Gear ratio question. ugh i'm confused.

[jakksincorpse]

Are you sure?

Also, about the 2400lbs lift at idle, how did you figure that out? I hope you're not underestimating how much brute force it takes to push air around. Some airplanes use up tens of thousands of horsepower doing nothing more than just pushing air around...

oooo. i think i did make a miscalc. i was doing this at 3am last night soo yea. i believe i divided 5280 by 24, thinking that the 24 was inches, i guess it was actually feet. whoops.

gotta fix that then

and yea i'll explain about the thrust, the main thing u got to understand is that tens of thousands of horsepower is moving hundreds of thousands pound of aircraft. most jets weigh over 100,000lbs.

this design would weigh 2000lbs with 900hp@6000, using the equation i was giving about converting hp to thrust

traveling 550ft/s with 1 hp=1lb of thrust, so 900lbs of thrust, this is at max rpm tho

so when you start the engine let's say your idle is 1000rpms

so we can assume if the engine is efficient enough for a steady powerband it will have 150hp@idle (note, i based this "engine" off 3 wankel motors with little modification)

so, ascending at 6ft/s with 1 hp=91lb of thrust, multiplied by your idle power= 13,650lb of thrust.

in theory this should be what happens. i know that looks like a tremendous amount of thrust but believe me, it goes away real fast when you want to go a little faster.

 

[jakksincorpse]

Well, yes, you can certainly swap-out a more efficient or smaller engine to gain fuel economy. Double would be tough, but it depends on what is in a car now. You seem to be assuming that if the engine is running at 1900 rpm, it is producing 149 ft-lb of torque. It has been explained to you before that this is not correct. 149 ft-lb may be the maximum torque, but it is not the actual torque.

The power at the wheels is the same regardless of the size of the engine, so the only real fuel economy difference is in the efficiency of the engine and drivetrain. The efficiency isn't likely to change much unless you go with a radically different engine. For example, I drive a Mazda 6 and Mazda currently offers a 170hp 2.5L 4cyl which gets 30 mpg and a 3.7L 6cyl that generates 272hp and gets 25mpg. So cutting the engine size (by peak power) by 63% only improves the fuel economy by 20%.

Note that 30 mpg is 2.0 gal/hr at 60mph. Calculating out using the energy density of gas and assuming about 30% engine efficiency yields about 30hp required at the engine to maintain 60mph. Figure at least a third of that is drivetrain loss and the power to the wheels to maintain 60mph on flat ground is about 20hp.
Nonsense. But by all means, if you have a reference, please share it. The GM ultralite weighed 1400 lb and was a concept car, not a production ready/street legal car: http://en.wikipedia.org/wiki/General_Motors_Ultralite

According to this link: http://peakoil.com/forums/viewtopic.php?t=24763
...the GM Ultralite had a drag coefficient of .19 vs a typical car's .32. That's a difference of 40%, which translates directly into 40% more fuel efficiency. Other features, such as tires unsuitable for a mass produced car probably also factor in heavily, as does the small engine. Since it is light, it can get away with a small engine and still have decent acceleration. That would be a big factor in city fuel economy, but GM didn't publish city fuel economy. No doubt, this concept car was also not tested with air conditioning running and I wonder if it even had power steering and brakes...

all i know is my mustang traveled 60mph@1700rpms and the same year model ect, factory gears travel 60mph@2250. factory says it got 24mpg hwy, mine got 27mpg hwy.

about the ultralite i apparently read an article online that was complete bs about the weight. what a jip. i was stoked when it said 420lbs i was like whattt?

how does 30mpg figure to be 2 gal/hr at 60mph. where did you find that from?

i don't understand that hp statement of efficency at all... almost every motor, even my hyundai, runs close to 30% efficient...it has 140hp...and gets 34mpg on highway. explain the equation you stated??

 

[jakksincorpse]

A gasoline engine, or diesel for that matter, will only produce its maximum torque with the throttle wide open. When you are cruising down the road your throttle is barely open you are only producing enough torque to maintain your speed - It is actually HP when you are moving because torque is a static force - horsepower is torque times the time or distance. HP = 33000 ftlbs per second.

but HP is a "side-effect" of torque. if you know about torque very well tho, can you please explain why the torque band depletes while the hp band keeps increasing through rpm. i never understood that.

