RV8 Engine timing

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I've been meaning to time my RV8 for quite some time ..

basically from the time I decided to replace the cap and rotor arm - ever since it's not run as well.

A local specialist suggested that the caps (old black, new blue) could be a few degrees different .. fair enought I thought.

So to do the job well - I market the flywheel up at 8 degrees and 28 degrees before TDC and the static engine marker with some white tipex.

What I found was that both old and new caps were both 8 degrees at tick over - so no difference but the new cap gave a mis fire .. the old one just sounded better.. (thanks Rimmer brothers - thats why I sold the TR6 cause the parts were rubbish).. anyway - I tried this several times and always prefered the old cap as it was not missing.. even though the posts and the coil contact button looked like they have had better days.

Anyway - to the point of my post.

With the vacuum pipe blocked (mole grips) the engine was set at 8 degrees at tickover and this equated to 28 degrees at 4000 rpm - so I was very happy with that .. but low and behold when I release the
vacuum pipe - the timing altered to around 14 degrees tickover and around 40 degrees (I guessing this figure) at higher revs.. to me this made no sence - and would be igniting the mixture to soon and possible causing detonation (and knock) ..

So - I followed my heart and V8Racings advice and blocked the Vacuum advance..

So - I'm happy and the car sounds great.

What do the experts thing of this tale ?

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Thanks for your suggestions..

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steve_D said:

Based primarily on the Chevy V8. The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation. At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph). _________________

Many thanks - that was just what I was after.

I was seeing just that and it was not what I was expecting - so I presumed perhaps my dizzy was on the way out or something (vac advance part anyway..).

That was really interesting.

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That's great Rob - must say the car runs brilliantly with it removed and now I understand why..