Planet Minis

Kurlon · 01-09-2006, 02:26 PM

Based on the theory that HP x 1.62 = CFM is reasonable, my 70 is coming up with a rather horrific 75% VE. I need real flow measuring gear...

In any case, assuming my VE doesn't change when I swap piston/crank/cylinder, I'm looking at a 10hp 110. Is it reasonable to assume VE will stay the same with a displacement change, as I'm keeping the head/cam/exhaust and plan to match compression ratios? I'll be recarbing to match, so intake and carb are variable.

If my math is right, there is a Kitaco SE 124 mill with 135% VE... wtf?!

thorwarrior915 · 01-09-2006, 02:29 PM

snap...those 124's are efficient. they are making power to spare..haha. Kurlon, you should work for the NASA of fiddies. Unfortunately so such place exists.

Assuming your math is right, can you just recalibrate 135 as 100% and adjust your other figures from there? I was never good at math....

FastMC · 01-09-2006, 02:37 PM

Holy crap Kurlon!!!!!!!!!!! This sh!t must keep you up at night!!!!!!!!!!!

xr1million · 01-09-2006, 02:43 PM

yea the flux capacitor on the muffler bearing of my momentus climatisc, isnt quite running as efficient as your 70 but its pretty close.

rida · 01-09-2006, 03:05 PM

^ weird i was thinking the same thing xR1MILLION.

Kurlon · 01-09-2006, 03:07 PM

Quote:

Originally Posted by thorwarrior915

snap...those 124's are efficient. they are making power to spare..haha. Kurlon, you should work for the NASA of fiddies. Unfortunately so such place exists.

Assuming your math is right, can you just recalibrate 135 as 100% and adjust your other figures from there? I was never good at math....

Greater than 100% VE is possible without a turbo/supercharger/etc, so I can't discount that info as false and try to re-calibrate around it. The odds that I'll match that stunt on this build... fairly low. I'm still working my way through the theory.

daddysmut · 01-09-2006, 03:18 PM

in english please

p3racer · 01-09-2006, 03:23 PM

Throw That Crap Out The Window And Put Your Ass On The Seat And See What The Hell Its Doing

Kurlon · 01-09-2006, 04:30 PM

Quote:

Originally Posted by p3racer

Throw That Crap Out The Window And Put Your Ass On The Seat And See What The Hell Its Doing

Heh, there in lies the rub. I have a fairly reasonable amount of spare time that I'd love to dedicate to tinkering and riding. I live in an apartment complex, and have to store my bike at a shop lest it get stolen out from under me. This limits the hours I can work on it such that I only get to toss an hour or two a week at it, tops. So I try to maximize my effort by nailing the theory down ahead of time.

And in english...

A motor is nothing more than a glorified air pump. Move more air, make more powah.

20CFM of air, with a 13:1 air/fuel ratio has approximately 12hp of power generating capability in a 4 stroke motor. In other words;
HP x 1.62 = CFM

Keep this in the back of your head.

Looking at my current motor, I know it's displacement, I also know the HP at any given RPM thanks to the handy dyno. My mixture should be close to 13:1 based on my plug readings. So, looking at the dyno output, I tagged 6.8hp at 11000 RPM. Based on the above, to generate 6.8hp, I had to be pulling in just over 11 cubic feet of air per minute.

"So what?"

Remember, I said a motor is an air pump. A lil math will tell us how much air that pump can move using;
( CI x RPM x VE ) / 3456 = CFM

CI = Displacement in cubic inches
RPM = if I have to explain this, stop reading
VE = Volumetric Efficiency, aka how good a pump are we talking?

Plugging in the numbers, using 11000 RPM, where my motor peaks, I get 14CFM.

"Back the truck up, the numbers don't match, you can't do math, go home!"

Yup, this difference between the theoretical CFM, and what you actually achieve is the Volumetric Efficiency. In this case, I'm flowing at 78% efficiency. Turbo motors and others that can PUSH air into the motor can flow at BETTER than 100% efficiency. Flow twice as much air (Which you measure as 14psi boost...) and you should generate twice the HP.

78% isn't bad, but I should be doing better. If I was actually flowing at 100%, using the formula for converting HP and CFM, I'd be generating 8.6hp at 11000 RPM.

The motor build I'm putting together puts me at 110cc. At 11000 RPM that flows 21.4 CFM in theory, or 13hp at 11000 RPM, in theory. If I only get 75% VE, that knocks me down to 10hp at 11000 RPM. A good number, but I'm hoping 12hp is reasonable for this setup.

