Jump to content

V8 Fuel Consumption


karman1970
 Share

Recommended Posts

This has been rolling around in the back of my head all week due to some other discussions on the forum.  I hear the Mustang guys say that they have the best shot at making a full stint by short shifting and keeping the RPM below 5,000.  I assume same for the GM crowd.

 

The question I have is: if you have two engines that make the same power, but one is more of a car engine and the other a truck engine, would they both still theoretically burn about the same amount of fuel?  My thinking being, if you only have a shot at making 2 hours by keeping the revs below 5K, why not run an engine that doesn't want to go any faster than that anyway?  Or does it have less to do with the RPM number itself and more that fact that the engine just never gets a chance to get into the peak power range by short-shifting?

 

One case: 350 Chevy.  A '73 Corvette makes 250hp @ 5200 and 285ft-lbs @ 4000.  A '91 Corvette makes 250hp @ 4000rpm and 340ft-lbs @ 3200rpm.  Keep the engines 100% stock, obviously.  If you are limited on fuel and all other things on your car of choice are equal, does one of these get better fuel economy than the other (running as hard as you can), and if it does can it still keep pace with the other?

 

I know there are hundreds of variables and it's fairly hypothetical until someone actually builds two cars to try it out, but just wondering what the engine whizzes on here think.

Link to comment
Share on other sites

The power (HP) generated by an engine at any given rpm is directly proportional to the air it is able to flow at that rpm and also corresponds directly to the amount of fuel it will burn at that rpm. (Assuming no overly lean or rich areas in the fuel map) Different engines have slightly different efficiencies, meaning that they can generate a higher specific horsepower per unit of air/fuel. Typical factors that affect the specific horsepower are compression ratio, combustion chamber geometry, ignition timing and quality, fuel vaporization quality/droplet size and the length of the power stroke (rarely a factor, see Miller cycle). Turbocharging and supercharging would appear to increase effective compression ratio, which would normally increase specific horsepower, but in practice they usually result in increased blow-by, richer than stoichiometric afrs, and if there is much of an overlap between the intake and exhaust valve (there usually is) more fuel will escape through the exhaust. This all leads to turbo and supercharged cars usually having a much greater power density, but a lower specific horsepower in terms of fuel efficiency.

 

To directly address your questions about the 350 SBCs, if one makes higher power at lower rpm, it will (probably) be flowing more air at that rpm and using more fuel at that rpm. Thus you would need to feather the throttle or reduce RPMs even further in the example '91 SBC than the '73. This assumes that the other factors  affecting efficiency as stated above are not solely responsible  for the difference in power at 4000 rpm.

Edited by jenoin
Clarification
Link to comment
Share on other sites

You might want to research "bsfc". That is brake specific fuel consumption, or how much fuel it takes to make each hp. 

 

Bsfc is heavily influenced by combustion chamber design, tuning, cylinder head design, intake\exhaust design and cam timing.

 

In general slow engine speeds help bsfc, but bsfc at high power output does not vary as much as you think(for na engines). Old head designs that require 12:1 afr wide open will be worse than modern 13:1 afr, but the engine weight i bet is a larger factor.

 

Edited by Black Magic
Link to comment
Share on other sites

1 hour ago, karman1970 said:

One case: 350 Chevy.  A '73 Corvette makes 250hp @ 5200 and 285ft-lbs @ 4000.  A '91 Corvette makes 250hp @ 4000rpm and 340ft-lbs @ 3200rpm.  Keep the engines 100% stock, obviously.  If you are limited on fuel and all other things on your car of choice are equal, does one of these get better fuel economy than the other (running as hard as you can), and if it does can it still keep pace with the other?

 

If everything else were exactly the same, the '91 would be more efficient because there's less friction loss at 4000 rpm than at 5200 rpm. 

 

However, there are about two decades of some rather significant development. The '91 will have EFI and that by itself is reason enough to pick it. It very likely also has a higher compression ratio and more efficient breathing and chamber design.

 

The '91 would be the engine to choose.

  • Like 1
Link to comment
Share on other sites

Another thing to add about EFI, while usually being more efficient in general, you also get DFCO or Deceleration fuel cut off, which a carb cannot do (or at least no factory carb designs can!) While time spent braking and the small amount of fuel sucked in through a closed carb on decell into a corner is small, it all adds up. When I'm at the point of tuning my next car build, it's one of the things I'm excited about having OBD2 for and a GM vehicle with the ability to use HP Tuners.

Link to comment
Share on other sites

I am familiar with BSFC, though my IC engines class was more than a decade ago, so saying I'm rusty on the subject and concepts surrounding that would be an understatement!

 

I guess by "all other things being equal" I was trying to imply things like compression, fuel injection, etc. are all the same.  Basically, identical engines except for intake and cam design being different and thus moving the peaks around and changing the shape of the curves.

 

If one's car is fuel limited, would there be a benefit to something like a "truck" engine with a big fat torque curve down lower in the range and could it still be able to turn similar times.  For example, 3rd gen F-bodies.  Why run a short runner intake and race-y cam when the Tuned Port (which I've seen some folks say are basically just truck engines) already makes a lot of torque and good power but all at lower rpm?  Keep the factory set-up or maybe swap in an truck/RV cam or something and then play with the gearing, you know?

