Since starting the discussions of torque VS HP and the rod length thread I got to asking myself...what about fuel consumption. Same car..small roadster/coupe. 406sbc with like a 3.08 gear 302sbc with higher gears to take advantage of higher revs. Stick or Lock up T/C for ******. Which one gets better gas milage at cruising speed? The 406 is turning fewer revs but the 302 moves less air per rev. Hey...some people actually care about mileage...really.
The key is to keep it right near the bottom of the power band at cruiseing speed. I doubt that you would see much difference in either mill milage wise. The SBC in the Pusher has a real broad power band I'm not going to shoot you torque numbers but they are more than sufficient. I start climeing up on the cam about 2K and cruise between 3K and 3500. The truck is a very heavy brick my milage is about the same between 2500 and 4K. My torque peaks @ about 4800-5200. The key to good milage or the best you can get from any mill is to keep it in the power band, you drop below and you loose and go go beyond you loose.
porkn****** has it right, I think it has alot to do with the powerband of you engine combination. I have a vivid example.....a friend of mine has a new truck, he *****ed about the bad gas mileage if his (new 1/2 ton Chevy) 5.3 engine, 4L60e trans with a 3.73 axle....he was only getting 13 mpg. He took it to a shop and had 3.25 gears put in it to "turn lower rpm at speed". His theory didn't work, it only got 10 mpg with the taller gears!! He put the engine below it's efficient powerband and was always "lugging" it. Since then he went back to the original 3.73 gears and his 13 mpg.......it ended up costing him $2000 to find out what I warned him about in the first place.
Go to a dyno that has some advanced measurement equipment, get some VE numbers for the combo you run - peak VE should give you peak fuel economy when geared properly. (VE=Volumetric efficency just in case you didn't know.) That said - properly matching components will go a long way - build an engine with a specific cruise RPM in mind, choose components that compliment that and you'll probably be close. Remember this - to push a car down the street at say 70 MPH it's going to take "X" horsepower. This is the horsepower needed to overcome friction, wind resistance, etc. With modern automatics and lockup converters nearly as efficent as manuals we can generally choose what we want and not "pay" for it in a mileage hit. That said - if your 302 is at .60 ve when cruising and the 406 is at 1.05 the 406 will get better mileage. Honestly - the best thing that you can do is match components and have the car jetted properly on a load cell dyno (so they can hold a cruise RPM) - if you're wanting mileage - have them tune the carb to run a little on the lean side at cruise, also have the spark curve optimized for cruise - it's amazing what properly tuning a/f ratios and spark timing will do to mileage. (and power!)
I'll risk taking a WAG here...I ran a 360" sbc w/a 3:08 rear in the A coupe. Usually netted 18~20 mpg. Lets ***ume you have the same "swept displacement" per mile, for the 302 vs the 406(?). Wouldn't the friction, as in, surface feet per minute of the pistons,(primary concern) the cam/lifters etc. mean that the overall efficiency of the slower turning "big block" yields a better return?
Sounds good in theory. but let me give you an example in the real world. The wife has an 05 Solverado with a 327 ( some litre ****). With the OD its turning about 1800 -2000 @ 70. She gets right @ 20 MPG at that speed on the highway. My mill is a 355 with a holly 4 bbl. 400 Turbo no lockup. My truck is 40 years older than her's and not as areodynamic they do weigh about the same. At the same cruising speed I'm snagging about the same milage maybe a little better if there is no head wind. Now here is the real clinker, if she keeps it at around 70 or below and turns off the OD she can increase her milage by alomst 1 mpg. The only place in the nation that she can really legaly take advantage if the OD is Wyomeing. Low revs doesn't equal good milage unless your engine is built to run at low revs.
you can play the efficiency game without a dyno...get a vacuum gauge and set your gearing at whatever mph you plan to cruise at to give you the highest vacuum reading and that will be you best mpg....but seriously...milage is OVERRATED when you are thinking performance, unless you want to get away from naturally aspirated.
The key is the engine has to be built for it...just like some said above. If you use a high rise intake then the torque will be better but lose HP until higher RPM. Torque gets you off the line and HP gets you to the other end.
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