Gas mileage, how to build an engine?
09-02-2006, 06:53 PM
#1
Team Owner
Thread Starter
Gas mileage, good one for the experts!
I see alot of hot cars driving in the slow lane even with overdrive transmissions. I know I took the Vette from Los Angeles to Willow Springs racetrack and back and it is a $100 bill in gas the way I drive. I have a second 1968 Vette that I am thinking about using as a driver to make every car event (Route 66, Coronado Vintage Race, Big Bore Classic, etc.) What gets you respectful gas mileage. I thought about dropping a 390cfm carb on the motor just for the trip. Don't know if it would run since everything I own has at least a 750 double pumper.
When I look at Hot rodding an engine vs mileage. Naturally you want the best of both worlds if possible! So what gets it done? Is it the cam grind, but all mileage cams I see sold look weak on the hp side. I know timing, good combustion (I have MSD everything, etc.). More advance, less, rpm's come in at what rpm (early/late?). What is important? What about engine temperature (cold- no atomization, warm, hot) what temp for cruising. Compression ratios?
Every article I see on mileage says to drop in an overdrive. Never anything more interesting! I was looking a HD truck motor for my Corvette tow vehicle (1965 Elco). I assume that they concentrate on some sort of respectful mileage when they assemble a truck motor and they also give you the Gross pulling ability (torque). So I figure put that HD truck motor in a lighter than planned vehicle with just enough Carb to run down the rode and you have better gas mileage. Does torque get it? Does optimizing the power band get it done? I mean extending the overlap of the rpm of the components that you use. Example intake manifold designed for 2500-6500 using a cam for 4500-8500 would be bad. Thus you would only have 4500-6500 operating range. Therefore you fall out of that, then you are burning major gas. Give me your thoughts. Sorry for writing the great American novel here!
When I look at Hot rodding an engine vs mileage. Naturally you want the best of both worlds if possible! So what gets it done? Is it the cam grind, but all mileage cams I see sold look weak on the hp side. I know timing, good combustion (I have MSD everything, etc.). More advance, less, rpm's come in at what rpm (early/late?). What is important? What about engine temperature (cold- no atomization, warm, hot) what temp for cruising. Compression ratios?
Every article I see on mileage says to drop in an overdrive. Never anything more interesting! I was looking a HD truck motor for my Corvette tow vehicle (1965 Elco). I assume that they concentrate on some sort of respectful mileage when they assemble a truck motor and they also give you the Gross pulling ability (torque). So I figure put that HD truck motor in a lighter than planned vehicle with just enough Carb to run down the rode and you have better gas mileage. Does torque get it? Does optimizing the power band get it done? I mean extending the overlap of the rpm of the components that you use. Example intake manifold designed for 2500-6500 using a cam for 4500-8500 would be bad. Thus you would only have 4500-6500 operating range. Therefore you fall out of that, then you are burning major gas. Give me your thoughts. Sorry for writing the great American novel here!
Last edited by TCracingCA; 09-02-2006 at 07:06 PM. Reason: Title wasn't good
09-02-2006, 09:00 PM
#2
Melting Slicks
I've wondered the same thing before.
What total package would give the most fuel economy in a V-8 rwd vehicle.
The short answer is to upgrade by installing a late model engine such as the LS1/LS2 along with appropriate gearing. Forum members who have done this report mid-20's for mileage. But, for the sake of discussion it would be interesting to map out the ultimate carbed V8 mileage miser engine package.
I also wonder if auto manufacturers were to disregard emission controls, how much, if any, improvement there would be in fuel efficiency?
To those who will inevitably post comments such as "If I cared about gas mileage I wouldn't drive a Vette!!!", this thread won't interest you. Move along.
Odds are, a fuel efficient engine won't necessarily mean poor performance.
What total package would give the most fuel economy in a V-8 rwd vehicle.
The short answer is to upgrade by installing a late model engine such as the LS1/LS2 along with appropriate gearing. Forum members who have done this report mid-20's for mileage. But, for the sake of discussion it would be interesting to map out the ultimate carbed V8 mileage miser engine package.
I also wonder if auto manufacturers were to disregard emission controls, how much, if any, improvement there would be in fuel efficiency?
To those who will inevitably post comments such as "If I cared about gas mileage I wouldn't drive a Vette!!!", this thread won't interest you. Move along.
Odds are, a fuel efficient engine won't necessarily mean poor performance.
