campinpoptop wrote:
I just installed an 091 transmission with 5.42 ring and pinion to compensate for larger tires.
I drive to the sierra mountains (tahoe) for Kayaking and camping primarily,
a) I have the 142 grind
a) on stock 78 Type 4 F.I. engine
a) with exception of larger valves, and extractor exhaust.
b) I'm also going to run heavier tires with more tread resistance.
So....should I look at stock rpm/mph and maybe just b) reduce tire size down to compensate for my application of mountain driving, and being heavy?
I believe a 29" puts me 6mph under stock at 4000 rpm's.
OR...should I use the 4200 optimum fan speed for cruising as a benchmark?
Should I put cam specs into the mix?
a) did you select these larger valves and 142 grind and extractor exhaust on this no-longer stock engine?
b) did you select or inherit this 5.42 (rear axle ratio from Europe) 091 because you had the larger tires to begin with, or did the transaxle get built for a car that had larger tires to begin with? Do you know the 4th gear ratio? Factory was .82 up until the 5-rib when it was raised to .89.
campinpoptop wrote:
Any help here is greatly appreciated as I'd love it if I can nail this calculation for reduced stress on upcoming summer trips.
I don't know if I can be of any help because you have an array of variables here. I have no idea how the engine is running. I have no idea where its torque or horsepower peaks are. I don't know where
the stress of driving to Lake Tahoe is to be found. If it is just a bunch of 3rd gear hills, well, that is nothing new under the sun for a VW bus, loaded or otherwise.
I have driven the Sierras and the Rocky Mountains with
a 1973 bus, a 1978 bus, and a 1970 bus,
they all had factory torque peaks at about 3,000 rpm,
with horsepower peaks at 4,800 rpm and 4,200 rpm, and 4,400 rpm,
with stock camshafts, stock exhaust systems,
stock rear axle ratios of 4.86 and 4.57 and 5.38 : 1, each with 27" tires,
which gave me 3rd gear pulls of 43 and 45 and 40 mph respectively,
and they all did/do/shall do fine.
The only thing I do is inflate the tires, tune them up, make sure they are clean, and drive.
Colin
(please note that once you have downshifted, it is YOU who determines the fan speed/heat generation most of the time because once you have downshifted, it is mostly up to you how fast you are going to go in 3rd gear.
On the highway, 4th gear, floored for 16 miles straight outside of Baker on I-15 north, then the engineering of your engine/cooling gear ratios/tire sizes becomes important because you know that you are going to have the accelerator pressed for most of the time)
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles