blowit

I am thinking some numbers are off here. 23cc measured volume - (approx) 6.6cc for piston = 16.4cc trapped. That would be enough to blow some shit in record time. Yeah, I would say testing that comp will be a good start. You do not need Orings to test the squish. Just shell it without the rings. They do not contribute dimensionally. 23cc trapped should be just fine all day long. Why don't you get Kev in on this? I am not sure you even have an issue. It looks like you might have just pinched an oring. Brandon

K, as I understand it, you have +4 timing, stock cyl, MX port, 91 oct? What is the squish clearance and how are the domes calculated? Ie, usually guys are offering the corrected number so if your dome displaces 26cc and you have say 7cc of piston sticking out of the hole, you have a 19cc dome. Squish and comp are good things to look at, at this point. B

The formula in Gram Bell books will work great. I recommend inputting that in an excel format and working it from there. I would start with ONLY the stock porting because you know what it is. Work your math until that calc comes out where is is supposed to. What gets tricky is the odd deck clearances for strokers. We use (-) for above deck. It is very important that you grasp the timing concepts. I promise that once you do, a LOT of other things will fall into place and make sense like rod angles, piston speed, etc. Hint here is rod angle and stroke have a LOT to do with port time area at a given angle. The change of port timing with a stroker is not a linear equation. Ie, there is no equation that says (example) .500mm up = 5 degrees of timing. That response is not linear. Beat your head on the floor if you must, then go get some Advil and work the numbers. If you cannot work the numbers, engine building is only a guess. I don't guess at anything!! Brandon

I cannot remember the exact figures off the cuff, but those numbers are not quite accurate. I apologize if I misled you or anyone else regarding any timings of a particular setup. The plate can certainly work for some things. I too may be mislead in the build if the OP is asking about a trail bike and then wanting a 421 (drag port) cub. To the OP, the cubs are near full drag spec out of the box so they should be reserved for more wide open riding or drag racing. It is just fine to ask though. Hopefully that will help you. If you want a trail bike, staying with your stock cylinders is the best bet. PM if you would like and I can lay down a setup for you that will work great. Slo, you may think I am trying to insult you but I am trying to get you to grab that calculator and figure it out. You need to learn it if you want to port. I never had anyone hold my hand in the learning process and I firmly believe that level of learning solidifies the foundation of tuning. Brandon

Agreed. Looks like separation. If the crank is good and you want to keep it, just send it in to be trued/welded. They should press it back together, true it, then weld it up. Problem solved. Rebuilding is not a certainty unless it is inspected and proved bad. True/weld is usually around 100 bucks. Mull Engineering

I was looking specifically at the OP and the riding requirement. Adding a plate will most definitely not create the type of engine he is looking for. For the record though, adding the plate and a 4mm crank adds more than just a few degrees of timing and most certainly will NOT create a power band that would please the OP. I was not concerned about which would run circles around another, I was focusing on the engine requirement, just as I would do with any customer. Give them what they want, not what I want.

For the record for the OP, my recommendation is to install a 4mm crank, mod the head or domes for the extra stroke, and if you do not want to afford it, leave the cylinder stock ported (no plate) and leave that as a future mod. Your bike will run better, will have improved bottom end from the extended transfer time and added stroke but may hit a little harder when it comes on. You will have about 15+HP hiding in there when you are ready. You can also have the cylinders ported for the 4mm without making them more top end happy. basically improve everything from bottom to top. Hopefully that helps. Note: You are going in the right direction if you want improved bottom end on a banshee. Stroking always helps. If you are looking for the "holy grail" of torque, you need a different bike. What you want is 4mm setup for MX. This will provide proper tuning to focus on the low end performance.

I would be happy to walk you through it. Just bring your pencil. B

This will calculate out the same as having a zero deck 4mm bike with raised port 2mm. Remember that rod angle certainly changes things if you are not at zero deck. ^^^^ UH, that is wrong. I apologize for my typo. That should be raised (0)mm. The spacer will certainly work to a degree without porting. I would agree with that.

Are you talking about the 4mm with a spacer?

Case and point. Many people can "port" an engine, BUT, who can fully develop a cylinder from scratch??? I will admit that I envy the engineers that do that every day. How do those Cheetahs run?? They did not have a template to work from. They had to "know" what they were doing when they started. I guess my point is there are leaders and followers. If you are only after copying other people's work, buy or borrow a good running set from a friend, copy them to every detail, and you have a port job. I am not looking to beat you down for your effort because that is how people learn. I just like to see people learn the right way. Fully understand it and you will be able to adapt your skills to any engine setup.

