Pinging CR500 Mid/top ported by Gorr.

[Firebalsocal]

Can't get into Forward Motions forum so I'll stick this in here.

The bike is a CR500AF that forward motion ported for me. Mid/top port job. Running straight pump gas, the bike pinged at the top end. I just bought a drum of 76 race gas at 110 octane and mixed this 50/50 with 91 octane pump gas. The bike still pings and I am wondering if I need to run straight race gas in it. The motor is stock except for the porting. Jets are stock pilot and one up on the main to a 178 (PWK carb). I believe as part of the porting, Forward Motion turns down the base of the cylinder to lower the transfer ports, in turn raising compression. I read on Forward Motions website that this particular porting should use 100 octane and Eric Gorr said the same thing over the phone. My mix should come out to about 103 octane.

The temp was about 65-70 degrees and it was very humid. Any ideas on why the thing is still pinging?

 

[Studboy]

Mixing half and half 91 octane and 110 race gas most likely won't give you 103 octane, it is usually less than that. Try the straight race gas and then get back to us. Also check your plug after a WOT run and see if it is lean.

 

[Firebalsocal]

Before I put the 178 main in, I had perfect jetting for mx. Cocoa brown. With long extended straights in this next race, I wanted a little extra insurance so I put in a larger main. I have not checked the plug since I installed a larger main or since I went to the 50/50 mix. I will get back to you on the straight race gas, maybe later this week.

On the octane, I was told you add the two octanes together, divide by two, and add two and you will have the octane. Not sure if this is good info or not.

 

[Studboy]

So, your jetting should be OK then, unless something else of the ordinary has happened like an air leak or ignition problem.

Mixing different formulas of gas together (especially pump) and expecting to calculate the octane is a guess at best.
I know that I've read a thread on this before, but I tried to find it again and I'm stumped.

 

[joereitman]

91+110=201, divided by 2= 100.5. That should be fine if Eric says so. Your jetting is off, could also be rich, being excessively rich can cause detonation due to concentrated fuel pockets in the cylinder (incomplete fuel/air vaporization), creating hot spots. Get your jetting correct per the techniques in Gorr's site and in how2ride.com and you should be good. If not, then you may be running too much timing. In such case retard it a little. At least all this theory aplies to dsm turbos, figure it should work here, someone whack me upside the head if I am all wet. Good luck. :)

 

[cujet]

Also make sure the squish is correct. The CR500 needs a proper squish to reduce the detonation problems that naturally occur with the stock large clearance.

Chris

 

[Rich Rohrich]

On the octane, I was told you add the two octanes together, divide by two, and add two and you will have the octane. Not sure if this is good info or not.

That's complete nonsense. Blending two different fuels together will not give a linear octane response despite what some well meaning but hopelessly misinformed soul might tell you. The only way to know if your mix will work is to TEST IT. :thumb:

Your jetting is off, could also be rich, being excessively rich can cause detonation due to concentrated fuel pockets in the cylinder (incomplete fuel/air vaporization), creating hot spots. someone whack me upside the head if I am all wet. Good luck. :)

You're all wet :)

It won't knock from being too rich but it could knock as a result of improper fuel choice. Fuel that won't vaporize can cause all sorts of tuning problems, especially if the bulk of the high octane components are in the portion of the fuel that hasn't vaporized. This is a common problem when the fuel has a distillation curve specifically designed for long manifold runner automotive engines (i.e. high 10% & 50% distillation point temperatures) .

I'd be inclined to agree with you on one point though. If the jetting is resaonably close the ignition timing is a logical place to start. Big two-stroke singles are very sensitive to small changes in timing. It's also worth giving Eric a call . :cool:

 

[Firebalsocal]

Thanks to all that replied.
Cujet: I'm not ashamed (OK, maybe a little) to say that I don't know what squish is. Lemme look that up using the search function and get back to you. I assume that as the engine was assembled by hand by guys that assemble them all the time and the porting was done by Erics shop and they do the same thing all the time, that the squish band is correct but again, that is only an assumption.

Rich: Your makin my head spin man! :worship: I think I understand about fuels that are blended exactly for 4 stroke automobile engines that use long intake runners. Do the long runners possibly atomize the fuel better than short runners because there is more rough surface area on the long runners? Therefore, the fuel is blended to take longer to atomize? I think I'm reaching real far here.

Specs on the drum of fuel:

Recommended for ASTM D 4814 Volatility class A
Product code 0290
Gravity API 63.4
Density @ 60 degrees F. .726
On the distillation temps, the initial boiling point is 100
10 is 158
50 is 215
90 is 224
Reid vapor pressure, lbs. 6.2
Research Octane number is 115
Motor Octane number is 106
Do the algebra equation and it comes out to 110 octane.

