performance and fuel economy?

[Dannyho]

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

 

[NormPeterson]

Start by tracking down a copy of David Vizard's softcover book "Performance with Economy". Written in 1981 in the wake of previous fuel "crises", but still mostly applicable except for its lack of EFI discussion.

Norm

 

[Warpspeed]

My very first step would be to try to find a lighter smaller car to put that engine into.

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.

 

[Dannyho]

thanks for the line on that book, I'll definately look into that. Also, I'm not getting a new car to go with my engine. and I am planning on doing something with a 5 speed trans, which is a whole new mess of problems that i'm not getting into here, and I know how to rebuild an engine and tune it, but the question was about that at all. I asked how I should plan the performance curve of the engine with the rpm range of my driving. I could add a dozen other things to what you listed, like taller tires, better lubricating oils, a completetly redesigned body to reduce drag, but these things are either trivial or unrealistic. A EFI is programmed to give fuel to the engine when it needs it, so you cannot simply change the maps to get better economy, you can try but you'll eventually lean out the mixture to an unsafe point. And I do plan on having the correct gauges to do this. I just want to know where in the powerband to design my engine to cruise at.

 

[Warpspeed]

Read Vizards book, I have it in front of me right at this moment. It is very highly recommended.

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.

 

[GregLocock]

Firstly, for good fuel economy in normal driving, merely note what rpm and MAP you use. Your aim should be to tune header lengths and intake to give maximum VE at that rpm (probably around 1800 rpm). Then use the EFI in closed loop at that MAP (probably -40 kPa) to set your mixture.

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.

 

[Dannyho]

yea I figured it might be a compromise, but thanks, that's what I was looking for. And yes I know the benefits of EFI over carbs, and that's why EFI is getting put on this engine.
thanks all

 

[SMOKEY44211]

I would recomend (in addition to the previous suggestions) raising the compression ratio to 9:1 - 11:1 depending on the fuel quality you intend to run. If you are on a low key budget see if you can score a set of heads from a sixties vintage 318 that has the quench pad in the same plane as the gasket sealing surface. Run a set of flat top pistons and shoot for zero deck clearance. (top of piston to block deck) This will promote chaos within the combustion chamber with the dual benifit of reducing the detonation threshold and reducing the burn time allowing for reduced amount of spark advance. I would also encourage you to consider reverse flow cooling the engine. This would further reduce the detonation tendancy by lowering the exhaust valve temperature. If you are interested in that I'll try and get some pictures of one that I did and post them. Let me know-------Phil

 

[Dannyho]

well I was planning to get a set of fast burn swirl port heads off of a 87-91 318. they have a smaller combustion chamber than all the other heads, and also, with porting and oversized valves, are very capable of performance. that's probably the way I'll go. I am shooting for a CR of 9:1, but I'd rather not replace internals just yet if its possible, so zero-decking the pistons probably isnt going to happen. especially since it would mean having the block sent out to be refreshed and get the top of the bore ground down, where the old rings stopped on the stroke there is almost a lip of material that isnt worn. I'm trying to do this cheap, because most of my money is going to go into the efi setup, but also not make any mistakes. However, any advice is certainly welcome

 

[Dannyho]

I am interested in this reverse flow cooling, do tell more...

 

[Rob45]

Good luck with getting good fuel economy from a 318 -Chrysler never managed to!

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.

 

[Warpspeed]

I agree with Rob, ideally you need to look at the whole package, not just the engine in isolation.

Modern engine management absolutely transforms these older engines for smoothness and economy.

 

[Dannyho]

I used to have a dodge stealth and it had a series of butterfly valves built into the plenum, that opened at high rpms, basically changing the runner length, its not quite a variable length runner, but it's something. I'm definately going to try and put a 5 speed transmission on for sure, but with mopars its not too simple and the only kit out there is about 4 grand, so i'll have to come up with something else. not that I dont realize i'm going to have to spend more than that to get this thing finished, but thats too big a chunk to do if I can manage for less. When i make my own plenum, I might try to incorporate the butterfly setup if i'm feeling adventurous (I have an extra stealth plenum lying around). but I know i'll have to and i am prepared to compomise power for economy and vise versa.
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.

 

[Tmoose]

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

 

[schmidtj86]

Combustion chamber has to be one of the most important areas (think fast burn and low ignition timing). Then, reduce friction and reciprocating mass wherever possible. Get the most from the fuel used also = drive like a chinese guy = shift at very low RPMs (I've been in China for over a year and a half, so can say that without criticism, I hope, hehe).

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).

 

[AboveRedline]

To answer the original question... for best economy select tuning bits which bring peak torque to your cruise rpm. The torque curve follows the same shape as your VE curve. The BSFC tends to be best at peak torque as well.

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.

 

[cudaracer]

You also might want to check out

http://www.moparts.com

it's a mopar only message board that should have some ideas for you...

 

[OP800]

IMO I think you need to get familuar with what modern fuel injection does before trying rettrofitting an early engine.
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.

 

[mpgmike]

Here are 2 sites that deal specifically with that "Performance & Economy" issue:

http://www.FuelEconomyTips.com

http://www.mpgResearch.com

Mike