I have a 79 bronco with a stock 400m wih a 425 impco propane setup.I remember reading somewhere that it is a good idea to block the "heat riser " to keep the intake manifold cooler, but I could not find any more info ie pictures or a more detailed description. I have the manifold off now to redo the head gaskets and would like to find out what is involved in doing this modification if it will gain performance or mileage..anyone know anything about it ??????

 

With your 400 (no M) it is easy to do. All you need is a bunch of those copper kitchen scrub pads available from the grocery store. Pack them into the heat riser passage in the manifold as far as you can from both sides with a metal rod. You can also stuff them into the head passages. I have heard there are also some gaskets that close the passage but they might rot out over time.

 

wont the "scrubbies" still let heat/air through, or is the idea just to restrict the flow??

 

When you pack them in there there will be close to zero flow. You could detect flow with some instrumentation but it is as close to zero as you need.

 

thanks, will try it

 

does this work with efi 351w
does it improve anything,

 

Do not block the crossover with a gasoline engine with computer controls etc. This discussion was about a propane fueled vehicle. Check with the 351W engine forum for performance questions.