performance and fuel economy?
performance and fuel economy?
(OP)
I'm going to be building up a small block mopar engine. a 1971 318 block. I have big plans for it. My goal is to get the best fuel economy with the most power. Simple and I know, that's everyone's goal. But my question is, as far as getting the best fuel economy, where do I want my powerband to be in relation to my cruising rpm. Should I design with torque in mind or horsepower. I plan on doing two rebuilds of it. one, moderately superficial, durring which I will be adding fuel injection, headwork, maybe a custom intake- but not much internal unless its absolutely necissary. The next summer will go deeper and possibly look for a possibility of turbocharging it. I've got a heavy car, but looking at what other people have done, I can use their experience as a guide of sorts. so my questions
how should I choose to build the engine in terms of cruise RPM and power for the best fuel economy. is it always a case of, lower rpm, better mileage, where would the limit be.
also I'm aiming for the range of 300-350 horsepower, which is attainable from a 318, but If i fall slightly short I'll live.
thank you in advance
how should I choose to build the engine in terms of cruise RPM and power for the best fuel economy. is it always a case of, lower rpm, better mileage, where would the limit be.
also I'm aiming for the range of 300-350 horsepower, which is attainable from a 318, but If i fall slightly short I'll live.
thank you in advance





RE: performance and fuel economy?
Norm
RE: performance and fuel economy?
Then I would fit a five speed manual transmission with overdrive fifth gear.
I would very carefully rebuild the engine using entirely standard parts, except I might do a little cleaning up of the cylinder head ports. Mainly around the seat/bowl area to improve low lift flow. Replace the possibly worn out original camshaft with an aftermarket cam of similar to original valve timing, but with slightly more lift.
The only really radical engine modification would be to fit a complete aftermarket EFI and ignition system that can be custom mapped, perhaps something like an Autronic or a Motec.
Keep the total exhaust back pressure as low as you possibly can, consistent with an acceptable noise level.
That should get you reasonable economy. If it is still not fast enough for you, I would start thinking about fitting a centrifugal supercharger.
RE: performance and fuel economy?
RE: performance and fuel economy?
You need to approach this whole problem from the perspective of efficiency. The good Mr Vizard goes through the whole deal, and the initial choice of vehicle is extremely important to the success of the whole exercise.
Modern EFI offers a vast improvement on using a carburettor, and the old centrifugal and vacuum advance distributor is now completely obsolete.
For a start, the EFI system can give much better control of air fuel ratio while the engine is warming up. It also completely cuts fuel flow when you back off on the throttle. Closed loop control is also an excellent feature, as well as far better fuel atomisation at low loads.
Don't ever assume that a carby can be tuned to work anywhere nearly as well as a properly tuned EFI system.
The ignition system too can be vastly improved by being able to more accurately map the ignition requirements under a wide operating range. All you can get with a few weights and springs is some sort of a curve. With a full ignition map it can go up and down any way you want to make it go under all conditions.
You will gain far more from using modern engine management than just building an old style hot rod engine with a big four barrel carby and a hot cam.
RE: performance and fuel economy?
That is going to hurt max power, which is achieved by getting max VE at the red line.
Such is life.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: performance and fuel economy?
thanks all
RE: performance and fuel economy?
RE: performance and fuel economy?
RE: performance and fuel economy?
RE: performance and fuel economy?
Keep in mind that good fuel economy is not necessarily the same thing as good performance, since minimum pumping a losses are found at low rpm, while maximum performance is generally achieved at high rpm. You might look for a truck camshaft, one biased toward torque at the expense of maximum performance.
You might also consider using a manual transmission without overdrive in order to reduce power loss in the transmission - 1:1 being a straight-through gearing, while all other gear positions are indirect and so have greater losses.
Finally, you can also try to minimize parasitic losses by foregoing power steering, deleting air pumps, driving a low-output (45 amp or so) alternator as slow as practical consistent with battery charging, and using a viscous-drive cooling fan.
RE: performance and fuel economy?
Modern engine management absolutely transforms these older engines for smoothness and economy.
RE: performance and fuel economy?
also, i looked into reverse cooling, and it seems like an awful lot of work for minimal gains. I'm not going to have extra heads around to cut apart and study the water jackets, so I'm never going to know where steam is likely to pool. maybe in the future it might be an option, but for now i'll stick with the tried and true regular flow.
RE: performance and fuel economy?
If you are not geared for >27 mph/1000 rpm you'll be in the ugly part of the specific fuel consumption curve. That is way more important than being at the "torque/efficiency peak" when the throttle is largely closed.
http://www.consult-g2.com/papers/paper12/F4.jpg
RE: performance and fuel economy?
On a 1L 3 cylinder, we measured fuel consumption to be the lowest at 2800 RPMs and 80% load, but torque peak was at 3600 RPMs, so I wouldn't try to match torque peak with cruise. Torque is an airflow relation. Economy is a fuel usage relation. It might even be an opposing relationship, to a certain degree (i.e. try to keep cruise RPM far away from torque RPM, without sacrificing drivability or emissions).
RE: performance and fuel economy?
For best economy you'll want to be deep into the throttle on take off and then coast for as long as possible. That's because the BSFC drops as the throttle is opened up. However a carburetor tends to not do so well here and needs an accelerator pump shot which wastes fuel.
Under steady state cruise a properly tuned carb won't give up much to EFI.
RE: performance and fuel economy?
RE: performance and fuel economy?
I have already done this some years ago.
There are things to learn such as how a EGR system can improve fuel milage, how decelleration saves fuel, what camshaft profiles tend to do well, how to size injectors, oxygen sensor operation and timing etc. All these areas are to be optimized and that is a big job for an independent.
It's a large subject and will take knowledge to acomplish over time not to mention expense.
Today if I were to do this it would be in reference to E85 fuel use trying to get that fuel to give up decent power and mileage as referenced to gasolene. But then even this presents even more challanges in addition to the retrofit.
However good luck with your project.
RE: performance and fuel economy?
www.FuelEconomyTips.com
www.mpgResearch.com
Mike