Don't know what it's got for engine management, but there is an idle speed hunt going on with neutral stability. Probably due to an air/fuel ratio issue. If it has active idle speed control, that subsystem is getting into a dance with the air fuel ratio. All of this is probably exacerbated by a long duration cam that is poorly optimized for idle rpm.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

Too much overlap in the cam timing combined with a common intake manifold shared by all cylinders (which is standard automotive practice). Obviously the cam timing is selected for wide-open-throttle conditions with scant consideration for part-throttle. The supercharger, in this case, doesn't play into it. A naturally-aspirated engine with big lumpy high-overlap cams, and a common intake manifold, will do the same thing.

The common intake manifold shared by all cylinders means that the pressure being below atmospheric in there (almost-shut throttle) will suck exhaust backwards through the cylinder during overlap, thus contaminating the charge for the next cycle, giving a high probability of a misfire. But after that misfire, the next cycle won't be contaminated as much, so it has a better chance of firing. When the engine revs go up, so does intake manifold vacuum, which makes reversion worse, then it tends to have a whole bunch of misfires, so the engine slows down, and then over the next few revs fresh charge gets purged through and also the revs drop, as does intake manifold vacuum, then a few firing events happen in a row, which makes the engine speed up, and now you have that unsteady rough idle.

Engines with milder cam timing with less overlap don't have nearly as much tendency for exhaust reversion at part throttle.

Individual throttle body per cylinder helps, also. The manifold vacuum at the end of an intake stroke mostly equalizes out to near atmospheric pressure by the time the next intake stroke starts, and also, even if the exhaust pressure tries to push flow backwards during the overlap period, it is against an almost-shut throttle very close to the cylinder.

In forms of racing other than drag racing, part-throttle operation matters, and you can't have it running rough like that.

Brian; Wasn't Hilborn fuel injection with individual throttle plates?

Keith Cress
kcress - http://www.flaminsystems.com

I don't know much about those, but photos clearly indicate that those are an individual-runner design. Fuel control is evidently rather primitive. Hilborn's own website suggests that they're not designed for street applications!

http://hilborninjection.com/faq/can-i-run-a-hilbor...

Simple answer, even with no super charger it will still do that sound. Its all about valve timing events.

A very general concept worth mentioning is that a control system with too much delay in the feedback loop can easily turn into an oscillator.

The general concept can arguably be stretched to simple carburetors, even where any idle speed control feedback loop is merely implicit.

Thinking along these lines suggests that perhaps the added volume of the intake system has created increased latency in the (implicit) idle speed control.

Acknowledge in advance, it's merely a very general concept. But I've found it useful to a wide variety of applications over the years.

Here's what's happening: as the RPM falls the intake manifold vaccumm drops(closer to 0 in-hg). When manifold vaccumm drops, the power valve opens up. When the power valve is open it adds fuel and richens the air fuel ratio. Rich AFR raises the RPM. As RPM rises, intake manifold vaccumm rises. As intake manifold vaccumm rises, the power valve closes and fuel is cut and air fuel mixture leans and RPM drops...repeat repeat repeat (As RPM drops vaccumm drops, as vaccumm drops the power valve opens)...


It's dieing on the idle circuit and then getting brought back to life by the (transition) power valve circuit every second so you hear that oscillating RPM phenomenon.


*Carb circuits*
-idle jet
-transition (booster or idle/main blend)
-main jet


The real culprit is the inadequate idle circuit. If the power valve was closed permanently, then you'd see exactly how the idle circuit is (not) working alone.

Don't know if your diagnosis is correct or not but it is interesting and sounds plausible so I gave your post a star.

je suis charlie

The question was specifically about supercharged engines- and I assume mechanically-driven superchargers, since that's where the hunting-idle most frequently occurs. I've been around blower engines a lot- competed for almost twenty years with a blown-methanol engine in mini-rod pulling. There are many different thoughts on the cause. All I can offer is my own theory, which only pertains to mechanically-injected blower engines. As mentioned, these use very crude fuel control, optimized only for full-power conditions. Some use tweaks- idle bypasses, etc., but are still crude. The only "tweak" I used was a manual fuel shutoff valve (between pump and barrel valve) with a knob remoted within easy reach of my throttle hand. For idling, moving around the pits, etc. I closed the fuel valve enough to bring idle speed up to 1,200-1,400 RPM- quite consistent, with no hunting. I think that hunting occurs when slower idling is attempted without appropriate control of fuel flow- the over-rich engine slows below the nominal idle speed until pump delivery drops enough to over-correct the richness, and engine "cleans out" and speeds up, and the cycle starts over. The cycle time is a few seconds due to the delay of small amounts of fuel moving through large-ID hoses. But it's only my theory.

