Interesting Hybrid - Nonhybrid comparison
 

I was on cars.com just now comparing the civic hybrid and insight gear ratios to the regular civic and corrolla. Given the stock tire sizes, here's what I found:

At 70 mph in od/5th the civic hybrid is running at 1,958 rpm; an '06 insight is at 2,396 rpm; the regular civic is at 2,947 rpm; and the corolla is at 2,767 rpm.

If you multiply the rpm by the length (inches) of the piston stroke, you get number that doesn't correspond to anything specific like actual piston speed, but it allows for a direct comparison among the four engines. The higher the number, the faster the pistons are moving in the cylinder walls.

civic hybrid: 6,167
'06 insight: 7,692
regular civic: 10,138
corolla: 9,961

So we can see that at 70 mph, the pistons in the regular civic are moving 164% faster than the pistons in the civic hybrid.

Now, according to my admittedly limited understanding of engines, as piston speed increases beyond the point at which it makes peak torque, the burn is actually chasing the piston down the cylinder. This requires greater and greater inputs of engery (fuel) inorder to make more power, which is why engines start consuming much more fuel as they rev higher and higher.

So the rpm x stroke numbers I just posted above should be a major factor in the improved fuel economy of the hybrids, which just goes to show that if the manufacturers would give us 6 speed trannies with really tall overdrives we could see huge FE gains in regular cars. But, no, evidently they want us to think we have to pay an $8,000 premium for fuel efficiency :mad:

Now, perhaps a straight comparison of the rpm x stroke numbers isn't a fair comparison, because the various engines might have very different cam timing and duration, as well as ignition timing. If someone knows more about this please chime in. However, I'm still struck by the huge differences in engine/piston speed between the hybrids and non-hybrids.

Naw, I think that your comparison is pretty fair. I think they just don't want to have a car the market perceives as sluggish. With the Hybrid they can have the engine turn over at the slower rpm and still give users the feel of power, that they feel they need.

Do you have the displacement numbers for each engine? You've basically made an offset comparison of linear distance traveled ;)

I think it would be interesting to compare engine speed to surface area displacement.

I've normalized your figures above to show how close everything is and did a percent gain on all compared to the civic hybrid.

How did you calulate piston speed? It is normally referred to in the total distance a piston travels in feet per minute. Your #'s seem too high to be in ft/min and too low to be in inches/min.

The aluminum limits the feet/min a racing engine can have its pistons survive and anything beyond about 5200 ft/min is considered unhealthy. I think you'll find that at 2900 rpm the standard civic is still well below this amount, but at redline is probably approaching it. Some racing engines go well beyond 5200 ft/min, but they also are replacing the pistons every couple hundred miles.

The ignition timing advance will make up for the speed the piston speed and is generally trying to keep the maximum cylinder pressure at the same crank rotation. For a far better explanation, see the article from Team-Integra.net below

https://www.team-integra.net/sections...?ArticleID=235

Like I said, I just multiplied rpm by stroke length. This results in the total distance (inches) a piston moves down the cylinder in one minute. Multiply it by two and you have the total distance a piston moves both up and down the cylinder in one minute. It is not a measure of maximum piston velocity (which, if my geometry is correct, would occur at some point partway down/up the cylinder when the connecting rod is pushing/moving tangentially to the rotation of the crank. I guess what my numbers actually correspond to is one half the average velocity of the pistons in inches per minute. It's half the average because the piston travels two stroke lengths -- down and then back up in one crank rotation -- and I simply multiplied one stroke length by rpm. Basically it's just a way to compare piston velocities relative to one another, since rpm alone doesn't account for the respective distances pistons travel in different engines, and so rpm doesn't give as precise an indication of piston speed, which is the more meaningful number.

To get total distance a piston travels in one minute, divide my numbers by six (divide by 12 to go from inches to feet, then multiply be two to account for both the up and down strokes).

I'm looking at the corolla and the difference between 4th and 5th gears. 4th is 0.89 and 5th is 0.73. If we added a 6th gear with the same percent change, it would be 0.60. With this extra gear, the 70 mph rpms would drop from 2,767 to 2,271. rpm x stroke number would be 8,177 (or to get the average feet per minute number that mrmad mentioned 1,363 ft/min).

