Blown BBC Idle

I have a blown (671) BBC with an Enderle bug catcher and eight 48 lb injectors setting on top of a tunnel ram manifold. Ignition is Ford wasted spark EDIS. Fuel and ignition is managed by a Megasquirt-2 ECU. I have a wide band O2 sensor. Initially, it would start right up but would only run for a few seconds and die. It would never stay running. With a lot of fuel and timing, it stays running but at 1500-2000 rpm. What needs to be adjusted to get the idle down?

That's a very open-ended question without a lot to work with.

Depending on many factors, such as camshaft, heads - ports and static comp' ratio, fuel, blower drive ratio, etc, you could easily be looking at 30-40 degrees idle advance.

Are the injectors in the manifold runners or the top hat? What do the lambdas, and spark plugs show? If you're used to N/A or turbo' engines, the engine is going to need significantly more fuelling than you might expect - this is because of the additional air and fuel required to overcome the drag of the blower, drive belt, etc.

If it's a big cam', big port, big valve build, you may be stuck with a high idle, but you may be able to get it down to 12-1500rpm, even then.

Oh, are you using throttle position, manifold pressure, or a mixture for the ECU?

Gord -

Here is a little more information:

Cam: mild roller

Heads: stock oval port

CR: 8:1

Fuel: pump gas

Blower drive ratio: 1:1 (will probably produce 5-10 psi)

Injectors: located between injector hat and blower

Spark plugs: not wet or sooty

Lambdas: ?

Using manifold pressurefor the ECU

Ignition timing is fixed at 30 degrees

VE in this part of the table ranges from about 60 up to 130

Not sure I understand this: Datalog shows MAP ranging from 25 minimum up to 95 maximum?? This seems odd for a blower motor.

Thanks

Jim

If the plugs are dry and clean, maybe a light tan, that indicates it's not being over-fuelled - you could try adding a little in case it's a lean condition causing the poor idle.

With a 'mild' roller, mech'or hyd', if it's got a lope the MAP is going to show a fair bit of variation as the engine will be trying to draw differing amounts of air through the throttle restriction - best move may to be to average it out as much as possible.

If you're using a referenced fuel pressure regulator - you should be fine without one with a minimum base of 50 PSI or so, if the injectors have the fuelling capaity under full load - there may be some hysterisis (delay) that could be causing problems, try without it referenced. I assume it's not a case of losing fuel pressure and you've checked that it holds pretty steady?

30 degrees is in the range of idle advance I'd expect - you say "fixed", that might be a bit much under boost, so be wise to keep an ear out for any det' noises and an eye on the plugs for signs there, too. Open chamber heads may benefit from a bit more than closed, but that'll come down to testing.

Is it a manual or auto'? If the latter does it drop to a relatively stable idle in gear - a stock or low stall may try driving the car quite strongly, so make sure you've both feet on the brake when trying. Should be less of a problem with a high-stall.

Oh, have you checked for a vacuum leak - spraying some brake cleaner or engine start around the gasket areas will show a leak as an increase in engine rpm. A faulty brake booster/servo' is also a common problem, as it can draw air from it.

Have you followed the 10 step process presented in the practical standalone course? There are a multitude of reasons why you'll have potential idle issues but you need to have the base tune dialled in otherwise you're just going to be chasing your tail. Gord has asked you about the lambda and you replied with a question mark - I'm not sure what to make of this? What is the lambda displayed on your wideband? If the fuelling is out of the ballpark all bets are off.

I'm also not sure what you mean by '30 degrees fixed'? Are you not using the megasquirt to control timing? If not, you're overlooking a pretty key advantage of a standalone. 30 deg is probably too advanced for idle and almost certainly too advanced under boost.

Lastly, with your injectors mounted above the blower you're likely to find that idle and cruise quality is sub optimal. In the perfetc world I'd run a combo like this with a multi point manifold and use auxiliary injectors above the blower to that are staged as you approach boost. It's important for blower efficiency as well as cooling the intake charge to have fuel running through the blower but it causes some unintended consequences at idle and cruise.