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Just been thinking about timing and MBT and wondered if there is a correlation between timing, MBT, and engine vacuum at part throttle? In other words a car cruising at any speed part throttle with a car setup to MBT or close to it if knock limited is the engine vacuum always highest than any other timing setting or is there no correlation at all?
No,there is no correlation between vacuum and MBT respectively any other point of ignition. The explanation is that because the intake valve is shut closed long before the spark occures. Having said this, the both events are not related.
I wondered this also... Im guessing you are doing abit of road tuning?
My theory was with "better" timing the engine would make more power and thus on the intake stroke be pulling harder...(more vac)
At cruise in the GTR I tried it and maybe it was just close to MBT already so was not noticable but I +/- 10degree's of timing and did not notice a change in the logs..
It works at idle tho, maybe the change is too small at higher power levels so hard to tell
You have nailed it Viper, I road tune, mostly Holden V8s. Thanks for your post it is food for thought. I know the backside dyno isn't accurate so I thought I'd ask here. When I organise a dyno I'll test it out but was wondering if anyone had seen any correlation.