Why can't we get this in regular cars!!!???:mad:

By the way, just out of curiosity, does anyone know the ideal average piston velocity in feet per minute, since that's apparently the standard unit used, at which a gasoline engine makes maximum torque? That would help determine how far off the manufacturers are when they gear cars for typical highway cruising speeds

I'm really surprised the regular civic is nearly at 3000rpm at 70mph.

The Civic hybrid has a 1.3 liter engine.
The Insight has a 1.0
The Corolla has at least a 1.8.
The regular Civic I believe has ballooned to 1.8 as well

I may have mistated this question, the rpm at which max torque is made does not necessarily correspond to maximum fuel efficiency. I found this page that states that max fuel efficiency occurs with a piston speed of 1,000 - 1,200 ft/min. https://books.google.com/books?id=mX1...UCPU#PPA378,M1 The derivation of this number on subsequent pages is over my head, but assuming it's accurate and restating my numbers, let's look at how the four vehicles compare:

Civic hybrid: 1,028 ft/min
'06 insight: 1,282 ft/min
regular civic: 1,690 ft/min
corolla: 1,660 ft/min

So again, the hybrid engines are designed to opperate at the optimal rpm for fuel efficiency, while all other cars are designed with entirely different priorities.

By the way, I chose 70 mph as a good compromise between the more optimal highway speed for FE of 55-60 mph and the typical speeds seen on highways these days of 75-80 mph. It also happens to be the speed I'd like to be able to drive and not take a huge FE hit.

Interestingly, my Duramax diesel truck has a piston speed of 1,098 ft/min at 70 mph, so evidently the diesel trucks get more FE consideration than the so called economy cars :confused:

Different engine cycle dynamics (makes direct comparison like this difficult) ;) The ideal diesel power stroke is isentropic (constant entropy) while the Otto power stroke is adiabatic (constant heat).

Efficiency of an Otto engine is typically demonstrated in terms of compression ratio whereas the diesel cycle is in terms of the cut off ratio... It is very likely that it's not so much an FE consideration, and more of a general diesel design consideration...

Yeah, fair enough, and I know that diesel fuel burns slower than gasoline, so diesel pistons should be slower. I guess my point is more that the truck seems to chug along nicely at 1,700 rpm on the highway, whereas a civic or corolla doing close to 3,000 rpm is very definitely well beyond it's FE "sweet spot". Of course, I base this assertion on the 1,000-1,200 ft/min figure I quoted and the fact that the hybrids are geared to run with these piston speeds. Maybe I'm wrong and for some reason like compression ratio or cam design almost 3,000 rpm is a fairly ideal engine speed for the civic and corolla. If so, I would love to hear why, so that I could feel better about possibly buying a new car and not think that I'm being manipulated by the manufacturers.

The regular Civic engine is designed more for performance then FE. They are setting the overal gearing and separate gear ratios so the engine can stay in the powerband between shifts. They could have designed it for better FE, which probably would have lowered the rom at 70mph, but I'm sure some marketing survey somewhere showed them they would not sell as many cars if they did.

2 inputs
 

2 inputs-

Interesting observations...

Not sure if anyone mentioned or factored the CVT in the Civic Hybrid, or if it was the older 5-speed used to compare (it probably doesn't change the final ratio at cruise, but holds another advantage up the speed range).

Finally, I think the only way we can get the cars we want, is to design them ourselves. To the mass public, Automakers don't have an incentive to make small, inexpensive, FE-oriented vehicles -- the profit margin is paper-thin compared to the truck-based SUVs.

It'll take a paradigm shift of customer education and demand through outlets such as these. First people have to care -- that generally translates to the wallet. Sigh, I'm preaching to the choir here...and in the wrong thread :o

RH77

EDIT: Based on the ideal piston travel speed, would there be interest in a custom calculator for ideal cruise (like a set of input questions in a web-based form or something?

Did the calcs
 

OK -- so for 1000-1200 ft/min piston speed, with a 3.5 inch stroke in my auto,

Then (accounting for some slight tire wear) ...I'd have to drive in 3rd gear at 60, since that translates to:

1000 ft/min @ 3428 RPM = 55 mph in 3rd, 85 mph in 4th (wind resistance starts to hit).

only went up to 4000 RPM for engine longevity...

1283 ft/min @ 4000 RPM = 64 in 3rd, 98 mph in 4th

Should I be driving around in 3rd? What else is at play here?

