Hey guys i have a stock carbd 300 in a 2wd f-100. With recent gas prices rising beyond stupidity levels i need to squeeze as much mpg out of it as possible. Ive read numerous aritcles about the swap, but most folks have other mods to go along with the swap. Will the engine still perform properly with just the manifold swap or will other mods be required? And also what is the average cost to have the pipes done at an exhaust shop? I can do the actually swap of the manifolds but dont have means to a pipe bender. Also is the swap worth it? im getting around 15mpg in the mountains(3 spd 3.55 rear end), which is a mix between city and highway.

 

The EFI exhaust manifolds do not provide for heat to the intake manifold like the stock log manifold. You will probably be going backwards with just the exhaust manifold swap.

 

What do u mean provide heat for the intake? What is the purpose in heating the air?

 

The heat from the exhaust is transferred to the intake manifold to assist atomization of the incoming charge. Without this, the fuel tends to condense against the intake manifold walls and causes bogging.

 

If you're looking for the best MPG, you're headed in the right direction.

EFI manifolds with a good, high flowing 2.5" exhaust, and then look into providing heat to the bottom of the intake. Attaching a plate with coolant flowing through it is the preferred method.

When I had this setup (1bbl carb with the EFI manifolds), I got my best mileage at around 20 - 23. However, I also have a 3.00 rear with a 4spd with Overdrive and had 29" tires at the time.

 

Who makes this carb spacer/ what do i need to look for? And also how much does the exhaust job usually cost? Thank you guys for help and info

 

The carb heat for the 1bbl is a custom job.
It's basically a plate cut, matched, and bolted to the bottom of the intake with two threaded holes in it for coolant pipes.





And bolted to the bottom of a 4bbl intake:



(They're covered in sealant, which is why they're all mucky looking.)



Exhaust jobs are all over the place, depending on where you go. One place offered to do my full exhaust for $450, another offered to do it for $900. I eventually did it myself by ordering pre-made pieces for about $350.

 

Thank u very much. Pics r great. I take it there are channels/tubes in the intake that the air/fuel mixture flow past to heat it. (haven't had one apart to look at)

 

i'm getting ready to do the EFI header swap, and the idea of the heated intake is interesting.

i'd be interested to hear first-hand from people who have before-and-after experience with the coolant heat. with the small amount of time that the intake charge spends in the intake, i'm wondering how effective warming just that small area up to 180 degrees actually is. i'd have thought that heat conduction from the block itself would be just as effective after a few minutes of running.

the stock setup warmed the intake with exhaust heat, which would have been way hotter than 180.

 

The bottom of the intake cooling down is why they added exhaust heat to the area in the first place. If just the block heat soaking out through the intake manifold would have been enough, engineers never would have messed with it.
The velocity of the fuel/air mixture through the intake is very cold (much colder than the ambient air), so it has quite a bit of cooling capacity.

As for the stock setup being warmed with exhaust heat and being way hotter, yes and no. The stock setup has a bi-metal spring that opens and closes a flap that allows exhaust heat in. This lets it warm up the intake much FASTER when the vehicle is first started (something that you just can't get with the coolant heated setup), but after it's warmed up, the flap closes and blocks off the exhaust. If the intake cools back down, it opens back up and lets a little more exhaust in to heat it back up.

So it never really heats up to full exhaust temperature. If it did, (which can happen if the valve rusts open, or the spring breaks), the fuel boils over and you have another problem.

In the long run, the only disadvantage of coolant heat is you don't get the fast warm up as soon as you start the vehicle. Other than that, it's much more stable than exhaust heat, since it has the capacity to cool the intake back down if necessary.

 

Well written, AbandonedBronco.

I have read that the HD version of the log exhaust manifold did not come with the flapper and bimetalic spring. I wonder why? I realize it was in large trucks which typically ran wide open, but why would they not want the feature? The HD manifold still was connected to the intake manifold, just like the normal log manifold.

 

that is really good info- thank you. looks like i need to start fabbing up a plate for my swap...

 

How did you plug the exhaust hole thats on the intake that gets uncovered and would let coolant go all over the place

 

This is the first I've heard of this spring and flap, and I'm wondering if it explains a problem I have with my 1980 F-150. My truck starts up great, and the first minute of driving is pretty good as well. But after that it begins to bog badly, and it's not until it gets good and warmed up that it starts running correctly again. Is it possible that my flap has stuck closed somehow, and my bogging is a result of the choke having come open but the intake manifold not being warm enough?

If so, is there a way to check or test this, without removing the manifold(s) from the engine?

 

I'm assuming you mean the port where the AIR/SMOG pump goes into the EGR spacer on the intake? There's normally a one way valve the screws into there.
I took the one way valve down and found a plug at a plumbing that matched the thread pattern. Then, got some of that thread sealant paste to put on the threads so coolant wouldn't leak out. It worked really well.