So, my question at this point, is weather or not increasing displacement but running the same head should improve my VE as well, ruin it, or not really impact it? That'll help me estimate where my HP will be when I go to run it on the dyno, as well as help indicate if I have a serious mismatch somewhere if the numbers DON'T line up.

For fun... Fatcaaat's 186cc motor is capable of 22.3hp if he can keep the VE at 100%. : )

3rdGearPinned · 01-09-2006, 05:03 PM

Well im trying to gather all this info in at once and try to let a little out at the same time here so bare with me if im totally off.

But shouldnt a larger piston along with a longer stroke "suck" more air/fue; raising the VE?

Could the 24mm carb be lowering your VE percentage with such a small bore? (motor cant keep up with the carb so it doesnt build up much velocity)?

Like i said im just taking in all the info, i really dont know if im on the right track or not?

jobobbjones · 01-09-2006, 05:24 PM

Peak hosepower is a false god. What you want is average power. How far and for how long would you have to drive to get to 11k rpm? Pretty far any way you do the math, If you ride in dirt and race you almost never top out. Even if your on the street you would need a lot of room to get up that high.

If I were you I would focus on your lower power range that is where it will matter. The bike/car with more tourqe wins because it takes off faster from a stop even if the person you are racing has more power if you get ahead first it is easier to stay there

What I mean is, Do your theory are rpms you will be using. Also doesn't your air fuel mixture change all the way through the powerband. Try to find your weak spots when you have a bad air/fuel mixture and your VE is down.

All I am trying to say is try to figure out what your power will be at a lower rpm, because I wonder how many times you are ever going to 11k rpm.

Post up your dyno if you have it.

jdmp · 01-09-2006, 05:29 PM

ummmmm ok.
helpful future info though, I cant really comprehend it all at once..haha

fastbmw · 01-09-2006, 05:30 PM

power is nothing if you weigh 300lbs. Lose some weight, it's free horsepower....

hahaha

I've heard for a car, every 10 lbs loss is approx. the same as a 1 horsepower gain.

thorwarrior915 · 01-09-2006, 05:38 PM

It does seem like there are a LOT of varibles that could mess the calculations up all over the place. You could get a general idea crunching numbers maybe, but the proof is in the puddin. Jason has a point with the different velocity with a bigger carb. What do you think about that?

xr1million · 01-09-2006, 05:48 PM

yea but combustion chamber size, port size and shape, valve size, valve timing and many other influx of things.

MadHumanDizease · 10-05-2008, 12:54 PM

In theory I would have to say that as long as the Intake, ports, valves, cam, head, and exhaust where all keep the same, it wouldn't change.. You would just be moving more air at 78% or whatever.

Unfortunatly Im still 2 classes shy of our performance clinic where we break out the dynos and bench flow tests, but if your still having this dilema in a few weeks, lol.

dax · 10-05-2008, 01:41 PM

I think the bigger cc's trying to suck the mixture through the same intake valve and intake port (and the same carb and aircleaner?) will result as a lower volumetric efficiency.

JJW Designs · 10-05-2008, 01:53 PM

This stuff keep you up at night Kurlon?

his section, we will look into three engine efficiency topics: Volumetric Efficiency, Thermal Efficiency and Mechanical Efficiency.

Volumetric Efficiency

Actual CFM

= Volumetric Efficiency

Theoretical CFM

Volumetic Efficiency or VE, I will be using from this point, varies depending on temperature and pressure.

From that, we know a normally aspirited engine will have VE of 100% or less. And force inductioned engine will have VE of 100% or more.

The actual calculation of VE is done by ECU using measured amount of intake air, with Mass Air Sensor measuring at intake pipe or Speed Density measuring inside the intake manifold (close to intake port of the engine).

* Theoretical CFM

theoretical cfm = rpm x displacement / 3456

* Engine Flow Demand

Engine Flow= (engine displacement) X (volumetric efficiency) X (engine speed) X (manifold pressure)

You can see the key to increase engine flow is to increase engine VE (volumetric efficiency). Reduce intake charge temperature is the easiest way to help increase engine VE. This is where air/air, air/fluid intercooler and water injection come into play.

Assuming VE at 100%, 1 atmosphere pressure, we have the following table for a 3.0L engine flow( CFM) at various rpm and pressure point.