RH77

I think you are multiplying the rpm by the stroke (inches) and then dividing by 12 to get ft/min. But the average piston speed must take into account two stroke lengths per revolution. Thus your piston speed is rpm x stroke x 2. Then divide by 12 to convert from inches per minute to feet per minute. Thus, to achieve 1,000 ft/min, your rpms should be half of what you figured -- 1,714. So according to your calcs, that means 27.5 mph in 3rd and 42.5 mph in 4th.

Okay, I'd be game for doing this. Give me a good late model car to start with like a civic, corolla, echo/yaris, or whatever -- just something with air bags, a decent drag coefficient, clean emissions, etc. And now help me figure out how to modify the 5 speed transaxle (tranny and ring and pinion) to get the gear ratios I want. Hopefully this could be done without wacking out the car's computer (I don't know much about newer car electronics). Hopefully there'd be an easy way to recalibrate the speedometer. I'm not a machinist and I'm sure getting custom machined gears would be pretty expensive, but if only say the R & P gear needed to be remade, then perhaps it wouldn't be too unrealistic.

Now I throw in a few simple aero mods -- grill block, rear wheel skirts, etc. and maybe I've got my 50 mpg at 70 mph.

Any ideas?

Something tells me there isn't 1 ideal piston speed.... I'll bet there's a relation with the torque curve, piston dimensions, intake tract length etc....

Ideal kind of car...
 

OK this makes sense: had to calc the complete travel of the piston...

Even more sense, around 45 is the perfect speed for FE in this car. My best segments are on a road with slight hills while using P&G/DWL between 40 and 50 mph.

OK, designing cars. Take something like the Pruis and ditch the drivetrain/batts. Now you have 4-doors + the use of a hatch + a slick Cd. Throw-in a 1.3L like the Yaris' x-US version: 2NZ-FE, VVT-i 10.5:1 Compression. 88 hp @ 6000, 89 ft-lbs @ 4400. There's also a 1.0L, but with a larger platform, it may not be efficient on the highway.

Gearing -- dunno. 6-speeds. You'll have some torque with the VVT and compression. To sell it to the U.S., it would have to be manual only in this version, then have a bigger engine and automatic for everyone else (or sport version with the big engine and 6-speed). One could argue all day about the auto being CVT or traditional, I suppose...

My ideal new car is a CNG 4-door Civic, using the older 1.6L variety with the unavailable standard shift -- maybe a 6-speed -- only one I can think of is from the Si, but the top gear might not be efficient, so a swap of sorts is appropriate, or throw in a 5-speed. Same for the Toyota example.

A CNG-Hybrid needs more research, but sounds pretty cool.

RH77

According to specs I got from metrompg's site and the team swift site, a 1.0L metro engine coupled to the 1.3L suzuki tranny (metrompg did this swap, and it's documented on his site) with 13" wheels would yield piston speeds of 1,192, 1,291, and 1,391 ft/min at 60, 65, and 70 mph respectively. This is the only non-hybrid gearing that I've so far found that could put you in the ideal-piston-speed ballpark at highway cruising speeds (still need to check out the CRX HF gearing).

By the way, the stock XFI trannies give piston speeds of 1,282, 1,389, and 1,496 while the regular non-XFI trannies from '89-'94 give piston speeds of 1,389, 1,505, and 1,621. The '95 and up trannies have a shorter R & P gear, so the piston speeds are even higher.

I think you're making a bit too much out of it. Just think in terms of load and yer set. This holds true for small and large, diesel and gas, AFAIK. The only notable difference comes from friction reduction in hybrid engines, which allows 'em to rev higher w/o the usual drop in efficiency. Every other BSFC map is pretty consistent in terms of efficiency. Something less than peak load at ~2-3k rpm.

Yah, but it's still fun to design your own car, eh?

Can you further explain what you're getting at, because I have no idea what you're talking about and I'd really like to understand this better. I know from my own experience with my car (and it's a diesel, so things may be a little different in a gasser) that there is this "sweet spot" where I'm barely touching the accelerater and the car just chugs along effortlessly. It happens to be about 40 mph in 5th gear which I've calculated to be approx 2,000 rpm or 1,200 ft/min average piston velocity. And when I've been able to maintain this speed for any length of time (unusual here in Colorado where I'm either in town or on the open highway), my fuel economy skyrockets. increasing speed beyond this point requires a lot more right foot. So it makes sense to me that there is an ideal piston speed for maximum engine efficiency, but like I said I want to understand this better, so can you elaborate?

There are more parameters that affect FE then just the piston speed. I think the best rpm for FE has as much or more to do with the cam, intake manifold timing, and port sizes then the piston speed. For instance, compare my Integra GSR and my CRX HF. The Integra has larger port sizes, longer duration cams, and longer intake runner lengths. Because of this, at lower rpm, the Integra's intake velocity is slower and the cam duration is too long such that the CRX HF is much more efficient in filling its cylinders at 2000-2500 rpm then the Integra is. At 1000-1200 ft/min piston speed (somewhere between 2000 and 2500 rpm), the Integra is anemic in power and is barely waking up. Because the Integra engine is designed for HP and not FE, it probably would give better gas mileage to operate it above 3000 rpm then lug it around at 2000 rpm.

I think you are probably right, the best FE is around the 1000-1200 piston speed, if the rest of the engine is designed for this.

I think you guys are simplifying things way to much, and looking at the wrong variables.

First of all, the sweat spot (if there is one for piston speed) would be created because of a number of factors lining up. The problem is that way to many of those factors (such as atmospheric pressure, ambient air temperature, the stroke/bore ratio, octane, spark timing, etc) are variable. The only one that is really constant is the burn-rate of gasoline. I'm sure there is some sweat spot where the speed that gasoline burns matches up with the speed of the piston travel, but I think it's effect probably less than all of the other factors going on here.

The biggest factor is just internal friction. Every time crank turns one rotation there is a given amount of energy lost to friction. So if two cars are going the same speed, and one is turning less RPM, it is loosing less of the energy it is creating to internal friction, and is therefore more efficient. That is true no matter what the RPM in question is, or what the piston speed is.

If you did your test at say 45 miles per hour the hybrids would now have a piston speed lower than your "ideal" while the regular Civic or Corolla would be right in the sweet spot. I bet the hybrids would still be more efficient.

The idea that less RPMs is better is always true unless the airflow and other factors in the engine have been tuned in such a way that the engines inefficiency at the lower RPM can overcome the natural efficiency benefit of taller gears. There is a point where that equation where that doesn't work anymore because of that constant gas burn-rate property, but I think it is significantly lower than a piston speed of 1,200 ft/min.

If the engine were being run with no car/gearbox attached to it then maybe piston speed would play a significant role. However once you add a bunch of weight, wind resistance and gear reduction into the equation overall ratio makes a larger impact I think.

northboundtrain, the reason you hit that sweat spot has to do with two curves interacting. Those curves are the engine efficiency, and the aerodynamic efficiency. As speed goes up, aerodynamic efficiency goes down. On the other hand going faster gets you into higher gears (more efficient as per my above statements), and gets you closer to your engines peak efficiency. Where those two curves meet is that "sweet spot" that you describe. The question is, if you had a 6th gear and were in that, turning say 1600 RPM at 40 mph, would you be using less gas? At that point you would be at 960 ft/min piston speed, and out of the piston speed "sweet spot" but I bet you would find you would lose even less gas. Also, the sweet spot might move to a slightly different speed.

The real lesson here is that taller gear ratios = better gas mileage. They also deliver worse acceleration characteristics, which is why you guys are right, they need more gears! I don't think that has anything to do with a sweet spot of piston speed though.

You make a strong case for the wide ratio CVT.

True, but then...
 

Very true, but then the argument becomes, "Does the CVT use more energy to operate than a traditional automagic"...

One of the car mags did a mileage test with a Nissan model with a CVT and one with a traditional automatic -- the auto was slightly more efficient. Probably not a proper test. More data is needed, I s'pose...

Also, I have nightmares of hypermiling a CVT (probably no EOC, even Neutral coast with the engine on = not sure :confused: about longevity, no bump-start...)

