trying to learn a little engine theory.

[trickedcarbine]

I'm trying to learn a little engine theory as far as tailoring after market cyiliners for my specific needs.

 

I know that the serval is pretty much a tall cub with the bottom ripped off to essentially lower the ports for the low/mid range increase, but at the expense of a little top end. Many builders would do that early on for the trail/duners by buying say a 7mill raw cub, mill the bottom off 3 mills, and adjust the transfers a little for a 4 mil crank, then get it plated and run a set of wsm pistons with little to no base gasket. Get some domes made from Noss, and there was a bike with a good strong curve with about 90 hp and say 40ish on the tourque.

 

So, for a midrange flat track/ice racer like me, I am thinking of taking the same concept of the serval cylinder, and using a super cub instead. I have talked to several builders about the concept, but vaguely. So I would like to buy a 72mm 7 mill cylinder raw, and have it under cut to give a deck height for a 4 mill crank. Hopefully get some where near a 192 exhaust.

 

I would imagine that it would produce a fairly torquey bottom with a way pipey mid to top. Maybe 45 in torque around 6500 and with my rockets and 35.5 TM carbs the power would be around upper 90's. Essentially a hybrid between a serval and large cub.

 

Anyone actually tried this and pioneered the process and found out how to wring every ounce out of a set up like this. Am I a little naieve as to how this works or am I on the right track?

[AKheathen]

so, i'm trying to understand what you are trying to do here......you want serval cylinders, but in 78 bore, not 68? if that is the case, i would start by first trying to get them the same timings as the servals themselves. run them, and then raise the exhaust some more if you want it to be a little more pipey......have you done all the research on the serval cylinders? even with the lower exhaust degree timing, the time/area is still great enough to pull peak power over 10k, but the whole design is quite broad. really, sounds like they are just about made for you out of the box....

[Larry's Shee]

Might want to actually make a "map" with what you're wanting to do . How far piston drops for desired ex open vs where port is at that point. Might end up taking 2mm off bottom and 1mm off top (just example numbers). If you haven't yet I highly suggest reading Gorden Jennings ,Bell, and anything else you can find. Not being knowledgeable on the cyls you want to use I would think it has to do with the port size/layout , reed cages , triple ex (?). Keep us informed on this I'm curios. Also what size will this be?

Edited October 24, 2010 by Larry's Shee

[sleeper06]

You can def accomplish your goal .Do you have a mill ?You need to get all the components together for mach up run everything on a degree wheel to figure out what to cut.Your prob looking at 50hrs of fab or better.The only thing that concerns me is the bottom of the exh/transf.7mil is allready lower than a 4mil to start,after milling you might be able to see a ring through the the ex port .I dont have anything to measure but a 4mil cub so it might be possible

[2Go-Fast]

.......I know that the serval is pretty much a tall cub with the bottom ripped off to essentially lower the ports for the low/mid range increase, but at the expense of a little top end......

 

I don't post in here much, but this is one of those threads that could "lead people" the wrong way if some "extra info" is not made available.....so here goes.

 

The Serval Cub is NOT a modified current Cub casting that was machined differently.........it is a COMPLETELY different casting, made with completely different internal tooling....made SPECIFICALLY as a 4mill stroke Serval Cub. I will try to explain why it had to be done the way we did it.

 

"Larry's Shee" was right on target when he suggested making a "map", what he was referring to is a "portmap" of where the ports are located, or a portmap of where you want them to be....(or in this case....BOTH) If you were to do the layout, you would see that you would be going the wrong way if you used the 7mill Cub casting to build your "Super Serval Cub"....... (OMG....not ANOTHER cylinder.....lol)....here is why.

 

When machining a cylinder, I first "set" the tops of the transfer ports in relation to TDC, this allows me to get the transfer port timing that I want. I have chosen to open the "main transfer" ports first because IMO, our test results using OUR PORT LAYOUT has shown better results using that port configuration. Many....many factors come into play when setting up a port layout, but for the sake of this thread, we will "skip ahead" to the setting of the exhaust height and it's relationship to TDC. If we look at a 4mill Cub portmap as shown on the CPI website, you can see the transfers are set at 130 degrees, which puts the highest port at 1.754 from the top of the cylinder, and with the top of the main exhaust port set at 197*, it would be located 1.123 from the top of the cylinder. The reason this is relevant is because this is the "blowdown" number....(BLOWDOWN - the difference between when the exhaust port first opens allowing the spent fuel to escape through the exhaust port, and the transfers first open.....OR the time that the pressure inside the cylinder "blows down" to a pressure which allows us to open the transfer ports)......in this case the measurement is given in thousandths of an inch instead of degrees of crank rotation). In the case of the 4mill Cub, the blowdown is 0.631.....whether you deck the top or the bottom of the cylinder, the "blowdown number" remains the same......it is SET during the casting process, and even if we machine it differently it remains whatever it was set at when the casting was made.