* Influence of the altitude above Sea Level on the Volumetric Efficiency

Influence of the elevation above Sea Level on the Volumetric Efficiency

Atmospheric pressure as given by average barometer reading
Height above sea level, ft. Atmospheric pressure, in. of mercury Atmospheric Pressure, in PSI (approximate) Relative volumetric efficiency
0 29.92 14.7 1.000
1,000 28.85 14.2 0.965
2,000 27.82 13.7 0.931
3,000 26.82 13.2 0.892
4,000 25.85 12.7 0.865
5,000 24.92 12.2 0.833
6,000 24.00 11.7 0.803
8,000 22.17 10.7 0.742
10,000 20.34 8.7 0.681
12,000 19.30 6.7 0.645

*
Engine Flow Measurement

Most engine control systems in production today utilize either speed-density sensors or air-mass sensors to measure engine air flow. Speed-density is very popular because of its low cost and high reliability. Speed-density systems typically calculate engine flow rate based on engine speed, intake manifold pressure and temperature. Some systems use barometric pressure sensors and inlet air temperature sensors to improve flow calculation accuracy for varying ambient conditions. Fig. 1 illustrates a typical engine with both speed-density sensors and an air-mass sensor.

fig001.gif (6059 bytes)

Figure 1: TBI Engine with Air-Mass Sensor and Speed-Density Sensors*

* Speed - Density

eed-density systems calculate an air flow rate that approximates the flow rate at At the intake ports, mdot_ao. Air-mass sensor systems usually measure the air flow rate near the throttle and thus approximate mdot_ai. During throttle transients (or whenever the manifold pressure fluctuates) the flow rates in and out of the manifold are different from one another, i.e., . This is illustrated in Fig. 2 for a rapid throttle transient.

fig002.gif (5644 bytes)

Figure 2: Air flow rates during a rapid throttle transient

The large in-rush of air during the throttle opening is often referred to as "manifold filling." During this time the flow rate into large manifolds can be several hundred percent higher than the flow rate at the ports.

Speed-density systems calculate air density in the intake using manifold pressure and temperature sensors and the perfect gas law. The air-mass flow rate at the intake ports, mdot_ao, is assumed to be quasi-steady and may be calculated as:

eq001.gif (2000 bytes)

The volumetric efficiency,eta_vol.gif (266 bytes), of the engine is usually mapped from steady-state air flow measurements and stored in the controller's Read Only Memory (ROM) as a table having inputs of manifold pressure and engine speed as shown in Fig. 3:

firepower354 · 10-05-2008, 03:23 PM

Some of us won't settle for under 100% VE on an N/A motor

Runner diameter and length tweaks can keep you from having to settle, too

01-09-2006, 02:26 PM	#1 (permalink)
Kurlon Baron Von BanHammer Join Date: Feb 2005 Location: Windham, ME Posts: 7,865	Motor Theory - Volumetric Efficency Based on the theory that HP x 1.62 = CFM is reasonable, my 70 is coming up with a rather horrific 75% VE. I need real flow measuring gear... In any case, assuming my VE doesn't change when I swap piston/crank/cylinder, I'm looking at a 10hp 110. Is it reasonable to assume VE will stay the same with a displacement change, as I'm keeping the head/cam/exhaust and plan to match compression ratios? I'll be recarbing to match, so intake and carb are variable. If my math is right, there is a Kitaco SE 124 mill with 135% VE... wtf?! __________________ http://www.x386.net/TTR 2004 ST3 - 'Duckie'?!?! 2005 CRF70RR - MiniGP Weapon 2005 CRF70 - Dirt Donk 2008 Pitster Pro X4 Motard 1991 GS500E - LRRS/CCS #771

01-09-2006, 02:37 PM	#3 (permalink)
FastMC Sporting the Merkin Join Date: Mar 2004 Location: boston area Posts: 4,467 Awards Showcase Total Awards: 1	Holy crap Kurlon!!!!!!!!!!! This sh!t must keep you up at night!!!!!!!!!!! __________________ The older I get...The faster I was !!!

10-05-2008, 12:54 PM	#16 (permalink)
MadHumanDizease 1st Gear Member Join Date: Sep 2006 Posts: 102	Re: Motor Theory - Volumetric Efficency In theory I would have to say that as long as the Intake, ports, valves, cam, head, and exhaust where all keep the same, it wouldn't change.. You would just be moving more air at 78% or whatever. Unfortunatly Im still 2 classes shy of our performance clinic where we break out the dynos and bench flow tests, but if your still having this dilema in a few weeks, lol.

