Stock appearing kt100

Who's ideal was 11cc? Wouldn't a 9cc head make more power and also save the rods from elongating so much? On a S/A it wouldn't matter anyhow, so the first thing I'd do was make a 9cc head...Cubic inches are king, followed closely by compression ratio and pressure... I like that nikasil idea a lot -- I was going to do it to a 8hp Briggs, but that was one of the many projects that got wheeled and dealed.
 
Ted Hamilton said:
Who's ideal was 11cc? Wouldn't a 9cc head make more power and also save the rods from elongating so much? On a S/A it wouldn't matter anyhow, so the first thing I'd do was make a 9cc head...Cubic inches are king, followed closely by compression ratio and pressure... I like that nikasil idea a lot -- I was going to do it to a 8hp Briggs, but that was one of the many projects that got wheeled and dealed.
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11 cc's is to the top of the spark plug hole, (IKF rules) the spark plug hole is two cc's.
 
Thanks for the reminder, Al. ;) If was unclear, I meant that 'd rather see a 9cc TOTAL head, or whatever is the same as an ICA or other Euro 2 cycle engine.
 
If you go to the "For Sale" section of the 2 cycle forum here there is a KT 107cc for sale. That engine sounds like its exactly what you want to build and its only $800. You probably couldn't build it that cheap. Might be to your advantage to buy it, dissect it and find out exactly what the builder did internally for future reference. Re-assemble it and go win races.
 
Ted Hamilton said:
Thanks for the reminder, Al. ;) If was unclear, I meant that 'd rather see a 9cc TOTAL head, or whatever is the same as an ICA or other Euro 2 cycle engine.
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ICA when it was running was down to 7.5cc min depending on track, that's what most engine homologations list when its listed in the specs. Most running around the 9cc region including myself. Until I run some more research that may lead me to run lower head volumes.
 
Guys,

Back in the early 80s IKF had a class called Controlled Stock for 100cc Reed Valve motors. The port layout was basically what you see in the HPV/KPV stuff which is similar in exhaust port height as a legal KT100. The class had a minimum head volume of 9cc with the plug hole included and we ran those engines with around .025" squish and a very slight taper to the band clearance. We ran the engines on 91 octane pump gas and never had any issues... in other words at 9cc the engines were still mild enough on compression that there was no need for high octane race gas. On the road race tracks we ran the engines up to 15,000 rpm for one hour enduros and there was never any problems with the big end of the rod or the big end bearings. The original rules for the KT100 were based on the idea that the engines should be run "out of the box"... LOL as if anyone that wanted to win would do that!
To establish the rules the tech committee folks got a bunch of engines and picked and chose the parts that would assemble to create the smallest volume in the combustion chamber and they set the minimum volume accordingly assuming that no one would get an engine with a combination of parts that could end up illegal out of the box. The 11cc to the top of the hole number is completely disconnected to any kind of performance target, engine life studies, manufacturers recommendation etc. It is just a number that was set back around 1977 and it has stuck for all these years. Unfortunately it is a BAD number that has led to the engine getting a bad reputation for reliability problems and poor quality that is undeserved.
If you take a KT100 and set the port heights at the current limits and then cut the head to set the squish at .025 and the volume at 9cc to the top of the hole the engine will run great on pump gas and the connecting rod issues virtually disappear. So do the crank journal wear problems. The problems with the big end of the rod and the wear of the main bearing journals are directly connected to the engines having too little compression for the rpm they are run. After I stopped racing and only went karting with a group of friends for pure recreation I set up my engine and others with 9cc and the engines just ran forever without problems.
Regarding the subject of cutting the top of the cylinder vs the head it is clear some here are not understanding what Al is saying. He is not suggesting a different amount of volume or a different squish clearance once the final setup is completed. He is simply advocating cutting material off the top of cylinder rather than reducing the step on the head. The stock cylinder head has a step that is around .065 deep plus or minus as much as .010" and the piston comes up flush with the top of the cylinder in OEM condition. If you add gaskets to raise the exhaust ports to the max height allowed the piston will sit down in the bore a little... possibly as much as .020" depending on the parts you get. In that case the first thing the engine builder usually does is cut the entire step off the head to get he squish to around .020".
With the stock dome the engine is now at around 9.5cc so the dome must be enlarged to get back up to the 11cc minimum allowed.
Now, if you take Al's advice and leave the step on the head and cut the top of the cylinder the same .065" instead of having the piston sitting down about .020" below the top the piston will be sticking up out of the barrel by .045". As Jon pointed out, the diameter of the step in the head is much bigger than the bore so there will be space around the top ring land of the piston and that is not desirable. The head alignment on a KT100 is determined by the studs that hold it on and there are no other alignment pins to assure it goes on in exactly the same location each time. The rules do not allow them to be added either. With the barrel cut and the step left on the head the clearance of the step hanging over the piston becomes a problem and there is no reliable way to assure the head is installed completely concentric to the bore. There is enough slop between the studs and the mounting holes that the alignment could be way off.
In a nutshell, while there is some theoretical value to the idea of retaining the 100% aluminum section of the head that is commonly machined off and cutting a like amount off the top of the cylinder that is composed of part cast iron and part aluminum, the benefits simply don't offset the practical negatives of doing it that way. Simply take a survey of the methods used by all the top engine builders over the last 30 years and you'll see that it's an idea that does not pass the smell test.
Steve O'Hara
 
