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MG MGB GT V8 Factory Originals Technical - Help please ... distributor timing

I'm just about done putting my conversion on the road ... I can't find any data out there at all on this: can someone tell me approx. what distributor timing is supposed to be used on a rover 4.2 liter motor? The instructions tell me that this distributor has 24 degrees of mechanical advance (and that it is fully mechanically advanced by 3,200 rpm) and G*d knows knows how much vacuum advance -- it doesn't say on the instruction sheet. It is a Mallory diss. Also, do I hook up the vacuum can hose to the manifold vac fitting, or to the "ported" fitting? I know, I'm supposed to know all this stuff to deserve to have a v8, and I probably would have known it long ago, but I don't.

Terry Buhl

Nice to see another person in Memphis working on a V8.
Tuning Rover V8's book says 6 degs static timing, max 32 at 4000 rpm. There should be a port on you carb for the distributor vac adv. You need to let me see your car when you're finished, we Memphians need to stick together:)

Mike
Michael Hartwig

Michael, will do, for sure. About this ported vs. manifold thing, I am desperately confused. Clearly, manifold vacuum's at max when idling or cruising ... i.e. when the throttle plates are closed. I thought ported vacuum was just the reverse. If that is correct, it seems wierd to me that when you stomp on the gas, the revs go up and the centrifugal advance weights advance the timing, only to then have it retarted again by the vacuum can? Or am I misunderstanding what type of vacuum the carb port provides? One thing I care about a lot is maxing gas mileage at cruise ... that's primarily how I'll be using the car. Does increasing the advance help mileage? (I know, you're thinking, this guy is an idiot and his conversion must suck, but actually it has come along very well ... bear with me here.)
Terry Buhl

Well, you have the part about the centrifugal advance correct, but the vacuum advance does just what it's name implies. When you crack open the throttle, the vacuum drops. The vacuum advance is typically set to advance the timing just a bit, so that you get more torque and more power. However, as the vacuum drops towards atmospheric (ie. when you really stomp on the pedal), the vacuum advance drops out to help avoid pinging of the engine. The vaccum advance works under a range of vacuum measurements.

The use of vacuum advance helps to increase power when you need it, but it also helps you to maximize economy. Why? When you are driving along the parkway, constant throttle, your engine will meet varying loads (ie. going up a hill). When you go up the hill, your engine is under load, your vacuum drops, and your vacuum advance kicks in more timing to increase torque and help you maintain speed. Vacuum advance also helps your off-idle transition. When you crack the throttle, the engine will increase speed via two means, more air-fuel, or advanced timing. Centrifugal advance only dizzies are not as rapid to react, so you can have a bog off idle. Many just increase the accelerator pump to help counteract this, but it decreases economy. The vaccum advance also helps with off-idle transition, since the mixture tends to go lean when you crack the throttle plates. The engine needs advance timing to ignite the lean mixture properly.

The best setup is to run vacuum advance for street use. If you are racing, centrifugal advance is all you need. The trick is tuning so that your engine doesn't ping when the vacuum advance kicks in.

Hope this helps. In short, run the vacuum advance off your carb port. During closed throttle, your vacuum advance reads atmospheric and advance is zero. When throttle is open, you read engine vacuum and you get advance.
Michael Hartwig

Wait, I think there is _plenty_ of reason to argue for hooking up your vac can to manifold, not ported vacuum. Especially with gas mileage in mind. At lower throttle settings, engines can use some more advance. Reason? The flame front moves slower as the mixture in the cylinder gets less dense (the opposite's also true, that's why you have to retard with nitrous and superchargers). So that the flame can get started earlier and catch the peak pressure just where you want it, you want to advance the spark. This helps the engine temperature, as the mixture is not burning late, helps the power, as the peak pressure is at the right place in the stroke, and helps the fuel economy as the fuel is being used effectively.

Ted

Ted and Michael, I think you are both missing part of the story. Either system (ported or manifold vacuum) has the exact same advance at all throttle positions above idle (whenever the throttle plates are open at all, both these ports are exposed to the same atmosphere ... you can easily prove this to yourself with a vacuum gauge). The sole difference is at idle (i.e. throttle plates fully closed, so the two ports are exposed to different atmospheres): manifold hookup has full vacuum while the ported setup has zero vacuum. For ease of starting if nothing else (easier if not too much advance), I'd stick with ported.

(Terry, I'd love to give you some specific numbers -- I have the same motor & similar diss, but there is no mark at all on my harmonic balancer ... should be pretty interesting when it comes time to re-time my engine!)
Bill King

This thread was discussed between 30/11/2000 and 01/12/2000

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