And heres how you can get a pretty good idea of where that begins.Put the trans in Neutral/ block the wheels. Start the engine, warm it up to operating temp. Hook up a vacuum guage, and a tach. Make sure all, thats ALL your timing systems have been previously bugged out and are working. Now slowly bring the engine revs up, while watching the V-gauge. When the vacuum stops climbing, note the rpms. Repeat several times. The lowest rpm that this occurs is the point in the revs that the induction system is first becoming efficient. If you are looking for economy,this would be the lowest place to gear your car at cruising speed. If you change anything in the top end or tune, this point can wander around some. Obviously the cam is likely the biggest contributor. But the intake/ exhaust systems move it around and not surprisingly, the ignition timing. Thats why its so important to have a well-tuned vacuum advance system on a mpg oriented combo.
There is nothing stopping you from gearing differently from the V-peak. If you stick close to it or higher, the cost in mpg is not that huge.And think about this; our aero dynamic bricks with their ancient motors, only require something like 50hp to run 70mph. So you will be running almost exclusively on the low speed circuits. If you are at least close on the V-peak thing, tuning those circuits and the cruise timing will get much greater results.Factory stock type 318s, V-peak at or near idle, and carry that peak for a long ways up. Thats why they are able to run 2.76s or even less. The 360 2bbl cam also peaks down low. My 360 with a 230* cam and Airgap dualplane, peaks at 2200rpm. I am geared for 2200@64mph. I also have tuned the snot out of; my cruise carb,( a 600V-sec Holley, making it run as lean as is safely possible), and also my distributor.At 2200 cruise rpm I have up to nearly 60* available of spark advance.If you think that might be excessive, put a scanner on an EFI vehicle some time.
So getting back to your question;Try the V-peak thing, gear it close, and concentrate on the tune.If she gets too sluggish down low, either live with it or get a trans with more gears or a lower first gear in it.
Yeah,one more thing; a carb with metering rods, can be tuned in the cruise circuits really well, and still deliver the goods when you nail it.To get the absolute most out of it though, you will need instrumentation. The Holley type carbs are great for quick and easy power adjustments. Not so much on the cruise circuits. Doable, but a pita.
Our street cars really need 5 speed transmissions.I solved the problem by buying a second car for the highway. Did you know that the GM(dont hate me) 3800 Series 3 with its O/D and loc-up, fwd, cruises at around 1600/65mph, and gets deep into the 30mpgImp? thats at least 30mpgUS.
PS. it really is in the tune. On a 12 hour trip to Shakopee in 2004, when I still had the 223* cam in my 360,and the cruiser carb 600, I was able to tune that combo into the 40mpgImp. Thats 32mpgUS, for you Southern friends.( Not everything is bigger in Texas,lol.) At that time I was running the Mopar O/D box, with the GV behind it.I was geared for 1500/62mph. It was too easy.
Awritenuffsaid.
