The Stovebolt Site Forums The Shop Area Engine Shop Dumb engine question

When an engine is going down hill and is being asked to work against the hill, why does it get so noisy in the exhaust?

There is no fuel or air going into the engine for combustion noise, so the pistons are simply moving up and down. Why the exhaust cackle? I've wondered this for a long time on my bikes cars trucks etc.

Good question. An extreme example of that would be the Jake Brake (exhaust brake) on a diesel. We have all heard that.

George

Why do you say there is no fuel or air going in? The valvetrain still moves and fuel is still drawn.

Most (not all) fuel injected vehicles will reduce the fuel in that situation (DFCO), but not completely stop fuelling.

If the exhaust gets noisy during compression braking coasting, you need a better design muffler. Most of the low restriction (glass or steel pack) mufflers that the hotrodders like to use on stovebolt engines produce a series of rapid backfires that are extremely annoying, at least to some of us. Coasting with a closed throttle produces an extremely lean condition that promotes backfiring. That's especially common with a "split" exhaust manifold or headers that produce a 120 degree separation between exhaust impulses alternating between dual pipes.

All well and good, but we haven't answered Ferris' question yet ...

Why all the noise when compression braking? And is it the same reason for why a Jacobs Brake also makes a lot of noise when compression braking, or is it something different (i.e., Otto Cycle versus Diesel Cycle, for you Thermodynamics fans out there )?

Free One-Year Subscription to Stovebolt.com goes to the first one to correctly identifies the inventor of the diesel engine brake (the "Jake")

Don't know why it took me so many years to realize it's a sign of too lean mixture screw adjustment, but richening it up always makes the backfiring or popping (whatever it actually is) go away. Supposedly stock exhaust system bought from Jim Carter's many years ago.

From the perspective of 50-something years of tinkering with Diesels, a jake brake is a totally different animal than compression braking with a gas engine. A Jacobs brake turns the engine into an air compressor- - - -a compression stroke, without a power impulse following. It does so by opening the exhaust valve just as the fuel injector would normally spray, and letting the compression pressure escape from the exhaust system. On a 4-stroke engine such as a Cummins, that happens as the center rocker arm that operates the injector plunger just begins to move- - - -it triggers an oil pressure solenoid valve that forces the exhaust valve(s) open. On a 2 stroke Detroit, there are four exhaust valves and NO intakes, so the compression gets released RIGHT NOW! That's why a Detroit with a Jake really rattles windows! Very little fuel gets wasted, because the throttle valve or the fuel rail in a Detroit is at the "idle" position when the Jake is working.

There's no way to relieve the compression on a gas engine, so during a closed throttle slowdown a little fuel/air mix is drawn into the exhaust while coasting. With the right exhaust temperature, some of that fuel gets burned in the exhaust pipe and muffler instead of all of it getting consumed in the cylinders.
Jerry

Jerry -- I got to drive Billy Marlow's '59 860 GMC for my birthday last year (took off by myself with it and was gone for about 30 minutes ) Hitting the Jake on that 6-71 was darn near a religious moment. Best birthday present EVUR!

Gas engine with F.I. or carburetor will still be allowing fuel into the combustion chambers, it will be only enough fuel for idle but as HRL has noted, that is a very lean mixture and produces backfiring on pretty much every exhaust stroke. If you were able to kill the ignition OR completely cut the fuel on coast-down, you won't have the noise. Next time you are coasting down, keep your foot off the throttle, cut the ignition and hear the difference, just don't turn the ignition back on while the rig is still coasting down, you'll be buying new mufflers.

Did that on my Dad's '63 Chevy in 1964 just to hear the bang. And yes, I had to buy a new muffler and put it on before he found out.

But it was lots of fun when I was a kid, at least until I blew the muffler apart on the International L160 truck Dad had.

I've shut ignition off on my goldwing and there is still considerable noise. I will do it with the truck next time I'm on a steep hill.

With the automatic transmission, it might not have the same effect if the trans disconnects.

So going down a steep hill which you still slowly accelerate in third gear with no fuel and ignition off there is quite a bit of engine noise. No fuel going in at all due to sniper.

What's the advertised compression ratio of your 383, and did you find the cam spec's?

Compression Ratio: 10:1

Camshaft: Hydraulic Roller Cam - .544 Intake / .555 Exhaust Lift and 230 Intake / 236 Exhaust Duration @ .050 - 110° LSA

Pistons: Hypereutectic

Crankshaft: Steel Crank - 3.750" Stroke and 5.700" Rods

That's an extremely radical cam for a street engine. Other than installing a set of very quiet mufflers such as the ones used on mid-1970s Cadillacs, there's not going to be much you can do to quiet it down. There's just too much unburned fuel getting into the exhaust system and being burned there during coasting to do much other than install super-quiet mufflers to stop the noise. One muffler for a big Cadillac engine, a 472 or a 500 with 2 1/2 inch inlet and outlet nipples, will produce very little back pressure, and duals with the same muffler will be quiet, and VERY low restriction- - - -the only thing they won't do is make noise.
Jerry

I was just curious why a gas engine did this. I do believe I went a little too much with this build from other thread info on bucking at low speeds. Just need to drive it the way it needs to be driven, and the next time go a little tamer.