Hi all,

I'm seriously considering converting my car to Multi-point EFI and, possibly, adding either a Supercharger or Turbocharger. I was wondering, everything else being equal, which would give better fuel mileage? My gut feel says a Turbocharger, since the parasitic drag associated with turning the belt would be eliminated. What do you guys think? Is there any real-life experience here on the board? Thanks in advance.

Roger

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If you can keep your foot out of boost,both will raise the efficiency of a given engine. Maybe slightly more with a turbo.

A turbo should "cost less" in terms of mpg; but put your foot into either, and both will suck down the fuel. Every horsepower requires a relatively fixed amount of fuel to make.

The turbo compresses the inlet air by scavenging energy from the waste heat in the exhaust. I'm not sure if I'd call it "free" energy (the turbo makes some backpressure, plus nothing in life is free)--but it certainly "costs less" than the energy that would otherwise be pulled (through a belt) to run a supercharger. The belt itself isn't the source of energy loss; it takes energy compress air.

The modern turbochargers, with their variable vane technology, should theoretically be able to be set into an economy mode, where basically the turbo is disabled. Similarly, a wastegate could be somehow manually set to full bypass, again preventing boost from occuring. Or, if computer controlled, one could limit boost if rpm's are below a certain rpm.

Superchargers though are more fixed, in terms of boost. Spin the supercharger faster (in terms of engine speed), and boost goes up (assuming it's not limited by the throttle plate; can't compress what isn't admitted). I'm not aware of a method of preventing boost (or othewise bypassing the supercharger). And you wouldn't want to simply waste compressed inlet air (bleed it off, so that extra fuel wouldn't have to be added), as that would represent wasted work (fuel burned to compress air, which is then vented w/o doing work).

My vote would be turbo, due to better efficency, and the possiblity of bypassing. Actually, what would be best would be to drop down the engine size (170?) and turbo that. Under part throttle (which is the majority of driving conditions), you are feeding a small engine. Under those momentary needs for extra power, the boost makes for an effectively larger engine. This is the real gain of a turbocharger: a small engine running at its nominal efficency, which can act as a larger engine *but* only when required.

But if engine size is fixed, and the goal is mpg's, then I don't think I'd bother. Every hp takes fuel to make. Every time you flog the motor it will burn fuel that much quicker. I'm not sure there is a case of a NA versus turbo car where the turbo version got higher mpg's -- unless if the turbo motor was a physically smaller engine to start with.

A longtime, well-experienced Chrysler dealer tech I know says exactly the same thing a longtime, well-experienced Volvo dealer tech I know says about turbocharged cars: at light throttle they give poor fuel economy because of the low compression ratio, and at heavy throttle they give poor fuel economy because of the turbocharger. In general they're right, as far as factory Mopar and Volvo turbo setups are concerned. Almost all the 2.2 and 2.5 turbo engines, for example, had lower compression ratios than the normally-aspirated 2.2/2.5, and around-town mileage is better with the nonturbo engines even if you're not "getting into" the turbo.

That said, long after the last of those cars (and the Volvo 240/740 turbos) left the line, makers started experimenting with small, low-pressure turbochargers in conjunction with not-very-low compression ratios and got some impressive MPGs out of setups like that.

It's difficult to imagine a supercharger retrofit setup that gives MPG as good as could be attained without the forced induction.

Seems to me a thoughtfully-built 170 with a thoughtfully-built MPFI setup and thoughtfully-selected ancillaries (exhaust, etc.) could be made to give good, peppy performance with relatively excellent MPGs.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Two words "Smokey Yunick" http://www.rexresearch.com/yunick/yunick.htm

You are speaking of his Adiabatic engine, right? Really cool stuff, too bad it never got developed to its probable potential.

I was talking about setups one could put together with a realistic budget of time and money.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Mr. Yunick was a very intelligent man, but I take a lot of his stuff with a grain of salt. Some of it is more 'legend' than fact.

He was also a cheatin' bastage.

Back on topic, I have owned 2 identical PT Cruisers, one N/A the other turbo. They got exactly the same fuel mileage, but the turbo was a ton more fun to drive.

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Official Cookie and Mater Tormentor.

Just for comparison purposes (and I know it can't really compare with a slant six in an A body), my other car is a 2001 Beetle 1.8 turbo with a 5 speed. Specs are: 1.8 liters (108.7 cu. in.), bore 81 mm, stroke 86.4 mm, compression ratio 9.5:1, hp 150, torque 155, curb weight approx 2900-3000 lb. It has five valves per cylinder (3 intake, 2 exhaust), so it breathes very well. It has good torque, and actually performs better at low rpms. It's a blast to drive, I drive it fairly hard and generally get 24-25 mpg in combined city/hwy driving. The highest I've gotten on the open road is around 28-29. It would probably do better with a six-speed trans. I can only dream of mileage like this in my Barracuda.

So, Dan, what type of build would you suggest for a high-MPG motor, starting with a 170 as the base? BTW, a 170's bore and stroke are almost opposite the VW 1.8: bore 86.4 mm, stroke 79.4 mm. What are your thoughts on raising the compression, and installing a small, low-pressure turbo? Do you happen to know of any cars that have such a turbo?