09-02-2006, 09:07 PM
#3
Burning Brakes
Im by no means an expert, but heres my thoughts on the subject. I think you would want the engine to be most efficient at the rpms you cruise at, this would mean a low rpm torque motor. obviously more cubes will mean worse gas mileage. as far as compression, use as much as you can with whatever grade fuel you plan on using. And I think going with a small carb may not do much, just dont go too big, err on the small side, but dont go ridiculously small. if anyone wants to add or disagree go ahead, as I said this is just my thoughts and im not an expert
09-02-2006, 09:08 PM
#4
Melting Slicks
Member Since: Mar 2006
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Look at the RPM range you will spend the most time in and use components optimized for that range (intake, heads, cam, exhaust, carb). While a 390 cfm carb will get you better milage than a double pumper, you will surely loose performance too. A proper size carb will give you the fuel savings without throwing away performance. Also, bumping the compression up a bit and using flat top pistons will increase efficiency, but don't go too far for daily driving. I think between 9-10:1 is where you want to be. Hope this helps. God bless, Sensei
09-02-2006, 09:08 PM
#5
Safety Car
An overdrive transmission is going to be where you biggest gains are. You can do some tuning on the engine to improve its efficiency(including as LS1 swap) but it's not going to be anywhere near as significant as the overdrive.
09-02-2006, 09:55 PM
#6
Drifting
fuel economy
G'day,
I've posted this before but it needs to be discussed again in light of the cost of petrol these days. BTW, don't whinge too much about the cost - you guys have the cheapest petrol in the civilised world!!! I know that in Iran it's about ten cents a litre (about one US quart) but who'd want to live there? Same for Venezuala and a few other oil producing states, but in every CIVILISED country, petrol is MUCH more expensive.
In Australia, the price varies from AU$1.25 to AU$1.45 per litre - multiply that by 0.76 to get US$ - while in the US it is about US$0.75 per litre or AU$1.00 approx. In Europe, petrol costs more than double that amount. So consider yourselves lucky.
Now to using it sparingly. Many years ago, I used to sell diesel engines mainly for industrial purposes and all serious manufacturers quote the "brake specific fuel consumption" or BSFC and that figure is generally LOWEST at the engine's torque PEAK. Spark ignition engines are basically similar in their BSFC, i.e., they use the least amount of fuel when they are running at the engine speed where they make their highest torque figure.
So, in order to get the best fuel econonmy on a long drive, you should run the car at the r.p.m. where the engine makes the most torque and if that means driving at 50 m.p.h. then that'[s the speed to drive at. If you drive faster, you will burn more fuel that if you drive at 50.
If, however, you fit an OD transmission that allows you to travel at say 65, but at the same ENGINE SPEED at which you previously travelled at 50, then the fuel economy will be similar, but you'll get there in less time. Of course, wind resistance, or drag, will come into effect and will increase your fuel consumption a little more at the higher speed.
Unfortunately, there is no "magic formula" that will calculate the exact mileage for every vehicle with every possible engine/transmission combo, so just use common sense.
Take out everything heavy in the car that you don't need on your trip, i.e. empty the trunk; keep the windows up to reduce wind turbulence, inflate the tyres (when COLD, before the trip) a few pounds above recommended, have a wheel alignment, check that your brakes aren't dragging, get a thin girlfriend, get thin yourself.
You could consider fitting headers, throw away the belt driven fan and fit an electric one, get your current carb tuned properly or replace it with a Qjet as GM intended and get a Lars tune-up on it. Finally, fit an overdrive transmission, manual or auto, but a manual one will give you better fuel economy than an auto behind the same engine.
Follow some or all of the advice in the above two paragraphs and you will get much better fuel economy in the long run than fooling around with a 390 cfm carb that was designed for a much smaller engine than was ever fitted to a C2 or C3.
Regards from Down Under
aussiejohn
I've posted this before but it needs to be discussed again in light of the cost of petrol these days. BTW, don't whinge too much about the cost - you guys have the cheapest petrol in the civilised world!!! I know that in Iran it's about ten cents a litre (about one US quart) but who'd want to live there? Same for Venezuala and a few other oil producing states, but in every CIVILISED country, petrol is MUCH more expensive.
In Australia, the price varies from AU$1.25 to AU$1.45 per litre - multiply that by 0.76 to get US$ - while in the US it is about US$0.75 per litre or AU$1.00 approx. In Europe, petrol costs more than double that amount. So consider yourselves lucky.
Now to using it sparingly. Many years ago, I used to sell diesel engines mainly for industrial purposes and all serious manufacturers quote the "brake specific fuel consumption" or BSFC and that figure is generally LOWEST at the engine's torque PEAK. Spark ignition engines are basically similar in their BSFC, i.e., they use the least amount of fuel when they are running at the engine speed where they make their highest torque figure.
