I can say this when my engine was being built I was clear with the builder I was running a metered carb... He had worked with them extensively .... If that had a bearing on what's he did I do not know because of his medical problems I am not bothering him...
Wheres APT Smartcarb ?
- Thread starter summitboy
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Interesting. So MtnDoo when you had your deto on your 500 did you send your head to get it relieved ? Or did u just treat it with fuel builder ? Was that one of your consistent motors ? Is there deto on that bike now with the Lectron ? Maybe the owner will chime in and let us know.
Thanks Corey for your post but how exactly does running a resquished 2 stroke running a Smartcarb benefit over a PWK ? Are you claiming more hp than a Pulse or PWK setup ? Better fuel mileage ? Seems like running the same fuel, oil and proper squish is what your saying for consistent results with a Smartcarb. Too me that doesnt seem to make the Smarty so smart ?
Thanks Corey for your post but how exactly does running a resquished 2 stroke running a Smartcarb benefit over a PWK ? Are you claiming more hp than a Pulse or PWK setup ? Better fuel mileage ? Seems like running the same fuel, oil and proper squish is what your saying for consistent results with a Smartcarb. Too me that doesnt seem to make the Smarty so smart ?
C
Re-squished only if a problem is identified, which in most cases it is not. However it is the current discussion here and as you clearly state this has not been your experience. Facts are facts though and without reservation we claim fully altitude air density correcting, no less than an 8-10% gain in Horsepower and Torque over any conventional carburetor, and in a lot of cases gains over modern small engine EFI systems as well. I don't know about pulse/pwk or fcr, perhaps Roosty would entertain a comparison. If tuned properly you can expect 30% or more gains in fuel economy on your two stroke. 4t's will net even more, and 50% is not unheard of. There simply is no other product out there that will do this that you can put on with a couple of hose clamps. Period. So you can quit questioning its validity and its value. These claims can and have been proven many times.
As far as Lectron's and SmartCarb's go this is an area where they are completely different. A powerjet lectron uses a second metering circuit and adds liquid raw fuel straight from the float bowl and squirts it down the throat. When the type of detonation conditions we're talking about present themselves, a Lectron will simply mask the issue and really acts no differently than a conventional Keihin or Mikuni providing wet mixtures. In fact it's a little wetter mixture yet because a Keihin/Mikuni is using the main jet/nozzle system. To cool down a big two stroke though it works well. The SC on the other hand is only using the main single circuit utilizing the same atomization potential.
What should be understood is that the float bowl pressurization circuit is the power-jet on a SmartCarb, and somewhere near the very top end-wide open the static pressures that have been steadily building in the float bowl now become dynamic pressures with actual fluid flow. Meaning the ram pressure against the scoop (approaching 230mph at high rpm) literally hammers fuel up the nozzle, but it is still finely measured and atomized by the metering rod; and the reason a SmartCarb, size for size, will make more HP and provide better fuel economy than a Lectron.
This is why we have stressed an SC equipped bike will suffer in auto altitude correction and top end performance if the vent scoop can't read MAP.
Corey,
What is the best way to get tuning advise and parts for my smart carbs at this point?
I have left messages on APT voice mail with no response.
What is the best way to get tuning advise and parts for my smart carbs at this point?
I have left messages on APT voice mail with no response.
C
Sent you a PM.
H
Interesting, so giving the large squish area of 91-01 CR500 head it seems that a head with a cut down squish band would be a better option for anyone attempting to run a smart carb? What would be a good target as far as squish height? I know most people shoot for about .055' on pump gas, adjusting the chamber volume accordingly. I'm sure you need to consider altitude also. The 89-90 head might be a better option, though Rootius claims no dice even when he cuts down the squish.
I'm in the middle of a CR500 build and have talked to both Cory and Rootius recently and really don't know what to think. I have heard too much positive feedback from Smartcarb users for it to be a fluke, it really does seem hit and miss. Talked to Jerry Hall in Phoenix and he does not have anything good to say about single circuit carbs, but then again I have never talked to an engine builder or suspension guy who hasn't said everybody else doesn't know what they are doing, lol. I traded in my long over due 38 cast order, that was originally planned for my 300xc, for a billet 40 that I'm going to try on the 500. The billet upgrade option Cory is giving to long over due 38 cast orders was hard to pass up given the cost savings. Hope it works out, or I'll try the 40 on the 300 with one of the new heads...
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H
Chambers
Here are some chambers I pulled off the old interweb just for comparison sake. Some are courtesy of Rootius posts on cr500riders.com.
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KTM chambers old and new
Early vs late CR500 chambers
Roostius head
Richard's engine development. This one is interesting because its got the narrowest squish band and added o-rings
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RPM Liger
Here are some chambers I pulled off the old interweb just for comparison sake. Some are courtesy of Rootius posts on cr500riders.com.
KTM chambers old and new
Early vs late CR500 chambers
Roostius head
Richard's engine development. This one is interesting because its got the narrowest squish band and added o-rings
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C
C
Thank you for the pictures, the shot of the KTM head progression gives good clarity to the discussion.
