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Spencer Uzri
Joined: 18 Apr 2006
Posts: 798
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 Posted: Wed Feb 10, 2010 8:16 pm Post subject: General moto tuning questions: octane, jetting, etc. |
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I'm up to my waist now, & soon, will be up to my neck.
I have a spece/stock moto CR125, PWM w/o pump-around (ie. fuel pump-only setup). For the moment, I think I have the basic chassis concepts down, but I'm still trying to understand engine tuning.
Fuel
What octane can I run? I've been told to run 98 octane to mitigate detonation, but it's ~$12/gal, & I know the CR125 was designed to run on pump gas for moto-X. Is there a detonation risk w/ lower octane?
Also, what are the hallmarks of detonation? I'm told that scoring/discoloration/shallow pitting on the perimeter of the piston dome is the sign, but have also heard (read here) that it occurs more at the dome center. What sign should I look for, & where? If detonation is occurring, should the main jet be changed, or is there something else to consider?
Jetting
Pilot: what's the method for choosing?
Needle: sounds like throttle transition through corners is the sentinel; use DJK/DJG needle, depending on barometric pressure/air temp; sluggish throttle respone = too lean; hesitation = too rich - correct?
Main: what's the tuning protocol using EGT - run it on the stand @ high RPM or drive it? If EGT is high does this mean indicate a too-lean condition/vice-versa (or is it the other way around)? I'm told the optimal EGT temp range is ~1100-1200 deg F.
On a side note, is an O-ring or non-O-ring chain preferrable?
Last edited by Spencer Uzri on Thu Feb 11, 2010 9:45 pm, edited 1 time in total |
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joseph hollinger
Joined: 12 Sep 2002
Posts: 9479
Location: United States, California, san francisco
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| Posted: Thu Feb 11, 2010 11:11 am Post subject: Re: General moto tuning questions: octane, jetting, etc. |
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| Spencer Uzri wrote: | I'm up to my waist now, & soon, will be up to my neck.
I have a spece/stock moto CR125, PWM w/o pump-around (ie. fuel pump-only setup). For the moment, I think I have the basic chassis concepts down, but I'm still trying to understand engine tuning.
Fuel
What octane can I run. I've been told to run 98 octane to mitigate detonation, but it's ~$12/gal, & I know the CR125 was designed to run on pump gas for moto-X. Is there a detonation risk w/ lower octane? |
You're fuel is probably specified by the organization you run with. At the very least, its best to stick with that so you're not testing on one fuel and then switching to something different on race day. All of the organizations I've run with spec something higher than 98 (more like 110). I've run 100 unleaded (left over TAG gas) but nothing lower than that. $12 per gallon seems incredibly expensive--our 110 leaded is something like $8 per gallon.
| Spencer Uzri wrote: |
Also, what are the hallmarks of detonation? I'm told that scoring/discoloration/shallow pitting on the perimeter of the piston dome is the sign, but have also heard (read here) that it occurs more at the dome center. What sign should I look for, & where? If detonation is occurring, should the main jet be changed, or is there something else to consider?
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change the main jet.
| Spencer Uzri wrote: |
Jetting
Pilot: what's the method for choosing?
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start with a 50 or 52. Blip the throttle on the stand, if the jet is too small it will bog.
| Spencer Uzri wrote: |
Main: what's the tuning protocol using EGT - run it on the stand @ high RPM or drive it? If EGT is high does this mean indicate a too-lean condition/vice-versa (or is it the other way around)? I'm told the optimal EGT temp range is ~1100-1200 deg F. |
There is no absolute target with EGT. You want it to be as high as possible (under load, not on the stand) and you lean the jets to accomplish that. If you encounter a situation where RPM is rising, but EGT is not (once again, under load) you are too lean.
| Spencer Uzri wrote: |
On a side note, is an O-ring or non-O-ring chain preferrable? |
Shifter chains are cheap. Buy the non-oring and replace it every other weekend.
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A lie gets halfway around the world before the truth has a chance to get its pants on. -- Winston Churchill. |
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John Denman
Joined: 19 Jul 2001
Posts: 4846
Location: United States, Texas, McKinney
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| Posted: Thu Feb 11, 2010 11:22 am Post subject: |
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Adding a little to Joe's answer;
The pipes used on shifter karts pump a lot more volume back into the cylinder; thats why you may need a higher octane. If you mix a strong leaded fuel like VP12 or T111 1 part with 2 parts 93 octane pump fuel it should be OK for track days. A little lead goes a long ways at first.
You don't have to have a pump-around just make sure the feed line is Tee'd and the Tee is above the carb. It still may not run perfect (some will) but that will get you around the track just fine.
We ran non-o-ringed chains and using a good Moly spray chain lube they lasted about 10-12 hours. Longer if we wanted but as they wear they begin wearing the sprockets too.
