OK Guys----I'm running a 350 small block, 260 hp stock cam, edelbrock manifold, 600 cfm edelbrock performer carb, 350 turbo trans. and a 10 bolt Chev. rear end with 262 gear ratio----All this in a 3,300 lb 48 Chev. pick up. Great truck-----goes any where----all the time. But LOUSY gas mileage. Average 10/11 around town and 14/15 highway (even on cruise control) The truck runs smooth and clean (no black smoke). I've considered that I may be under geared, a 305 gear would be a better compromise, but the 262 should be great at high way speeds, and you can see, that ain't so good. The distributor (stock GM HEI) reads right, va*** advance works normal. I'm leaning towards the Carb, but it runs so good I'm reluctant to tear into it. Whats your take on this? BILL RINALDI
For starters I'd take another look at trying to advance the timing , rechecking the carb and maybe a hotter dist. set up over the stock HEI. If I can run my double tunnelram with 2 500 edelbocks and a 4:33 rear at roughly 55 mph and get 16 mpg , I'm either one lucky s.o.b. or yours just isn't quite right. It's possible that your rpms are maybe too low ( depending on tire height ) to be efficient .
The compression on these motors is approx 7.9 to 1 .I have bumped up timing a lot and taked 1 spring off the weights.Timing to manifold vacuum.With low compression this motor needs lots of advance
Yea, I used to drive a Chevy pickup-350, small cam, aluminum intake, 3.08 gears. It got 14-15 mpg on the highway. I think that if you really want good gas milage, you need to build an engine that REALLY works between 2,000 and 2.500 and run an od transmission thatwill put you in that cruising range. Larry T
at cruise..in top gear what is your highway RPM? reason im asking is you might be lugging the engine, to low an rpm isnt good either. and a 48 chevy truck isnt very aero-dynamic. what do your spark plugs look like?
Bill, I'm right there with Von on the gearing possibly too HIGH { low numerically }. The engine is always laboring and it needs to be in its sweet spot in the torque curve. Which Edelbrock intake is it, I'm ***uming a Performer? How about air in and air out. You see some guys complain about mileage and they have way too small of a air filter on thier engine. Same thing with exhaust could be way to restrictive, conversely easy to be too large too. If your running headers the primary's can be way too big. Even the pipe size out the back, too large a pipe and you loose all the velocity, and efficiency suffers greatly. A 3" exhaust might look and sound cool, but its killing your engines performance. There are 2 ways for maximum volumetric efficiency, that is volume and velocity. Its very easy to make a wrong selection on parts and kill performance and mileage.
You should consider a 700R4 ****** behind the 350. I did this (gear ratio is 3.07) and I get over 20mpg/hwy,13 city. Car weighs over 5000 lbs.
That truck just needs a little more rear gear to get the motor cruising in the most efficient RPM range. The rear gear change is going to be cheaper than an OD trans.
With his rearend gearing, it's like having a 700R4 and 3.73 gears. Definitely not undergeared. Play with the timing, think about going to a Qjet carb on a small dual plane intake. Should be a few mpg higher than your getting, but don't expect miracles, as mentioned it is a truck.
I shot off at the mouth before doing the math..yeah he's close to what it would be on a 700r4..so probably not the gear..still would be interested in knowing what RPM he's at on the e-way. still pushing a brick..that hurts in the MPG arena, and worse if your running against the wind
get yourself a vacuum gauge. that's a good way to tell if your motor is in the proper RPM range while driving.
yup...you want low vacuum, to get good mileage. That happens with low rpm and the throttle open a ways. Think about it.
Not what you want to here. But that milage sound's about right to me. I have a late model SUV with all the new computer controlled everything and i get 16 mpg on the highway.
You're already doing better than my '95 van with all the computer stuff on it. Put an air dam below the bumper to deflect some air around rather than under the truck. Make sure the trans isn't slipping. And you're not running like a TBI engine cam with a carb, are you? That might make a difference since the TBI is computer controlled and it advances and retards the timing as needed.
I thought about it and that's contrary to EVERYTHING I have ever experienced or been told regarding tuning and gearing for mileage. Or was this a subtle attempt at sarcasm and it was a little too subtle for me?