 

[russ_watters]

all i know is my mustang traveled 60mph@1700rpms and the same year model ect, factory gears travel 60mph@2250. factory says it got 24mpg hwy, mine got 27mpg hwy.

My car can get anywhere from 29-34mpg depending on conditions, with the EPA rating being 32. So it really doesn't tell you a whole lot that you're getting more than the EPA rating. What would be more useful is a before and after comparison at similar driving conditions (which would be difficult to do accurately).

about the ultralite i apparently read an article online that was complete bs about the weight. what a jip. i was stoked when it said 420lbs i was like whattt?

If you read the link I posted, the "shell" weighs 420lb, which sounds kinda like an exoskeleton, but in any case would be a substantial improvement over a steel frame.

how does 30mpg figure to be 2 gal/hr at 60mph. where did you find that from?

60 mph * 1 hr = 60 miles. 60 miles/ 30 mpg = 2 gal

i don't understand that hp statement of efficency at all... almost every motor, even my hyundai, runs close to 30% efficient...

Yes, exactly the point. The [peak] efficiency is mostly a function of compression ratio and most gas cars run at similar compression ratios. Diesels run at substantially higher compression ratio and thus have a higher thermodynamic efficiency. Turbochargers also add substantially to the efficiency of either. Other things, like electronic/variable fuel injection can add a few percent.

it has 140hp...and gets 34mpg on highway. explain the equation you stated??

What equation?

 

[russ_watters]

but HP is a "side-effect" of torque. if you know about torque very well tho, can you please explain why the torque band depletes while the hp band keeps increasing through rpm. i never understood that.

This is getting frustrating because you are repeating the same misunderstanding over and over and we are correcting it over and over. You're just not listening.

Torque is not horsepower.
Torque is not horsepower.
Torque is not horsepower.
Torque is not horsepower.
Torque is not horsepower.
Torque is not horsepower.

The horsepower keeps going up because the horsepower is not just a function of torque, it is a function of rpm. Power is torque times angular velocity. The simplified equation (taking into account the time difference and Pi to get the units right) is: Hp = T * RPM / 5252

So please: do the calculation and prove it to yourself:
If an engine generates 150 ft-lb of torque at 5000 rpm, what is the horsepower? If it generates 140 ft-lb of torque at 6,000 rpm, what is the horsepower?

Note that as this applies to an electric motor, the torque is relatively constant over a fairly broad operating range and as a result, the power increases in a straight line over that range. For example: http://farm4.static.flickr.com/3393/3272966085_fa928de4ce.jpg

 

[jakksincorpse]

My car can get anywhere from 29-34mpg depending on conditions, with the EPA rating being 32. So it really doesn't tell you a whole lot that you're getting more than the EPA rating. What would be more useful is a before and after comparison at similar driving conditions (which would be difficult to do accurately). If you read the link I posted, the "shell" weighs 420lb, which sounds kinda like an exoskeleton, but in any case would be a substantial improvement over a steel frame. 60 mph * 1 hr = 60 miles. 60 miles/ 30 mpg = 2 gal Yes, exactly the point. The [peak] efficiency is mostly a function of compression ratio and most gas cars run at similar compression ratios. Diesels run at substantially higher compression ratio and thus have a higher thermodynamic efficiency. Turbochargers also add substantially to the efficiency of either. Other things, like electronic/variable fuel injection can add a few percent. What equation?

what you just said above with the 60mph and 30mpg equals 2 gallons. what does that even mean?

 

[jakksincorpse]

This is getting frustrating because you are repeating the same misunderstanding over and over and we are correcting it over and over. You're just not listening.

Torque is not horsepower.
Torque is not horsepower.
Torque is not horsepower.
Torque is not horsepower.
Torque is not horsepower.
Torque is not horsepower.

The horsepower keeps going up because the horsepower is not just a function of torque, it is a function of rpm. Power is torque times angular velocity. The simplified equation (taking into account the time difference and Pi to get the units right) is: Hp = T * RPM / 5252

So please: do the calculation and prove it to yourself:
If an engine generates 150 ft-lb of torque at 5000 rpm, what is the horsepower? If it generates 140 ft-lb of torque at 6,000 rpm, what is the horsepower?

