MSefi.com read only forum (Jan.15,2018)

I have about 7 feet of rubber vacuum hose connecting my intake to the MAP potentiometer on my ECU. At idle, my car gives about 64kPa, which seems high.

If I connect a 3ml syringe to the vacuum line as per the test in the MS manual, I can get 84kPa of vacuum. If I connect the same syringe to a 6-inch piece of the same hose, I can get 42kPa. I don't understand this, since the manual also says there should be no issues regarding the length of the hose.

Therefore I presume my engine is actually idling a lot lower 64kPa, and I would like to get a more accurate reading. Would using a solid vacuum line (for example, copper brake line, or some plastic vacuum hose) give a more accurate reading, or is the issue with the length of the pipe? I could remote-mount my pot inside the engine bay, but I'd rather avoid this as it would be a lot more work.

Advice much appreciated!

James

I have spent so long thinking about this and you just explained it in two sentences - thanks!

Still don't understand why my engine only produces a vacuum of 64kPa at idle though... If I shortened the hose, would I see a lower reading (in the same way it is easier to suck through a short straw as opposed to a long one)? And if so, would I get a better tuning map by having a broader range of MAP pressure?

It's a 36 year old 911 - maybe that has something to do with it! 60-ish is the best I could get with any tuning.

Thanks for your help - much appreciated.