In the mean time, my test of a 50/50 mix didn't turn out so unless Rich doesn't like my fuel, I'm going to try 100% race gas. Unfortunately, I won't have much time to test as the race is Sunday and I work all this week. :whiner:

Ben Brittain

 

[Firebalsocal]

Couldn't really find much on squish using the search function. Maybe I'm looking in the wrong area? Plenty of advice on where to set the squish but I didn't find out what squish really was. Can someone help a guy out? I don't mind looking so even a link would help me out.

 

[joereitman]

I am not sure how wet I really am. :lol: Here is some car stuff- :)

"There are several items that can cause or contribute to pre-ignition detonation. They include anything that creates higher than "normal" cylinder temperature or pressure. Advanced ignition timing, rich fuel mixture, high jacket water temperature, high boost pressure (from any form of supercharging), high compression ratio (tends to increase with increasing cylinder deposits), a too-high of valve lift (allows more air in, thus creating higher pressure during compression), etc." http://www.eng-tips.com/gviewthread.cfm/lev2/16/lev3/58/pid/816/qid/78557

I knew I didn't dream this. :)

Squish: "Turbulence generally, but not always helps to improve the homogenious nature of the mix, and in some instances it can detract from it by causing seperation of the heavy fuel from the light air dur to centrifugal force sending the fuel to the outside.

Squish also helps control detonation and propogates flame travel by greatly increasing turbulence just after ignition.

Squish or quench or whatever you call it only realy works by the piston rapidly displaceing gasses towards the combustion chamber as the piston gets very close to the head, hence the need to go as close possible without causing actual damage from impact. "

"So it looks like the consensus view is:

1. Make the squish gap as tight as possible w/o mechanical damage from contact.

2. Make the squish area as large as possible except do not exceed ~50% of bore area."

http://www.eng-tips.com/viewthread.cfm?SQID=77431&SPID=71&newpid=71&page=1

And some more-
"Start at 0.075 with an unproven combo. As the engine developed we would push toward 0.030 'ish.

Like the KZ900 example, the latest dragbike I work on is now running with light contact when using the "small motor" which is well developed. The new bigger engine is back to 0.035 just to be safe. It too will progress towards light contact.

Even with the same compression ratio, we ceratainly see power gains by getting the quench right. No doubt about it."

"The point of squish bands is to increase the mixing of the charge through increased lateral velocity. A smaller squish band gives more velocity and generally improves combustion. But keep in mind two things.

One, as you decrease the squish clearance the compression ratio goes up pretty quickly. Improvements in performance may have more to do with this than they do to tighter squish clearance by itself.

Also, I have seen situations where the piston crown and the head were parallel through the squish band creating what was in effect a ring land crevice volume. In other words the flame front couldn't travel into the squish band so the charge trapped in this area didn't burn. This not only hurt power, but also increased HC emissions. The problem was fixed by tapering this zone at about 3 degrees toward the center of the bore. This lowered the compression ratio very slightly but resulted in better power, due to improved combustion, and lower HC emissions. A greater taper - 5 deg - did not had no additional positive effect, and greater than that dropped the compression ratio enough that a power loss was noted."

"It is my understanding that a engine with large? squish areas are also less prone to detonation.In that case The slight increase in compression(and the risk of detonation)is more tolerated. This is assuming a properly designed combustion chamber. Current streetbikes are 12:1 compression ratio and tolerate 93 octane pump gas with no detonation problems..2ND.. I and slowkaw setup up engines at no less than .040 piston to head. I have tried .035 on a 1325cc Kaw., Piston was hitting the head(to hard) and fear of a crack starting between gas ports had me set it back up to .043. Absolutely the minimum for a large dragbike engine.Whats interesting is how a car drag engine can run so close a piston to head when there slinging around components twice as big and three times heavier!!As an example a JE flatop for a 1000cc motor weighs 190 grams.Very light,however max r.p.m. on this engine is a skyhigh 13000..."

"One point which no-one seems to have mentioned is that the squish area needs to be above a critical percentage of piston area to make any difference. I can't remember the exact figures but I think its around 20%. Below this figure(whatever it is) squish does not create enough turbulence to make any difference to preventing detonation. As some have observed, the improvement from having a small clearance is probably due to increased compression ratio and smaller crevice volume. So the criticism that people are too obsessed with squish is probably valid where small squish areas apply."