Note that as this applies to an electric motor, the torque is relatively constant over a fairly broad operating range and as a result, the power increases in a straight line over that range. For example: http://farm4.static.flickr.com/3393/3272966085_fa928de4ce.jpg

142hp and 159hp...sooo...what does this prove...? i don't understand where I'm going wrong with any of this...i knew that already.

bleach is mostly water, and we are mostly water. so therefore, we are bleach. =] i love metalocalypse XD.

 

[russ_watters]

what you just said above with the 60mph and 30mpg equals 2 gallons. what does that even mean?

The point is that it is an easy way to calculate (measure) the horsepower required to propel a car at highway speed.

142hp and 159hp...sooo...what does this prove...? i don't understand where I'm going wrong with any of this...i knew that already.

You said:

"but HP is a "side-effect" of torque."

...which it isn't. And:

"can you please explain why the torque band depletes while the hp band keeps increasing through rpm. i never understood that."

Which (together with the previous) implies that you think horsepower should increase as torque increases and decrease as torque decreases.

 

[jakksincorpse]

The point is that it is an easy way to calculate (measure) the horsepower required to propel a car at highway speed. You said:

"but HP is a "side-effect" of torque."

...which it isn't. And:

"can you please explain why the torque band depletes while the hp band keeps increasing through rpm. i never understood that."

Which (together with the previous) implies that you think horsepower should increase as torque increases and decrease as torque decreases.

well i knew that torque decreased after a certain point as hp just kept rising but i never knew why torque started decreasing after a point... like, it makes no sense to me. if you're constantly adding fuel to an engine why does torque decrease?

 

[russ_watters]

well i knew that torque decreased after a certain point as hp just kept rising but i never knew why torque started decreasing after a point... like, it makes no sense to me. if you're constantly adding fuel to an engine why does torque decrease?

It has more to do with airflow. You can easily make a fuel pump to pump a specific amount of fuel into the engine per cycle, but getting the airflow to be constant per cycle requires an ever-increasing intake manifold size because the same amount of air (per cycle) has to flow through the same pipes faster as the rpm increases.

...but anyway, do you see why the horsepower keeps increasing even as the torque starts to decrease?

 

[jakksincorpse]

It has more to do with airflow. You can easily make a fuel pump to pump a specific amount of fuel into the engine per cycle, but getting the airflow to be constant per cycle requires an ever-increasing intake manifold size because the same amount of air (per cycle) has to flow through the same pipes faster as the rpm increases.

...but anyway, do you see why the horsepower keeps increasing even as the torque starts to decrease?

really the manifold controls that? wow you learn something new everyday. that's awesome. so even if a turbocharger is put on the engine it won't increase the torque past that rpm? because the manifold doesn't change in size?

 

[xxChrisxx]

really the manifold controls that? wow you learn something new everyday. that's awesome. so even if a turbocharger is put on the engine it won't increase the torque past that rpm? because the manifold doesn't change in size?

That's right, the fastest air can flow through an orifice (into the combustion chamber) is Mach 1 [see: choked flow]. So there is a limit to the amount of air a cartain cylinder head will flow. This is why turbocharged cars can have flat torque curves, they have the ability to deliver the same amount of air at low rpm as they can at high rpm.

This is why naturally aspirated cars are 'ported' or use straight thottle bodies on racing engines to improve airflow.

Getting more torque and therefore power is basically an exercise of getting more air into the cylinder so more fuel can be injected and combusted.

 

[jakksincorpse]

So to come closer to an optimal motor the first thing that should be revised is a manifold that can support a line of torque to redline correct?

I've felt loss of torque before. It's like bad turbo lag at 5000rpms... Is that right though? I'm guessing with a bigger manifold to reach redline you'll need a lot of air flow to even produce significant results at lower rpms correct?

 

[xxChrisxx]

So to come closer to an optimal motor the first thing that should be revised is a manifold that can support a line of torque to redline correct?

