clutch engagment rpm vs gearing purple plate

sachilles

Member
From what I've read so far, clutch engagement can be trial and error, and driver experience dependent.
Jr champ, with a freshly rebuilt wka stock purple plate animal.
Track is approximately 1/20 of a mile narrow oval. 7 second lap times are normal-ish. 10 year old driver on his 4th year of competing.
In the previous 2 seasons, rookie crew chief dad, didn't realize the motor was deteriorating to the extreme. When engine re-builder believes even when it was healthy, it likely was not built with the purple plate in mind.
Last season. Varied between running 12/72 and 11/72 gear ratio. Hilliard clutch with either 2 black/2 white springs, or 4 black. The 11/73 was preferred, but the lack of roller bearing in the 11 could cause pre-mature lock up in long races or races with many stops. In that time I never hear the motor hit the rev limit.

This year with the new rebuild, the motor is noticeably stronger. We ran 12/72 (2 black/2 white) for the first race, as I had done a million things to the kart over the winter, and just wanted a reliable known starting point. To my surprise, my son won the feature, but 2nd place finisher was tight on his bumper. The track is not easy to pass on due to the small size. 2nd place finisher is known to be quick, but a very clean racer and did not push to pass, though I think he was quicker.
In practice the kart actually bounced off the rev limiter at the end of the straight, two rev cuts. In the heat and feature race, I couldn't hear the rev cut, but my son believes he heard it.

In my brief history of karting, I know the goal is to get close to rev cut at the end of the straight, but do you actually want to hit it?
Gear and clutch engagement are variables that I can control, I assume it's wise to only mess with one variable per race to see if there is a gain.
My plan is to try all black springs next race, however part of me thinks that will just make it hit the rev limiter sooner. Do you all think that is a wise first step?
After I see what the black springs do, then my thought is to adjust the gearing. If it hits the rev cut sooner, then I drop the axle gear down to 70. If it doesn't hit rev cut, then raise the axle gear, or lower the clutch gear to 11. If possible I want to avoid the 11.

It's probably time to invest in a mychron 4 as well, otherwise I'm just guessing, as a stopwatch isn't consistent enough with such a small track, and knowing what the revs are doing consitently would be better than using my untrained ear.

One other concern that might affect things, and I don't have the experience to know. Chain tension. Likely was a touch tighter than I normally run. It was a new chain and figure it might work in a bit and loosen some, however it didn't seem like it. As a result, spinning the axle didn't seem as free. I feel like that resistance has to factor in.
 

flattop1

Dawg 89
Is this dirt ?
I may try the 4 black .
Is it an animal or lo206 . Why a rev limiter ?
Set the clutch one way or the other then concentrate on gearing .
Of course the clutch needs to be in good condition .
 

sachilles

Member
This is pavement
It's an animal not lo206
Maybe a rev limiter is the wrong term. Valve float? Engine revs, then cuts.
 

paulkish

old fart
Clutch engagement in karting is the only thing which is not trial and error. It's a cut and dry thing.

You want your clutch to engage at peak torque.

You either get your peak torque set point from your engine builder or you find it via what I think are common karting methods.

How you fine tune how your clutch engages is a trial and error thing per the type of kart racing your doing and track conditions.

Make it engage at peak torque and then go from there with any trial and error or initially how hard or soft it engages.

_______________________________

Gearing is set so you can maximize the use of your engine between the start of acceleration and where you end accelerating, There is trial and error in gearing because it may be more advantageous to tweak maximized engine use for racing purposes at the start or end of acceleration.
 

flattop1

Dawg 89
This is pavement
It's an animal not lo206
Maybe a rev limiter is the wrong term. Valve float? Engine revs, then cuts.
That imho is not right .
 
You should NEVER float springs in a purple plate animal.
The stock Briggs springs are good to 8000+ rpm.
There is no way that a purple plate (legally) will even turn that rpm.

Now, if someone put the LO206 (6100 rpm) rev limiting coil on your purple plate animal, then I could see you hitting that rpm and limiter.

Always set your clutch to engage at peak torque - your engine builder will know exactly at which rpm this is, (or if he provided that information with his dyno sheet, etc you should have that at available to you.)

Depending how much rpm you're scrubbing in the corners on this tight little track, you might be better to engage slightly lower than peak torque just to keep the engine out of the clutch (keep the clutch from disengaging if the engine rpm drops below peak torque each lap.)
I'd start with 4 black springs and work on your gearing.
Get a tach on that kart and watch your lap times.



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Carlson Racing Engines
Vector Cutz
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sachilles

Member
If for whatever reason it has an Lo206 coil with a rev limiter, is that necessarily a bad thing for a purple plate motor?
I have two clutches. So I have one set with 4 black springs installed for the next race and one with a pair of white and a pair of black as back up. I'll try to run both in practice to see the results.
I'm trying to scrounge up a used mychron 4, but I suspect I won't have it before the next race, so I'll have to rely on a stopwatch(which may not be reliable with me at the helm).
Looking at my receipt from the engine build it says 45-4800 max, which I assume is where the max torque was from the dyno, if that is logical.
I'll report with what I learn. Thank you for the help.......I have much to learn.
 

