Thread: 20+mpg small block with carb.

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Im trying to build a small block 327 with fuel efficiency in mind. I currently have a 327 short block setup 30 over with flat top pistons.

Im wondering what heads to run combustion shamber size, valve size, cam size lift and duration, intake and carb as well. i was told a 500cfm carb and an edelbrock eps manifold.

Thanks

I'd start with a 305, Edelbrock Streetmaster intake manifold (version 2-spreadbore), Cliff Ruggles-tuned Quadrajet and Crower 00902 cam....
EDELBROCK STREETMASTER SBC 327 350 400 CHEVY ALUMINUM 4 BBL INTAKE MANIFOLD 3025 | eBay
Crower 00902 - Crower Mileage Beast Performance Level 1 Camshafts - Overview - SummitRacing.com
I'd build the motor at 8.0:1 to 8.5:1 static compression ratio with a 0.035" to 0.045" squish.

It wold sure help to know what vehicle you are trying to get 20 MPG out of. My own experience is that the you can't build an engine in an information vacuum. You need to take the engine, transmission, gear ratio, tire size vehicle weight and type, and speed you want to travell all into consideration before you start.

For instance if you're building it for the 64 C10 you list in your signature 20 MPG might be doable at 65 MPH, but with the amount of air you have to push out of the way at say 75 MPH you may not see that type of mileage.

Its going in my 1964 C10 with a 700r4 tranny i have not yet picked rear gear size for this truck i was thinking something between 2.73 to 3.42

2.73 times the .7 overdrive will produce a final rear gear ratio of 1.91
3.42 times the .7 overdrive will produce a final rear gear ratio of 2.40
3.73 times the .7 overdrive will produce a final rear gear ratio of 2.61
4.11 times the .7 overdrive will produce a final rear gear ratio of 2.88
The spread on the 700R4 is so dramatic that with the 2.73 gear you'd have a nearly unuseable 4th gear and with the 4.11 gear you'd have a nearly unuseable 1st gear. 3.42 or 3.73 would seem to be a good compromise, or use a 2.73/3.08 rear gear with a TH350.
Rear tire diameter will make a difference also.

One useful number I came across during review of the 700R4 transmission is that the combination of rear gear and tire size (look up rev/mile) needs to be such that the engine is turning 2000 rpm or more in the 4th OD gear at 70 mph. I can just make the rpm at 2023 rpm with a 3.55 Ford 8" rear and large P235R/75R15 tires (698 rev/mile). Since almost every dyno chart you see starts at 2500 rpm you need to worry a bit over whether the car body is sufficiently streamlined at 70 mph and that the engine has enough torque to move the car in 0.7 final OD gear at 70 mph. As Tech said the best rear ratio might be in the 3.4-3.7 range and my setup might better have been a 3.7. The formula for rpm is

(70mph)(tire rev/mile)(0.7)(rear ratio)/(60min/hour) = rpm in OD at 70 mph

I am using a stock stall converter at 1650 rpm so that is another worry, the stall rpm has to be less than 2000 rpm as well.

Don Shillady
Retired Scientist Teen Rodder

Originally Posted by Don Shillady

I am using a stock stall converter at 1650 rpm so that is another worry, the stall rpm has to be less than 2000 rpm as well.

Don, that's the STALL rpm for the converter. It will only slip at that rpm when you have the loud pedal to the mat in DRIVE with the brakes locked. If the converter slipped at that rpm all through the range, millions of grandmas would be at their dealerships, raising hell about engine noise on the way to bingo.

OK Tech, can you estimate at what low rpm the converter will slip based on a 1650 stall rpm in the OD gear? Normally the trans will downshift before this happens so this rpm determines the shift point?

Don Shillady
Retired Scientist/Teen Rodder

OK this post is supposed to answer a general set of questions by 1972Chevy regarding setting up a SBC for higher mpg. While we have Tech1 available for discussion maybe I can learn something and help 1972Chevy at the same time? The STALL rpm has to do with how high the rpm can go against a locked no-go situation but I am not aware of a rpm rating for when a locked-in gear selection will either slip at the converter or stall the engine. I am sorry to say my brain was frozen in the 1950s relative to a "road test" site near my home. It was/is a short (about 100 yards) steep hill (about 40% grade) approached by a sharp left curve so you could not accelerate much at the bottom before attempting the grade. The "test" was would your car be able to reach the top of the hill in high gear and many a Model A could make it due to the heavy flywheel and low rpm torque. Later model 59AB flatheads would barely make it and the engine would "buck" and almost stall near the top of the hill unless the clutch would slip. Today most automatic transmissions just downshift on that hill and few notice the problem except that the hill is steep. So with a 700R4 in OD locked in either electrically or by an internal hydraulic condition about what rpm would the transmission downshift? A 305 SBC with a Crower 000902 cam suggested by Tech1 is a good idea but the question is whether the shift point for the OD will be low enough to shift into OD at say 55 mph where higher mileage is desireable? I have sweated this out trying to match tire size to my 3.55 rear ratio and 700R4. My engine (350 sbc) now runs but I have yet to set the shift points. A local transmission guy says my 3.55 rear and 235/75R15 tires are in a good range with less wheelspin than with a 3.73 rear but still good traction and potentially reasonable mileage with the 700R4 shifting into OD somewhere around 55 mph but I have been checking the rpm values at various shift points and it looks close to me whether the shift point into OD will be low enough to help the mpg. I will only know after the trans shop sets the shift points for me. Perhaps my worrying and the formula I have provided will help 1972Chevy? At the very least 1972Chevy might want to worry over tire size as well as the rear ratio.

Don Shillady
Retired Scientist/teen rodder

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