Some of these head designs I hope are benefiting from a different piston crown profile than others. Not that anybody asked and again fuel selection is always key, but of all these heads, at least based on appearances, I would choose the Richard's engine development head over the others. It seems to have the proper surface area and perhaps more importantly nice smooth radii from the squish band into the combustion pocket. Unfortunately any transitions with sharp edges in the combustion chamber are very unforgiving, no matter how well intended, and create heat risers that glow red hot while the engine is running and without question will contribute to detonation issues.
Here are some chambers I pulled off the old interweb just for comparison sake. Some are courtesy of Rootius posts on cr500riders.com.
KTM chambers old and new.
Thank you for the pictures, the shot of the KTM head progression gives good clarity to the discussion.
Some of these head designs I hope are benefiting from a different piston crown profile than others. Not that anybody asked and again fuel selection is always key, but of all these heads, at least based on appearances, I would choose the Richard's engine development head over the others. It seems to have the proper surface area and perhaps more importantly nice smooth radii from the squish band into the combustion pocket. Unfortunately any transitions with sharp edges in the combustion chamber are very unforgiving, no matter how well intended, and create heat risers that glow red hot while the engine is running and without question will contribute to detonation issues.
C
Not necessarily. Remember we are working with two separate yet interrelated processes here. Squish (angle of the area), which is designed to apply a controlled turbulent state to the air/fuel charge with the idea of mixing it really good and creating a faster burning mixture. Quench (size of the area), to trap, what is basically liquid fuel to stabilize the end gases that surround the combustion pocket. Keeping these trapped gases from superheating by the advancing flame front and pre-igniting.
This also may as well be as good a time as any to note that pre-ignition and detonation are also two separate things but display the same characteristics of knock and piston damage. Both can be caused by localized hot spots, improper fuel selection or not enough area to provide proper cooling by the quench action. Detonation or unstable combustion however are more closely related to too much turbulence (meaning squish velocity or what we call MSV) .
The short answer may very well be a head that has a relatively wide squish/quench area, say 50% of the bore diameter, but a steeper squish angle to slow down the MSV. In fact that is where I would start, I would shoot for a large quench pad and an increase to the step angle to bring the MSV to around 25 m/s. This may also require a clearance change to around .065"+, which may diminish static compression slightly, but a good pipe like a genuine Scalvini (Not that House of Horsepower knock off) or some of the other quality exhausts that will more than bring the running pressures back up where they belong.
If working with a SmartCarb I would open it up to .057"-.058" as a place to start and use the earlier head with the traditional wide quench pad to provide a little more stability for the finer atomized fuel.
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M
Thanks for the schooling. With modern super atomized fuel charges we have to really give the engine and especially the head mod's some thought. I found consistent success by trial and error. Now it makes sense. Thanks
H
That seems counter intuitive when you look at the progression of the KTM head. I'm not saying its wrong, it just doesn't follow what KTM did as far as the quench area. Though it does kind of follow what Roostius did with his head mod, maybe he can chime in. Working within the constraints of the stock casting there is probably only so much you can do without weakening the chamber, so that may be the way to go.
Looks like the quench face is liad back some
This head, I think its a KX500, looks like the ticket, nicely blended.
Sorry pic are huge.
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R
Both carb versions I ran were 38mm, The 40 wasn't available.
The smaller carbs (not taling apt here) have been crisper and stronger mid range power on the CR5, the bore at 89mm and stroke at 79mm combination creates a shorter duration but higher velocity signal than a kx5 which is square at 86mm stroke and bore. The Kx's longer lower velocity draw does work better with the bigger carburetor.
Heads, I ran the billet carb on a stock 1991 engine, bone stock. The cast i tried on both a stock engine and a ported one with a modified chamber. ML3-R head castings on the engines.
Having the carbs with a known compatibility issue yet continuing to sell them for that application without knowing a solution or alteration is irresponsible.
I'm at 1000'
The smaller carbs (not taling apt here) have been crisper and stronger mid range power on the CR5, the bore at 89mm and stroke at 79mm combination creates a shorter duration but higher velocity signal than a kx5 which is square at 86mm stroke and bore. The Kx's longer lower velocity draw does work better with the bigger carburetor.
Heads, I ran the billet carb on a stock 1991 engine, bone stock. The cast i tried on both a stock engine and a ported one with a modified chamber. ML3-R head castings on the engines.
Having the carbs with a known compatibility issue yet continuing to sell them for that application without knowing a solution or alteration is irresponsible.
I'm at 1000'
R
I can also o-ring the head, normally only needed like with this project where the cylinder was a big bore and would require a custom gasket.
C
Ok let me take off my IMAX glasses and we’ll go deeper.
Let’s be perfectly clear, we are talking about CR 500’s and the first rule of thumb, when it comes to quench type combustion control, is that the larger the bore diameter the more critical all this becomes. With that said we are also dealing with an over square engine, as delivered by the factory. Meaning the bore diameter is larger than the length of the stroke. 89mm (3.503”) X 84.5mm (3.326”) bore and stroke. A 300 KTM on the other hand in stock configuration is 72mm X 72mm (2.834”).