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John Denman
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Erik Maxfield
Joined: 16 May 2004
Posts: 1069
Location: United States, California, Vacaville
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| Posted: Thu Feb 11, 2010 9:54 pm Post subject: |
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the 50-52 idle works with a PWK at sea level.
I have not found that lean to work well with a PWM.
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Erik
The early bird gets the worm.....
The second mouse gets the cheese....
Stock CR125.
Chassis-FrankenKART
Intrepid/ITAL combination |
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Spencer Uzri
Joined: 18 Apr 2006
Posts: 798
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| Posted: Thu Feb 11, 2010 9:58 pm Post subject: |
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Joe - What do you mean by "...as high as possible"? I've read on kartweb to keep it around 1350, & been told at the club to keep it in 1100-1200 deg F. Shouldn't there be an optimal T range, not?
John - I've learned that the club spec fuel is Chevron 92, which I think is their premium pump stuff. I understand that lower oct = higher lead content = increased det. risk. True? If so, how can I mitigate this, if possible? |
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Erik Maxfield
Joined: 16 May 2004
Posts: 1069
Location: United States, California, Vacaville
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| Posted: Thu Feb 11, 2010 10:09 pm Post subject: |
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| Spencer Uzri wrote: | Joe - What do you mean by "...as high as possible"? I've read on kartweb to keep it around 1350, & been told at the club to keep it in 1100-1200 deg F. Shouldn't there be an optimal T range, not?
John - I've learned that the club spec fuel is Chevron 92, which I think is their premium pump stuff. I understand that lower oct = higher lead content = increased det. risk. True? If so, how can I mitigate this, if possible? |
Spencer,
Search EGT and you will find a bunch of information on EGT tuning. In short the absolute value of the temperature (IMHO) is meaningless. Constant upslope in the temperature trend is the thing that you target.
TEL (tetraethyllead) is cheap octane. You can get decent octane by blending in benzene, Tolulene, alkylate, or a bunch of other stuff in the "gasoline" mix to get octane. Generally race gas has lead in it because it is a lubricant and it is a cheap octane blend part.
To sum up:
octane and lead are....kind of independent. Lead is not in standard motor fuel (as in out of the pump....non high octane....legal for motor use)
Search EGT and read posts. Form your own opinion.
Personally I do not get all that excited with a EGT of 1400+ KNOWING that the rough melting temperature of aluminum is 1100.
Gas temperature and metal temperature are not always the same.
_________________
Erik
The early bird gets the worm.....
The second mouse gets the cheese....
Stock CR125.
Chassis-FrankenKART
Intrepid/ITAL combination |
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al nunley
Joined: 13 Nov 2006
Posts: 3037
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| Posted: Fri Feb 12, 2010 7:20 am Post subject: |
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| Spencer Uzri wrote: | Joe - What do you mean by "...as high as possible"? I've read on kartweb to keep it around 1350, & been told at the club to keep it in 1100-1200 deg F. Shouldn't there be an optimal T range, not?
John - I've learned that the club spec fuel is Chevron 92, which I think is their premium pump stuff. I understand that lower oct = higher lead content = increased det. risk. True? If so, how can I mitigate this, if possible? |
If the temp gets "Too" high, you'll get detonation and the temp will drop. This is when you want to go richer and keep the temp just below that number, no matter what that number is.
As the ambient conditions change, that maximum number could change, keep an eye on it. Because detonation is caused by excessive heat and/or pressure, on the low end, coming out of the slowest corners with the engine at peak torque, (maximum heat and/or pressure), that number will be lower, but the same rule applies, keep it as high as possible, and if it goes down, richen the low speed.
Octane is a measure of a fuels ability to resist detonation; why then would you believe there’s more lead in “low” octane fuel? “Lead” is used to prevent detonation. Use the lowest octane fuel you can without getting detonation.
There are many ways to raise the octane of fuel, but most would not pass the fuel test most tracks use to keep the competition close.
The whole point of the EGT, (unlike the CHT and water temp) is to get the highest reading you can. Maximum EGT means maximum HP.
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If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting |
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Spencer Uzri
Joined: 18 Apr 2006
Posts: 798
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| Posted: Fri Feb 12, 2010 11:09 am Post subject: |
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| al nunley wrote: | | Spencer Uzri wrote: | Joe - What do you mean by "...as high as possible"? I've read on kartweb to keep it around 1350, & been told at the club to keep it in 1100-1200 deg F. Shouldn't there be an optimal T range, not?
John - I've learned that the club spec fuel is Chevron 92, which I think is their premium pump stuff. I understand that lower oct = higher lead content = increased det. risk. True? If so, how can I mitigate this, if possible? |
If the temp gets "Too" high, you'll get detonation and the temp will drop. This is when you want to go richer and keep the temp just below that number, no matter what that number is.
