bigger is not always better

[speedy1125]

Just read this at rebels and it makes perfect sense. this is mainly for those who are thinking of big carbs and intakes.


'......the 50cc GY6, for all intents was designed as a 72cc (80cc) engine.

18mm is the optimal intake diameter for a 4T, 72cc, 1 cylinder, peak torque @ 6500rpm, engine.

22mm is overbore

while 4mm does not seen like much, intake air speed is inversely proportional to the square of the intake radius.

18/2=9 9*9=81 22/2=11 11*11=121

1/81 (0.0123) v 1/121 (0.00826)

a secondary effect of lower intake air flow speeds is poor fuel air mixes.

lower flow rates cause uneven distribution of fuel droplets ( and larger droplets) in the intake charge and uneven combustion.


bigger is not always better.'

[90GTVert]

I always had better luck with the stock carb vs the 150 carb on my GY6. With some setups I could get a little more pull at higher revs, but I always lost response. I think the lack of an accelerator pump on the larger carbs plays a big role in that too.

[thunderkat59]

I learned this building hotrod cars in the 70's.
The guys who went straight for the 850 dbl pumpers w/ mechanical, solid lift -540 duration cams were running hi-13's or 14's and the guys running a 327 or 340 built right off the shelf with 650 vac / Holleys- hydro lifters were running 13.3's (Shout out to Cecil County, yo!! )

If you have the time and patience to read what Harley drag bike guys do, they have this stuff down to a "T". They have to run the old dinosuarous rex motors almost to the point of melting when the get the HP they need sp the fuel-air / air-velocity-through-all-4-cycles stuff is big with them. Interestingly, they are not big fans of super hi-octane. I forget why, but I think it has to do with flash temp, lenghth of burn time, etc . . .
Ok, I need to switch to decaf . . .
Im going out now to the Ducati shop to get my lil hot rods first jug of synthetic. A proud day for a dad !

regards,
LM in Ky

[rocketdog]

"The guys who went straight for the 850 dbl pumpers w/ mechanical, solid lift -540 duration cams were running hi-13's or 14's and the guys running a 327 or 340 built right off the shelf with 650 vac / Holleys- hydro lifters were running 13.3's (Shout out to Cecil County, yo!! )"

Almost always a mistake and I agree with you Thunder. I had a bored 440 Dodge and I ran the stock Carter AFB.

"A secondary effect of lower intake air flow speeds is poor fuel air mixes."

Another reason not to polish the intake port to the inth degree. The rough texture of the surface helps keep the fuel suspended. The exhaust benifits more from that treatment. And as I have seen GT say before, going overboard on the valve size deceases velocity as well.

RD

[reveeen]

For a conventional carburetor it's true.......... but a CV (or vacuum) carburetor acts/behaves/works differently. The air flow through a CV carb is solely controlled by the vacuum operated slide..... the throttle may be wide open, without the ability to pull the slide open it (the slide) isn't going anywhere. A CV carb is a very forgiving carb, able to operate smoothly where a conventional carb wouldn't, and able to moderate air flow as the demand dictates.

Combined with the fact that we all go for lighter rollers in the transmission (to keep the RPM up), air flow figures based on 6500 rpm are not too realistic in my garage.

I am not disputing anything here........ just trying to point out hat everyone has different needs/solutions, what works for one does not necessarily work for another.

[rocketdog]

No offense taken Reveeen. And I agree with you about the CV Carb., up to a point. They still require a strong vacuum signal from the engine in order to operate well. Putting a 24mm throated carb on an engine that only has an 18mm intake, or intake valve for that matter, doesn't increase your horsepower or help it's driveability.

RD