Economy and heat riser valve on pass. exh. man. questions

[83GMCK2500]

I was reading one of the quadrajet tuning threads and I noticed that the vehicle in the thread had a vacuum operated valve down on the outlet of the pass. exh. manif. I have one with a counterweight and clockspring on my truck. Does one work better than the other?

Driving ~75% city/25% hwy I was at 10 MPG on 91 octane with my last tank of fuel. (On hills the truck pings with anything less.) Those are 25 & 35 zones in town and 55 & 60 zones on the hwy. Details not in my sig are 4.10 gears and 265/76-R16 tires, speedo is very nearly dead-on at 60mph.

Is 10 MPG reasonable? If I posted detailed pics of the topside of my engine can we verify that all of the vacuum lines are hooked up correctly? The truck runs very well right now, is just thirsty. After reading that other tuning thread I would like to verify that everything is in the right place.

[C-10 simplex]

Well, we can try---i have several detailed pictures as well as an 83 service manual. But don't expect any miracles as no-one gives a plop about all that emmisions stuff, and i don't have time or the ability to really get into it even though the stuff fascinates me.


2) i have idea if one is better than the other as far as the heat riser valve goes.

3) Depending on the setup 10mpg may be reasonable, but there is probably big room for improvement.

[ray_mcavoy]

Hi Devin,

Reguarding the heat riser valve, the vacuum operated style is likely better for emissions purposes. That's because it's hooked up to a thermal vacuum switch that opens the valve once the engine reaches a certain temperature. As opposed to the spring type valve that simply opens with increasing exhaust temp.

When closed, that valve forces extra exhaust flow through the crossover passage in the intake manifold under the carburetor for improved cold engine driveability and reduced emissions. If the valve gets stuck closed (can happen with either style) the resulting exhaust restriction will cause a lack of power and reduced fuel economy.

[ppbr2001]

Mine's wired open.

[RandyP]

I took my valve out on my 79 K5, used a torch and blew out the butterfly valve, and reassembled it. At the same time I took the EGR off and removed about a mile of vacuum hoses that it didn't need. Truck actually runs even better without it all.