 

The two Corvette engines just came to mind because sometimes I think about building one when I forget all the other projects I have and money I don't, so I already knew their peak power numbers were close and displacement and architecture are the same.  I don't have a fuel-limited V8 nor do I have plans to build one, the idea popped in my head the other day and curiosity has just been gnawing at me.

Link to comment
Share on other sites

Unfortunately fuel economy and racing are two mutually exclusive events.  You might as well lock an astronaut and a geologist in a room and expect them to come up with solutions for peace in the middle east.

 

Try looking for headers to see which ones deliver the most fuel efficiency - everybody talks about hp increase.

Search which heads will give decent hp and max fuel economy - everybody talks about hp increase.

Intake, same thing.  Same for cams.

At any rate none of the above will burn less gallons per hour, they will burn less 0.XX gallons per hour.

 

What is Fireball the fuel heavy driver doing different than the other three?  What am I doing when I am in between Fireball and the other two for fuel consumption?  What is JJ doing when he is between Fireball and the other two?  Without full telemetry on the injector pulse width, throttle, brake data I don't know.  We have consistent driver results based on the style of track, I have that figured out, it took some thinking on that one.

 

I keep a binder of tracks, the RPM run, and lap time versus fuel consumption curves.  The difficulty comes in when we go to a track we have never been to and zero data.  There is no reference for lap time and fuel consumption, nobody tracks that.  Once the first stint is over I only have one data point.  If we don't have any car problems and there are minimal yellows I will have a decent set of data (4 data points) after 8 hours of the upcoming 24 hour race at Utah.

Link to comment
Share on other sites

49 minutes ago, karman1970 said:

I am familiar with BSFC, though my IC engines class was more than a decade ago, so saying I'm rusty on the subject and concepts surrounding that would be an understatement!

 

I guess by "all other things being equal" I was trying to imply things like compression, fuel injection, etc. are all the same.  Basically, identical engines except for intake and cam design being different and thus moving the peaks around and changing the shape of the curves.

 

If one's car is fuel limited, would there be a benefit to something like a "truck" engine with a big fat torque curve down lower in the range and could it still be able to turn similar times.  For example, 3rd gen F-bodies.  Why run a short runner intake and race-y cam when the Tuned Port (which I've seen some folks say are basically just truck engines) already makes a lot of torque and good power but all at lower rpm?  Keep the factory set-up or maybe swap in an truck/RV cam or something and then play with the gearing, you know?

 

The two Corvette engines just came to mind because sometimes I think about building one when I forget all the other projects I have and money I don't, so I already knew their peak power numbers were close and displacement and architecture are the same.  I don't have a fuel-limited V8 nor do I have plans to build one, the idea popped in my head the other day and curiosity has just been gnawing at me.

I like this chart because it shows quite a few trends:

image.png.3a6fe5c89226eaba022c542d94ece44e.png

 

Pay special attention to the scale on the right of the chart. There are lines that go through the chart that indicate a specific hp level, and you can see the effect that rpm has on, say, the 80 hp line. At 4000 rpm, the engine is able to make 80 hp and at a BSFC of about 0.430, it's only 5% off the peak efficiency. The same hp at 5200 rpm requires less throttle so no longer requires the max power AFR but the effective compression ratio is dropping and the internal friction is climbing, both contributing to lower efficiency. Moving over to 6500 rpm, those effects lower the efficiency even more; the BSFC of about 0.540 is 25% worse than at 4000 rpm for the same performance.

 

The max efficiency island (red) and the next best (orange) cover a fair bit of the total area. Ideally, the engine would spend most of the operating time in those two. Note that the red area isn't at peak torque but instead is just below that. If you can control your right foot and use about 80-85% of the peak torque through that range you'll get the most out of your engine. However, most people needing to save fuel settle for the full throttle part of the chart (orange) and shift before it gets very far into the yellow area.

 

 

Edited by mender
Link to comment
Share on other sites

Reading your examples i bet the 91 motor has better bsfc in its powerband, but probably burns more fuel due to higher power (flatter curve) across the powerband. 

 

I bet pissing in the wind difference all things considered. People saving fuel shift early because it guarantees a lower power output, primary factor in fuel consumption.

Edited by Black Magic
Link to comment
Share on other sites

1 hour ago, Black Magic said:

Reading your examples i bet the 91 motor has better bsfc in its powerband, but probably burns more fuel due to higher power (flatter curve) across the powerband. 

 

I bet pissing in the wind difference all things considered. People saving fuel shift early because it guarantees a lower power output, primary factor in fuel consumption.

If you do the math and factor out the rpm difference, the early engine has more area under the curve.

 

'73: 250 hp @ 5200 rpm, 285 ft lbs @ 4000 rpm = 217 hp.

'91: 250 hp @ 4000 rpm, 340 ft lbs @ 3200 rpm = 207 hp. (4000/5200) x 4000 = 3076 rpm, so for the same proportional drop in rpm the '91 engine is making 199 hp vs 217 hp for the '73 or 9% less hp.

