Topic: HEAT RISER VALVE
in Forum: C3 Body
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the engine on my 74 had been rebuilt to 350 hp. specs (but i don't know exactly what may have been done internally, other than a cam with unknown specs, edelbrock manifold and holly 750 d/p carb) the rebuilder had taken the butterfly out of the hear riser body and plugged the holes but one came out. so i bought a stock heat riser valve and had it installed, it made it sound better (not to mention no exhaust fumes!) except during hard acceleration, it flutters and sounds terrible, so i have plugged the hole on the old valve body by tapping the hole and threaded a bolt in.
I am going to try putting the old one back on and wondering what the heat riser valve does and if any ill effects running without it.
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I don't want to overstep my boundaries here...
but joel (aka adam's apple) taught me that the heat riser valve is originally supposed to stay closed after start-up to help your motor warm up more quickly and then it opens up. but it generally just makes a lot of noise (probably the fluttering you are hearing).
unless you are going for stock (which it sounds like you aren't with a 350 hp motor) it doesn't make any difference in performance with not having it.
you'd probably be better off without it.
joel, please correct me if i am mistaken on this.
As I remember engine 101, the heat riser allows the engine to warm up the intake manifold so as to allow the mixture to warm and allow the choke to open from that heat. If you eliminate the heat riser you will generally experience cold run issues related to the lack of fuel mixture in the cold run times. If you are in Florida not a problem if you are in the Northeast spring and fall runs will be a problem. If you had the wrong heat riser or an inferior one from China, you may experience that flutter noise you spoke of as the heat spring is weak and won't allow it to open properly.
Lifetime MemberAh yes, the heat riser.
I lives on the passenger side exhaust manifold. When the engine is warm it opens and has no affect on the engine.
But when the engine is cold, it closes, restricting the exhaust flow out of the right side. This forces the exhaust through the passages in the cylinder head and across the intake manifold, and out the other side.
By creating this flow the intake is warmed up by the exhaust flow, and the warm intake in turn heats the intake air and fuel the create better cold drivability and faster warm up.
These same passages allow exhaust flow to the EGR vavle when the EGR opens.
When the engine warms up, the exhaust flow is now equal as the heat riser opens, and with even exhaust pressure on both sides of the intake manifold, it stops heating the intake.
If it does not open or is not there, the car will warm up a bit slower, and may hesitate when cold as the choke is opening. However the choke can be adjusted to correct the running problem by slightly tighten the choke spring, and reducing the pulloff opening just SLIGHTLY. This will make it run okay, but won't decrease the warm up time.
In reality the difference is only a few minutes difference warm up time. However without the heat riser, the emissions when cold will exceed the legal limits. But with cars our age that doesn't really make any difference that most anyone cares about. Including the EPA
If the heat riser sticks shut when the engine warms up, the performance will suffer due to the restricted exhaust when the 4 barrel opens. In addition, the extra heat in the manifold will cause fuel boiling and lead to vapor lock. It will also cause the engine to knock/ping, and may create backfiring.
When you ask Dr. Ken "what time is it?" you will completely understand the inner workings of a clock.
Nice job professor.
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