10-05-2008, 01:41 PM	#17 (permalink)
dax 1st Gear Member Join Date: Jun 2006 Posts: 113	Re: Motor Theory - Volumetric Efficency I think the bigger cc's trying to suck the mixture through the same intake valve and intake port (and the same carb and aircleaner?) will result as a lower volumetric efficiency.

10-05-2008, 01:53 PM	#18 (permalink)
JJW Designs Advertiser Join Date: Feb 2006 Location: La Center, WA Posts: 2,408	Re: Motor Theory - Volumetric Efficency This stuff keep you up at night Kurlon? his section, we will look into three engine efficiency topics: Volumetric Efficiency, Thermal Efficiency and Mechanical Efficiency. Volumetric Efficiency Actual CFM = Volumetric Efficiency Theoretical CFM Volumetic Efficiency or VE, I will be using from this point, varies depending on temperature and pressure. From that, we know a normally aspirited engine will have VE of 100% or less. And force inductioned engine will have VE of 100% or more. The actual calculation of VE is done by ECU using measured amount of intake air, with Mass Air Sensor measuring at intake pipe or Speed Density measuring inside the intake manifold (close to intake port of the engine). * Theoretical CFM theoretical cfm = rpm x displacement / 3456 * Engine Flow Demand Engine Flow= (engine displacement) X (volumetric efficiency) X (engine speed) X (manifold pressure) You can see the key to increase engine flow is to increase engine VE (volumetric efficiency). Reduce intake charge temperature is the easiest way to help increase engine VE. This is where air/air, air/fluid intercooler and water injection come into play. Assuming VE at 100%, 1 atmosphere pressure, we have the following table for a 3.0L engine flow( CFM) at various rpm and pressure point. * Influence of the altitude above Sea Level on the Volumetric Efficiency Influence of the elevation above Sea Level on the Volumetric Efficiency Atmospheric pressure as given by average barometer reading Height above sea level, ft. Atmospheric pressure, in. of mercury Atmospheric Pressure, in PSI (approximate) Relative volumetric efficiency 0 29.92 14.7 1.000 1,000 28.85 14.2 0.965 2,000 27.82 13.7 0.931 3,000 26.82 13.2 0.892 4,000 25.85 12.7 0.865 5,000 24.92 12.2 0.833 6,000 24.00 11.7 0.803 8,000 22.17 10.7 0.742 10,000 20.34 8.7 0.681 12,000 19.30 6.7 0.645 * Engine Flow Measurement Most engine control systems in production today utilize either speed-density sensors or air-mass sensors to measure engine air flow. Speed-density is very popular because of its low cost and high reliability. Speed-density systems typically calculate engine flow rate based on engine speed, intake manifold pressure and temperature. Some systems use barometric pressure sensors and inlet air temperature sensors to improve flow calculation accuracy for varying ambient conditions. Fig. 1 illustrates a typical engine with both speed-density sensors and an air-mass sensor. fig001.gif (6059 bytes) Figure 1: TBI Engine with Air-Mass Sensor and Speed-Density Sensors* * Speed - Density eed-density systems calculate an air flow rate that approximates the flow rate at At the intake ports, mdot_ao. Air-mass sensor systems usually measure the air flow rate near the throttle and thus approximate mdot_ai. During throttle transients (or whenever the manifold pressure fluctuates) the flow rates in and out of the manifold are different from one another, i.e., . This is illustrated in Fig. 2 for a rapid throttle transient. fig002.gif (5644 bytes) Figure 2: Air flow rates during a rapid throttle transient The large in-rush of air during the throttle opening is often referred to as "manifold filling." During this time the flow rate into large manifolds can be several hundred percent higher than the flow rate at the ports. Speed-density systems calculate air density in the intake using manifold pressure and temperature sensors and the perfect gas law. The air-mass flow rate at the intake ports, mdot_ao, is assumed to be quasi-steady and may be calculated as: eq001.gif (2000 bytes) The volumetric efficiency,eta_vol.gif (266 bytes), of the engine is usually mapped from steady-state air flow measurements and stored in the controller's Read Only Memory (ROM) as a table having inputs of manifold pressure and engine speed as shown in Fig. 3: __________________ Graphic designer by trade, rider by heart.

