Hey guys my mild 318 9:5 Cr with a rv cam 1,500-5000 Rpm range pulls about 16-17 in vaccum at idle and at cruising light throttle pulls about 20-21 how is that? should i be pulling more vacuum and light cruise 2,500 rpms? or am i fine.
In what gear at what speed?
And what could you possibly do to change it?
If you are wondering at what rpm to cruise at, that's easy;
rev it up slowly in Neutral, until the vacuum peaks and no longer rises. Fix it there.Then advance the timing until the rpm no longer rises.
Then back up the rpm until the vacuum starts dropping. Then go back up about 100rpm. Now put a timing lite on it and see what you got. Write it down; both the total cruise timing and the rpm.
Put everything back to the start point.
What you have there is an rpm adjusted to max efficiency. And that is your target lowest cruise rpm with it's attending cruise timing.
With a small cam, your cruise plateau could be several hundred rpm wide, and higher than the minimum target. So say all this above doing got you 1800 rpm. That would be the minimum. The cruise window could be 1800 to 2400. But if you need to cruise at 2400 you will have to optimize the timing for that, as before. Rev it up to 2400 and jack the timing around until 2400 has the highest amount of cruise-timing that the engine likes. BadaBoom!
On the roadtest, you might find that the engine likes a couple less or a couple more degrees than what it liked in Neutral.
Of course, now you have to figure out how to hit that cruise-timing number, while still maintaining the correct amount of power-timing. With a factory distributor, that cannot always be done. In fact, hardly ever.
For instance, My 367 likes 2240 rpm to cruise at, and likes up to 60 degrees there. I can only get 22* in the Vcan, and 2240 is about 23* of power-timing, for a total of 45*. So I had to figure out how to get 15 more.
So I bought/installed a dash-mounted, dial-back, timing-delay box with a range of 15*. Badaboom there was the 60*.
But I needed some timing retard at idle to drive slower with my 4-speed. So I reset everything for 9 retard/6 advance, and so my cruise timing ended up at 51*. Too bad so sad, I biased the timing to be able to drive at 4.0 mph, and let the cruise timing be what it would be.
Hope that helps,lol.
