RV8/supercharged intake manifold keeps blowing up, why?

[GreenV8S]

I've got a 4.6 RV8 with a M112 supercharger fitted using a custom manifold. There are some pictures here:

http://www.tvrsseries.com/sowners/greenv8s/greenv8s.htm

The basic layout is a throttle upstream of the blower, quite a long manifold leading to a plenum feeding all eight cylinders via very short ports, port injection (but very close to the top of the ports), wasted spark ignition, vacuum operated recirc valve takes the load off the blower when the throttle is closed.

There are a couple of teething problems with it, but the most worrying problem is that from time to time the intake manifold seems to light up, which gets quite exciting.

Opening the throttle suddenly from idle can be enough to do it; sometimes it happens much higher up the rev range after the engine has been on full boost for a couple of seconds.

The symptoms are that the engine completely dies (no power output at all). In gear there is a lot of engine braking; out of gear the engine spins down very quickly indeed. The blower drive belt squeals like a pig (it's quite tight so that implies there is a massive load on the blower), boost goes off the dial, masses of smoke from every orifice around the engine. I think what's happening is that the charge is burning in the intake manifold. Closing the throttle stops it after a second or so, the engine knocks like mad for another couple of seconds and then (assuming it hasn't stalled by now) starts running normally again.

I'm running wasted spark now, but I also had the problem when it was running on a distributor. I'm running a very short period 'supercharger' grind cam now, but previously I was running a kent 214 fast road cam and had the same problem. It's got a static CR of about 9:1 now, but previously it was up at 10.5:1. I have water injection, it doesn't seem to make any difference to this problem.

Anyone seen a problem like this before? I can see how there might be enough fuel and pressure in the manifold to sustain a flame, but puzzled that this is not a more widespread problem if it's that easy to light. I'm only running 7 psi boost, which doesn't seem much in the scheme of things.

[Eliot]

Have you tried plugging or eliminating the recirc valve out of things?

[GreenV8S]

I've tried locking the recirc valve open so there's no boost (very little boost, anyway - it can still build up a small amount of boost because the recirc valve is a bit restrictive) and it didn't seem to cure it.

[bill shurvinton]

Strange. Do you have a BOV at all there to vent excess pressure?

Thoughts on possible causes.

1. crankcase breather blockage. Unlikely but possible

2. Lean misfire. Either not enough fuel or too much advance. Engine spits back out the inlets. Large bang occurs, then if lean condition continues engine dies. Carbed cars used to do this if badly tuned with sheets of flame coming out the inlets.

[GreenV8S]

Strange. Do you have a BOV at all there to vent excess pressure?

Thoughts on possible causes.

1. crankcase breather blockage. Unlikely but possible

2. Lean misfire. Either not enough fuel or too much advance. Engine spits back out the inlets. Large bang occurs, then if lean condition continues engine dies. Carbed cars used to do this if badly tuned with sheets of flame coming out the inlets.

No BOV and nothing else to limit manifold pressure. I know it would be a sensible precaution to have some sort of pressure relief system, I'm justifying the current setup because there are several hoses which *should* hopefully burst to relieve the pressure before metal gets mangled.

The crank case breather is a simple hose to atmosphere and seems to be working as normal.

The second explanation seems like a good bet (especially since it doesn't seem to need much/any boost to provoke it, it even happens with very small amounts of manifold depression). I can't figure out how the flame gets from the firing cylinder to the intake, is it from a misfire leading to unburned charge being expelled in the exhaust stroke, lighting in the exhaust and the flame spreading back to the inlet during the overlap? There's very little overlap but there is *some* so that might explain it. If so that should be good news, I 'only' have to get rid of the misfire?

[bill shurvinton]

Misfires are funny things. But basically combution has not completed by the time the inlet opens so you get mixture that is still burning coming back out.

Either more fuel or more advance needed. Your symptoms suggest that it is the former, although I could be wrong.

[JP.]

but the most worrying problem is that from time to time the intake manifold seems to light up, which gets quite exciting......

......The blower drive belt squeals like a pig (it's quite tight so that implies there is a massive load on the blower), .

.....Anyone seen a problem like this before? .

Yess and according to your pic I found your problem, I don't see anny support brace on the blower snout. By the tightness of your belt its pulling the snout down while going into boost meaning the oposite side is lifting up.

If you look close to the nect pic you'll see a support brace mounted to the waterpump. The blower snout is fitted to this plate with an exhaustclamp.

What sort of tensioner do you use?? It must be an automatic tensioner with some sort of spring if you'r using the serpentine belt to drive the blower.
I am using a Gilmer drive with a fixed idler running it with atleast 1" slack on the longer side of the belt with a fully warmed engine. Thats almost 2" slack on a cold engine. Don't forget you are runing it on a Alloy engine which is expanding far more than a cast Iron block.

[350matt]

Hello Pete

Is there any way of fitting any sort of trumpet into the very short ports?

As with your inlet manifold there is a large flat area under the the charger where fuel can collect and potentially 'wash' into a port. If you could have a small pipe leading into the 'chamber' with a 0.6mm drain hole on each trumpet to slowly drain any excess fuel. A la RV8 plenum / trumpets
Alternatively lower the floor of the chamber so the ports are relatively higher.

My reasoning behind this is a mate's 2.8 V6 carb'd granada used to suffer from carb fires quite frequently and its manifold was a similar design to yours.


Best of luck with it
Matt