Author Topic: Semi off topic technical questions  (Read 1513 times)

Blake

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Semi off topic technical questions
« on: June 07, 2005, 12:11:52 PM »
A couple ?Quick? questions for all of the fellow engineering/designing minds out there.

?Theoretically??

If I have a single 4 stroke cylinder of 1,025cc, max RPMs is about 4,500 rpm, with most use around 1500rpm - 3000rpm, what would be a good (or best/ideal if you choose) carb size?  I figure the cfm for this cylinder to be around 100cfm (if I figure it correctly) with a little bit of head room.  In other words, I have no clue about flow ratings for mikuni/keihn carbs and was trying to figure it out.  HOWEVER..A dirtbike site I found the following:

?With all racing engines, the more air you can flow into the cylinder at a high velocity, the more power can be obtained. Below are measurements of different reed valve assemblies measured at 10" of pressure drop, with all figures corrected for temperature and humidity. Note that CFM is cubic feet of air per minute.
80cc engines ? 26/28mm carb
125cc engines ? 34/35mm carb
250cc engines ? 35/36mm carb
500cc engines ? 36/38mm carb
These are estimated Flow Rates of a popular carburetor.
34mm 109.15cfm
35mm 114.70cfm
36mm 109.33cfm
28mm 129.50cfm?

now I assume ?popular carb? is a mikuni round slide. So lets say with the 500cc engine it revs to about 8 or 9k or so.  Wouldn?t the cfm be the same if the cylinder was 1000cc going half the rpms? (4,000 or 5). So in that case according to their recommendations, it should be a 36/38mm carb (but also I know it?s a two stroke engine they talk about, so that throws it all off).



Also,

For Driveability (comfort) yet good power on hard acceleration, would a CV be the way to go?


Now this next part is where im having the most difficult time.  Port length.  Is there any significant difference as to the port length between the carb and engine?  Does shorter port length move the power band up, longer down? Or is putting the carb as close to the intake as possible the best way to go? Or does it even matter?  I know ?tuned port length? tries to distance match the reverberation of the intake pulse so it gets back to the valve at the right time, but honestly im not that in-depth with what I want to know.  Basically im thinking best response is the goal, but im just wondering if I put the carb right on the intake (or an inch or so off) would there be any drastic effects?

Finally, (last part)

Now don?t call me crazy.. this is ?hypothetical.?  Now I know we?ve all said it many a times, that the vision carb is basically a down sized car type carburetor used on a motorcycle, and also many harley guys use car carbs all the time ?weber, etc.?  the question is, if people frequently use car-intended carburetors on a motorcycle, should there be any reason why you cant use a motorcycle-intended carb (round slide, flat slide, keihn down draft, CV, etc) on a car? (if cfm requirements were the same).  If the air requirements (low, mid, high speed circuits) THAT much different than that of a motorcycle engine?

If anyone has any insight I?d greatly appreciate it.
(I hope all my feeble attempts at keeping it ?hypothetical? doesn?t give away my thinking J )  ;D

Honestly just wondering.


Thanks,
Blake
"At first it's like a new pair of underware... Frustrating and constrictive.  But then, it kind of grows on you..."

Coil Coyle

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Re: Semi off topic technical questions
« Reply #1 on: June 07, 2005, 06:51:09 PM »
Blake,
? ? I the 70's i recall Gordon Jennings musing that if you are considering the entire length of the intake tunnel from the valve to the open end, that he thought you should put the carburetor's venturi about two/thirds of the way from the valve.
? ? Your torque peak is at the maximum cylinder filling RPM. I think that maximum fill occurs at a crossover between increasing charge velocity and increasing drag of the charge against the port wall- Then there is a spike of increased drag as the flow needs to become supersonic but cannot.
? ? ?Your horspower peak is going to be at the intersection of the best cylinder fill (torque) at the highest RPM.
? ? ? If you put a big enough carb bore you can move the RPM that will carburate clean so high that your engine is limited by the port or the mechanical pieces.
? ? ? ?If you decrease the carb bore you will move the RPM that will carburate clean down, but you will not increase the cylinder filling until you decrease the port diameter (increase the charge velocity)
? ? ? :oAs long as you have not changed the cam timing ;DWhat are you thinking? An 8.5 Liter V-8 with individual carbs?

coil
 

« Last Edit: June 07, 2005, 06:52:51 PM by coilXZcoyle »

Blake

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Re: Semi off topic technical questions
« Reply #2 on: June 07, 2005, 09:34:29 PM »
500cui v8 cadillac to be exact  ;D   I got a "new" toy. :)  granfathers 76 fleetwood brougham..just musing.... thought itd be an interesting idea (scenereo) to figure out.

so if i understand you correct about the runner legnth.....

are you saying take 2/3's the legnth from the valve to the opening and make that the distance from the opening to the carb? so lets say its 6" from valve to intake opening. so the carb should be 4" from the intake opening? 


like i said.. just musing.  with my friend the machinist, with what im thinking i dont think itd be all that difficult to fab up mounts for the carbs.  most difficult thing would be making a throttle shaft to run the legnth of the 4 carbs and making sure all 4 (on each side) are exactly alligned..   i think ill lead up autodesk one of these days and see if i cant scribble something up for a few different ideas already have a few..

thanks for the help,

Blake
"At first it's like a new pair of underware... Frustrating and constrictive.  But then, it kind of grows on you..."

Coil Coyle

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Re: Semi off topic technical questions
« Reply #3 on: June 09, 2005, 05:56:12 AM »
Blake,
      Gordon was saying that the "Venturi" should be at the 2/3 point, if you were dealing with fuel injection (no venturi) you would just worry about the length.

      Your intake length should reflect a reverse (positive) wave back down from the tube opening to arrive at the valve just at the time the valve is closing and the piston has begun increasing the cylinder pressure (overlap, the valve is open while the piston reverses). This wave gives the final fill to the cylinder for the torque peak.

      So, you take the RPM you want the torque peak...
divide it into a minute...Time per revolution... Times 2... Time for 4 cycles...
multiply that time by the ratio between the 4 strokes (720 degree ) and the intake timing to get the time the intake is open ( I think Gordon said to use the point the valves were open 10 thousandths because before that there is really no flow) so knock 20+/- degrees off of your cam duration for this number...

OK, with your intake flow duration time now in hand you have to figure what the speed of sound is in your intake tract.  Any of you engineers got a number for
the speed of sound in say a slight vacuum and slightly warmer that atmospheric standard??

Take this speed in distance per time and you will have the distance the pressure wave will travel while the intake tract is flowing.

Divide this distance in half (the negative wave travels to the end of the tube-a positive wave reflects back when the negative wave leaves the open end)

Thats your best port length for that RPM

Or if your vision peaks at 9000 RPM and you want your Cadillac to peak at 4500 RPM just trust that Yamaha allready did the math and measure your vision intake from valve seat to intake opening in the air box and double it. (1/2 RPM = 2 times the distance for the wave to travel, the speed of the wave stays the same )

On Trans Am sports cars in the early 70's Chaparral had 454 big blocks where they tuned 4 cylinders for high RPM (short 8-10" intakes) and 4 cylinders for low RPM (14-16" intakes) to get a wide torque peak that the tires could put down to the pavement, rather than a high peak that just shredded the tires.

Have Fun,

coil

Lucky

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Re: Semi off topic technical questions
« Reply #4 on: June 09, 2005, 11:07:02 PM »
Ouch Coil! you gave me a headach! lol
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