JPMKarting

Premium User
Your clutch engagement isnt going to change your RPM. Once its engaged, its engaged.

Setup, gearing, speed, etc will determine your top RPM, but clutch engagement will not
 
If for whatever reason it has an Lo206 coil with a rev limiter, is that necessarily a bad thing for a purple plate motor?
I have two clutches. So I have one set with 4 black springs installed for the next race and one with a pair of white and a pair of black as back up. I'll try to run both in practice to see the results.
I'm trying to scrounge up a used mychron 4, but I suspect I won't have it before the next race, so I'll have to rely on a stopwatch(which may not be reliable with me at the helm).
Looking at my receipt from the engine build it says 45-4800 max, which I assume is where the max torque was from the dyno, if that is logical.
I'll report with what I learn. Thank you for the help.......I have much to learn.
That is max RPM, the torque will be a smaller number, probably around 3000 RPM give or take as im not a motor builder. Sounds like you have a 206.
 
If for whatever reason it has an Lo206 coil with a rev limiter, is that necessarily a bad thing for a purple plate motor?
I have two clutches. So I have one set with 4 black springs installed for the next race and one with a pair of white and a pair of black as back up. I'll try to run both in practice to see the results.
I'm trying to scrounge up a used mychron 4, but I suspect I won't have it before the next race, so I'll have to rely on a stopwatch(which may not be reliable with me at the helm).
Looking at my receipt from the engine build it says 45-4800 max, which I assume is where the max torque was from the dyno, if that is logical.
I'll report with what I learn. Thank you for the help.......I have much to learn.

It will most certainly make a difference.
An animal engine is allowed to be blueprinted, ie cam, cylinder clearanced, low tension rings, machine work to the head and deck to raise the compression, etc etc. The rev limiting coil may or may not be an issue.
45-4800 max sounds really really low. One of those numbers may be max CHP, but certainly not max rpm, and certainly not max torque either.
Your max torque will be considerably less, like 2800 rpm. Our purple plate animal engines need to turn 59-6200 rpm max.

Give the clutches a try, but until you get a tach on the kart that'll monitor rpm and lap times, you're really guessing.
 
Your clutch engagement isnt going to change your RPM. Once its engaged, its engaged.

Setup, gearing, speed, etc will determine your top RPM, but clutch engagement will not

I think in general, I'd agree with this JP, but your clutch engagement can certainly change your top rpm (ex. slip the clutch too long, or engage it too low, and your corner exit rpm will be considerably lower than normal, which translates to lower speed and rpm at entry to the next corner.)
We see this moreso on road courses where you are in and out of the clutch every lap. Some real tight tracks (like the OP's - 1/20th mi narrow track) may cause that issue as well.
 

sachilles

Member
For what it's worth I have a mychron coming.....and true to fashion I put the kart before the horse. Ordered a data key, mount and beacon from a vendor.....then all the mychron 4's seemed to be sold off or not responsive. Ended up with a mychron 5 for a reasonable price.
Pretty sure I don't need the data key for the 5. I know the 5 uses gps, can it also use IR? I'm thinking the track is tight enough there might not be enough resolution in gps. Karting was rained out tonight, so I could have it set up before next event.
 

sachilles

Member
by way of follow up. Bought a mychron 5 from a user here. They didn't ship it timely....bought on june 24, received last night. Had to dispute the transaction to actually motivate them to ship. Arrived without leads. arg. So I'm ordering leads now.
Following up, without much data.
I went with all black springs, and it seemed faster. This week I dropped from a 72 gear to a 70, and he gained 0.5 seconds a lap. Unfortunately some how the axle shifted and/or the right rear hub...sliced a tire at some point in the race. Noticed he slowed down in the race. Completely flat right rear be end of the race. Stopwatch indicated it was faster. Going to try a 69 this week. I'll use the mychron at least as a timer for now, if the leads don't show up in time.
My son said he could hear the engine stutter at the end of the straight like before, but I was unable to hear it. I'm not sure what to believe there. I'll have to wait for the data when I get it......which at the rate I'm going will be the end of the year ;). Anyways I think I'm going the right direction, but I don't know that for certain. He's still having fun and competitive, which is the name of the game, so I'll just have to wait.

Wish I had the motor rebuilt ages ago.....it's a different world, and more enjoyable night at the track not having to fiddle with the thing constantly.
 

JPMKarting

Premium User
by way of follow up. Bought a mychron 5 from a user here. They didn't ship it timely....bought on june 24, received last night. Had to dispute the transaction to actually motivate them to ship. Arrived without leads. arg. So I'm ordering leads now.
Following up, without much data.
I went with all black springs, and it seemed faster. This week I dropped from a 72 gear to a 70, and he gained 0.5 seconds a lap. Unfortunately some how the axle shifted and/or the right rear hub...sliced a tire at some point in the race. Noticed he slowed down in the race. Completely flat right rear be end of the race. Stopwatch indicated it was faster. Going to try a 69 this week. I'll use the mychron at least as a timer for now, if the leads don't show up in time.
My son said he could hear the engine stutter at the end of the straight like before, but I was unable to hear it. I'm not sure what to believe there. I'll have to wait for the data when I get it......which at the rate I'm going will be the end of the year ;). Anyways I think I'm going the right direction, but I don't know that for certain. He's still having fun and competitive, which is the name of the game, so I'll just have to wait.