 

10% better efficiency for the same power usage by driving smart can be the difference between making a full stint at full pace and having to shift early and lose 1-2 seconds/lap.   

Edited by mender
Link to comment
Share on other sites

13 hours ago, mender said:

If everything else were exactly the same, the '91 would be more efficient because there's less friction loss at 4000 rpm than at 5200 rpm. 

 

However, there are about two decades of some rather significant development. The '91 will have EFI and that by itself is reason enough to pick it. It very likely also has a higher compression ratio and more efficient breathing and chamber design.

 

The '91 would be the engine to choose.

plus the 91 is going to be a roller motor... FWIW we routinely run our 305 to 6k on the long straights. Our car has a 15.6 gallon tank we can get 1.5 hours a tank...give or take depending on traffic and yellows. if you kept it to 4k you'd get passed by every thing on the track.  We have the stock Quadrojet (pretty big carb) Since we h ave points to burn now with the new system we might go with an Air-Gap intake and Race Hollley 2bbl (500cfm) it SHOULD have better throttle response and could save some gas. Of course we could also go with a cell, but not too many 17.6 gallon cells out there. 

Link to comment
Share on other sites

Still kicking after 2 years...

 

https://www.austinkayak.com/products/1134/Closed-Cell-Foam-12in-x-24in-x-4in.html

 

And yes regular ping pong balls will deteriorate, I tested them for my home made discriminator valve. If you only need to take up a little space you can also make PVC "pills", but I wouldn't want to put a bunch of those in the fuel cell due to them not being compressible in a severe impact. 

Edited by pintodave
Link to comment
Share on other sites

2 hours ago, pintodave said:

Still kicking after 2 years...

 

https://www.austinkayak.com/products/1134/Closed-Cell-Foam-12in-x-24in-x-4in.html

 

And yes regular ping pong balls will deteriorate, I tested them for my home made discriminator valve. If you only need to take up a little space you can also make PVC "pills", but I wouldn't want to put a bunch of those in the fuel cell due to them not being compressible in a severe impact. 

That's what I was thinking of. I knew you had been doing some testing on homemade displacement devices.

Link to comment
Share on other sites

The TPI can make lots of torque and stay under  5200 RPM while making the needed power.   Quite a bit better fuel use VS the old shcool  carb and 6000RPM. You need a taller gear and thus also save parts by slowing down the drive shaft /trans speed. Pretty much everything last longer going slower. 

With the allowed Microsquirt, you can shut off the fuel on over run, you can trim the AFR to way lean on part thrott le, etc.   Closed loop is an option that compensates for density/ (temps/alt.) 

 The stock ignition has a knock sensor  sensor system that will also help fuel burn and reduce the problems with detonation. 

Link to comment
Share on other sites

I have a 80 camaro and we have been debating if  a 305 would get better gas milage than a 350 because it might need more rpm to get the same results ? And about the fuel displacement blocks i had a 22 gallon cell in a late model that i ran for 7 years stuffed with gallon  antifreeze containers to get to 16 gallons and never had any issues with it.

  • Like 1
Link to comment
Share on other sites

I would say to oversimply menders pretty chart....

 

You look at the rod stroke ratio...  what you are trying to compare is essentially the rotating mass of the two engines, but because of the formula for rotating mass is logrythmic...  even a light rotating mass or slightly lower piston travel speed wont change the HP wasted at a given RPM that much inside the same engine family...  (assumption on my part)...

 

For me its really mostly about AFR and how that works (air and fuel in a given volume per revolution), again as mender points out, you dont always need the Full fuel if you arent asking for full power, but at WOT, you are filling the engine with 1/13 its displacement in fuel per RPM...  so this is where the sense of WOT at a lower RPM is generally more fuel efficient than part throttle at higher RPM...  (most people tune their car to around 14:1 or a bit higher at part throttle, and 12 or 13:1 on WOT for NA motors...  so there isnt that much to be saved for part throttle)   the more scientific way to get these numbers is looking at your fuel maps, because there are lots of factors...  but in general short shifting is a good way to save fuel.

 

As for the truck vs car engine, mostly they have different air intakes (and sometimes cams), designed to help torque, but that choke off the engine at higher RPM...  so that issue is more subtle...  but due to the AFR statement above, more air and more fuel at a lower RPM is generally favorable to more fuel and more air at a higher one (for fuel economy).

 

The only way to really change this is to get lean on your AFR's at part throttle, and then you have to get super lean...  like 20:1 because you need to control the heat down to the point that there isnt enough extra heat to melt anything...  its expensive and time consuming to try to tune an engine this way, and generally only makes sense for a narrow speed / load band (like highway cursing)...  as we add aero loads to our race cars, we keep using more and more relative throttle, and we get further away from being able to use these really lean AFR's

 

One thing to consider might be a ratio change, can you change your final drive or add a sixth gear to give you that room for better fuel economy without needing to change the engine at all... maybe even tire diameter; although that has ride height implications which are generally unfavorable.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...