I've felt loss of torque before. It's like bad turbo lag at 5000rpms... Is that right though? I'm guessing with a bigger manifold to reach redline you'll need a lot of air flow to even produce significant results at lower rpms correct?

Well it's not an optimal motor per se, as if you have peak torque at max rpm, it shows you can rev the engine higher and produce more power.

You could make an engine that does that (many racing engines that are rpm limited do it) but if you didn't have to it wouldn't make sense to limit the rpm, unless you had a material concern for the engine internals.

It's also not as easy as that, large ports to not support flow at low rpm very well. So it's a trade off if you had huge ports you would me making masses of power top end, but the low end drivability would be awful. It'd likely have a very lumpy idle and stall if you went too big.The first thing you have to remember with an engine is: everything is a compromise you gain top end torque (and therefore power) you lose bottom end torque and vice versa.

 

[jakksincorpse]

Well it's not an optimal motor per se, as if you have peak torque at max rpm, it shows you can rev the engine higher and produce more power.

You could make an engine that does that (many racing engines that are rpm limited do it) but if you didn't have to it wouldn't make sense to limit the rpm, unless you had a material concern for the engine internals.

It's also not as easy as that, large ports to not support flow at low rpm very well. So it's a trade off if you had huge ports you would me making masses of power top end, but the low end drivability would be awful. It'd likely have a very lumpy idle and stall if you went too big.The first thing you have to remember with an engine is: everything is a compromise you gain top end torque (and therefore power) you lose bottom end torque and vice versa.

i see, well I've been looking at tdi jetta's and I've noticed that its torque does stop at half the maxium rpm. since this number is 150lb/t if you'd double it you'd produce 300lb/t@3800rpm. so with this said, could you make a flat torque line(like stated before) with the turbo already on the motor so it will supply enough airflow to reach that high end torque. this would need a smaller turbo but is it possible? if so what would the statistics look like?

and another thing. since diesels are very thermo-efficient, is it possible to turn it into a 2 stroke? i have an idea that's so simple i can't say what it is until i patent it in the automotive world.

 

[xxChrisxx]

i see, well I've been looking at tdi jetta's and I've noticed that its torque does stop at half the maxium rpm. since this number is 150lb/t if you'd double it you'd produce 300lb/t@3800rpm. so with this said, could you make a flat torque line(like stated before) with the turbo already on the motor so it will supply enough airflow to reach that high end torque. this would need a smaller turbo but is it possible? if so what would the statistics look like?

and another thing. since diesels are very thermo-efficient, is it possible to turn it into a 2 stroke? i have an idea that's so simple i can't say what it is until i patent it in the automotive world.

As a note of interest I wouldn't even bother trying to think you've invented something new to patent it for any 2 or 4 stroke engine (conventional engine). Pretty much everything that works that you think is innovative has already been concieved of, probably by the 1930's. It's possible, but unlikely you've come up with something unthought of before.

I was astonished at just how much they patented but couldn't implement due to materials, or technology of the time. But everything on a very modern engine, that you think is cutting edge, like variable turbines, variable inlet geometry, variable compression, twin spark, direct injection (gdi), exhaust gas recirculation, etc etc was all concieved of years ago.


If the torque is trailing off top end, it's becuase the turbo is loosing boost, or it's not big enough or the cylinder head just can't flow enough. Probably a combination of the last two. Putting a smaller turbo on will likely create a flat torque curve if the boost is controlled, but the torque across all the range will be lower.

So if a big turbo boosting to say 14psi gets 150f.lb torque, a small turbo boosting to 8 psi will create less torque but will spool quicker meaning it will boost across a larger range. You will still get the tail off. If you used a bigger turbo (assuming the head could handle it). You'd sacrifice low end torque and power (as the turbo wouldn't spool up to full boost) for top end.

The answer to that is either twincharging (super and turbo charging), twin turbos, or variable geometry turbos.

 

[jakksincorpse]

As a note of interest I wouldn't even bother trying to think you've invented something new to patent it for any 2 or 4 stroke engine (conventional engine). Pretty much everything that works that you think is innovative has already been concieved of, probably by the 1930's. It's possible, but unlikely you've come up with something unthought of before.