Steve, when you say a KT with a 9cc head will run great on pump gas, you should qualify that statement with " if you retard the stock timing" We ran around .085 BTDC on the Controls you mentioned and like you we never bought high octane race fuel, but the KT fires at .136 BTDC with the stock keyway in place, more advance than we set the Control engines to run on methanol. The stock KT's low compression is the only thing that lets it get by with that much advance. Jon
 
Jon,

Not true on the KT100 ignition timing for me or any of my gang. I ran mine and set up a half dozen friends with the same setup and never touched the timing. One thing that I really liked about the setup is that the engines ran fast and consistent with a generally rich mixture that produced a dark brown plug read and a dark brown header. CHT would run around 475 on the new style heads and the low 400s with the older heavy casting heads. Our group ran a variety of pipes. I used a Delta something, several of the guys had C3s and one ran the DD87 direct drive pipe. The DD87 equipped kart ran best with about 4 more teeth on the axle and the clutch set to hook up in the low 8s. It turned about 15,500 on top and the rest of us with the peakier pipes ran 14,500 to 14,800. I quit counting when my engine passed 20 hours without touching it. When I bought a TAG Leopard powered kart in late 2007 I re-built the KT with a new piston and rings, main bearings and wrist pin and pin bearing. Kissed the cylinder with the hone and used a NOS 51.99 piston... still under .004 clearance with all those miles on it. No crank savers and never changed a big end bearing.
Set it of the shelf and planned to sell it but it sat there until last summer when I finally decided to sell it to a new member of our group who wanted to start karting but not with a 125cc kart. The motor is back in action and it runs the same lap times as my best HPV with the 3 pipe. I set it up with the DD 87 pipe and low stall clutch for the new guy because that setup is so easy to deal with.... very little attention needed to the carb settings and the clutches last so much longer with the low 8s stall speed. Carb is set up the way I have described on this board often. Starts easy, idles steady, requires very little attention. Jets run 1-1/2 on the low and 1/2 to 5/8 turn on the high. Using the popular AR51 Autolite plugs and they last forever!
Will the engines run faster lap times with the timing retarded??? Perhaps they will but we were not concerned with getting the last couple tenths off the lap times. Our whole focus was on getting them to run consistent with as little attention as possible. Basically the same approach as the TAG stuff that came along and changed the face of karting about ten years ago.
It may not work for everyone but it sure worked for me and the group of guys I was mentoring back in the era from 1994 up to around 2007.
Steve O'Hara
 
Not sure why I had a different result, interesting you mentioned one setup with a direct drive pipe, that's the setup I had some serious detonation with turning 16,000. Backed the timing up and so far so good.
 
Maybe a difference in elevation and atmospheric pressure between your location and Steve's helped cause the detonation. Also could have been differences in the fuel, not sure about TN but I know here in NY the fuel has all kinds of junk added to it to make it legal. I imagine California with its strict emission rules may require different specs on its pump gas, that could possibly have an effect on detonation. I do agree that 11cc seems to be a rule that could be looked at and possibly changed to make the Yamaha an even better race engine. If the decision making people at IKF , WKA etc. listened to some of the top engine builders and big names behind the sport I bet a lot of great things could come of it. Not only with the Yamaha but with many of the engine packages currently being run.
 
Fuel and tune would certainly have an effect, also my engine was built as a stock appearing so more intake timing, more exhaust duration and much more carburation, maybe that was a contributing factor.
 
Jon,

Something else to consider: the length of the exhaust pipe (at least from the piston face to the mean point of the rear/converging cone) may be too short once the engine is modified.

Since that length is primarily determined by *time* from exhaust open to transfer close (and somewhat exhaust close), the exhaust actually needs to be *longer* when the exhaust is raised. (i.e. when port "open time" duration is increased).

To many, this can be counter-intuitive, since most people equate raising the exhaust and modifying the engine to be things that move power upward in the rpm range. It's true that power may very well move up the rev range, but when the math is done on duration/time, it usually ends up showing the pipe actually does need to be longer (and sometimes signficantly) vs. a stock port timing engine.

And that... may be why you are seeing detonation.

PM
 
Thanks for the input Pete, that's possible, it's a pipe that I am unfamiliar with, but my associate handling the kart is very experienced with tuning and the pipes he uses. Looked to be about 8.5 to 9 inches, I can say that testing so far has shown that backing the timing up seemed to have solved the issue. I've never experienced that kind of pre-ignition before, but then again I've never ran 28 degrees of advance on a high compression two cycle before either. Another factor I didn't consider when comparing to Steve's engine , he mentioned he was slightly under 52 mm, with my engine at 54.5 I had a little higher comp. because of the added swept volume. Jon
 
My S/A had a 55.35 piston and never had any issues with detonation. I ran a TS64 pipe on it at 9 5/8" flex. Stock ignition.
 
I wonder how much squish band width has an effect. By my quick calculations a stock band is about 65% of area. Reading Jennings book it is suggested 50% . If max squish velocity is to high would that also contribute to pre ignition and detonation by over heating the fuel charge?
 
I'm guessing the spark curve has more to do with it. I don't know what the kt100 has for a curve but with the increase in compression and pressure the ign should retard more at high rpm.
 
rooster said:
if he says gas he's lying i know better lol
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That was on Alky with Bullers atomizer walbro. I know it had 30 thou squish. Can't really say how the dome was cut cause my dad did the head. I know it had tons of port work done. Blow down was high also.
 
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