Adam knows, as do most quality engine builders, that once you get over a 3” bore diameter a lot of things change dramatically and likely the reason he has spent a great deal of time working with these big bores. We also know that combustion technology, fuels, fuel delivery systems and a host of other items have changed since 2001 when the last CR500s were built. In the most conventional sense it’s long been widely embraced that increases in compression ratio, which can be accomplished simply by milling a few thousandths from the cylinderhead can work minor miracles with a 2 stroke engine. The down side of course is if the compression ratio is high enough, heat input into the piston may raise the piston crown temp to the point where detonation and pre-ignition occur. And where this gets really crazy with a 2 stroke is that our static (cranking) compression numbers are vastly different than actual running compression numbers with an effective pipe because of supercharging. And here is where most run into trouble.
Engineers work within a theoretical level of efficiency in terms of converting heat into work, referred to as “air standard efficiency”. In this theoretical level it is assumed that the cylinder is only filled with dry air, and then heat added, which completely ignores the fact that in practice this air contains some moisture and a considerable percentage of hydrocarbon fuel. What is important in working with air standard efficiency (ASE) is that it does provide a way to gauge the way actual efficiency can be measured and tells us a lot about the effects of power output relative to compression ratio. For example, at a compression ratio of 5:1, ASE is 47.5%, while if we bump that double at 10:1, ASE is now 60.2%. Which, of course is a large gain and the results measured at the output end of the crankshaft are the reason many tuners are fixated on raising the compression.
One way to work around this is to raise the compression by removing several thou from the bottom of the head and then open up the combustion pocket to balance it out. And this is normally what most do and all well and good until you start getting into 3.5+” cylinder bores and then we have to start taking into consideration flame speed (Which is what we were talking about when this conversation started, in regards to highly atomized fuel burning faster than heavy wet fuel droplets). This is furthermore where fuel selection and octane rating become very important, and in the case of our discussion here, adequate quench combustion control for large bore engines that display certain anomalous characteristics.
Flame speed as defined by Wikipedia is “the measured rate of expansion of the flame front in a combustion reaction". Of note also is that fuel and explosives are the same measured relationship, however combustion engines in general and 2 strokes in particular must differentiate between laminar flame speed (explosives) and turbulent flame speed (squish) combustion. And as one last thing the flame speed is also dependent upon stoichiometry or ideal air/fuel ratios.
By now you are either completely confused or you can see where I am going with this. A large bore engine that has a long path of flame travel the design of the combustion chamber becomes more and more critical to avoid detonation issues, while achieving good usable power.
Bore diameter isn’t the only difference between the new KTM heads and older CR 500 and modified heads. Understand KTM is getting ready to release a DFI engine, someday anyway, and to inject a plume of fuel in through the top of the cylinder head requires feature changes to provide a more even distribution, and if you look closely at Ski-Doo's E-TEC cylinderheads you will see that they use a very large curving combustion chamber cylinder with no sudden step changes. Same thing KTM appears to be leading up to.
In short, there is a threshold of bore size that leads to an increase in quench area to accommodate the longish burn time that occurs in larger engines. What exactly that is for a modified CR 500 I do not know, and it appears that for stock Honda was a little unsure as well. Also remember these are loop scavenged engines and during the exhaust phase, the shape of the cylinder head is also required to provide a gently curving surface to help whisk the exhaust residuals out of the combustion chamber while the fresh charge is coming up the back wall of the cylinder and pushing it out. This is partly the reason we used to polish the combustion chambers to a mirror shine to eliminate boundary layer stiction and provide a smooth surface for the gases to travel along.
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C
Well we are a little slow and didn't realize what the problem was as not all CR500 SmartCarb owners were or are having a problem. Hell I'm still not sure if what we are talking about here is even the problem with some of these bikes. Frankly, we now know most of the problem is with the airboot.
EDIT: Just out of curiosity I went and looked to see how many carbs we have actually sold for CR500 applications. We have sold 59 carbs total. 14 cast and 45 billets. Your Cast 38 that I offered to buy back is now happily running on a 300 KTM I understand.
Also might be a good idea to use Irfanview or some other free utility so you can bring these pictures down to 600 X 800 and we don't have to scroll the page over to read the text.
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When I look t the gss porting pics of their head it looks pretty sharp... That's what I have and it works don't know...
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C
Not trying to be stupid here but by sharp, do you mean as in attractive or do you mean an unbroken edge? Under the plastic, I can see a decent radius on the step from the squish into the combustion pocket, also the squish band looks to be nearly half of the bore diameter. Not sure but it also looks to be a bathtub design with a flat top and overall a good looking head. Compare that with the pictures above of cylinder heads with squared off unbroken edges leading into the chamber. These sharp edges do nothing to promote uniform squish action, create snap-off because the gases will shear and tumble off the sharp edges, plus those very narrow lines will heat up much faster than the surrounding metal.
These transitional areas require a proper radius and without it will glow and radiate localized heat right at the top of the piston....
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And that's why your the expert all I know is the smarty and this head talk well together
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C
And from looking at it I'll bet they do.