I was astonished at just how much they patented but couldn't implement due to materials, or technology of the time. But everything on a very modern engine, that you think is cutting edge, like variable turbines, variable inlet geometry, variable compression, twin spark, direct injection (gdi), exhaust gas recirculation, etc etc was all concieved of years ago.


If the torque is trailing off top end, it's becuase the turbo is loosing boost, or it's not big enough or the cylinder head just can't flow enough. Probably a combination of the last two. Putting a smaller turbo on will likely create a flat torque curve if the boost is controlled, but the torque across all the range will be lower.

So if a big turbo boosting to say 14psi gets 150f.lb torque, a small turbo boosting to 8 psi will create less torque but will spool quicker meaning it will boost across a larger range. You will still get the tail off. If you used a bigger turbo (assuming the head could handle it). You'd sacrifice low end torque and power (as the turbo wouldn't spool up to full boost) for top end.

The answer to that is either twincharging (super and turbo charging), twin turbos, or variable geometry turbos.

its nothing spectacular. just a reverse supercharger, if there's already something like it, i haven't came across it yet. i mean ppl have always targeted on how much air goes in. why not innovate how it goes out? that could make a 4 stroke a 2 stroke and still be clean, instead of relying on the cylinders vacuum pressure alone, if you have 14psi of air/fuel being shoved in, why not 14psi of exhaust being sucked out.

so I'm going to take a wild guess and say the jettas 4 banger seems kinda pointless to modify if I'm going to use variable turbos. maybe a cummins straight 6 or fords v8 diesels would be a better choice, deff the cheaper choice. i can't find a freakin tdi for under 5 grand!

i'm really interested on the manifold still. i know the "anatomy" lol of a manifold, but how do you determine what size the pipes should be and how much they taper? could i take the stock manifold from a turbo'd diesel and port them to the correct size or are we talkin way bigger than the plans intended.

what about the exhaust ports. do they have to be changed as well? and if this all works out correctly and has a full rpm torque band like a hp band, how much more efficient will the motor be?

 

[Lsos]

its nothing spectacular. just a reverse supercharger, if there's already something like it, i haven't came across it yet. i mean ppl have always targeted on how much air goes in. why not innovate how it goes out? that could make a 4 stroke a 2 stroke and still be clean, instead of relying on the cylinders vacuum pressure alone, if you have 14psi of air/fuel being shoved in, why not 14psi of exhaust being sucked out.

It takes a certain amount of power to push or pull the exhaust out of the cylinder. Whether it is provided directly by the piston, or by a reverse supercharger (which is inevitably powered by the piston anyway) doesn't really matter. Except, I would imagine a supercharger that sucks the air to be less efficient (and much more expensive) than if it was pushed by the piston...and this is disregarding the inevitable losses incurred by transmitting the power from the crankshaft to the supercharger.

 

[xxChrisxx]

Exhaust valves are usually sized to be as big as possible, and inevitably flow is choked at higher rpm across the orifice so sucking any harder won't make any difference anyway.

At low rpm there is more than enough pressure differential and time for all the exhuast gas to be evacuated.

 

[jakksincorpse]

Exhaust valves are usually sized to be as big as possible, and inevitably flow is choked at higher rpm across the orifice so sucking any harder won't make any difference anyway.

At low rpm there is more than enough pressure differential and time for all the exhuast gas to be evacuated.

what do you mean the orifice is choked? like there's just not enough time for all the exhaust to spill out because the exhaust valve closes to fast?

 

[xxChrisxx]

what do you mean the orifice is choked? like there's just not enough time for all the exhaust to spill out because the exhaust valve closes to fast?

Choked flow (I linked to the wiki article earlier) is where the flow reaches mach 1 across an orifice. At this point no amount of pressure differential will make it flow more.

So sticking something to 'suck' the exhuast gas out would be pointless at rpm where it would be useful.

 

[jakksincorpse]

It takes a certain amount of power to push or pull the exhaust out of the cylinder. Whether it is provided directly by the piston, or by a reverse supercharger (which is inevitably powered by the piston anyway) doesn't really matter. Except, I would imagine a supercharger that sucks the air to be less efficient (and much more expensive) than if it was pushed by the piston...and this is disregarding the inevitable losses incurred by transmitting the power from the crankshaft to the supercharger.

i don't think there would be a huge loss. you can freely spin a turbocharger with ur hand. i figured running the hot side of a charger through the auxilery belt to drag out the exhaust would be pretty nifty. maybe if there were some changes in design of the motor this could be useful at high rpms.

