Thread: 20+mpg small block with carb.

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Ok, I’ll take a stab at this even though I’m not Richard. Please bear with me as I will also try to help answer the original question.

First off, if the transmission is running a functional lockup convertor the question of slippage is mute, there is no slippage when it’s engaged, ( I believe this is usually in at least 3 and 4th gear on a 700R4).

I’m assuming the stall speed you are quoting it the rated stall provide by the manufacture. Unfortunately in real life (when the convertor is actually in the car) the manufactures rated stall is usually only a best guess. There are too many variables such as vehicle weight, real engine power and what RPM it is made at etc for the manufacture to know. The same convertor will react differently in a different car or with a different engine installed in front of it. The figure is good however in comparing the same manufactures convertors to one another. In general terms the higher stall (looser convertor) has more slippage.

If you don’t have a lock up convertor, the key is to take into consideration the amount of slippage……how much percentage the convertor slips (the RPM the engine would be running at if it were locked to the drive train as in a manual transmission or w/a lock up convertor compared to the RPM it is actually running due to the convertor slippage. Unfortunately as the convertors will react differently in each vehicle it’s virtually impossible to do that until you are actually driving the car/truck to find out.

A good example might be taking a drivetrain and tire combination from a lightweight aerodynamic car and putting it in a 60s pickup truck. In the lightweight car there will be less slippage in the convertor than there would be trying to move the truck with a heavier weight and all the aerodynamics of a piece of plywood thru the air.

All that being said in either case the key to efficiency is to match the engine components to produce the most power (torque) at a target RPM, which for mileage would be the lowest RPM the engine is efficiently making sufficient power to move the vehicle at a given speed. Where a non-lockup convertor comes in to play is taking the amount of slippage into consideration when choosing the final gear ratio and tire size to put the engine at the most efficient RPM for a given MPH. But again as the convertor will act differently in each vehicle, there is no way to determine the amount of slippage until the car is complete and driving.

As far as the original question, I think you will find that a mild RVcam, intake with small runners and heads with small valves (that give about an 8.5:1 compression ratio) will be the most efficient combination. Personally I would be shooting for making peak torque at around 2500-3000 RPM and setting the gear ratio/tire /convertor combination to be doing the posted interstate speed limit at somewhere between 2400-2500 RPM if the speed limit is 70-75 or around 2200 RPM if the posted speed is 65.

Hopefully I haven’t confused the question too much.

MikeP,

Thanks for your reply. There is a page with a lot of good information on the 700R4 at the PATC site.

700 Raptor 700R4 700R4 Transmission 700r4 700R4 700r4 PATC 700R4 700r4 700r4 700r4 700r4 700r4 700r4 700r4 700r4

It mentions that the STALL rpm depends on weight of car and engine power and a BBC may have a stall rpm as much as 700 rpm higher than the same converter with a SBC. It also mentions that 1985 and up 700R4 trans will shift OK without the electrical connection but 1984 and earlier need the electrical switch to achieve lockup. I purchased my 700R4 from PATC because I wanted their conversion to a hydraulically controlled OD lockup so that is why I am trying to figure out at what (pressure) rpm the lockup releases. In principle the electrical lockup is controlled by the switch not the rpm. So now I understand that it is the downshift rpm that is important with the hydraulic lockup. The PATC site mentioned above does say that the rpm should be at least 2000 at 70 mph so my formula given above can be used to see if a given rear ratio and tire size can meet that condition. Ideally I would like to shift into OD at about 50-55 mph and so far my rear and tires seem to be in that range. Well I have learned about the OD lockup but I guess I will just have to wait and see where my 3rd-to-OD shift point turns out to be.

Don Shillady
Retired Scientist/Teen Rodder