As the ambient conditions change, that maximum number could change, keep an eye on it. Because detonation is caused by excessive heat and/or pressure, on the low end, coming out of the slowest corners with the engine at peak torque, (maximum heat and/or pressure), that number will be lower, but the same rule applies, keep it as high as possible, and if it goes down, richen the low speed.
Octane is a measure of a fuels ability to resist detonation; why then would you believe there’s more lead in “low” octane fuel? “Lead” is used to prevent detonation. Use the lowest octane fuel you can without getting detonation.
There are many ways to raise the octane of fuel, but most would not pass the fuel test most tracks use to keep the competition close.
The whole point of the EGT, (unlike the CHT and water temp) is to get the highest reading you can. Maximum EGT means maximum HP. |
Thanks for the clarification. Would a detonation counter be a worthwhile investment, & if so do you know of one that can interface w/ a Mychron 4? |
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Sonny Van Hook
Joined: 14 Sep 2001
Posts: 955
Location: United States, California, Nor Cal
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Spencer Uzri
Joined: 18 Apr 2006
Posts: 798
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| Posted: Fri Feb 12, 2010 12:18 pm Post subject: |
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| Sonny Van Hook wrote: | Spencer:
I think you should just get a safe baseline setup and drive the thing. You're thinking too much, man. 
Sonny |
I'm sure you're right, though I just saw someone stick his engine a few wks back for slacking on his jetting setup. Stuff happens, I guess. |
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Michael Taksa
Joined: 25 Jul 2001
Posts: 1399
Location: United States, Massachusetts, Boston
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| Posted: Fri Feb 12, 2010 12:21 pm Post subject: |
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When in doubt go two jet sizes larger  |
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Sonny Van Hook
Joined: 14 Sep 2001
Posts: 955
Location: United States, California, Nor Cal
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| Posted: Fri Feb 12, 2010 12:42 pm Post subject: |
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Spencer: I understand. I'm guilty of the same pretty frequently. It's much easier to see when other people do it.
When I first built my 125, I did a bunch of research, too, and had ideas about jetting and watching the gauge, etc, etc. It was so damn fast and violet to drive that during the sessions, I couldn't think about anything except trying to keep it on the road. You do, of course, eventually adjust. In the beginning though, jet it rich (like Michael says) and try to keep it on the pavement. Don't worry too much because the stock stuff is pretty damn bulletproof...especially when you're starting out and are only able to drive at 80% of its potential.
So...take the advice given in this thread. You will eventually need it, but my guess is, not right away. Good luck and report back.
Sonny
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Erik Maxfield
Joined: 16 May 2004
Posts: 1069
Location: United States, California, Vacaville
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| Posted: Fri Feb 12, 2010 9:16 pm Post subject: |
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If you are learning to drive a 125 I would hazard a guess that a couple of jets is 10% or less of the total performance package with regards to lap time....
Jet it rich enough to not blow up, and drive the snot out of it. When you get the driving close, then fiddle with the jets.
Just my $.02
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Erik
The early bird gets the worm.....
The second mouse gets the cheese....
Stock CR125.
Chassis-FrankenKART
Intrepid/ITAL combination |
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al nunley
Joined: 13 Nov 2006
Posts: 3037
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| Posted: Fri Feb 12, 2010 9:49 pm Post subject: |
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| Spencer Uzri wrote: |
Thanks for the clarification. Would a detonation counter be a worthwhile investment, & if so do you know of one that can interface w/ a Mychron 4? |
I've never used one and I've no idea if they're good or not. I do know that the EGT will tell you everything you need to know, and it will do it in real time, while you're driving the kart.
If you have the money, and the time to test it, why not, the more you know, the better off you are.
Read my sig, it tells you a lot more than most know.
_________________
If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting |
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Chris Parks
Joined: 09 Sep 2001
Posts: 1504
Location: Australia, not USA state,
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| Posted: Sat Feb 13, 2010 7:23 am Post subject: |
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| al nunley wrote: | | Spencer Uzri wrote: |
Thanks for the clarification. Would a detonation counter be a worthwhile investment, & if so do you know of one that can interface w/ a Mychron 4? |
I've never used one and I've no idea if they're good or not. I do know that the EGT will tell you everything you need to know, and it will do it in real time, while you're driving the kart.
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Never let the truth get in the way of a good story. Al reckons that EGT is the total story. There used to be someone here that reckoned compression was the total story. Use what makes you happy, I use Lambda but Al can't spell it let alone know what it does. I get a bit P'd off when one eyed people try and shove their views down other's throats. Sure EGT tells a story but there are other tools to do the job including det counters and Lambda sensors. They do cost more and at an early stage they might not be necessary and certainly won't make or brake a lap time. On the other hand it is useful to learn how to read the tool of your choice and with guidance you can begin early on even if it does not make you faster.
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understeer happens on road courses and usually is pronounced with a British accent. Push happens on ovals and usually is pronounced through a wad of chew. Other than that, I don't think there's a difference - Jim Derrig
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