Stock appearing kt100

pko said:
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?
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pko,

In my experience, squish band width needed to be fairly related to what type of load the engine was going to be under and in what rev range.

The general trend I found: Tight sprint track where there is lots of quick acceleration (tight track, low gear), the squish band needs to be wide with a small diameter deeper bowl. On wide open tracks where the engine spends a lot of time at higher revs (like a long road race track), the squish band needed to be quite narrow with a wide/shallow bowl.

These observations are not "set in stone", but that's what I found worked best.

PM
 
A couple thoughts.... 91 octane gas from the local gas station in California back in the early 90s was probably much different than what you get today. For one thing, out here MTBE was added to the gas blends during part of the year until they started finding the crap in the underground water supply. Ethanol was not part of the blends back then. Another thing to consider... all of the motors we were running back then were fitted with the original OEM Yamaha TCI box. I have no direct experience with any of the later (smaller) Yamaha versions or any of the aftermarket replacements... they may behave different. Interestingly, I found the DD87 pipe was the most user friendly of all the pipes. Of course, the reason may be that it's power curve was such that it liked to rev higher if teeth were added and with the extra teeth and wider power band it worked well with the clutch stall 1,000 to 1,500 rpm lower. Take those two factors and consider what Pete mentioned.... how fast the motor moves through the power band and it makes sense. Geared lower and locking the clutch at a lower speed tends to make the setup move close to the same range of speed and rate of acceleration as you get with a direct drive setup... just won't spin up quite as high with the clutch hanging on the end of the crank. But, compared to the other pipes the DD87 setup moves up through the wider range at a faster rate. The Delta pipe I used was the touchiest even though it was a larger volume than any of the others. It's multiple angle scavenge cone made carb tuning much more critical. With that pipe the fastest setup was to gear for around 14K on top and stall the clutch at 10,500. I didn't like running it that way as it just used up clutches so I ran the head pipe about an inch and a half shorter than RLV recommended and geared for 14,500 or a little more and stalled the clutch at 9,500. Not quite as fast but the clutches could handle that setup without burning up so fast.
Much has changed over the years so it would come as no surprise to me if results were different these days with the current fuels, pipes, ignition parts etc.
Steve O'Hara
 
Twinenginekart said:
walt nice build thanks for the link
is pete muller site no longer up?
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I pulled it offline recently because I was having some difficulty answer questions I received through it in a timely manner. If anyone from here is looking some particular information, I can email you any articles that I used to have online.

Pete
 
I know this is an old topic but I used this as a reference to try to get more performance out of my KT100 and I'm left scratching my head after following some of the advice here.

When you guys are running this 9cc head what compression numbers are you seeing on the motors?

I had a spare head cut to minimize squish and have a 9cc chamber per populare advice here. The first motor I put this on within 5 minutes of running I stuck a piston. I chalked this up as me having carb tuning too agressive and running on 89 octane pump gas.

The second motor I put this head on was a fully built WKA legal motor. It seemed no faster if not a little bit slower than the legal 11cc head immediately out of the pits. After a few laps it started to detonate and I shut it down. I did not stick a piston this time but it did start to burn the top of the piston. This time around I was running the carb extra rich to keep temps under control AND I was running on 110 octane fuel. Squish was set to .032. Compression on this motor with the 11cc head was 160psi, with the 9cc head it was like 235psi.

So what am I missing trying to get more power with this 9cc head? I read opinions that the timing needs retarded and then counter-opinions that it doesn't need retarded and in fact many can get the 9cc head to run on pump gas. I figured with the 110 octane gas and the carb set rich i was as safe as I could be but the results still don't add up.

Any opinions would be appreciated.
 
picture of the piston after 4 laps
42395503555_62782a86bc_c.jpg
 
In my many years of experience with the KT 100, I found the EGT (exhaust gas temperature) to be the most valuable tuning device available. If the engine gets lean, with the octane fuel you're using, the exhaust temperature will go down, caused by detonation. Also, the temperature readings come much faster than with a CHT. You always tune to the maximum EGT readings, or just a little below. In Sprint racing, coming off a tight turn, because the engine is at maximum torque, (detonation occurs when the heat and/or pressure exceeds the fuels ability to prevent it) you will see a lower temperature than you would at the peak rpm. Some people see this and think it's the temperature probe that takes a long time to reach maximum temperature, but it doesn't. It's a little bit different with a muffler versus an expansion chamber. You get more heat and/or pressure, thus more torque, with an expansion chamber.
 
I'm using an EGT...that's how I knew it was detonating and why I was able to shut it down before it did more damage.

Using standard timing (meaning it hasn't been slotted or modified) and AR51 plug.

is the 235psi cylinder pressure just too high? I haven't cc'd the head myself but the engine builder that cut it for me told me it was right at 9cc. Like I said, I had it setup soo fat to keep temps low in the chamber that it wasn't any faster than the legal head.