01-09-2006, 02:29 PM	#2 (permalink)
thorwarrior915 3rd Gear Member Join Date: Dec 2003 Location: Malone, NY Posts: 4,483	snap...those 124's are efficient. they are making power to spare..haha. Kurlon, you should work for the NASA of fiddies. Unfortunately so such place exists. Assuming your math is right, can you just recalibrate 135 as 100% and adjust your other figures from there? I was never good at math....

01-09-2006, 02:43 PM	#4 (permalink)
xr1million 3rd Gear Member Join Date: Dec 2005 Location: Just south of hell, FL Posts: 4,596	yea the flux capacitor on the muffler bearing of my momentus climatisc, isnt quite running as efficient as your 70 but its pretty close.

01-09-2006, 03:05 PM	#5 (permalink)
rida 2nd Gear Member Join Date: Apr 2005 Location: NH Posts: 341	^ weird i was thinking the same thing xR1MILLION.

01-09-2006, 03:18 PM	#7 (permalink)
daddysmut 3rd Gear Member Join Date: Dec 2004 Location: O-town florida Posts: 3,167	in english please

01-09-2006, 03:23 PM	#8 (permalink)
p3racer Founding Member Join Date: Sep 2003 Location: FLORIDA Posts: 5,861 Awards Showcase Total Awards: 2	Throw That Crap Out The Window And Put Your Ass On The Seat And See What The Hell Its Doing

01-09-2006, 05:03 PM	#10 (permalink)
3rdGearPinned 3rd Gear Member Join Date: Dec 2003 Location: Annapolis, MD Posts: 5,729	Well im trying to gather all this info in at once and try to let a little out at the same time here so bare with me if im totally off. But shouldnt a larger piston along with a longer stroke "suck" more air/fue; raising the VE? Could the 24mm carb be lowering your VE percentage with such a small bore? (motor cant keep up with the carb so it doesnt build up much velocity)? Like i said im just taking in all the info, i really dont know if im on the right track or not?

01-09-2006, 05:24 PM	#11 (permalink)
jobobbjones 1st Gear Member Join Date: Jan 2005 Location: El Cajon CA Posts: 124	Peak hosepower is a false god. What you want is average power. How far and for how long would you have to drive to get to 11k rpm? Pretty far any way you do the math, If you ride in dirt and race you almost never top out. Even if your on the street you would need a lot of room to get up that high. If I were you I would focus on your lower power range that is where it will matter. The bike/car with more tourqe wins because it takes off faster from a stop even if the person you are racing has more power if you get ahead first it is easier to stay there What I mean is, Do your theory are rpms you will be using. Also doesn't your air fuel mixture change all the way through the powerband. Try to find your weak spots when you have a bad air/fuel mixture and your VE is down. All I am trying to say is try to figure out what your power will be at a lower rpm, because I wonder how many times you are ever going to 11k rpm. Post up your dyno if you have it.

01-09-2006, 05:29 PM	#12 (permalink)
jdmp 3rd Gear Member Join Date: Jun 2005 Location: pinellas FL Posts: 4,475 Awards Showcase Total Awards: 1	ummmmm ok. helpful future info though, I cant really comprehend it all at once..haha

01-09-2006, 05:30 PM	#13 (permalink)
fastbmw 2nd Gear Member Join Date: Sep 2005 Posts: 604	power is nothing if you weigh 300lbs. Lose some weight, it's free horsepower.... hahaha I've heard for a car, every 10 lbs loss is approx. the same as a 1 horsepower gain.

01-09-2006, 05:38 PM	#14 (permalink)
thorwarrior915 3rd Gear Member Join Date: Dec 2003 Location: Malone, NY Posts: 4,483	It does seem like there are a LOT of varibles that could mess the calculations up all over the place. You could get a general idea crunching numbers maybe, but the proof is in the puddin. Jason has a point with the different velocity with a bigger carb. What do you think about that?

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01-09-2006, 05:48 PM	#15 (permalink)
xr1million 3rd Gear Member Join Date: Dec 2005 Location: Just south of hell, FL Posts: 4,596	yea but combustion chamber size, port size and shape, valve size, valve timing and many other influx of things.

10-05-2008, 03:23 PM	#19 (permalink)
firepower354 3rd Gear Member Join Date: Oct 2006 Location: Flint MI Posts: 6,266	Re: Motor Theory - Volumetric Efficency Some of us won't settle for under 100% VE on an N/A motor Runner diameter and length tweaks can keep you from having to settle, too Last edited by firepower354; 10-05-2008 at 03:30 PM.