air flow

Hey buchay are u going to be at Selinsgrove Friday then penns creek Saturday can u please let me know thanks iceman oh and by the way cfm's on dual 360's are all they can get lol sorry.
 
That's real close Wes, that's about the number for a stock 360A, it can go up from there depending on the level of modification up to about 150 cfm for the pair. Jon
 
If 140cfm is a good number for an engine, is there any thought that it is more beneficial to get that 140 cfm from the dual setup, than it is to get the 140 cfm from a single carb (obviously a big 'un) :)
 
It really depends on the engine efficiency and the application the engine is going to be run in., there is a different calculation for the type of racing, for instance a drag car that stays at wot, vs. a car that goes from off idle to wot., (turns). the object is to stay as close to the optimum air/fuel ratio for max power and still provide enough cooling from the fuel.. while maintaing a good fuel signal..

Read Burnillis (spelling) law.. A good example of this is it easier to suck thru a straw or a garden hose, or another way to look at it.. in a venturi when press. increase's, velocity decrease's. and vice/versa.. I know there is no such thing as suck... LOL..

Basically in most any circumstance on a kart, where you have turns, the carb is going to be most responsive and easier to turn with multiple throttle bores..

I'm pretty sure Jon, has done some t/b- venturi ratio calculations, I have, and it;s not just the size of the 360 carb, it's the t/b-venturi ratios.. in another words, you may be better with a 380 with stock throttle bore and 360 size ventures. they may flow the same, at wot, but flow different from off idle to wot.. another couple combos that have been tried, 360/380, 360c/360a., dual 33mm's with different t/b-venturi ratios, dual 34mm's with different t/b-venturi ratios..

Having a dyno is a good tool for for seeing this..

Do a search on carb theory, there are different calculations for different engines, type of racing, type of fuel., etc.. if I remember for gas, the multiplication factor is .85 for a engine that will be run with turns, and 1.1 for a drag car..
 
A SBN pumps about twice as much as the old round top BN carbs so that isn't the problem. There is quite a bit of hardware in the throttle bore that can restrict flow, but like I said the amount of fuel they can deliver it makes it very viable for what we do.
 
Brian, It's been a long time since I flowed a 38 SBN and I don't have the data, but from memory I believe it was right around 140 CFM, Mike says he's flowed them maybe he can confirm that number or dispute it one. Jack, the alky double stack will move enough fuel for a big thirsty motor, a lot of the early HR conversions were done without doing the necessary mods to get enough fuel past the needle and seat, that's where the bad rep came from and why the grose jet became popular as a solution to that problem even though they would eventually start leaking and cause a tune problem, but with the cross hole drilled in the bottom of a stock alky seat and the appropriate mod to the carb body, fuel starvation there is no longer a problem. I built an HR196 for a GX390 running 25% nitro that worked fine, nitromethane will definitely test the carbs ability to move enough fuel. Ya Mike, I've calculated all of Tillotson's V. to TB ratios and have found them to be very consistent from the hl's to the HR's. The hl360A and the HR191 have exactly the same ratio of 1.12 to 1 in fact along with the Mikuni 34, the largest is 1.25 to 1 in a HR190A that has a venturi the size of a 181 and a shutter that's .020 bigger than a 191's. I'd say you'd always want a throttle bore that can out flow the venturi so that it doesn't become a restriction point in the carb, that's the whole point of the ratio, but once you reach that point then going any bigger isn't necessarily going to be better, I prefer to reduce the profile of the butterfly and shaft to pick up flow and velocity. Whether a larger V to TB ratio is gonna be helpful is really gonna depend on what your flowing into. If your flowing into a manifold that's gonna taper down to a 1 inch port, then smaller is better so that your manifold taper is shallower and can be shorter, both are good for flow. If your dumping into a big reed cage more ratio can be a good thing as you can pick up some flow without reducing the shutter and shaft profile. Jon
 
One thing is a certainty. (4 cycle). If the engine is well made, and sound, then the carb becomes THE factor in the engine's performance.
Do not, in any way, underestimate the importance of the carb. To max it's performance, a LOT of work must be done to it.
 
Jon I wish you would break it up in paragraphs, it's hard to follow for me, being just a ole redneck hillbilly... LOL.. I think I understand what your saying, and I agree..

Basically I like a carb that has a good size tb, and a smaller venturi than stock., this is mostly on my big opens and my 250...

Something comes to mind that always has come to mind with carbs, at one time I got a buller sudam that was used on a road course.. it had a 191 single ally carb, I tried, as well as a few other notable builders to get this carb work on dirt, this is stupid and I hate to admit it, but it virtually had no venturi... we just didn't notice it... it would scream wot, but had no low/mid response.. I still have it sitting on the shelf somewhere..

Flow numbers on the sbn, I can't remember, but it really don't matter, I;ll always run dual carbs on my engines, I have a set on my flat track framer 03 cr250 with my kart pipe.. it rocks..

BTW, did you ever talk with superhunky?? he's here in az., getting into some vintage mx.. also.. he was a friend of EC's
 
I might add.. a single carb, whether it's a sbn or a flat slide on a kart engine, is going to be marginal, most dirt bikes are made to run within a certain power band, with gears, from my testing a dual pumper assy, will be more responsive and easier to tune on a engine put on a kart with no shifting.. and it seems to work pretty good on a flat track bike with no shifting:)
 
You must log in or register to reply here.
Back
Top