does the 30 series limit rpms?

[gran_pann]

Hey everyone. I recently raced a buddy on my modified briggs 5 hp flathead, and have been kind of bummed that the engine doesnt break 3800 rpms on a flat surface. Here is some background (before you ask 'did you remove the governor?') :thumbsup:

Briggs5 hp flathead with steel sleeve i built from the ground up. dual bearing block, 94ss cam, stock billet conrod, 19 lb springs (as cam card indacated, also set lash to card specs), indexed plug, billet flywheel, lapped valves, broken in now with 10 hours runtime on it. Has 1" thin stainless exhaust (~.87" ID), and a 1" 'race' muffler (i forget the name of it)... basically engine is in great shape.

I feel like (as another member said before here) that the torque converter is limiting the rpms, where i should be reaching 6k+ rpms, im only getting 3800 before it planes out. I was driving downhill slightly and it slowly (and i mean slowly) eventually got to 4300 rpms.

So i eventually am going to have to race my friend again and I want to school him in the race. (I beat him off the line, but he catches up and beats me at the end). i have an 8t on the jackshaft and a 50 (stock?) sprocket on the axle. baja mini bike -the one in my avatar

Is there any hope with the 30 series of going into the 5k rpms? I also have a cheetah clutch and am considering using that with a gear reducer (13, 14, or 15 on the clutch the clutch lets you swap) 20 on jackshaft in, 8t jackshaft out 50t axle, and 19" tires.

Thoughts?

 

[minikenny]

Those 19" tall tires make your gear ratio too tall. Can you install a larger sprocket on the rear wheel?

 

[minikenny]

To clarify, your gear ratio clearly isn't too tall for cruising. It's just too tall for racing or whatever you are trying to do.

Your engine isn't making enough power at 3800 rpm to get over the hump to go any faster. If that makes any sense. :shrug:

With 19" tall tires, your 6.2:1 gear ratio is like having a 4:1 gear ratio on a normal small wheel mini bike. Too tall. Ask the other guy who was blaming the TC for limiting his rpms how a 4:1 gear ratio works out. :laugh:

 

[gran_pann]

kenny, the bike pulls wheelies from start. This makes me seriously doubt that its a gearing problem. If i got a bigger sprocket i think id end up with my ass on the pavement and my bike somewhere in front of me. correct me if im wrong here

Also, i got a cheetah clutch and will be doing some testing. I also got a 20t jackshaft sprocket (clutch to jackshaft).

 

[minikenny]

Well I really don't want to argue the laws of physics with you, they really aren't up for debate :laugh:

Of course when the TC is in low gear you pull wheelies. Your gear ratio is 2.7 x 6.25 = 17:1 !

When it shifts into high gear your gear ratio is 0.9 x 6.25 = 5.6:1. With 19" tall tires that is too tall of a ratio to allow your engine to reach it's power peak. 5.6:1 is like 3.8:1 on a small wheel bike.

I'm sorry but we just went through several pages of this with xbrent275. If you wanna go faster, drop your gearing.

Yes, you will have to baby it out of the hole. Or get wheelie bars. Or lose the torque converter. :thumbsup:

 

[ole4]

here is a Jr dragster gearing with much smaller tires than you have and it goes 60MPH in the eighth mile.
, Salsbury clutch, 3,500 RPM stall, 11 / 80 Gearing, 7.27 to 1 ratio. Bill - Georgia: Stall 3,900 18 mm—jetting 180 main, ...

 

[joshua. c.]

changing the springs in the torque converter can help a lot. I replaced the stock springs in my primary with ones that engage the clutch at 3100rpm and replaced the stock spring in the secondary driven clutch with the stiffer yellow spring. these mods slow the upshift and allow the engine to rev higher. it takes off like a rocket. if the bike is pulling wheelies it sounds like you need a smaller rear sprocket to trade some of that off the line power for more top end speed.

 

[ole4]

not true just because it does wheelies has nothing to do with the overall gear ratio. The TAV30 when it shifts to high has a .8 to 1 overdrive so the engine will rev on take off because of the massive gear reduction in low but when it shifts to high it will load down the engine and restrict top speed and it gets worse with a engine that generates power at a higher rpm. If you cant deal with the wheelies put a weaker spring on the driver so it engages at a lower rpm so it will reduce the amount of torque initially. It will still probably wheelie but maybe it will soften the hit so it does not just flip over on you.

 

[gran_pann]

Here is another piece of information based on actual testing i have done so far with the driven unit. Stock (green) spring on driven unit.

Position #1 (from low to high fastest) allows my mini bike to rev up to 4100 rpms, all else the same. It takes off ok, and then revs high super fast, then it feels like the engine is under a load again (i think the belt probably slips here somehwere)/transitioning ratios, RPMS then dip down to the 3000 range, then it fully engages into overdrive.
Position #2 (factory setting) I keep it on this, allows mini bike to rev up to about 3900 rpms, dont have the surge at beginning, and it seems like it transitions better overall.
Position #3 (for most torque) Feels similiar to #2 setting, but takes longer to upshift (as expected) however, engine only gets up to about 3500-3700 rpms

So this is what makes me ask the question in the first place about the 30 series torque converter.

YES i understand physics (took the course in college). But that is not the point. The point is that I personally believe (based on spring settings testing) that the torque converter causes enough parasitic drag to keep my engine from revving higher. Yes friction, drag etc all come into play, but I believe that the torque converter causes enough drag for it to suck out enough power flipping the belt around to keep the engine from revving much higher.

PS. Does anyone know where to get a bigger sprocket for this mini bike?

Ole4, jr dragster? Ehhh my engine isnt even close to the power that a built jr dragster has. I dont use methanol either, no decked block, only minor porting.

Ill do some more testing with the race clutch and get back to you all.

 

[ole4]

It was a 12.90 jr dragster. They are the slowest class but still fun.

 

[minikenny]

This sentence is 100% correct:

I believe that the torque converter causes enough drag for it to suck out enough power flipping the belt around to keep the engine from revving much higher.

Yes your engine is out of power. But it is because of your tall tire/ tall gear ratio!



A little bit oversimplified and where that says "higher gear ratio" it should say "taller gear ratio" because they way we express it numerically it is a smaller number. Same thing with "lower" it should say "shorter." Or it is okay if you think of it as "low gear" and "high gear." :thumbsup:

I have a barstool racer i built with a bsp engine and tav2 knockoff and i have been 58mph gps with my phone and it still had throttle left. I ran out of room cause i was in a church parking lot and had to let off. I was about 3/4 throttle or so.

11" shifter kart tire. 60 tooth #35 split sprocket. And i think a 10 tooth front.

I guarantee that engine was spinning a lot faster than 3800 rpm at 58 mph and 11" tires!!

 

[minikenny]

I also agree a torque converter isn't helping for what you are trying to do. A clutch and jackshaft will more give you easy ability to get the gear reduction you need to go faster. :thumbsup:

But in fairness to the torque converter, you should do a trial with 5 or 5.5:1 gearing (whatever the effective gearing is with the TC in high gear, 0.8 or 0.9 x 6.25.) I'll bet your top speed and top rpm are similar, it will just take a lot longer to get there and be hard on the clutch. Then you could see it isn't the torque converter's fault.