 

[xxChrisxx]

i don't think there would be a huge loss. you can freely spin a turbocharger with ur hand. i figured running the hot side of a charger through the auxilery belt to drag out the exhaust would be pretty nifty. maybe if there were some changes in design of the motor this could be useful at high rpms.

Yes but a turbo is DRIVEN by exhuast. If you want to use it to EXTRACT them you are assentially using a pump. Whch requires power.

 

[jakksincorpse]

Choked flow (I linked to the wiki article earlier) is where the flow reaches mach 1 across an orifice. At this point no amount of pressure differential will make it flow more.

So you can stick something that will try to suck more exhuast out, but it simply won't flow any more fluid.

so...at this point ur not gaining any hp or torque yea? when does this usually happen in most vehicles?

 

[jakksincorpse]

Yes but a turbo is DRIVEN by exhuast. If you want to use it to EXTRACT them you are assentially using a pump. Whch requires power.

it couldn't be any more harmful than what your waterpump and oil pump do. far less power extraction than an alternator or an ac. plus, if the cylinder is capable of expelling all the exhaust by itself then why arent we seeing 2 stroke motors now? seems kinda pointless to have a intake and exhaust stage if all the exhaust can be pushed out in 1 stroke

 

[xxChrisxx]

so...at this point ur not gaining any hp or torque yea? when does this usually happen in most vehicles?

This is a very difficult answer to try and convey. Exhuast gas itsself has nothing to do with gains in torque and power. However it's pressure pulses that allow exhuasts and inlets to be tuned, which is why you get such elaborate exhaust shapes.

You've got to remember the ONLY way you gain torque (and therefore power at a given rpm) in an engine is by burning more fuel, and to do that you need to cram more air in the cylinder. Everything tuning wise has that goal in mind.

 

[jakksincorpse]

This is a very difficult answer to try and convey. Exhuast gas itsself has nothing to do with gains in torque and power. However it's pressure pulses that allow exhuasts and inlets to be tuned, which is why you get such elaborate exhaust shapes.

You've got to remember the ONLY way you gain torque (and therefore power at a given rpm) in an engine is by burning more fuel, and to do that you need to cram more air in the cylinder. Everything tuning wise has that goal in mind.

well i ment as in, if no exhaust is leaving the cylinder, then obviously an air/fuel mixture isn't going to combust right? which would result in crappy performance.

 

[xxChrisxx]

well i ment as in, if no exhaust is leaving the cylinder, then obviously an air/fuel mixture isn't going to combust right? which would result in crappy performance.

Pretty much all the exhaust gas does leave the cylinder. Once it's gone its gone. I don't see what it has to do with making more power.

The only way to get more exhaust out at very high rpm is to have bigger exhaust ports. In all engines, they are made as large as possible. If you look exhuast valves are always larger than intake valves to ensure a clean cylinder.

Also 2 stokes have dreadful emissions, as they burn oil and throw fresh charge straight out the exhuast due to them having no valves. 4 strokes for production engines don't have this problem.

 

[PaulS1950]

Jakksincorpse,
I am no physicist but I have been an engine builder and mechanic for 35+ years so take this for what it is worth;
Placing a partial vacuum on the exhaust is likely to make less torque and horse power due to the lean condition that will result by drawing part of the air fuel mixture out during the overlap period (that is when the exhaust valve is closing and the intake valve is opening). Not only will that increase HC emissions and reduce fuel mileage but it will also lower the pressure in the cylinder.
The reason compressors are put on the intake side is so more fuel is drawn in with the fuel raising the cylinder pressure and thus making more torque and more horse power.
What you are proposing has been done before - even without a mechanical pump. Headers with very good extraction can cause an engine to go lean enough that the mixture has to be made richer to compensate for it. Properly tuned exhaust and intake systems can provide for volumetric efficiencies of over 1.2 - essentially super-charging the engine but this only works at a very narrow rpm range when the effects gets to the exhaust valve before the intake valve opens.
I suggest you take a class on engines in a community college to get the basics of how and why they work.

