[Amc-list] Re; Suggested maximum long-term cruise RPM for 195.6OHV?

["John Elle" <johnelle@xxxxxxxxx>]

SNIP

Any ideas? The new AMC six seems most happy (max. mileage etc) at 2500 rpm, 

but it's oversquare, so I assume the old undersquare (long stroke) six would


be happiest somewhat less than that. 2200rpm at 65mph?

 

The factory setup (tires, gears) for a twin-stick powerpak 63 American puts
it 

at 2250 rpm for 65mph (year 2007) which I hazard a guess meant 60mph at 2000


might be the olde tyme design cruise spot (it was a long time ago).

SNIP

 

Tom, 

To my way of thinking the way you have asked this question, the answer is a 

more based upon the performance curves of the engine than a mechanical
limitation

based on bore and stroke. Most of the American I6 engine designs used a 

cam shaft that was ground to give a maximum amount of torque at a very low 

rpm. That being the case, they would not be functionally happy at higher
rpms's. 

They also would not be capable of developing a whole lot of H.P. at higher
rpm's 

either. Between that and a carburetor that was too small for the engine, 

performance in traffic would be fair with the low rpm torque peaks and the
small 

carburetor would have a tendency to give sort of decent fuel milage under
those 

conditions and that is what over drive was for. To reduce rpm's at cruising 

speeds. Most overdrive cars that I drove from the late 50's into the middle 

60's were faster straight up than in overdrive but would go all day long at
state 

speed limit getting fairly good gasoline mileage. Using your numbers, 2250
for 

65mph makes it 22.9 mph/1000 rpm which is about where my 1980 Spirit is 

geared at including taking into account the size of the rear tires
(P245R60-14).

Actually the Spirit and the AMX are taller at about 33 mph/1000 rpm

That is very tall gearing. 

There used to be a number floating around that was called "Piston Speed"
that 

one did not want to exceed when building an engine and it was mostly
important

when building an undersquare (small bore long stroke) engine. I do not
remember

what that number was now, but I remember running calculations out and 

the best builders kept the engine rpm at a number that did not let piston
speed 

limits be reached. This of course is a function of the stroke, two engines
of 

the same size, one under the other over would have higher piston speed with 

the undersquare configuration. 

 

All the I6 engines I have built I have installed a cam shaft in it that can
be 

considered a horse power cam, that is it develops horsepower at higher 

rpm's and there for torque peaks are raised also. These engines do not 

go dead at higher rpm's. 

My modified 258 cruises quite happily at 3200 rpm and will do so all day. 

Where as my Stock 258, same year transmission and final drive is decidedly 

unhappy going much above 2200 rpm under the same conditions 

(about 75 mph) as it is out of power and almost incapable of going much 

faster. 

 

Actually the Spirit Wide Open Throttle 2/3 shift out of passing gear is set 

for 75mph and about 4200 rpm, the Stock 258 will not even get close to 

this number and can only approach it once the transmission has shifted 

up to 3rd or final drive.

 

To my way of thinking, it will be a function of the cam shaft primarily and 

not necessarily a comparison of how happy a 195.6 with twin stick is 

compared to a 4.0 with a 3 speed or a 258 with a torqueflyte. I see that 

as more of an apples and oranges comparison and has very little merit. 

Your particular engine will have a "Sweet Spot" so to speak and when 

you find it, the next question to be answered is does it have enough 

power to pull tall gears to obtain a maximum cruising speed with the 

weight of the car in mind or not. Or will you have to short gear it a bit 

to let it climb up the Grape Vine and settle for a less that legal

speed limit cruising speed on the open road. 

Everything is some form of a trade off. 

 

John.

 

Of course I do know some one with a Turbo sitting around that at present

is not being used!

 

 

 

 

 

 

 

 

 

 

 

 

 

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