Wish I had the motor rebuilt ages ago.....it's a different world, and more enjoyable night at the track not having to fiddle with the thing constantly.

All you need is some 18 gauge wire for the RPM lead
 

sachilles

Member
I'm still learning how to use the mychron effectively learning the track. The kart track is part of a larger track already in the mycron database....so it didn't enable the autolearning at first.
For this event I went back to the baseline setup for a few reasons, though I know it's a touch slower, but with all black springs.
12 t clutch sprocket and 72t main.
From what I can tell it hits mid 4700(4759) rpm peak, does 1 dip to 4500, and back to 4700's at 24.9 mph prior to the corner. Dips to 3500 rpm in the corner and 17.3 mph.
It does seem to hit some sort of barrier right below 4800 rpm. I don't have enough experience to tell if that is typical for a purple plate briggs, or if this is something of concern with the health or setup of the engine. It performs flawlessly otherwise.

My plan is to change the main gear to 69 for the next race. I had used 70 for the previous race with good results.
 
From what I can tell it hits mid 4700(4759) rpm peak, does 1 dip to 4500, and back to 4700's at 24.9 mph prior to the corner. Dips to 3500 rpm in the corner and 17.3 mph.
It does seem to hit some sort of barrier right below 4800 rpm. I don't have enough experience to tell if that is typical for a purple plate briggs, or if this is something of concern with the health or setup of the engine. It performs flawlessly otherwise.

Could it be too rich? A jet size too rich on a small plate will make it shut off for just a second a peak rpm. Try going down .001" or .002" on main jet and see if this "barrier" as you call it goes away. Also, simply dropping a tooth on the rear should accomplish the same thing because you will drop your rpm just a touch as well, which will keep it out of that overly rich condition at peak rpm.
Now, with that said, you should be turning more rpm on a track where you are dropping that much corner rpm. I would like to see 5900 or so on a purple plate for max rpm. Again, consult with your engine builder for his preferences on rpm - he knows your engine better than I do, I'm just going off of my experience with the purple plate animals that I've built and worked on personally.
 

alvin l nunley

Premium User
From what I've read so far, clutch engagement can be trial and error, and driver experience dependent.
Jr champ, with a freshly rebuilt wka stock purple plate animal.
Clutch engagement is not a hit and miss thing, it's an exact science. You want to have the clutch, while it slipping, to hold the engine at peak torque. Horsepower is a calculation, RPM X torque / 5252.1 = HP. So if the engine is being held at peak torque, it transfers that torque through the chain to the axle and then HP is calculated "at" the axle. There's some losses in the process, but we can't worry ourselves with that.
Get a bathroom scale, put it between a strong wall and the front of your kart. Start the engine, warm it up, stand on it and read the pounds on the scale, along with the RPM. Adjust the clutch, up or down, until you get the highest reading on the scale. It can take some time, I know that, but it's worth it! This has to be done relatively quickly, don't burn up the clutch.
 

alvin l nunley

Premium User
On the subject of jetting; what jets do you have in your carburetor? When I've asked this question before, I was told high .038 and low .020. Calculating the area of the hole, a .039 jet would be 5.379% more area. 2 sizes bigger would be a 10.874% increase in area. Interestingly, the difference in area between a .020" to a .021" is 10.191%. But the difference between a .020" and a .038" jet is over 250%. I'm wondering if changing the low-speed jet, not sure you have one, would give you a much finer percentage change in area. I'll have to work on that.
 
Should be about a .052" main jet and .022" pilot jet in a purple plate animal, Al. The percentage change in area will be even greater.
BUT, I don't care if his change in jetting 5 or 10 or even 20% change in area....if the engine is shutting off at peak rpm due to fouling the plug, then he needs to start leaning it. Dropping a thou or two on the main would be a good place to start.

Also, that gearing appears (to me anyway) to be considerably high for a 1/20 mi. track.
Generally you need to be on an arena clutch (9 or 10 tooth) or use a jackshaft to get the ratio right for tracks this small.
That might explain the OP's low peak rpm. It still doesn't explain the "barrier" the engine hits @ 4800.
 

alvin l nunley

Premium User
Should be about a .052" main jet and .022" pilot jet in a purple plate animal, Al. The percentage change in area will be even greater.
Thanks for that, I was speaking about a gas animal, I guess they don't run those anymore. I'm unsure what you meant by when you said
"The percentage change in area will be even greater."
I'm interested in the reasoning for increasing the the big jet by 100% but not the low-speed jet. I don't race 4 cycles, no experience, looking to learn.
 
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