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What are bore/stroke for the VW? 28 MPG should be doable on your Barracuda, but not easy.

I have a relatively large turbo on my '68 Dart and it gets the same mileage as before the turbo (21-23 MPG hwy). The turbo does not spool under normal driving. I just installed a large airdam, which I anticipate will give substantial MPG improvement. The car went from mid 16s in the 1/4 to high 14s, and it is very lazy off the line. I believe it will run well into the 13s without major further mods. My other main street car (64 Dart) gets 24-26 hwy and runs the 1/4 in the mid 14s. EFI, 5spd, NA...

I am contemplating a 170 with 198 crank build, probably with a turbo, that should give some nice mileage and power.

Your Cuda will have much better aero than my car too.

Lou

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Home of Slant6-powered fun machines since 1988

Erik (mpgfanatic) had a 64 Valiant 4 dr (car is now mine, engine is his.) with a 170 3speed on the columm, that he reported upper 20 mpg on the highway. The engine was bored and honed .060 over with a block plate, deck cut to .005 hight. Don't remember the head cc's, but was a true 9.5 compression ratio. The mild MP cam, and balanced. Stock intake and exhaust manifolds, with a 1920 one barrel carb and a low restriction exhaust.
If he doesn't take the engine back (if he does, I will build a similar one), I plan on installing a A833od, and doing a little head work, and see if I can get over 30mpg. Maybe electronic timing control, and/or FI, but that might not happen.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Roger, my carbureted 69 with a stock 225 and Carter BBD 2 bbl, with an OD manual tranny and all electricals upgraded to new quality or better got 22-25 always, and an occasional 28-29 MPG on the highway. No lie. And that was before working on the aero aspects of the car, which reportedly can yield 5-10% with modest changes. YOur car is already tons better than mine could ever be.

I have never done that well with the EFI turbo. The best highway mileage so far as been 23 mpg and that is with the front air dam. Much tuning has taken place since, so maybe it would be better now. I have never had this car with EFI and no turbo, so cannot comment on that. I'm just saying that a good NA slant and an OD tranny can be amazingly efficient.

Honestly, if I cannot get the bugs worked out of this turbo/EFI setup, I will be very tempted to remove the turbo and tune it for MPG with the EFI. I think it would be amazing in that regard.

So far I have not gotten the stray RPM signal feed back out of the system under boost, and it just runs like crap.I am working on it, but it is a slow process and you have to remember that the OEM stuff has millions spent on its development. With home developed stuff it is sometimes two steps forward and one step back. Or sometimes one step forward and two back.

Lou's tubo set up seems to be running great, but he is controlling only fuel, and has apparently built his engine right, since he is not battling preignition at modest boost and fairly advanced timing levels. Remember, at this point his timing is just a fixed number set at initial, where it stays. So there is no complexity there at all.

The point to all this is that if you want speed with aftermarket, simple is better, or you will spend a lot of time tuning and working out bugs. No one builds a bolt on set up for your car. If you want economy with after market, go with proven, applied technology and don't try to stretch the development envelope yourself on your limited budget. I would keep your stock 225, and add a proven EFI with good support.

Here is the sobering fact: while right now I know maybe 1000 times more about EFI than the average hot rod car hobbyist, I only know about 10% of what I need to know to eventually understand everything needed to just get where I am trying to go. Not a week goes by that I do not get a fresh insight.

And, I do not have a dyno, which is the ultimate tuning tool, IF you know what you are doing. And in my experience many guys with commercial dynos do not know what they are doing. If I am painting a bleak picture, it is so you will realize that it is easy to dream, and a long, challenge filled path. Be prepared to spend more time working on you car than driving it for awhile. And, you burn a ton of gas tuning it up.

Sam

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Nice writeup, Sam.

Small correction: I was running fixed mech advance at about 17 deg, with about 18 deg of vac advance under part throttle cruise. 2 months ago, I switched to the MSD 6AL-2 box, which I have programmed by laptop to have more aggressive advance under light throttle and even into light boost. I have not checked mileage yet (new big airdam too), but hope to get some numbers on a trip to NC tomorrow...

Lou

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Home of Slant6-powered fun machines since 1988

Thanks for the update on ignition Lou. I missed that. I think keeping the timing control out of the efi fuel control is actually a smart way to go. If I had to do it over again, I might very well follow your lead there. I would like to hear more about your set up and how you tuned it. Does it use a MAP sensor?

Sam

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Thanks very much, Sam. I appreciate your insight and advice. Since I am primarily interested in more efficiency and better MPG, i may just go to EFI and leave it at that. As I said, i don't want to make a career of trying to tune it. Still lots more reading to do, but I'll keep you guys posted on my progress. Good luck with your car.

Roger

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I believe Sam Powell's comments open a window to the true meaning of "hot-rodder". The thrill of victory comes only after the agony of defeat, not of man but of machine. It's a lonely, lonely place sometimes.

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'63 Valiant V200 Convertible
225 Aluminum Block