... you could probably model it with a few gains, delays, adders and integrators.

Steve

There was a time in "the good old days" when it seemed that every modified engine with a "big" cam made this type of sound. I am not sure but I don't think modified EFI engines do it.
Note that carb engines with the choke fully on also had this same hunting idle sound - so it would be reasonable to think that maybe a very rich mixture had something to do with it.
There is an effect at idle where if the idle speed really drops off the small throttle plate opening is enough to let the cylinder pressure become almost the same as WOT - that is; full atmospheric. This is the same phenomenon that allows "running on" to occur after the ignition is turned off.
Possibly a very rich mixture needs a certain level of compression pressure to allow the spark to ignite it - when the idle speed becomes very slow maybe the cylinder pressure is enough to allow the cylinder to fire which then speeds the RPM up - dropping the pressure and stopping the firing - and the cycle is repeated.

Usually its a combination of excessive lift, excessive ramp rates, and excessive pressure pulsation in the manifold at low rpm.

First reading into the original comments, it seems that the thinking is that these motors have a modern EFI. Not true.

Here is the gearhead term for the condition in question. For many years it’s been called Detroit Roll.

First I’ll hit on the race fuel injection then we’ll look at carbs.

Jack Gilford hit on some good points. I think the OP was noting cars with GMC 6-71 blowers. These are positive displacement blowers derived from the early Detroit diesels. These have been very popular on hot rods since the mid ‘50’s. The racers used the Hilborn, Enderle,Kinsler and a couple other mechanical fuel injection. These all operate pretty much the same. There is idle fuel control and full throttle control. Nothing in between. These systems were never designed to be run on the street.

You have to understand how these systems work. When the throttle is opened, there is a huge gulp of air brought it. This requires a large shot of fuel to adjust the AIr Fuel Ratio. The fuel pump has high pressureand due to it being positive displacement too. Essentially a hydraulic pump. There is a calibrated idle bypass valve that prevents hydraulic lock so the fuel system is pressurized. The throttle valve called a barrel valve, has an idle slot leading to a straight through hole. When the hole is fully opened all the fuel goes into the motor. This represents the carburetor accelerator pump.

At idle there is vacuum in the intake manifold. When the throttle quickly opens it can go to boost in less than a tenth of a second. So the motor requires a lot of fuel very quickly.

The race motors have pretty radical cams so the proper way to smooth out the idle is to simply speed it up. It’s not uncommon to have a 14-1500 rpm idle on NA motor. The supercharged motor responds about the same. We ran 1800 on our blower alcohol motors and 2200 on the nitro motors.
I never had a supercharged motor that had Detroit Roll. Generally when this happens there is too much cam and too slow of idle speed.

Now we’ll go into carbs and Streetrods. I just sold my blown sbc Willys. I drove it 24k miles and it never had had the DR.

Using carbs it’s standard to use 2 750 or 800 cfm carbs on blown street motors. These will have 50 cc accelerator pumps on both ends. So 4 50 cc pumps instead of a single 15 cc pump. These big supercharged motors need all the fuel you can put in them to prevent backfire when you step on the gas.

Now the carb has to adjust the AFR throughout the rpm range, just as they normally do. However you need to be able to adjust the idle and high speed restrictors and the idle and high speed fuel restrictors. Then there is the power valve and adjustable PV channel restrictors. And you also have the standard carb jets. The carbs are to Taylor the AFR across the operating range. Idle and most cruising there will be substantial manifold vacuum. But even small throttle opening can cause boost. The vacuum drops and boost come on, again very quickly.

I haven’t hit everything because there is a lot to fine tuning these motors.

When you hear DR you know something is wrong.

Now modern supercharged motors are EFI controlled and have very precise fuel and air control so you will never hear the DR. The entire motor is designed to run correctly at all times even with their relatively hot cams.

Fascinating. LPS for the excellent tech blurb.
I stand by my original remarks.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz