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What will get better MPG?

Old 02-06-2013, 12:41 PM
  #76  
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Default RE: What will get better MPG?

Back in the 70's, the Lotus Élan got very good mileage despite being a high-output engine; DOHC and very light weight yielded great efficiency.
The tv show Top Gear pitted a Prius and a BMW against each other on the test track. The Prius got horrible mileage, as it was flogged trying to keep up. Small engines can get poor mileage when run outside their optimal range.
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Old 02-06-2013, 05:20 PM
  #77  
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Default RE: What will get better MPG?


[quote]ORIGINAL: Jetdktr

Dump them both, get a VW Jetta sedan or wagon with the TDI Diesel

/quote]

AMEN TO THAT!! I have 2 Jettas, TDI, one auto, one straight, straight gets 65 mpg, auto gets 60, 175K + mileage!!! We always cruise at 80 on interstate, 40 50 around town, hills, wind, a/c on or off, You just can't beat 'em. BUT, when you have to get 'em worked on!!!! You'll pay big $$$.
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Old 02-07-2013, 12:26 AM
  #78  
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Default RE: What will get better MPG?


ORIGINAL: baronbrian

No one is disputing that smaller engines can help with mileage, but engine size Is not the biggest part of the equation. Other factors, such as weight, size, and drag are larger factors. The most efficient cars in the world are going to maximize all these factors, but that doesn't tell you what one is the most important. A large engine can be offset by other criteria and produce an efficient auto. And yes, mashing the throttle on a small engine IS NOT efficient.
Indeed, it's amazing how people forget that the only real relationship between your car and your fuel economy is the weight of your right foot.
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Old 02-08-2013, 09:57 PM
  #79  
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Default RE: What will get better MPG?

aerodynamics and effective powerband play a HUGE role in fuel economy, heck, modern corvettes get pretty good fuel economy, because 1- the car is streamlined and 2- the engine is in its sweet spot at highway speeds.
chevy tends to make larger cars fuel efficent, up untill recently dodge made the worst, my mothers durango drinks gas at a rate of 10mpg or 15hwy, while a trailblazer (though butt-ugly and slower) gets around 15-18

my miata gets 30-33mpg no matter how I drive, where I drive or the temperature, she SCREAMS on the highway, maybe 4000RPM @75mph but the car is fine humming allong at that speed.

the car does not accelerate unless you shift it above 3700RPM, it always sounds like your thrashing it, but thats just how the cams are cut and how its geared, feels like a turbo kicks in after 4500RPM
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Old 02-10-2013, 12:01 PM
  #80  
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Default RE: What will get better MPG?

' mashing ' the throttle on a small engine is not efficient, not true.
petrol engines work at maximum thermal efficiency ONLY when the throttle is wide open, unless you know something engine designers dont? try googling it if you dont believe me,
i wouldnt, i dont believe anything, i prefer to know .
the reason wide open throttle produces max efficiency is simple, that allows max air into the engine which produces max compression ratio which equates to max efficiency,
thats one of the reasons a diesel will hammer a petrol on mpg, it doesnt have a throttle so it operates at max compression ALL the time, the other reason of course is the compression ratio is about twice as high as a petrol.
mike.
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Old 02-10-2013, 06:46 PM
  #81  
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Default RE: What will get better MPG?

ORIGINAL: goodvibes
the reason wide open throttle produces max efficiency is simple, that allows max air into the engine which produces max compression ratio which equates to max efficiency,
thats one of the reasons a diesel will hammer a petrol on mpg, it doesnt have a throttle so it operates at max compression ALL the time, the other reason of course is the compression ratio is about twice as high as a petrol.
mike.
A diesel is more efficient than a gasoline engine for two very specific reasons: 1. the energy density of diesel fuel is higher than petrol, and 2. Diesel fuel burns at a higher temperature than gasoline promoting a more complete burn while in the combustion chamber allowing for more energy capture by the piston. you can stop with the "diesels dont have a throttle and that is why they are more efficient" nonsense.

I do not know what you are using as your definition of compression ratio, however if you think your compression ratio is changing while you drive your car, you may have to look that one up. Compression ratio is determined by the ratio of volume in the combustion chamber when it is at TDC compared to BDC. Displacement is the difference in volume of the cylinders from TDC to BDC. While it is true that tuners use term such as Static Compression ratio and Effective Compression ratio, SCR and ECF are only used with regards to turbocharged engines where the pressure of the air entering the cylinder is greatly different than atmospheric pressure. compression ratio and Static compression do not change. while effective compression ratio can change at different throttle settings on a forced induction engine, you did not specify ECR so I am assuming that you don't know what you're talking about, I suspect everyone else assumed the same after reading your first post of misinformation on page 2. I think they were being polite by not mentioning it, as I was until you posted a second time stating the same thing.

as was also mentioned in this thread, on the BBC TV Program Top Gear, the Stig was sent out to lap the track as fast as possible in a toyota prius, while Jeremy casually pursued in a BMW M3 at the same rate. after several laps guess what one got better MPG? hint: it was the one that was heavier, had more drag, had a bigger engine, and had self-admitting fat Jeremy driving it (everyone should be aware that the Stig is weightless) why did the BMW get better MPG? because the power of the bigger engine kept the beamer in the engines' efficient rpm range, while the Stig had to keep the revs high in the small engined toyota, which was NOT efficient.
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Old 02-10-2013, 09:21 PM
  #82  
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Default RE: What will get better MPG?


From HowStuffWorks.com:
 
Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules (147,000 BTU), while 1 gallon of gasoline contains 132x106 joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines.

From another site:
 
A high expansion ratio is also one of the two key reasons for the efficiency of Diesel engines, along with the elimination of pumping losses due to throttling of the intake air flow.

I agree compression ratio doesn't (actually, can't) change with throttle position. It's a mechanical value. However, the text on limping losses would support goodvibe's statement about not being 'throttled' per se. Right intent, sideways explanation. 
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Old 02-10-2013, 10:12 PM
  #83  
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Default RE: What will get better MPG?

dont' forget power to weight ratio... prius is heavy for it's engine (ugh, the word prius makes me shudder in disgust)

its no use to argue, there is tech out there for 100 and even 150mpg engines/hybrids and there has been for a while, but instead of making all cars obsolete by releasing one that gets triple the fuel economy, slowly make them more efficent so people buy your outdated designs.

hybrid tech is useless unless its diesle, and EV's are useless unless your using lipo/life with a CVT transmission.
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Old 02-11-2013, 06:50 AM
  #84  
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Default RE: What will get better MPG?

Smokey Yunick (Nascar engine builder and crew chief) took an interest in improving fuel economy after the first Arab oil embargo in the 70's. He mated a small japanese industrial 3-cyl diesel with mostly off-the-shelf parts and created a 70 mpg Pinto. He did weld up engine mount adapters and sold kits IIRC.
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Old 02-11-2013, 06:02 PM
  #85  
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Default RE: What will get better MPG?

its late ,im tired and have to get up at 5 inthe morning,but its time to initiate anyone who likes to learn , some little known secrets about how engines really work,but first,
well done, yes diesel has about 8 percent advantage re energy density and probably burns at a higher temp, havent looked into that one, but this dont explain the massive advantage in mpg and incredible
increase in torque at tickover speed that the diesel has.
i live on a steep hill but my peugeot 1527cc will pull away in first gear effortlessly without touching the accelerator and with a run up will climb this hill in 3rd gear, still at tickover speed, why? [ sorry about talking nonsense again] because there is no restriction on the amount of air that the engine is try ing to 'suck' in.
so, the ACTUAL compression ratio will be close to the mathematical one of about 22 to1 , in other words
if the engine was a 4 litre the induction stroke would pull in almost 1 litre of air at roughly atmospheric pressure an d cram it into the tiny combustion chamber of 1 divided by about 22 of a litre, loads of compression, lots of torque all at tickover, and all achieved at minimum fuel injection.
but , what would happen if we,' throttled' the air intake like a petrol engine? simple to find out, just remove air filter and fit a carburettor from a 1500 car to the manifold intake. now temporarily jam the throttle wide open and drive,you wouldnt notice any difference , a 1500 diesel will suck in roughly the same amount of air as a 1500 petrol,but now unjam the throttle and jam it in the 'idle' , tickover position.
id be amazed if the engine continued to run, why? because with the throttle shut' and 'idle' speed of about 1000 rpm the cylinder has only roughly[ im tired]30 thousands of a second , top dead centre to
bottom dead centre to 'suck' in 1litre,roughly' of air, if youve ever looked into a carburettor intake at
idle position the actual gap between the throttle disc and intake tube probably equates to the area of a drinking straw, ever tried breathing through a straw? how the hell can 1litre of air be sucked through the equivelant area of a straw in 30 milliseconds?, the answer is, IT CANT, by the time the piston reaches bottom dead centre ,there will be a PARTIAL VACUUM in the cylinder , exactly the opposite of what we want, as the piston travels up on compression stroke the partial vacuum will decrease , eventually reach atmospheric and by the time tdc is reached, it might actually reach a low compressio ratio .
im hammered its almost time to get up, but in a nutshell its very high compression ratios that produce good torque for small fuel input, this can only happen with throttle wide open or no t hrottle at all[diesel],
the petrol engine is aVARIABLE COMPRESSION ENGINE because the throttle, unless its wide open,prevents the cylinders from filling up with air at atmospheric pressure ,producing lousy' volumetric efficiency,' [look it up]
you may be right about some one not knowing what their talking about, but im happy to let others decide which one of us that is.
im asleep,
mike.


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Old 02-11-2013, 06:43 PM
  #86  
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Default RE: What will get better MPG?

  the petrol engine is aVARIABLE COMPRESSION ENGINE

You're misusing the term (while shouting), and confusing it with pumping losses. All while asleep. Impressive. 
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Old 02-11-2013, 06:47 PM
  #87  
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Default RE: What will get better MPG?


ORIGINAL: goodvibes

WALLOFTEXT
WALLOFTEXT
WALLOFTEXT
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Old 02-12-2013, 11:47 AM
  #88  
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Default RE: What will get better MPG?


ORIGINAL: goodvibes

its late ,im tired and have to get up at 5 inthe morning,but its time to initiate anyone who likes to learn , some little known secrets about how engines really work,but first,
Never good to imagine you know more than the next guy...even if you do.
Baronbrian laid out why the diesel has better fuel efficiency. The diesel turns at a slower RPM and produces more torque....it uses the fuel more efficently for those tasks. If you want high RPM and more power the petrol is not so efficient ..but burns faster
Is the debate about a comparison between a petrol engine at WOT and a diesel...or the fact a petrol has a throttle?
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Old 02-13-2013, 03:55 PM
  #89  
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Default RE: What will get better MPG?

well, im amazed anyone would want to reply to someone who ' talks nonsense and
'doesnt know what their talking about', you sound like you have an open mind, excellent!
re:
never good etc i couldnt agree more, but perhaps you should be aiming that in bb's direction in view of the above comments?
its very late again, and i need some sleep, ill elaborate when i get some time, its very
precious stuff, but if you start at post 40 you should get the idea.
mike.

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Old 02-14-2013, 05:11 AM
  #90  
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Default RE: What will get better MPG?

Not to drain the mood here, but this thread has nothing to do with diesels actually. It was simply asked, what gets better mpg. It's obvious that some diesels i.e VW TDI vehicles get exceptional mileage. this thread is not to debate how or why and argue the difference. Carry on chaps!!
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Old 02-14-2013, 12:12 PM
  #91  
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Default RE: What will get better MPG?

ORIGINAL: goodvibes
well, im amazed anyone would want to reply to someone who ' talks nonsense and
'doesnt know what their talking about', you sound like you have an open mind, excellent!
re:
never good etc i couldnt agree more, but perhaps you should be aiming that in bb's direction in view of the above comments?
its very late again, and i need some sleep, ill elaborate when i get some time, its very
precious stuff, but if you start at post 40 you should get the idea.
mike.

Elaborate away when you have some time....
I read again from post 40 (twice), yet I still do not get the point?
Petrol cars use a throttle to achieve maximum efficiency..diesels do not...if I were looking to get economy on either I would not "mash" the pedal.
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Old 02-14-2013, 12:58 PM
  #92  
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Old 02-14-2013, 02:00 PM
  #93  
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Old 02-15-2013, 07:46 PM
  #94  
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Default RE: What will get better MPG?

cummins power and darkfire
sorry its drifted away from , who gets, etc, the only reason i mentioned a diesel was to show in a simple way why a small petrol engine at full throttle works more efficientlly than a big engine at small throttle, sorry to have bored you with the 'how and why' happy motoring, mike.
baron brian, eddie c and any one else who stated that ' the compression ratio'
cant change are , of course correct, stupid mistake, what i meant to say is that
' the petrol engine is a variable compression engine' which it most definitely is.
the compression,[ psi] is high only at wide open throttle, because there is no restriction to the cylinder filling with air at [almost] atmospheric pressure.
at lower throttle openings , the compression drops sharply , untill at tickover
there is virtually no compression at all , but fortunately the spark can still ignite
the mixture.
baron brian, you said ,'a diesel is more efficient due to fuel energy density and
higher burn temperature' not true, these are contributory factors not the main one.
put, 'what is the major factor in diesel engine efficiency ' into 'google' and every reply ive seen all state , ' that its the very high compression ratio' which is responsible.
you also said,' you can stop with 'diesels dont have a throttle and thats why they are more efficient' what i actually said was' thats ONE of the reasons a diesel will hammer a petrol on mpg, it doesnt have a throttle so it operates at max compression ALL the time, the other reason of course is the comp ratio is about twice as high as a petrol'. both of these reasons are true.
you mentioned my ' first post of misinformation on page 2' apart from my stupid mistake about saying 'compression ratio' instead of using the word compression, id like you to point out anything else in that post which is not true, feel free to prove me wrong , im a lover of the truth.
racing is about power and acceleration and is virtually the opposite of driving for efficiency, it bears little relation to driving on the road.
driving through town at slow speeds the bmw's big engine would have the throttle barely open, sounds good for economy, but wait, if the throttle's barely open, the cylinder compression will be very low, thats low efficiency.
the small engined car following , provided its in the highest gear it s comfortable with, will need a larger throttle opening, sounds bad for economy, but wait, if the
throttle has a larger opening, the cylinder compression will be higher,
that means higher efficiency.
at moderate speeds, provided the small car always uses the highest gear its comfortable with,the throttle opening wil have to be opened a bit more, again it sounds bad for economy but this will further increase the cylinder pressure increasing the efficiency again.
at some higher speed, the smaller engine will reach max torque rpm,[this is usually in the handbook] for max efficiency, this rpm must not be exceeded because the cylinder compression starts to drop after this point, and efficiency
drops, but the power will increase if the throttle is opened more.
up to the point where the small car reaches max torque , mr bmw will always
be operating at a relatively smaller throttle opening, hence lower compression
and lower efficiency.
big petrol engines [and small] at low throttle openings are effectively 'low compression engines' they only turn into' high compression' high efficiency
engines at high throttle openings, but how often would you use full throttle
on a really big engine? .
mike.
















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Old 02-15-2013, 08:28 PM
  #95  
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Default RE: What will get better MPG?

To the OP, is it possible to get better mileage with a heavier car with a bigger engine? The answer is that it depends.

To goodvibes; mike, I did not say that you didn't know what you were talking about, I said that by misusing a term, it made you sound like you didn't know what what you were talking about. You have since admitted that it was a stupid mistake on your part to use that term in your explanation. I admit that I am not an expert on diesel engines. I know the theory, the difference in compression ratios, the immense pressure that the injectors run. I know about the chemistry of the fuels. I know that the principal of compression ignition is similar to our alcohol model engines. I do not know about throttle bodies, air intake on carburetors (all my cars are fuel injection) or other details about diesel engines. I've never owned one, never worked on one. For me their higher initial cost as well as higher maintenance costs too far offset the savings at the pump..

As far as I know my engine allows 1 part fuel per 14 parts air at all throttle settings.. there must be a similar ratio for diesel, or does this change with thottle since air is never restricted??

Anyway, Mike, no hard feelings, but I gotta razz ya...
"You're so vain, you probably think this thread is about you"
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Old 02-16-2013, 01:59 AM
  #96  
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Default RE: What will get better MPG?

cummins power,
ive just had a thought, you yanks have smaller gallons than us brits, that means over here, your dads car would be doing almost 41 mpg
not bad for a 3.8 litre petrol, im wondering if your dads cars fuel consumption meter is a bit like our politicians over here, economical, but only with the truth.
seriously, if it genuinely did that to the gallon , id buy it!
why not swap cars for a month, then make up your mind, something ive learned is ,never sell or swap, a car to someone you know, something major usually goes wrong with it soon after.
mike.

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Old 02-16-2013, 03:53 AM
  #97  
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Default RE: What will get better MPG?

hi,
i dont think ive been as clear as i should have, ill have another go, the point is, any size of engine , big or tiny that has its air [ oxygen ]supply restricted, is bound to be
operating at lower efficiency than its capable of , if you reduce ,[ throttle], the air entering the engine, youve reduced the level of compression in thecylinder, its now operating as if it had been designed to run at a very low compression ratio, we all know that low compression engines cant match high compression ones
for economy, torque and power.
im , obviously [that dangerous word again] not saying that people should drive around flat out in every gear ,thats good only for winning races ,not economy,
ideally, for max economy, the designer would make the smallest size engine, which at full throttle and at max torque engine speed would , with the right gearing
just attain the[ car] speed reqd, about 70 over here, but we dont live in an ideal world, most drivers want a fast car so the designer has to install a much more powerfull engine than is actually needed, but the problem then arises that much of the time ,in traffic, low speed limits etc that that you only need small amounts of power, so what do you do, yes youve probably guessed it, you have to close the throttle, and what does closing the throttle do?
it reduces the cylinder pressure, which is fatal for economy, torque and power.
of course, there is a solution to most of these problems, its called a diesel engine, diesels are not throttled, so they always work at max very high compression
levels and obtain the max amount of energy from the fuel, but dont they ever STINK, but with 70 to the ' galleon' [when the sails up] on a 'run' and a genuine 50
miles to the 'galleon' [without sails, no wind in the city] through town, thats a price im more than willing to pay. this is getting boring, time for a change in subject.
anything dropped out of the sky down in somerset? im off to the motorbike shop to buy ,me the missus and 'scruff' some crash helmets, do they make em for little ****s,sorry i meant to say **** zoo's, isnt my spelling atrocious, they should keep us safe in case one of these massive 'hemorroids' comes down on us. the governments useless, they should fire missiles packed with anti coagulent at em before they enter r atmosphere. im gettin ungry, so im gonna trot of to tesco's for sum be sorry orseburgers, apparently their 'galloping' off the shelves.
cant see what all the fuss is about, the french ave bin eating em fur donkeys years.
and my mates fink im mad, cant think why.
mike.


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Old 02-16-2013, 05:07 AM
  #98  
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Default RE: What will get better MPG?

the 'stoichometric' ratio used to be 15 to 1 by weight, so sounds about right to me, ithink it means ratio reqd for complete combustion,but dont quote me on that, its a ,long time since i read
anything about petrol engines. their trying to make them run on as lean a mixture as possible ,partly for economy and maybe for emission control.
if you take that to the extreme,lean mixture that is, you would end up with,guess what? a petrol engine that didn't have a throttle, the power would be varied by just changing the amount of fuel injected, no throttle means operating at max compression all the time, very efficient. id buy one tomorrow just one snag, isnt there always? with all the brains on the planet
no one seems able to make massive air to petrol ratios ignite with a spark , have a think about it, if you can crack that one, you could write your own cheque!.
as to fuel air ratios on a diesel, i dont think there is one, my guess is because of the very high compression , constant, because theres no throttling of the air, you simply inject as much or as little as you want , up tothe point where the smoke becomes unacceptable, diesels would be much more powerfull if you were allowed to inject as much fuel as you wanted, but the smoke produced would prohibit it.'
this is all from what i remember from many years ago when i used to bury my head in all sorts of technical books, im a' walking question mark' basicly, how about some clever engine designer out there stepping in and clearing up any mistakes or doubts, dont be shy, some of us want to know, dont keep it to yourself.
vain? give me a minute to stop laughing on that one, trust me , if you could see me, that is the last thing you'd say.
mike.[humble].






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Old 02-16-2013, 08:35 AM
  #99  
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Default RE: What will get better MPG?

ORIGINAL: goodvibes
anything dropped out of the sky down in somerset?
I have not seen anything.
I am still unsure of your point.....but you seem to be having a nice discussion with yourself. If a petrol engine without a throttle is what you are looking for then look up gasoline direct injection.
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Old 02-16-2013, 09:13 AM
  #100  
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Default RE: What will get better MPG?

Thanks for the info, Mike. And the vain comment was just to lighten the mood. Since you're laughing, mission accomplished
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