base gaskets & heat riser; Most never vapor locked and most never iced. Today's GM engineers are pretty dang smart and were then as well.
Do y'all recall just how dang hot an bay can get with plugged up cats or crushed tailpipes?
FWIW, I recall Fords with weber downdraft carb had a small cast heat exchanger between carb and intake manifold ...but it was supplied by engine coolant, not exhaust.
I love a carb, a distributor and open headers but I'm dang glad my daily driver has neither; thank God & engineers for computerized engine management.
As for hypereutectic pistons; look how reliable & powerful LS3 is. Yes there're some crappy pistons out there but not because of material. I'm not a fan of most UEM pieces.
Blocking the crossover is just insurance and a way to avoid vapor lock and boiling the fuel in the bowls.
Blocking the crossover will not change the operation of the engine and will not add performance.
Most people will block the crossover to avoid being stranded on the side of the road.
I've owned my share of GM cars from the 70's and vapor lock has always been a concern.
I'll be blocking the crossover on my 73 as soon as I replace the intake.
It sounds like there is a middle ground here that some people do in partial blocking. So just having a small hole for some exhaust crossover might still allow for a quick start up in cool weather but not allow so much crossover that could cause the vapor lock?
Seems Xover designed to work in concert with properly functioning heat riser valve. If riser sticks closed, seems there'll be constant over pressure pushing more (and hotter) exhaust back across Xover toward non-riser side and out tailpipe. Seems too much hi-heat, hi-pressure exhaust through or toward Xover gasket or partial will soon burn through. Seems then there's no Xover restriction and with riser stuck closed there'll be too much heat pushed through Xover and then overheating plenum. Seems many carb overheating problems could be traced (?root cause) to sticking heat riser valve.
If there's a heat riser valve, seems it'd be wise to ensure its intended range of motion is both complete and free. I've certainly had them stick shut.
Going to roll with the felpro 1256 gasket as it has the crossover not wide open but not blocked rather just restricted. It does appear that the 2101 has slightly larger ports than the stock heads...hence recommendations of 1205 and edelbrock 7201 gaskets (2.09” x 1.28”) for the intake whereas the 1204 and the 1256 gaskets (1.23" x 1.99") are recommended for the stock head side. From everything I have read here is sounds like the difference isn’t a problem rather more about the material. Anyone had trouble with the slightly smaller gaskets (1204/1256) on the Performer intake?
You can't add material if a gasket is too large obviously, but you can always trim open a too small gasket port with an Exacto knife. It s hard to port match gaskets to intakes, much easier to match gaskets to heads.
I hope you won't be sorry in running a partial opened crossover gasket next summer. If you have spent any time on this forum, you have seen the many complaints about C3 fuel bowl percolation / vapor-lock. And no complaints about idling an extra two minutes for the car to warm up.
You can't add material if a gasket is too large obviously, but you can always trim open a too small gasket port with an Exacto knife. It s hard to port match gaskets to intakes, much easier to match gaskets to heads.
I hope you won't be sorry in running a partial opened crossover gasket next summer. If you have spent any time on this forum, you have seen the many complaints about C3 fuel bowl percolation / vapor-lock. And no complaints about idling an extra two minutes for the car to warm up.
Good Luck.
hoping with the heat riser gone, high flow side pipes, and the restrictor (instead of wide open) it is enough. I run the car on some pretty cold mornings in the fall and spring. I am going to put a nice big carb gasket on too. We shall see.
Once the heat riser is disabled, you can block the non-choke side of the passage with a thin stainless steel shim when using an aluminum intake. The choke stove still works. The aluminum manifolds heat up fast enough so it is hard to tell. On a cast iron intake it might be better to keep it partially open or you could be washing the cylinder walls with wet fuel for at least a half hour. Might want to keep the heat riser valve working there. If you get fuel percolation on an iron intake engine, restrict the passage.
The heat riser valve is what sent the hot gas through the passage. Once the car warmed up the heat riser valve the valve opened to the exhaust system and the passage became for the most part inactive. Then the vast majority of hot gas goes out the tailpipe.
I finished my intake swap completely so I thought I would follow-up.
First here is what all I did:
Swapped stock cast iron intake with 2101
Used Felpro 1256 gasket
Swapped stock thermostat housing with custom black metal one
Added "Hose Candy" kevlar to radiator house
Replaced water pump
Had carb rebuilt by Lars (learned I actually have a 800 cfm carb)
Lessons learned
Use Lars for carb rebuild, the guy knows his stuff, is fast and does spectacular work (car runs much smoother now)
Follow HeadsU.P. advice in this thread regarding gasket/sealant work
Consider whether you want to keep your heat choke before making a decision on intake gaskets (if you block crossover you can't use the heat choke)
1256 gasket has been great (no overheating, sealed strongly, perfect fit on stock heads)
Take your time on scraping the old gasket off the heads.
It is a long, slow process and there was a lot of crap in the coolant passages.
Flush coolant completely after swap to make sure there aren't pieces of old gasket floating around
Diligently mark alignments on the distributor and take your time on getting on the right tooth on reinstall (the long screw driver down the hole to move the oil pump teeth slightly works great)
Here are some pictures of before work, during work and after
12-06-2018, 07:06 PM
varied intake gasket Xover configurations