 

Blowdown numbers for some of the Cubs and SuperCub castings:

CPI-6854........ 0.550

CPI-6858........ 0.631

CPI-6861........ 0.650

CPI-6864........ 0.679

 

 

We have not put the Serval Cub cylinders on our website yet, so the portmaps for the 4mill Serval Cub is a little harder to come by, I believe Andy from M&M ATV may have posted the portmap, but it may be easier to post it here rather than search for it, so I will attach it to the bottom of this reply. What it will show is the blowdown on a 4mill Serval Cub is exactly the same as on a stock stroke Cub...... 0.550, the problem is the stock stroke Cub would need about 0.162 more metal on the top of the cylinder to make it have the same numbers as a Serval......believe me, if I could have built a Serval port timing Cub without the expense of new tooling, I would have.

 

Sorry to get so long winded, but sometimes these things are hare to explain.....hope this helps,

Calvin Pollet

Port layout for CPI-S6858P_Serval_Cub.pdf

[trickedcarbine]

so, i'm trying to understand what you are trying to do here......you want serval cylinders, but in 78 bore, not 68? if that is the case, i would start by first trying to get them the same timings as the servals themselves. run them, and then raise the exhaust some more if you want it to be a little more pipey......have you done all the research on the serval cylinders? even with the lower exhaust degree timing, the time/area is still great enough to pull peak power over 10k, but the whole design is quite broad. really, sounds like they are just about made for you out of the box....

 

Ak, not neccacarily looking for a big bore serval. I have read anything I could find.(love google) but what I am looking for is a motor with a useble power range similar to the serval, yet still put out more torque as well as upper 90's in hp. It would be similar to the motors that pikes peak builders have ran.

[trickedcarbine]

I don't post in here much, but this is one of those threads that could "lead people" the wrong way if some "extra info" is not made available.....so here goes.

 

The Serval Cub is NOT a modified current Cub casting that was machined differently.........it is a COMPLETELY different casting, made with completely different internal tooling....made SPECIFICALLY as a 4mill stroke Serval Cub. I will try to explain why it had to be done the way we did it.

 

"Larry's Shee" was right on target when he suggested making a "map", what he was referring to is a "portmap" of where the ports are located, or a portmap of where you want them to be....(or in this case....BOTH) If you were to do the layout, you would see that you would be going the wrong way if you used the 7mill Cub casting to build your "Super Serval Cub"....... (OMG....not ANOTHER cylinder.....lol)....here is why.

 

When machining a cylinder, I first "set" the tops of the transfer ports in relation to TDC, this allows me to get the transfer port timing that I want. I have chosen to open the "main transfer" ports first because IMO, our test results using OUR PORT LAYOUT has shown better results using that port configuration. Many....many factors come into play when setting up a port layout, but for the sake of this thread, we will "skip ahead" to the setting of the exhaust height and it's relationship to TDC. If we look at a 4mill Cub portmap as shown on the CPI website, you can see the transfers are set at 130 degrees, which puts the highest port at 1.754 from the top of the cylinder, and with the top of the main exhaust port set at 197*, it would be located 1.123 from the top of the cylinder. The reason this is relevant is because this is the "blowdown" number....(BLOWDOWN - the difference between when the exhaust port first opens allowing the spent fuel to escape through the exhaust port, and the transfers first open.....OR the time that the pressure inside the cylinder "blows down" to a pressure which allows us to open the transfer ports)......in this case the measurement is given in thousandths of an inch instead of degrees of crank rotation). In the case of the 4mill Cub, the blowdown is 0.631.....whether you deck the top or the bottom of the cylinder, the "blowdown number" remains the same......it is SET during the casting process, and even if we machine it differently it remains whatever it was set at when the casting was made.

 

Blowdown numbers for some of the Cubs and SuperCub castings:

CPI-6854........ 0.550

CPI-6858........ 0.631

CPI-6861........ 0.650

CPI-6864........ 0.679

 

 

We have not put the Serval Cub cylinders on our website yet, so the portmaps for the 4mill Serval Cub is a little harder to come by, I believe Andy from M&M ATV may have posted the portmap, but it may be easier to post it here rather than search for it, so I will attach it to the bottom of this reply. What it will show is the blowdown on a 4mill Serval Cub is exactly the same as on a stock stroke Cub...... 0.550, the problem is the stock stroke Cub would need about 0.162 more metal on the top of the cylinder to make it have the same numbers as a Serval......believe me, if I could have built a Serval port timing Cub without the expense of new tooling, I would have.

 

Sorry to get so long winded, but sometimes these things are hare to explain.....hope this helps,

Calvin Pollet

 

Calvin, thanks for being johnny on the spot. I definitely am glad you took the time to drop solid knowledge. I didn't mean for it to sound like your serval was a cub just chopped in a mill. What I meant was just how close the designs were together in design, yet how different the out put is.

