Heat Riser question
Thread Starter
Instructor
Joined: Nov 2005
Posts: 142
Likes: 0
From: Warwick New York
Heat Riser question
I recently had my 327/300 rebuilt. When I installed it I installed the heat riser and it moved freely. I didn't heat it up to test it to see if the spring would open it by itself. I drove the car on a few short trips which may equal 300-400 miles (Odometer doesn't work). I now noticed that after I dove the car and the engine reaches operating temp the heat riser is still closed but does move freely. I wired it open and went for a ride. I have two questions:
1. Could I have damaged anything driving with it not opening?
2. When driving with it wired up the car seemed to have more power, I'm I crazy or could the restricted exhaust cause a lose in power?
Thanks for any comments.
6coupe4
1. Could I have damaged anything driving with it not opening?
2. When driving with it wired up the car seemed to have more power, I'm I crazy or could the restricted exhaust cause a lose in power?
Thanks for any comments.
6coupe4
Race Director
Joined: Jun 2006
Posts: 17,901
Likes: 751
From: Inverness FL
St. Jude Donor '07
the heat riser is used to pass more exhaust gases through the passage in the intake manifold under the carb in order to heat the carb in cold weather and improve driveability. it might be possible to burn the valves or overheat the engine if the riser stays closed, but i THINK exhaust gas flow might keep it open enough while driving if the spring doesn't. it has a counterweighted butterfly that helps the spring open it. and a certain amount of gas will continue to pass through the intake manifold passage and out the opposite side exhaust manifold.
i suggest you buy and install the heat riser spacer that doesn't have a valve in it and replace your present one. never have to worry about it again...
Bill
i suggest you buy and install the heat riser spacer that doesn't have a valve in it and replace your present one. never have to worry about it again...
Bill
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,271
1. I doubt it.
2. Old heat risers often don't perform as new even if the valve is not frozen. If the valve is closed when the engine is hot, it should open if you blip the throttle hard.
If not it could definitely be costing power since you essentially have a single exhaust and all the RH bank exhaust has to travel through the relatively small heat riser passage in the inlet manifold. This will also lead to carb percolation.
If it doesn't open when hot except at high load it will also lead to premature rust out of the RH exhaust system due to low flow and water vapor condensation.
Unless you drive the car in subfreezing weather, it's best to leave it wired open. Some guys just install the FI spacer and some actually remove the valve, but leave the spring and shaft installed, so it doesn't look modified.
Duke
2. Old heat risers often don't perform as new even if the valve is not frozen. If the valve is closed when the engine is hot, it should open if you blip the throttle hard.
If not it could definitely be costing power since you essentially have a single exhaust and all the RH bank exhaust has to travel through the relatively small heat riser passage in the inlet manifold. This will also lead to carb percolation.
If it doesn't open when hot except at high load it will also lead to premature rust out of the RH exhaust system due to low flow and water vapor condensation.
Unless you drive the car in subfreezing weather, it's best to leave it wired open. Some guys just install the FI spacer and some actually remove the valve, but leave the spring and shaft installed, so it doesn't look modified.
Duke
Melting Slicks
Joined: Jan 2003
Posts: 2,817
Likes: 21
From: Somerset, NJ
Mine was wired open when I got the car. I ended up taking it off a few months ago when I dropped one of the side exhaust. I found it was frozen opened anyway so I tossed the wire away. I then decided to just take my torch to it and cut away all that was not needed. I then took a brazing rod and filled in both side holes, ground it down, painted it and reused it like a replacement heat riser spacer.
Melting Slicks
Joined: Oct 2004
Posts: 3,345
Likes: 87
From: Northern MN
St. Jude Donor '13-'14-'15-'16-'17
Originally Posted by vettes1st
Make sure you have it installed so that the weight moves down. Sounds basic, but I've seen them that way.
This may sound nuts but I did exactly what vettes1st is talking about. I installed a new heat riser in my car when I built it. I installed it upside down. Can't open if the weight is going the wrong way. Reverse the heat riser and see if that works first. Mine has operated fine ever since I learned to put it on correctly. Not saying this is your problem but its a good thing to check.
Steve
Team Owner
Joined: Oct 2000
Posts: 38,897
Likes: 1,926
From: Washington Michigan
The "top" side of the heat riser is flat, and mates to the "flat" outlet on the exhaust manifold with a steel-reinforced flat gasket. The "bottom" of the heat riser is chamfered at an angle, to match the beveled surface of the "donut" on the downpipe. 
Racer
Joined: May 2006
Posts: 268
Likes: 0
From: Farmington Hills Michigan
Originally Posted by JohnZ
The "top" side of the heat riser is flat, and mates to the "flat" outlet on the exhaust manifold with a steel-reinforced flat gasket. The "bottom" of the heat riser is chamfered at an angle, to match the beveled surface of the "donut" on the downpipe.
Wouldn't it be impossible to install upside down then - without a heck of a leak.
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