From- http://www.eng-tips.com/viewthread.cfm?SQID=18058&SPID=71&newpid=71&page=1

Man this stuff is cool- now if someone would just explain it to me, and confirm whether it applies to 2 smokes... :lol:

 

[billyghog]

Here's the general info on 2 stroke squish bands -

If you look at the combustion chamber in your cylinder head you will see a flat spot on the outside and a bowl on the inside. This flat spot is the squish band. As the piston nears TDC, the fuel/air mixture in the squish region is squished out at a high velocity towards the bowl. This homogenizes the fuel air mixture to help promote complete and stable combustion. As an example my 2001 CR250 uses a flat top piston. The squish band is nearly flat - it is at about a 1.75 degree angle (relative to the piston). Changing the width of the squish band will have an effect on the powerband of the engine. A wider squish will give more low end power and a skinnier squish will give more top end. My CR's squish is at about 50% of the bore area. The squish clearance is the gap between the piston at TDC and the squish bands closest point (at the edge of the bore). The min squish clearance for a 250 is 1mm (.040"). In general as the displacement increases, the minimum squish clearance increases. By decreasing squish clearance to the minimum amount, you will create more horsepower because the amount of fuel/air in the squish band will be decreased. (the fuel/air displaced by this clearance decrease will contribute to more power during combustion.)

That's the basics. If you want more info read Graham Bell's book 2 stroke performance tuning or maybe somebody else will post more info.

 

[Firebalsocal]

That was exactly what I was looking for. Thanks for posting. I didn't quite grasp the whole concept but now, I understand what the squish band is and why it is important.

On another note, I was able to get in touch with Eric Gorr today. He expects that my jetting is too lean, even though the plug shows a little darker than cocoa. Eric did turn down the cylinder to lower the transfer ports but raised the exaust port and worked the squish band a little. He said there is almost stock compression as he raised it very little. He said I could try running higher octane but seemed to think I just needed to run a larger main jet. I've just changed out the 178 that was in it for a 180 so I'll see how it goes. I haven't heard about discounting the plug color when jetting but Eric is the man and if he says the bike is lean, I'm not gonna argue. :worship:

 

[Rich Rohrich]

I haven't heard about discounting the plug color when jetting but Eric is the man and if he says the bike is lean, I'm not gonna argue. :worship:

The reason Eric has you ignoring the color of the NOSE of the plug is because he knows it's more of an indicator of the plug running temperature rather than the jetting.

Here's a link to a thread that explains it in more detail and shows some pictures of fairly specific situations that might help : http://dirtrider.net/forums3/showthread.php?t=92818

BTW, the specs on the fuel you listed should be a good starting point if you decide you need to run a higher octane fuel.

 

[Firebalsocal]

Rich: Why is there such a misconception when it comes to reading plugs? My auto shop teacher had the plug chart on the wall that showed the insulator tips color and what caused that color. Most anyone that jets carbs will look at the insulator tip. Why are we all wrong? Is reading the ring at the base of the plug a fairly new developement in tuning? I ask because this is the first time I've heard of it and I've been around alot of hot rodders and offroaders. Is this the laze mechanic syndrome?

On another note, I am a moderator for a small website on Glamis. I would like to put up the link to your plug thread for our members to access as it's the best main jet tuning guide I have come across to date. Will you or dirtrider/net have any problems with this? (Tried to PM you this but I don't have the ability. Still working on that.)

 

[splatt]

Why is there such a misconception when it comes to reading plugs? My auto shop teacher had the plug chart on the wall that showed the insulator tips color and what caused that color.

Those spark plug charts are based on research done on automotive type 4-strokes which don't use an oil mixture. Some tuning tecniques for car engines don't always directly translate to 2-stroke engines.

Steve

 

[Rich Rohrich]

Rich: Why is there such a misconception when it comes to reading plugs? My auto shop teacher had the plug chart on the wall that showed the insulator tips color and what caused that color.

Those charts are nothing more than a guide for plugs that have been in an engine for 30,000 of more miles of moderate speed running in a fairly narrow rpm band. Not exactly high performance tuning. Champion had a little performance tuning pamphlet back in the 60s and 70s that described the proper way to read a plug. Needless to say it didn't look anything like those silly charts in auto shop.

Is reading the ring at the base of the plug a fairly new developement in tuning? I ask because this is the first time I've heard of it and I've been around alot of hot rodders and offroaders. Is this the laze mechanic syndrome?