Paul, the 60 year old student

 

[xxChrisxx]

Paul is so right, it is a good idea to get the basics down. Many of the ideas you are coming up with are creative, but with a more in depth understanding you will see why they will not work.

It would also make sense to get a course specifically about engines and transmission systems, as the best we can do here is give you some reference material which doesn't fully convey the fiddly pita nature of designing and optimising an engine.

 

[russ_watters]

Something missed before:

...instead of relying on the cylinders vacuum pressure alone, if you have 14psi of air/fuel being shoved in, why not 14psi of exhaust being sucked out. [emphasis added]

You didn't fully grasp what Chris was telling you here and though he said it, it wasn't completely made clear to you:

Exhaust valves are usually sized to be as big as possible, and inevitably flow is choked at higher rpm across the orifice so sucking any harder won't make any difference anyway.

At low rpm there is more than enough pressure differential and time for all the exhuast gas to be evacuated.

Since the exhaust is the burned fuel and air that just powered the stroke of the engine, you already have vastly more than "14psi of exhaust being sucked out." I'm not sure how much, but "choked flow" requires a minimum of something like 75psi, but I'd expect the initital pressure of the exhaust gases is far above that.

 

[xxChrisxx]

This acutally reminds me that I need to take my reference books to work! I really miss working on problems with things that acutally move. Static stuff is really boring.There is a formula to work out the critial pressure drop for choked flow, but I can't remember it at present.EDIT: Before we go any further. OP I suggest you get the Haynes "4 stroke performance tuning". by A Graham bell. It's a great book that's aimed at the enthusiastic amateur. So it only assumes a minimal amount of technical knowledge. You'll prbably be able to pick one up dirt cheap 2nd hand off amazon.

There was another book I was going to reccoment that outlined the basics of engine and transmission operation too, but I can't remember that. (memory like a seive these days)

 

[jakksincorpse]

Jakksincorpse,
I am no physicist but I have been an engine builder and mechanic for 35+ years so take this for what it is worth;
Placing a partial vacuum on the exhaust is likely to make less torque and horse power due to the lean condition that will result by drawing part of the air fuel mixture out during the overlap period (that is when the exhaust valve is closing and the intake valve is opening). Not only will that increase HC emissions and reduce fuel mileage but it will also lower the pressure in the cylinder.
The reason compressors are put on the intake side is so more fuel is drawn in with the fuel raising the cylinder pressure and thus making more torque and more horse power.
What you are proposing has been done before - even without a mechanical pump. Headers with very good extraction can cause an engine to go lean enough that the mixture has to be made richer to compensate for it. Properly tuned exhaust and intake systems can provide for volumetric efficiencies of over 1.2 - essentially super-charging the engine but this only works at a very narrow rpm range when the effects gets to the exhaust valve before the intake valve opens.
I suggest you take a class on engines in a community college to get the basics of how and why they work.

Paul, the 60 year old student

oh, well thanks for clearing that up, its fine, i took an autotech class, didnt really care for it. speed is more my thing.

 

[jakksincorpse]

like i said above guys, I'm not really into the mechanics of it, I am just ready to make a motor that's wicked fast and gets awesome gas mileage. like the cars in the SEMA competition. i would do anything to figure out how that mustang guy got 400hp out of a v8 and still got over 100mpg cruising the highway.

 

[PaulS1950]

He got the power with boost and got the mileage by not using the horse power.
The amount of power used is proportional to the amount of fuel burned.
Build a light and aerodynamic car with a relatively small engine with a turbo-charger on it and then cruise at 60 mph using a light throttle, no boostand an electronic engine management system and you can easily duplicate or even exceed those results.
The biggest factor in mileage is the nut behind the wheel (steering wheel that is). Just by changing your driving habits you can get an increase of up to 50% in your mileage. Add to that skinny, hard tires running on low friction bearings (loose but not sloppy) and a completely locked up drive train (rotating as a single member) and you begin to understand that we as drivers waste a lot more fuel than our cars do.