So, in order to get the best fuel econonmy on a long drive, you should run the car at the r.p.m. where the engine makes the most torque and if that means driving at 50 m.p.h. then that'[s the speed to drive at. If you drive faster, you will burn more fuel that if you drive at 50.
If, however, you fit an OD transmission that allows you to travel at say 65, but at the same ENGINE SPEED at which you previously travelled at 50, then the fuel economy will be similar, but you'll get there in less time. Of course, wind resistance, or drag, will come into effect and will increase your fuel consumption a little more at the higher speed.
Unfortunately, there is no "magic formula" that will calculate the exact mileage for every vehicle with every possible engine/transmission combo, so just use common sense.
Take out everything heavy in the car that you don't need on your trip, i.e. empty the trunk; keep the windows up to reduce wind turbulence, inflate the tyres (when COLD, before the trip) a few pounds above recommended, have a wheel alignment, check that your brakes aren't dragging, get a thin girlfriend, get thin yourself.
You could consider fitting headers, throw away the belt driven fan and fit an electric one, get your current carb tuned properly or replace it with a Qjet as GM intended and get a Lars tune-up on it. Finally, fit an overdrive transmission, manual or auto, but a manual one will give you better fuel economy than an auto behind the same engine.
Follow some or all of the advice in the above two paragraphs and you will get much better fuel economy in the long run than fooling around with a 390 cfm carb that was designed for a much smaller engine than was ever fitted to a C2 or C3.
Regards from Down Under
aussiejohn
09-02-2006, 09:57 PM
#7
Safety Car
Here is a cheap but good one:
Get another vacuum switch--like the one used to manually raise the headlights--and run a line to the dashpot that allows the butterfly valve on the secondaries to open.
This would keep the secondaries from opening--thus saving gas.
When performance is desired, just engage the vacuum switch to allow the secondaries to function normally.
I never tried it, but it ought to work. In effect you would have a small 2-barrel carb...
Get another vacuum switch--like the one used to manually raise the headlights--and run a line to the dashpot that allows the butterfly valve on the secondaries to open.
This would keep the secondaries from opening--thus saving gas.
When performance is desired, just engage the vacuum switch to allow the secondaries to function normally.
I never tried it, but it ought to work. In effect you would have a small 2-barrel carb...
09-02-2006, 10:05 PM
#8
Race Director
I have a 550HP motor and I get over 20MPG on the highway at 80MPH. The .64 overdrive tranny helps but the LM-1 wideband is the key. If you can get the A/F mixture to where it runs good and is lean enough to give you good gas mileage then you have the best of both worlds, high performance and good gas mileage. YOu also need alot of torque so a bigger motor is benificial so you can cruise at lower RPMs in 5th 
09-02-2006, 10:23 PM
#9
Burning Brakes
Gas mileage ??
Mind you mine is not a daily driver so when it comes out of the garage its for amusement.
Mind you mine is not a daily driver so when it comes out of the garage its for amusement.
09-02-2006, 10:43 PM
#10
Race Director
How about this? 3 - 381 CFM carbs. one allen screw adjustment disconnects the front and rear carb. You run on the center 2 bbl. for performance reset the allen screw. If you really want to go kick in the nitrous too. 
09-02-2006, 11:28 PM
#11
Burning Brakes
gas mileage
I have a 1974 equipped with a stock motor [350] out of a 1981 pickup. Originally equipped with a 2 barrel rochester I pitched all that and went with a weiand 8004 intake and edlebrock 650 cfm 4 barrel. Dual chambered exhausts with shorty headers and a stock th 400. It runs pretty good for a 350 stock and gets 19mpg [highway] dragging the th400 around. I wonder if I could get 22-23 mpg with a 200r4 in front of that stocker?
09-02-2006, 11:43 PM
#12
Race Director
One guy said he averaged 28mpg going to Carlisle with one of these
http://www.popularhotrodding.com/eng.../photo_01.html
http://www.popularhotrodding.com/eng.../photo_01.html
09-03-2006, 09:27 AM
#13
Le Mans Master
Optimize the thermal efficency for the RPM where you will spend most of your time at when cruising. 
That is the easy answer.
-Mark.
P.S. - A smaller engine would be better for economy at any given MPH but a larger one would be better for WOT. Maybe just keep it where it is?
Fuel injection is probably better at controlling air/fuel ratio so that is probably the way to go. Carbs can be made to work too if you can tune them right.
Late model factory performance engines offer a great compromise. Drop one in and go. The factory spent a LOT of time and resources to get the best compromise for power and performance. It is pretty difficult to do much better on your own given the same objectives.
-Mark.
That is the easy answer.