Yes,in theory maximum fuel mileage is obtained from low rpm and open throttle.....but in reality...low vacuum will have the power valve opening anytime you hit a slight grade .Need to watch that... And at part throttle vacuum helps a lot to vaporize the fuel mixture,especially an issue on an unheated intake manifold.FI doesn't have these issues of course... I messed around a lot looking for fuel mileage gains on performance inline 6's and a SBC.The best results were with a zero deck engine and maximum quench,lean part throttle mixture and a very aggressive spark advance,heated intake manifold,low cruise rpm as possible while maintaining 10 inches of vacuum average..The gas mileage was impressive compared to similar engines built without these details.Throttle response was excellent...The bad was the engine always ran on the edge of detonation.
Don't you want a higher vacuum? That's what I have always heard and seen oon those gauges that are supposed to help you get better milage. As for you gas milage, cutting the top off is about all you can do to improve the aero on those boxes. /they push a lot of air.
A lock-up torque converter should be worth 4 to 6 mpg if your running a standard converter now. I ran a simular set-up in a OT '79 Chev PU with the lock up converter, about 19 hi-way mpg. It too had the same slippery shape of a cinder block.
I get the low RPM (too a point) but I'm still having a hard time grasping the open throttle part of it. I'm not trying to argue, I'm tryng to understand.
Using a vacuum gauge is a great tool to tell you how the engine is running and how you are actually driving it as well. 30 years ago my dad taught my mom how to drive her 2300cc Pinto Wagon with a 4 speed using a vacuum gauge. She managed to get it up to 42mpg average on one tank on a trip from Southern California to Oklahoma once. The overall average for the entire trip was up over 35mpg. Just something to think about.
The engine produces the most power for the least amount of fuel .Think of an aircraft needing 60 percent of it's maximum power at cruising speed.Of course with a car going down the road only needing 20 hp from a 300 hp engine,it doesn't work out so well. Real life tests show that accelerating using a lot of throttle and short shifting the engine gives the best fuel mileage in faster suburban traffic.
One part of the equation is missing for the actual FINAL drive ratio, or Revolutions per Mile - tire size/ tire O.D". Big diffrence from a 50 series 14" and a 10 hundred 16". Like already stated that mileage number is not that bad. TR
Bill, do you drop your tailgate down to parallel with the road when you travel at highway speed? Works for a new rig, should work on a 48 too.
Not sarcasm. Think about a modern car with overdrive. How much vacuum do you think it pulls at highway speed in OD? How much at highway speed in the next lower gear? Think about it this way. When you have your car parked, with the engine running, and you rev it up to 2500 rpm and hold it there...what happens with the vacuum? should be very high. What's the mileage? should be zero.
350 chevy's with the 76cc heads will not get good mileage. The quench is horrible and the cams have retarted valve timing for lower emissions. I suggest switching to a cam like the comp 268 (short duration high lift), and if your on a budget use a set of HO 305 heads (they have the largest valves) and go to 1.94/1.60 valves. This will bump up your compression about a point and give you a decent quench. People will tell you that 305 heads wont flow but the ports are the same size as all the other 350 heads the just have smaller valves. These mods with your gears should get you around 20 mpg.
Car not moving,zero fuel mileage Modern engines have electronic fuel injection and spark controlled by a powerful computer to deal with very low manifold vacuum.
That's backwards to everything I was ever taught about getting fuel mileage Jim. This is the first time in three years that I choose to differ from what you suggest but everything I have ever come across says that the higher reading on the vacuum gauge equates to better mileage. That also included operating the engine through the whole speed spectrum so that you maintain the highest reading possible while going through the gears. That isn't anything like the spirited driving that the majority of us are used to doing. Maintaining the highest reading in top gear on the highway usually equates to the best mileage. That often doesn't equal the road speed one is used to driving nor what one wants to drive at though Driving habits often are the big factor in fuel mileage. Using less spirited driving though the gears and finding a road speed that gives the best overall highway fuel mileage even though one might get p***ed by other vehicles on a regular basis. In my 71 GMC running 58 on the road equals 3mpg better mileage than 70 does and that is just above 20% better mileage. 3 mpg when you go from 30 to 33 mpg isn't much but it's a big jump when you go from 10 to 13. Still just about everything on the rig factors in on fuel mileage. engine in tune. Meaning the whole engine package is working at it's best to get optimum fuel mileage. gear ratio that has engine working at best road speed tires that have less rolling resistance and are inflated correctly. Front end aligned correctly to allow for the least drag on the front tires. Not hauling a lot of extra stuff (weight) in the vehicle all the time. Weight of vehicle it's self. Wind resistance. Some you can improve on some you can't and don't have much control over.
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