 

I am just trying to under stand exactly what is being done to the cylinders on outlaw tt/pikes peak bikes. I have briefly spoken with some pretty well known builders the concept of buying raw cast cylinders for larger stroke DM or Super Cubs, and actually cutting them down and adjusting the ports for the 4 mil. I know some builders have taken cubs and milled the base, ran no base gasket and moderate inframes and carbs for a bit more bottom, but they give up just a bit of the mega pull associated with a cub. I have seen one built and it ran, just looking for info on how they are doing it and if it is worth the effort.

[AKheathen]

Ak, not neccacarily looking for a big bore serval. I have read anything I could find.(love google) but what I am looking for is a motor with a useble power range similar to the serval, yet still put out more torque as well as upper 90's in hp. It would be similar to the motors that pikes peak builders have ran.

oh, calvin here would be the best to point you in the right direction, but i do believe he stated that abnother 10hp can be squeezed out of the servals, but that would change their broad nature. idk, for sure, but i would immagine increasing the blow-down would get you there. you mentioned that you desired a little more "pipey" power-band. you may be on to something with using the 7mill super cub on a 4mill crank, but, you would need to recess the domes an extra 1.5mm to accommodate removing 3mm from the bottom.

 

damnit, i've never had interest in playing with kitties, but with the new servals, and your ideas, my wheels are starting to turn to the tune of a purr, lol. i just have one funamental question......i've been asuming, but i would like to know- are the 4mill, and 7mill cylinders made taller to accommodate the extra stroke?

one thing you should be paying more attention to, is inreasing torque, over peak hp. i think there are a lot of differnt ways you can go with the differnt cylinders, but the biggest differnce with the cut-down 7mill, and a serval is the blow-down. if you get time/area close, the will just be a lower rpm pipey cub. if you really want serval-like power, you could start with servals, and even mill the bottoms and recess the domes, then bring the exhaust up some, or take the cub and bring the transfers up to broaden the power, but you will have the issue of port bottoms going that rout. i think your best bet to prevent the obtrusive piston crown in the port near bottom dewll is to start with a serval that will have more meat there to manipulate..........this is all just thoughts in my head, since i do not actually have hands-on experience with these, and just now am jumping into the kitty hype

[shanYE west]

why not a cheetah PV motor?

[trickedcarbine]

why not a cheetah PV motor?

 

 

I would love to do a 485 cheetah, but those fags at trinity will never get my business again. I bought a set of pipes, paid for two day shipping, waited a month before they would give me my money back after I still had not recieved my pipes.

[No Limit Powersports]

oh, calvin here would be the best to point you in the right direction, but i do believe he stated that abnother 10hp can be squeezed out of the servals, but that would change their broad nature. idk, for sure, but i would immagine increasing the blow-down would get you there. you mentioned that you desired a little more "pipey" power-band. you may be on to something with using the 7mill super cub on a 4mill crank, but, you would need to recess the domes an extra 1.5mm to accommodate removing 3mm from the bottom.

 

damnit, i've never had interest in playing with kitties, but with the new servals, and your ideas, my wheels are starting to turn to the tune of a purr, lol. i just have one funamental question......i've been asuming, but i would like to know- are the 4mill, and 7mill cylinders made taller to accommodate the extra stroke?

one thing you should be paying more attention to, is inreasing torque, over peak hp. i think there are a lot of differnt ways you can go with the differnt cylinders, but the biggest differnce with the cut-down 7mill, and a serval is the blow-down. if you get time/area close, the will just be a lower rpm pipey cub. if you really want serval-like power, you could start with servals, and even mill the bottoms and recess the domes, then bring the exhaust up some, or take the cub and bring the transfers up to broaden the power, but you will have the issue of port bottoms going that rout. i think your best bet to prevent the obtrusive piston crown in the port near bottom dewll is to start with a serval that will have more meat there to manipulate..........this is all just thoughts in my head, since i do not actually have hands-on experience with these, and just now am jumping into the kitty hype

 

 

All of the cp cylinders have different deck height per stroke. Servals are no different. At this time the 54 and 58 are available and both have different deck heights. Calvin usually makes it even easier on us by posting up the blueprints so we can get it perfect.

[trickedcarbine]

Nobody familiar with this type of build. I know there used to be quite a few guys that would shave cubs from the bottom and run wsm pistons and use rtv or permatex instead of a base gasket to keep ports lower. Just trying to understand exactly what is being done and how it works.

[J-Madd]

I think snopzynski has some information regarding that.

[trickedcarbine]

Right on, I was hoping he would chime in. There are a few others as well that I would like to hear from. I just don't like to call shops and waste to much of their time asking them to school me on engine theory. Cam at Redline was way cool as well as John Stallworth at Jsr.

Just trying to really get a deep understanding on what type of things guys are doing to cylinders to get the most from them.

 

By the way J-madd that is one helluva stable of bikes you have.

[trickedcarbine]

Really, no body willing to dish out the details? Cam, Snop, Andy.................