Myth or Fact? (Reving engine before shutdown)



OK Experts, is this Myth or Fact...

Does reving an Engine prior to shutting it down preserve the engine upon startup?
8) no. it may at best clear out excess fuel in the combustion chambers but even then not likely.
It is the worst thing one can do... On a Carbed engine it pulls extra fuel into the engine and this is not burned because the key is shut off.. The extra fuel then condenses on the cylinder walls, Washing the lubricant off. This leads to "DRY" starts..
Don't ask what bad it does, But also ask what good does it do?? :oops: :roll: :shock: :LOL:
Agreed. Revving your stock engine before shutdown is a waste of gas at best, and harmful to your engine at worst.

One sees this done on carburetted race cars because they are often grossly over-carbbed for low RPM use. Fuel accumulates in the intake manifold while they're idling, so the driver blips the throttle enough to suck this gas out of the manifold before shutdown. Otherwise the plugs may foul on restart. Since these engines are rebuilt quite often anyway, the effect of the fuel on the cylinder walls is not considered a problem.

Bottom line? Don't do it to your street of even hot performance engine unless the engine is grossly over-carbbed.
Yes!.. I laways hea peple saying you should do that...but i always tell them not too..but they never understand. Theres no piont in doing it on an EFI system, and it just dumps extra gas into a carbed the Honda owners manual of my CR 125 dirtbike, it says to properly check the spark plug for how the engine is running, run at WOT going down the road, then cut the throttle and the Ignition at the same time....but i guess cutting the throttle may prevent extra fuel form entering the cylinder....
Yah, but when you cut the throttle and ignition, you should also clutch it so the engine dies. Same thing you'd do on a car/truck to check WOT mix.

This is something that was done on aircraft engines. You revved it, then cut the fuel, to make sure no unburned fuel was left in the cylinders. 8)
Doing this on a carbureted auto engine only insures that fuel is left in the cylinders, to wash away any oil from the cylinder walls and promote wear. :shock:
54Ford":2o73pfxs said:
Bottom line? Don't do it to your street of even hot performance engine unless the engine is grossly over-carbbed.

Well, is having a Holley 650 on top of a Offy manifold considered "Grossly Over-carb'd" for an engine with an RV cam and otherwise stockish?
Hard to say, since I don't know which carb or what engine you have this setup on. Is it a Holley 4150 'double-pumper' or a vacuum secondary carb? If it's on a small-block, then I would say definitely not. If it's a medium-six, then probably not. If it's a small-four, then probably yes.

Lemme give you a 'fer instance'. The car you see in my avatar is powered by a 97 CID (1600cc) engine, equipped with DUAL Weber DCOE 48's. That is to say, four 1.9" bores feeding one cylinder each. THAT's grossly over-carbbed... :roll:

The 514 in my '73 Mach 1 has a single Edelbrock 750 to choke it, thereby keeping the ET and trap speed NHRA friendly. :D

I don't mean to sound like a smart-aleck, but until I know more about your car I honestly can't answer your question. ;)
Yep, fair question, as I ommitted some critical info...

Its a 300cid engine, RV cam, Offy Intake and EFI manifolds. The carb is a 4150 with an "Adjust-a-jet" on it (which I am still unsure how to use properly). I assume the previous owner installed that to compensate for the overcarburetion of the 650.
By 'EFI manifolds', I assume you mean the exhaust manifold, right? No headers? Auto or manual trans? Rear gears? Street or strip?

In any event, this doesn't sound like it's grossly overcarbbed to me. In fact, it sounds like the setup that came on 302 Mustang GT's of the mid-80's, before Ford went to EFI (different engine, I realize, but if anything the Big 6 has a better vacuum signal than the 302). If this setup is in a pickup turning low RPMs, then it might not work all that well right now, but certainly can be made to work. I am no expert on 4150's, and am not familiar with the "Adjust-a-jet", but in my experience, tuning a carb is a lot more than dropping in a bigger or smaller main jet.

I realize this is getting pretty far off your original topic, but if you are having challenges with getting this setup to work right, give us more details and we'll see what we can do to help you. There is a lot of collective and individual experience here, and I am sure we can get you moving in your desired direction.