RH77

Instructive diagram
 

I just found this: https://www.isuzuengines.com/products...=5&model_id=13 It's some specs for the 7.8 liter Isuzu 6H I-6 diesel engine, made for medium duty trucks. The thing that jumps out right away is that the engine is capable of making 850 ft-lbs of torque while "only" having a top end hp of 300. This is a 2.8 : 1 ratio, compared to an '06 Jetta TDI which is rated for 177 ft-lbs and 100 hp, a 1.77 : 1 ratio. This is obviously because the engine is designed (cam timing and duration, injection timing, etc) for low-end pulling torque and fuel efficiency, as opposed to high reving power/acceleration. But it is interesting that the ratio of bore to stroke, 1 : 1.09, is even closer than in the jetta, which is 1 : 1.20. (longer stroke relative to bore creates a more torquey engine, but reduces top-end revs).

The isuzu could probably be "hopped up" if it were allowed to rev beyond 2,000 rpm (piston speed: 1,640 ft/min), but I'm sure it's governed to stay below 2,000 rpm for longevity purposes. The Jetta, otoh, can rev at 4,000 rpm (piston speed: 2,507 ft/sec). So, the relatively low hp rating of the Isuzu is somewhat misleading, but nevertheless, even with it's shorter stroke relative to bore, the Isuzu makes 17% more torque per liter than the TDI. Again, this suggests the top end of the Isuzu is designed for low-end torque.

Check out the fuel consumption graph on the bottom of the page: The 6H engine runs most efficiently at just under 1,600 rpm or 1,300 ft/sec piston speed. But the fuel consumption curve stays really flat (variation is less than 2%) from about 1,250 rpm all the way up to 1,875 rpm, or 1,025 ft/min up to 1,538 ft/min. Beyond 1,875 rpm, the fuel consumption curve starts to climb, and presumably would continue a steep upward path if the engine were allowed to rev higher.

Another interesting thing jumps out at me: The 6H engine consumes approx 1 gallon of fuel per 24 hp-hr. If I assume a 0.010 rolling resistance coefficient, a toyota prius with one or two occupants requires about 20 hp to cruise at 70 mph. So assuming 20% driveline and accessory losses, an engine opperating as efficiently as the 6H in a prius could get 70 mpg at 70 mph!

This makes me want to mess with the cam -- timing duration, etc. -- in my little jetta. I have an engine that I'm rebuilding so if I destroy this engine, it wouldn't be the end of the world for me. I should also do the 8v gasser tranny swap, which would give me 22% taller gearing.

Engine efficiency, for everything from semi engines to small gasoline engines is a function of load and engine speed. High load and low engine speed are typically where efficiency is the best. Although, low load hurts efficiency more than high engine speed does IMLE. Efficiency is given in terms of grams of fuel used per kWh generated, so lower is better. Here are some examples

-Mercedes I4 Circa 2000
-1.9L TDI circa the lates ninties
-Heavy duty diesel engine circa (?)
-Toyota V6 engines circa the early nineties
-Unkown, likely gasoline
-The Prius' engine

Due do friction reducing design, newer hybrid engines are unique in that they can rev relatively high w/o hurting efficiency as much as most engines do. Diesel engine efficiency tends to peak at about half of their designed operating speed, and gasoline engine efficiency tends to peak at about a half to a quarter of their designed operating range. This is just a byproduct of the different air utilization rates, which limits the speed of diesels compared to gassers. All engines operate better at around full load, usually a little less than full load, so all things being equal more pedal is better.

Given a certain car that needs X kW at N mph, in order to get best mileage we want to optimize load such that the engine is working as efficiently as possible. Low output (compared to displacement) TDI diesels are nice in this respect since engine efficiency doesn't vary compared to load nearly as much as it does in other engine types, so gearing isn't as crucial for good economy, although it can help. For instance, the 1.9L TDI has fairly good efficiency of ~300-200g/kWh from ~10-90hp. Unlike the mercedes I4 gasser which has efficiency of ~500-250g/kWh from ~10-150hp.

Here's the thing, most cars today have decent highway efficiency given gearing and speeds of ~70-80mph, so there really isn't much to improve from the factory in this respect. However, if we don't mind driving 55, then with an appropriate OD ratio we can see the same engine efficiency w/ much less kWh needed by making sure the engine is at similar load. Here's a crappy edit of the Toyota V6 I linked earlier. My engine is the one w/ the dotted lines, and each square corresponds to a different OD ratio, w/ admittedly high numbers for the power needed to go down the freeway at 55mph. The bottom one is where the gearing is at now, with BSFC of ~350g/kWh(?) which corresponds to ~35mpg@55mph. If I drop the engine speed to ~1500rpm by a taller gear, BSFC drops to ~260g/kWh since engine load increased, and my mileage would increase to ~45mpg@55mph. Going to 1000rpm doesn't increase efficiency, but it does reduce available power in that gear, so it's probably no worthwhile unless I can drop the energy needed to move the car enough, likely cut it in half ala Basjoos.