-Mark.
P.S. - A smaller engine would be better for economy at any given MPH but a larger one would be better for WOT. Maybe just keep it where it is?
Fuel injection is probably better at controlling air/fuel ratio so that is probably the way to go. Carbs can be made to work too if you can tune them right.
Late model factory performance engines offer a great compromise. Drop one in and go. The factory spent a LOT of time and resources to get the best compromise for power and performance. It is pretty difficult to do much better on your own given the same objectives.
-Mark.
09-03-2006, 09:51 AM
#14
Melting Slicks
Drive faster, to get there quicker, with engine running duration shorter, and achieve some fuel savings. 
09-03-2006, 09:58 AM
#15
Safety Car
Originally Posted by noonie
One guy said he averaged 28mpg going to Carlisle with one of these
http://www.popularhotrodding.com/eng.../photo_01.html
http://www.popularhotrodding.com/eng.../photo_01.html
09-03-2006, 10:01 AM
#16
Race Director
Small displacement reduces pumping losses. Use a cam that runs clean at less than 2000 rpm (small overlap). Gear the car to turn sub-2000 rpm at your cruising speed. Carefully tune your carb/EFI to about 15.5:1 AFR at cruise. Leaner if it will take it without missing. Run the max timing advance it will take without surging.
The above setup will have no problems getting high-20's on the highway.
My 454 w/ EFI and 5-speed gets just over 20 mpg highway.
The above setup will have no problems getting high-20's on the highway.
My 454 w/ EFI and 5-speed gets just over 20 mpg highway.
09-03-2006, 10:30 AM
#17
Tech Contributor
Back when gas hit $1.25 a gallon( I was making $4.50/hr) I set up my 69 as a daily driver. I kept the M20 and 336's and installed a new 350-250 crate engine. I use headers, a lean Q-jet,an SP2P intake only good to 4500 but low end was good. It was ok driving, not a drag car but it still performed well,around town it averaged about 15 and highway @65 was 18-19 mpg. Ran on 89 octane and never pinged or ran hot. If I had super tuned distributor like Lars does and fuel injection I would think the number would have jumped up about 3 mpg. Back in late 70's Popular Hot Rodding magazine ran a lot of articles on a 70 Monte 350 and were able to get it over 20 mpg at the time. Now 30 years later the OD trannies and computers that can be done, but you have to figure your use vs that of the upgrades. If you only drive it 1000 miles a year it's not worth it.
If you follow up on this keep us posted as gas will be high forever. Don't believe the current propaganda on dropping prices, here in CT regular is a deal at $2.99 at the same time in TX it is $2.55 and Carlisle was 2.73.
Gary
If you follow up on this keep us posted as gas will be high forever. Don't believe the current propaganda on dropping prices, here in CT regular is a deal at $2.99 at the same time in TX it is $2.55 and Carlisle was 2.73.
Gary
09-03-2006, 11:43 AM
#18
Burning Brakes
Member Since: Oct 2001
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With a ZZ4, both with EFI and previously with the 625 cfm Road Demon I get 20 MPG.
TH-400 kills me there. I'm hitting about 2700 RPM @65 MPH. Overdrive manual with ~3.55 rear should get me to 25 MPG.
TH-400 kills me there. I'm hitting about 2700 RPM @65 MPH. Overdrive manual with ~3.55 rear should get me to 25 MPG.
09-03-2006, 01:55 PM
#19
Race Director
Tuning the carb properly has the most affect on mileage in my opinion. When I got my engine back from the dyno shop I thought I had a leak in the gas tank that's how bad the mileage was.
When I hooked up my LM-1 wideband I could see the A/F was pig rich at idle and cruise. I tuned the carb and now the A/F is where it should be. I could go out and play with the carb now and it a minute or two "fix" it to where I would be getting 10MPG again no problem. If you don't have a A/F gauge you are shooting in the dark
When I hooked up my LM-1 wideband I could see the A/F was pig rich at idle and cruise. I tuned the carb and now the A/F is where it should be. I could go out and play with the carb now and it a minute or two "fix" it to where I would be getting 10MPG again no problem. If you don't have a A/F gauge you are shooting in the dark
09-03-2006, 02:07 PM
#20
Drifting
Most efficiency will be from short stroke for less parasitic drag loss and big bore for better breathing. Check out the Big Bore or Long Stroke article in the tech section of Reher-Morrison's website. He explains it pretty well. That would equate to building a 3.5 or 3.25 stroke in a 400 block or the 3.750 x 4.600 engine I'm building for mine. Can't really say that mine will be "efficient" though, since my right foot will remain planted most of the time. Good luck!