too much duration

JRW31526

JRW31526

Member
how much is too much duration, say you had a isky 315 that cam has 260 duration, say you also had a 25mm carb and 28.5 27mm valves, would their be too much duration for the motor to run at its best?
 
Generally speaking, and I mean generally, 260 dur is the standard for big Hp kart engines, 260 dur will still give you some low end power to get you off a corner,it's really more a matter of engine operation range then anything else, just remember you will have to feed the engine, so good port job and carb are a must if you want to use the cam to it full potential. I personally don't build many small blocks, I work on mainly big blocks so I don't no what the right size carb is best for you to use, but I another general rule is try to keep carb same or a little smaller then valve size, for best engine response.
 
Not that I’m any kind of expert on cams, but I’ve had some experience with them. Before karting I was kind of heavy into drag racing.
My experience is that duration is only one part of grinding a cam. Lift rates are another. Our 320, (or was it 360. I’m talking mid 60’s) degree Buz Schaller roller cam, in our 389 inch small block Chevy, would run really rough below 5000 RPM. Not something you would want in a kart engine. How fast you open and close the vales makes a difference. The faster you open them, the more time they’re wide open.
Bigger carbs are, to a limit, good, but again, you increasingly lose low end the bigger you go. The bigger the hole, the more time it takes to get that column of air moving. There is a tuned length in the carb, and the bigger the hole, the more time it takes to get the column moving, the higher up in the RPM range the ideal tuned length gets.
There is a “tuned” length all the way thru the engine, and that “tuned” length is broken up into several sections, and there is a point in the RPM range where those sections all become “in tune” with each other at their most ideal. Changing the length, or the cross section, of any of those sections, can change their tuned length, and this can change the RPM where all the sections work together best.
One thing is apparent to me; it’s far more complicated than I can explain.
Comments, Compliments, Criticisms and Questions always welcome.
 
It really depends a lot on corner speed , in my opinion. A cam that's 260 @.050 wont make much power in lower rpms. The bigger the engine, the more duration can be used (to a point). Also, the worse a head flows, the more duration it will need (to a point). I would say with what you have posted, you will need to be turning a minimum of 6500 to 7000 off the corner.
 
that's a big cam should make good power up top!!! but I don't think it will like any kind of rpm under like 4,000rpm... im running his black mamba sr. with over size lifters and 1:3 rockers in a 212 and im still working on my clutch lock up... I know it will chug at 4,500rpm... but its a monster!!! an you have more cam then I do!!! so I bet it doesn't clean up tell 5,000+... what rpm are you shooting for??? 9,000rpm???
 
JRW31526 said:
well considering that i am more of a "top end" guy i dont think i will have a prob with this cam
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Don't under estimate what Barry and Stubbie are saying..."IF" you are dealing w/ the need for good low end power. I too like too buzz them and I am pulling very hard at 8K with a Dyno '275' (not f275). The reality is that it is telling me that it wants about 42-4500 stall rpm and thats tough too get w/ a drum-clutch.
 
im hitting full lock up at 4,500 an I think im still a hair low screaminclones... id like to know what kind of rpm hes shooting for??? that's a monster cam an you don't hear much on iskys big cams
 
i would like to turn 9500+ 18/53 gearing i know its really low, but my current gearing 12/53 is too high, my motor now with a 265 cam pulls from about 4000+ with that high gearing to about 6k and quits pulling at ~6500-7000 because of the carb im using,on a normal day, on a good day about 7500-8k or close to that,
 
18/53 is 2.9 ratio never heard anyone using that it will be a dog for the first lap or so, better hope you don't have to use your brakes, or have any corners.
 
yeah, i know, i run a very large oval or i race on a 1/4 mile strip, im dragracing my friend who has a briggs flathead with a 19/61 gear ratio, with how much more hp i will have i dont think ill have a problem with getting him off the start, and i meant 16/53 is what gear im using, which i think is good enough at the start and for top end, his motor runs about 8500 max if that with the cam he has, more of a torque cam
 
Well 12/53 to 16/53 is a big difference and then jump to 18/53 massive. If you are drag racing and want 9500 RPM I don't think going to 18/53 is your answer you are going the wrong way. I still say at a 2.9 ratio a cat will walk past you on the start.
 
hopefully with that cam 1.2 rockers and big valves, will help, also running a tilly 304 pure power methanol the carb is from isky, probably just jetted not re worked or anything, hopefully i can get great top end with this set up, i think i will be surprised when i go to gun it for the first time
 
If your big duration cam isn't performing like you want it down low, you can always up the stall on your clutch. 260 duration = 5k stall
 
I stall my clutch up too 5400+ depending on what the track will hold, I've enven went as high as 6200. I think 9500 is gonna shake your motor apart plus is gonna pump all the oil out. I would shoot for somewhere in the 8500-9000 range. If your looking for top end get away from that small rear sprocket stick with somewhere around a 60 tooth then adjust the front driver. Taller front gear equals more top end.
 
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