Heat riser passages...

[COBALT]

I'm starting to question the whole heat riser passage thing in the intake manifold. The replacement manifold has two open passages. I got a feeling that the one still on the engine has two open passages as well. I have no idea how stock 350 heads are set up. I think I read somewhere that 350's 79 and up are blocked on one side. Mine's a 1980 block.

The gaskets I got are open on both sides, but I started thinking about leaks and how much i HATE them, so I got a set of pro seal intake gaskets from Mr. Gasket. Well, the heat riser passages on both sides are blocked. What the hell? What's the significance of the heat riser passage and if I block them or leave them open what would the effects be?

Remember this is a 350 with stock heads sporting a 327 manifold and a 2bbl carb.

[tom hand]

These trucks origally had a valve in the exhaust pipe that forced the exhaust thru the intake for faster warmup. Ussally these valves are MIA if the exhaust has been replaced. Later model GM stuff had a restricter in the intake gasket but it was still not blocked off. I would not block one off on a street driven truck, it can cause to many problems in the winter, and you will probably lose a mile or two per gallon. Some people will tell you it will give you more horsepower, but unless you are super slick at carb tuning you will never see it.

[mikep]

You live in Seattle. You dont need a choke. You dont need an exhaust crossover. There is no winter here.

[COBALT]

Well, I disagree on the choke. Cold oil and air is just that: cold. I know because I did without a choke during last winter. Now it wasn't cold, but it was a pain starting the truck.

Screw it. I'll leave them open and not worry about it.

[Sinister]

get a choke, electric preferably...
so much easier to start even in warm weather,
like we get down here in Los Angeles,
(where everbody says "dude" alot)...

i just rebuilt my top end and blocked them heat risers off...
they were all crusty from carbon and crap...
wanted to keep the intake charge nice and cool...
the choke will make your cold starts much more enjoyable...

[mikep]

I havent used a choke in 12 years. Never have had a problem starting. I figure all they do is wash down the cylinder walls with gas when the engine is cold and can least stand that sort of abuse. Takes 10'000 miles off the life of an engine . Maybe more.

[Longhorn Man]

I have always blocked those holes. I think my Fell-Pro instructions told me to...but it's beena while since I read the instructions in the gasket kits.

[ddsmith]

If you have the valve in one side of your exhaust and your going to block the crossover you may as well get rid of the valve also. I see pro's and con's to blocking the crossover. Pro's include cooler intake charge because the manifold is not heated(although when the valve opens on the exhaust pipe there is more resistance to flow through the intake so it would head out the exhaust instead), another is that the oil won't cake on the bottom of the intake cooked from the heat you've probably all seen this when you take off the old cast iron manifold. Con's include poor cold performance. This passage along with the heat riser valve force a large amount of exhaust from one side of your engine through the intake crossover passage quickly vaporizing gas that would otherwise redeposit on the manifold and flow toward your cylinders as liquid. When the exhaust heats up the riser valve opens fully allowing unrestricted flow out the exhaust for that side. As for the choke issue I think that the choke is an effort to help bring air/fuel ratio up to where it should be for proper cylinder firing. I'm pretty sure that if you don't use the choke then you have fuel injection, your running in warm weather all the time or your running to rich for proper operation at temperature. The cylinder wash with gas thing could easily be taken care of by using the original crossover as designed and this would allow the minimum time the choke is used.

[mikep]

I am running in warm weather all the time. I'm in Seattle. warm is a relative thing. Maybe we get 2-5 days a year that it dips below freezing but the rest of the winter it hovers in the low to mid 40's. Thats not cold to this former midwesterner and I have never noticed any problems starting of warming up to temp without a choke. I will admit that I go to extraordinary efforts to make sure my vehicles are in tip top condition especially concerning carburetors.

[COBALT]

Well, I have no valve off the exhaust.

No, mikep, 40 isn't cold and you're right it doesn't really get cold here. However, but this is a motor with at least 50K miles left in it, and last winter it was a ***** to keep the thing running with that little 2bbl and 327 manifold on a cold start. I rebuilt the carb in May, and now I could go without the choke for most of the year probably...

What I'll do is simply see what setup is in place when I pull the manifold. My gut instinct is to leave them open, but I do have a couple of other gasket sets with them closed.