Drag race kart

mariner12

mariner12

New Member
#1
#1
Looking for some outside ideas on how to get a kart to leave harder from a standing start. 20℅ of passes the engine bogs from the dig at WOT, but otherwise does well.

A group of friends and I race a 233' dirt drag we built. I pull like a train mid range and up top. Looking to put more power down from the dig. Drive set up follows:

26 HP Predator 212

22cc head, 32/28 valves, .050 off, 60lb dual springs 1:3 billet rockers
Minkuni 28 mm alky
Robertsons h loop
Ultra light flywheel 38 degrees timing
+.020 rod with Wiseco piston
Nr .252 torque cam
Clements high Flow fuel pump.

Bully 2 disc 4900 rpm lock up
6.5:1 drive

12" tall tires

Tires have to stay for clearance issues. No room for a TC

Video below shows what we do. Any ideas would be appreciated.

https://m.youtube.com/watch?v=xf8048O66NE
 
trinik7597

trinik7597

Active Member
#2
#2
That's a new one to me ... A plus 20 rod with a 490 wrist pin for a wiseco ? But any how .. is 4,900 rpm where you are making peak torque ? You said 26hp if that is based on a dyno what rpm was peak torque made and what was that number
 
mariner12

mariner12

New Member
#3
#3
trinik7597 said:
That's a new one to me ... A plus 20 rod with a 490 wrist pin for a wiseco ? But any how .. is 4,900 rpm where you are making peak torque ? You said 26hp if that is based on a dyno what rpm was peak torque made and what was that number
Click to expand...

Arc sells the rod and piston kit. HP is off the dyno at JRPW. 5,000 rpm was his suggestion for clutch setup. Didn't get a max torque number from him just HP. Launching lower bogs the engine more. Don't know if I need to be higher.
 
trinik7597

trinik7597

Active Member
#4
#4
Well if jrpw told you to set your stall there then that's where I would keep it ... Have you played with your gearing maybe trying a 7:1 ratio it might get you out of the hole quicker but it may also affect you at the top end
 
trinik7597

trinik7597

Active Member
#5
#5
I just read your post again... Are you running a tach on your kart ? I am just curious if you are reaching max rpm at the line .. if not and you are still pulling past the line I would definitely go with a gear change maybe even 8:1 it will be a little rougher on the drive set up but you should rip out the hole and if you have to give it wide open throttle to lock up at 5,000 rpm I might play with the stall just a little and go maybe 4,600/4,800 you may very well be past peak torque at 5,000 and on your way past your power curve jmo
 
H

Havasu Dave

Moderator
#6
#6
Hey, and excuse my stupid question, but wouldn't a heavier flywheel be beneficial for torque off the line? Seems to me that the stored energy from the inertia would add torque at that initial 4900 RPM launch, while the engine is starting to load and drop RPM. Trade off being further down the track trying to build RPM, but maybe worth it?
 
H

Havasu Dave

Moderator
#8
#8
trinik7597 said:
Actually Dave a lighter flywheel with faster spin up makes more torque . Theoretically ...
Click to expand...
Yes, and I am familiar with the various calculations to derive HP and Torque and RPM factors in that equation. However, it takes very little power to spin a heavier flywheel, and according to the tractor pulling guys, a heavier wheel does give them more torque. Certainly, the faster you can develop RPM, the faster you can develop torque.

I keyed in on the original post where Mariner says "ultra light flywheel" as opposed to billet flywheel. I do know a lighter rotating mass accelerates faster, but the torque as measured at the wheels will be greater at the same RPM with a heavier rotating mass, as "work."

The average torque will be exactly the same, however a heavier wheel provides more torque at lower RPM.

Obviously, I am just musing here based on my own experience, and while it seems that dyno's for these small engines are all over the place, I'd not care a whit where my horsepower was, if I didn't see at what RPM I was developing maximum torque.
 
mariner12

mariner12

New Member
#9
#9
Turning into a good thread. The flywheel is a billet ultra light.

I swapped to a 11/77 this morning, and not too much change. Anything past that I'm out of sprocket clearance. Only other option is to drop to an outboard mounted 9 tooth driver but I will run into lateral clearance issues on the seat.

I tach 8200 at the line, the engine has spun to 9800. Gearing is the right idea but I'm limited there.

I appreciate the thoughts, it's easy to get tunnel visioned on your own stuff and usually an outside idea helps.
 
trinik7597

trinik7597

Active Member
#10
#10
Agreed Dave !!! You notice I added theoretically !! And HP on a dyno means kind of jack to me ... I know the guys at jrpw and they are very professional reputable engine builders ... In this application I would want to know at what rpm I was making peek torque and I would set my stall just under it ... I would also want to know at what rpm my power started to drop off for gearing ...
 
trinik7597

trinik7597

Active Member
#11
#11
Limited gearing is always an issue ... The only other thing I would make sure of is the carburator is tuned properly ... A 28mm carb is very large for a stock bore stock stroke motor .. your bogging may not be your wheel load at all .. just a thought
 
mariner12

mariner12

New Member
#13
#13
Jimmy has done wonders for tuning the carb and engine. All in all I'm trying to make the kart do something it wasn't supposed to. Dig racing in dirt is quite a different application. He suggested a bit more jet, so I'm giving that a shot tomorrow, if I'm still not happy then bigger bore will be next.
 
DeadPixel

DeadPixel

Member
#14
#14
I don't know if they work or not but they sell a cable operated clutch on Ebay so you can dump the clutch at what ever RPM you like. It looks like it is hand operated and might have a lock and release on it but might be wrong. Just looking at the pictures of it on Ebay right now. Its not cheep either, $230. If you look for manual clutch kit for gokart you will find it. I would find a few people that have ran this thing before I tried it though.
 
ole4

ole4

#15
#15
couple of questions, you said it is a hi torque NR 252 cam. NR has 5 different 252 grinds and I think the only difference is LSA. They are 230 degrees duration and 252 lift. with 1.3 rocker your max theoretically is .327. I am wondering why the clutch is set so high if it is the hi torque 252-207 version, and also why it is being reved so high? Also the 60 lb springs are overkill I would use 37lb springs and free up a little more power. What was your in the hole measurement? and head gasket thickness. Trying to figure out your compression. depending on which version of the 252 cam you have the 207 wants at least 10 to 1. With a bully clutch and 4900 rpm it should take off like a rocket. The heavy flywheel would also help with a clutch lockup that high, kind of like popping a clutch at 4900 rpm. Bully clutches lock up hard. That aside the 4900 rpm seems way past your torque peak for that particular camshaft (if it is a 207).
 
ole4

ole4

#16
#16
I also agree with Tim 28MM seems large for a torque motor. If it was a long duration drag motor maybe but it could cause a bog although the 4900 rpm clutch engagement should be moving enough air to carburate OK, are you checking lockup rpm with your tach?
 
massacre

massacre

#18
#18
I had a similar problem with the centrifugal clutch on my drag bike. Can you get a torque converter that will handle 26HP? It sounds like the motor is running fine, have you tried any clutch tuning?
 
mariner12

mariner12

New Member
#19
#19
Sorry gents, haven't been on here in awhile. Carb ended up being the issue. I sent it out to Snyders (jr dragster side of the sport), and they re-jetted the carb and cut the slide. I played around with the bathroom scale and 5200 was the sweet spot. Set the clutch, raced last Sunday and destroyed my buddies. Thanks for the posts.
 
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