Would really like to figure out why this isn't working so that I can utilize the lower cc head for some extra power. I'm practicing vs. my brother who I out-weigh by 35 lbs and looking for a way to not get pulled in the straights so as to keep us glued together for some more fun while we're out there....ideally with only the head change so that I can put the 11cc head back on there for race days when i run the old fat guy class :)
 
I've always understood that those marks on the top of the piston are from preignition. Detonation usually shows up as a, what I call sandblasting, marks around the edge of the piston. Show me a picture of that AR 51 plug. With that much compression I would run a retracted gap plug. You know you have to cut the head just to get to 11 CC's? Some people measure to the bottom of the spark plug hole, (9 cc) some people measure to the top. The spark plug hole holds 2 CC's. I've tested some TT75's with my expensive Snap-On compression tester and never saw that much compression. 215 psi was the highest. A (good) stock KT (blueprinted), 11cc's to the top of the spark plug hole) measures about 160 PSI. You might check with your builder as to his methods. Not that there's anything wrong with them, just wondering what they are.
Ignition timing; totally retarded the stock KT is at 25° BTDC. Way too much timing for that compression ratio. Just trying to coordinate our nomenclature! lol
 
AudiOn19s said:
Using standard timing (meaning it hasn't been slotted or modified)
Click to expand...

Forgotten tricks of the KT.
You can vary the timing by running more, or less gap between the rotor & stator,
using different ignition modules, and re-magnetizing the rotor once in a while.

Also, I always went for a lot tighter squish, like .020".
As Gordon Jennings explained in his book, fuel/air trapped in the squish area never burns, or burns so late that it doesn't add to the power.

Take that tight head, close up the squish, then enlarge the diameter of the combustion chamber, preferably to the front, away from the exhaust.
(you will need a large lathe, with a 4-jaw chuck to do this)

When we started racing here in Texas, WKA mandated centered round combustion chamber, and a squish band.
The next year they dropped the 'centered' part, and we offset the chamber all the way to the front.
Following year they dropped the 'round' part, and we went to a wedge chamber.
Every change made us faster.

Al, good to see that you are still around, been missing your posts !

Oh, and Pete, find yourself one of Al's Mayko pipes.....
 
That's interesting, claiming there is no burning of the fuel in the squish band. Every time I've gotten detonation, I could see the telltale "sandblast like" marking around the edge of the piston. Something is burning, exploding really, in that squish band.
Strange how the Europeans have never discovered this moving of the combustion chamber trick!
Keep this a secret, but instead of trimming the head, removing all that nice heat absorbing aluminum, trim the top of the barrel. Looking at the K71, factory fresh, it's piston comes right to the top of the barrel. And the combustion chamber is right in the center of the head. You think the factory knows something?
 
By definition, detonation is not burning.
Burning fuel makes power, detonation makes holes.

They did use offset chambers, JLO (many high power sled engines), and Rotax did.
JLO:
s-l300.jpg

Rotax:
rotax_28.jpg


The offset moves combustion heat away from the exhaust side of the piston also.
And the transfer flow path will more effectively scavenge the chamber, with less mixing with the burned combustion gases from the previous cycle.

Reason why more engines don't use it is plain economics. It's much easier to machine a concentric chamber/squish band/gasket surface.
 
GoFast said:
Reason why more engines don't use it is plain economics. It's much easier to machine a concentric chamber/squish band/gasket surface.
Click to expand...
I don't know, I can only wonder, but do you really think that Komet, Pirelli etc. etc. care about economics when it comes to World Competition? Obviously I could be wrong, but where's the evidence?
 
Al - My apologies for mis-using terms...I threw out detonation as a general term without using it correctly, you are absolutely right I'm getting pre-ignition.

Go-Fast - any pictures of these ports that are not circular? I'm assuming you're cutting these combustion chambers by hand when not circular? I'd love to see reference pictures. I have a couple of talented engine builder friends who could easily do this for me with some reference. I clearly need to open up that chamber a bit anyhow based on what it's doing now.

Al mentioned a "better" target for compression would be in the 210psi range. Is that about where it needs to be to be "safer" yet make some additional power?

Thanks again guys.
 
I have done several KT100's for stock appearing classes, not all out open's, but modified to suit the original parts.
add 2 boost ports each side of exhaust ports oval shaped ( don't cut into barrel counter bores )
do not raise exhaust ports
raise intake ports to .380" blowdown
enlarge intake port passages and match to cases
set compression 8.3cc to top of plug hole
set timing 21 degrees
pipes=steeply angled cones best acceleration---fat pipes best power-- gently angled cones broadest power curve highest rpm slowest acceleration--set clutch and pipe to same rpm 10,000 to 10,500 with quick engagement no lingering slip add rear teeth max rpm 15,000 to 15,500 rpm
inlet timing at 80 degrees btdc and atdc
use good gas maybe 100 to 105 octane racing gas for consistent performance add 6 to 8 ounces synthetic or castor oil per gallon
hi 5/8 to 3/4 low 1 3/4 pop off 10 to 15 psi fulcrum flush to .015" up change pumps diaphram often
this is simple affordable, fast, and easy to maintain
if you can not add the boost ports just add eyebrows maybe .375" long each side do not widen exhaust ports and do not narrow ex port web
Good Luck.

Mike Berg
 
One question; 80° before Top Dead Center? Adding exhaust port area? Are you familiar with "area/time"? Actually, most of your advice is the same as I would have given 40 years ago. In those days nobody was racing modified Yamaha's. We did race modified McCullough's (91s) and most of your advice is in the direction we took.
 
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