It's been around a lot longer than me, and I'm pretty old. :cool:

I won't even begin to speculate as to why myths refuse to die, and why people feel compelled to argue their validity. Like you, I heard all the standard things when I first got involved with race engines. I was fortunate enough to meet a few scary smart folks who steered me in the right direction early on. I'm eternally grateful to those guys. :worship:

On another note, I am a moderator for a small website on Glamis. I would like to put up the link to your plug thread for our members to access as it's the best main jet tuning guide I have come across to date. Will you or dirtrider/net have any problems with this? (Tried to PM you this but I don't have the ability. Still working on that.)

I don't see any reason why that would be a problem. We're happy to share info. :thumb:

 

[Firebalsocal]

Rich: I was at a 178 main jet while the engine was pinging at wide open throttle and high RPM's. I have gone up to a 180 (Currently my largest main). Any idea how many jet sizes I may have to go up to be safe? I will be racing a Grand Prix on Sunday with extended WOT straights and the bike shop where I get my jets is quite a ways away.

What is your opinion on running straight pump gas if I can get the bike to stop pinging by richening the jets?

 

[Rich Rohrich]

Rich: I was at a 178 main jet while the engine was pinging at wide open throttle and high RPM's. I have gone up to a 180 (Currently my largest main). Any idea how many jet sizes I may have to go up to be safe?

There really is is no way for me to know, sorry.

What is your opinion on running straight pump gas if I can get the bike to stop pinging by richening the jets?

Pump gas belongs in your tow vehicle NOT your race bike. Purposely lowering the performance of your race bike so you can run junk gas seems silly to me, but I guess you knew I would say that. :cool:

 

[motometal]

regarding minimum squish clearance, it sounds to me like there is additional horsepower to be had by "blueprinting" the squish clearance on a given engine, using clay or solder to measure the clearance. Since a tuner such as E.G. can't be sure of the exact clearance due to mfg. tolerances, etc., he is forced to set this clearance on the conservative side.

Am I on the right track? Maybe the tolerances of the components affecting this don't vary more than a thou. or two and the opportunity for power increase here is minimal?



Regarding the post mentioning an engine set up to have "light contact" does this mean what I think it means? Sounds like a recipe for disaster!

 

[joereitman]

Yes I think it does mean that. And there is no way I would build an engine that way. Except maybe a dragster, but that's too expensive of a hobby for me!

 

[motometal]

any updates? did the pinging go away? If it were my bike, i'd be tempted to try carefully marking the current ignition timing setting, and then retarding it a bit (i'm assuming it's adjustable). On some bikes, this will support better high rpm power, and it sounds like that's what you were after. Also, there is a good chance that the pinging will go away (if it hasn't already). With a measurment and a bit of easy math, you can figure out how far to move the stator plate to give X degrees.

 

[Firebalsocal]

I wasn't ready to reply till I was sure the problem was fixed but since you asked.....

This bike has a keihn PWK 39.5mm carb like the kx500's instead of the 38mm on cr500's. It came with a 175 keihn jet. I broke the bike in on this jetting and didn't remember it pinging though I was fairly easy on it. I took it directly to a dyno after break in with the same jetting. It didn't ping during the dyno run. They pulled the plug and advised one step higher on the main jet if I was headed to Glamis (Sand Dunes at sea level) or if I was to spend much time at wide open throttle.

I purchased two larger main jets from my local bike shop stamped RD and installed the 178. That's when it started pinging. I mixed 50/50 110 octane and 91 octane and it still pinged. For a race, I went with straight 110 and a 180 main jet. The race was at aprox. 3000 feet and the damn thing still pinged! Growl!!!!!

On the website CR500Riders.com, I read about a guy who siezed his 500 in Glamis. Turns out he had the same RD stamped jets as I do. These jets aren't factory keihn jets and are almost half the cost of the keihns so shops sell these unless you ask specifically for keihn jets. While they were stamped 178 and 180, they were actually smaller than the 175 I had taken out of the PWK, making my bike run lean and causing it to ping. I have 5 new keihn main and 5 pilots on their way from mxbike.com. I haven't ran the bike since the race but when I get the new jets, I'll report back here on how the bike runs. I expect the pinging to be gone.

Thanks for everyone's help guys. Being it's a brand new bike and me coming off a thumper, I was pretty rusty on my two stroke knowledge.

Ben Brittain
GlamisDunes.com

 

[motometal]

ahhh, I get it.

Seems like Master Rich once mentioned that even the genuine jets' flow can vary significantly (within the same number/size). Even if the hole is exactly the same, the lead in angle, chamfer, etc. may vary, which affects the flow.

Personally, i've found that moving one jet size can result in a very noticeable change or apparently no change (depending).