Here's some more on it, and like I said, the road load line is how efficiently the engine operates in a certain gear over that gear's range of speeds. If you put a taller gear in that engine, the line would go through BSFC ovals with smaller values, and you would get better mileage. That's the VW 1.5L IDI diesel, so more or less the same as our diesel engines. At the end of my diesel swap post I calculated approximate mileage via that BSFC map assuming certain conditions.

Here are a couple BSFC maps that show the difference between Turbo/NA and 4/2 valve engines, althought I don't have any info beyond that.

If you can pull 50mpg@55mph in the Lambo, I think yer golden in terms of gear ratios. P&G is pretty much simulating taller gearing by loading up the engine for a shorter duration than at a cruise to improve overall efficiency, so I suppose it's a poor man's taller gearing... ;)

Meh... 50mpg is better than 25mpg. :D Why not grab another trans and crack it open to see if you can swap stuff around?

22s playa1

Pardon the Interruption
 

Sorry to interrupt the wheel discussion fun...

I'm pretty certain that larger wheels and tires may become more costly to purchase/maintain and even power...

Back to the unsprung weight argument I made earlier (in some thread :o ) -- lesser-expensive wheels generally become heavier as they get larger, which translates into more energy given during acceleration unless momentum is conserved. I guess it depends on the type of driving...

But also, to fit larger tires on most cars, you have to go with a more aggressive aspect-ratio -- then you're back to the original total diameter -- plus more $ for those types of tires.

I'd rather swap a transmission or gear, albeit more difficult...

RH77

Sounds like a gud exkuse to get mas t00ls! :D

Thank you for the explanation. This is very instructive. One question: I understand that the vertical axis represents load on the engine, but is load the horsepower being demanded of the engine or just the torque? If I'm reading the maps correctly, I'm assuming that the vertical axis is only the torque component since horsepower is torque x rpm and rpm is represented separately on the horizontal axis. Am I correct?

So if a car needs say 20 hp to cruise down the highway at a given speed, then lowering the rpm (with taller gearing) increases the torque required in order to maintain the 20 hp output.

Now since the fuel consumption circles represent fuel quantity (grams, gallons, etc.) per hour of power output (hp, KW, etc.), then running the engine at a lower rpm that demands more torque (i.e., load), up to a certain point, to make that power gives the best efficiency.

So let's say again that you want to run at a speed that requires 20 hp. Since hp = torque x rpm x 1/5250, then at 2,000 rpm the engine would have to make 52.5 ft-lbs. If you run at 3,000 rpm, then the engine only has to make 35 ft-lbs. Now let's look at your fuel consumption map for the prius engine, which has the best high rev fuel efficiency of any of the engines you've shown. Making 20 hp at 2,000 rpm and 52.5 ft-lbs (71 Nm), fuel consumption is less than 240 g/kWh. At 3,000 rpm and 35 hp (47.5 Nm), fuel consumption goes up to 270 g/kWh. So even though the prius engine has very "flat" fuel consumption lines across the rpm range, as rpm increases, the engine load required to maintain a given horsepower drops, so engine efficiency drops. In other words, if you plot 20 hp on the map at different rpms, you get a line sloping down from left to right. So plot a given hp at various rpms on any of the fuel maps that you've provided and you find that the best efficiency comes in around 2,000 rpm or less.

Am I reading things correctly here?

The X axis is engine speed and Y-axis air/torque, with peak hp corresponding to peak torque at whatever rpm. Everything you wrote seems to be accurate. Regarding the gearing line for different ratios at a given power/speed output and efficiency, realistically, it can be anywhere from 1000-4000rpm depending on displacement and engine. An older small displacement SOHC engine designed to rev high and make power may have it relatively high, while a newer large displacement DOHC may have it around idle.

How do you convert from bmep [bar] to ft-lbs torque?

I just find the engine's peak torque and scale it from there. I suppose we could also use the eff, energy content of fuel, stoich ratio, and percent of oxygen at sea level for gassers, but I'm not sure about how we could do the same for diesels w/o knowing the amount of fuel injected...