AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 26,608
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This is how I've been selecting cruise rpms, when trying for best mpgs. With the engine well tuned, I just rev it up in neutral until the vacuum peaks.Then I gear my car to hit at least that rpm at my cruise speed or I cruise the car at that rpm or better with whatever gears are in it. My thinking is, this is the first rpm that reversion has probably ceased,and I stand the best chance of calibrating the carb and cruise timing, for best lean running.
A stock type engine will vacuum peak very early,and might be ready to cruise at 1300rpm. A 250* cammed engine will vacuum-peak quite late and might not be ready until after 3000.
This has served me very well.
Of course these are my starting points, and there are trade-offs.
I have had a combo that wanted to cruise at 2200.And did very well there. But with an overdrive, did better with a smaller carb and made best mileage at about 1600.
Another combo wanted to be up around 2800, and didn't do well at all. Not there, not higher, nor lower. It was just a pig no matter what I tried.
At one time I wanted to relate the cruise rpm to the overlap period. I tried a (50% x 100) ratio. If the overlap was 50 degrees then the cruise should be 2500rpm.At first it actually seemed to work pretty close.
Using this formula;
the 340 cam has 44* overlap,so the minimum cruise rpm = 2200.
A stock teener has 16* o/lap, so the minimum cruise rpm = 800.
The 292/508/108 cam at 76* overlap,might cruise at 3800.
Obviously the formula needed work,lol..........I cruised that 292 cam at 2800 and it sucked gas bad.And just as bad at 2600. And at 3200 it was still bad. But I couldn't stand it any higher, so maybe by 3800;who knows!
A stock type engine will vacuum peak very early,and might be ready to cruise at 1300rpm. A 250* cammed engine will vacuum-peak quite late and might not be ready until after 3000.
This has served me very well.
Of course these are my starting points, and there are trade-offs.
I have had a combo that wanted to cruise at 2200.And did very well there. But with an overdrive, did better with a smaller carb and made best mileage at about 1600.
Another combo wanted to be up around 2800, and didn't do well at all. Not there, not higher, nor lower. It was just a pig no matter what I tried.
At one time I wanted to relate the cruise rpm to the overlap period. I tried a (50% x 100) ratio. If the overlap was 50 degrees then the cruise should be 2500rpm.At first it actually seemed to work pretty close.
Using this formula;
the 340 cam has 44* overlap,so the minimum cruise rpm = 2200.
A stock teener has 16* o/lap, so the minimum cruise rpm = 800.
The 292/508/108 cam at 76* overlap,might cruise at 3800.
Obviously the formula needed work,lol..........I cruised that 292 cam at 2800 and it sucked gas bad.And just as bad at 2600. And at 3200 it was still bad. But I couldn't stand it any higher, so maybe by 3800;who knows!
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