That is a good example of "technically" not going to work, but does... I would say at 2 yrs, you would have found if you had an issue. If you want to know your actual CR, give me a buzz just before you tear down and will get you a tool list so you can check it out. Or you can just put it back to bed and ride....

I think I might have misunderstood your post. As I said, if you are not going to touch the cylinder porting for a 4mm, you are better off adding the plate, BUT neither will preform close to optimal. I certainly do not try to make porting any hidden art. However, as an aerospace engineer, I have a pretty firm understanding of air flow, structures, fluid dynamics, and math. I will admit that there are many builders that talk more than they walk but they get sorted out soon enough. Most engine builders have learned from trial and error. Very few have a formal education in performance mods. That does not mean their engines do not haul. They found what worked. Most of them found it through someone else's port work. I think that the reason you find some resistance in getting port info is most builder have put MANY hours in the learning process. That learning did not come over 30 days. Personally, I have been building engines for 13 yrs. My first port job was a drag port Banshee that still runs and makes 65HP with more to go. I have never ruined a set of cylinders in my career. I never stop learning. I always say, if I knew then what I know now. This level of knowledge is cumulative and the only person that will have enough time to gather most of it from me is my son. I guess what I am getting at is trying to get all the info out on the table from a competent builder will take more than a 12 pack.... I think I have preached enough on this site about education. I really feel before one can properly port an engine and "know" (key word) what they are doing, you need to go buy books fluid dynamics, 101, 201, 301, Aerodynamics 101, 201, 301, Mechanical design 101, etc. I am not saying, go be an engineer, I am saying that when I see someone razor back an intake bridge in a sub sonic air stream, I know what I am dealing with.... I now sound like a cocky asshole but I am trying to convey that as soon as you fully understand how that air is moving through the engine, you will "know" what to do instinctively. Even with all the knowledge, you will still go through a trial and error process. That is part of learning. If you maintain your drive to learn this stuff and read the books, you may be one of the best engine builders in the country in 10 yrs, or 3 if your port every day... K, I have rattled on enough by now... B

Not trying to be an ass here but that is just poor advice. I will put any of my 4mil builds against a spacer plate job any day. Indications above are that without the plate, the bike will only perform on the bottom end. Simply NOT the case! I have several drag ported 4mils with OOFs out there. Not one uses a plate. I can prove my concepts on paper rather easily why I do not condone the use of a plate. This plate is simply offered as a bandaid for people that want to just assemble something in the backyard without grinding on any ports. NOTE: A plated bike WILL out perform a stock port 4mil without plate. Neither will touch a properly ported, non-plated 4 mil. Mull Engineering

I agree with most of this post. By looking at the actual ratios, you can see that there are some concerns at 20cc on pump fuels. However, the trapped volume does play in. I tend to agree strongly that looking at trapped volume is much more realistic. However, you must consider the air/fuel density when trapped to get an accurate estimate of octane. It gets damn tricky and I still think it is more or less a guess. When we do head work, we look at both numbers, pipes, timing, etc to arrive at what should be used. Ie, a 14:1 engine with poor fuel/air density when trapped will have no problem with pump fuels... Density is key but calculating it ties us in knots all the time. Mull Engineering

Because you are only facing the head (leaving all kinds of power on the table), this just become simple geometry. Measure your head and subtract what you remove and you will know exactly where the squish and volume will end up. Brandon

^^^ +1 You really should be measuring things thought. I get the impression you plan to just bolt up and run. You really need to invest in a caliper at a minimum before playing with head dims. That is a good way to get in trouble. Know what you got, then work from there. B

Don't you have a teen aged daughter that types with two thumbs??? Seriously, the damn text crap really gets tough to read. I think that has taken bad grammar to a WHOLE new level... I will admit though that the older I get, the more I realize how screwed up the English language really is. I am about to invent the "New English" where one letter ONLY has ONE sound... Trying to help my son with school. What am I supposed to tell him?? "Yeah, that C make about 5 difference sounds depending on what word it is in"....

Tyler has not come through as of yet. Probably a bit busy. If you have time, that would be great. The YFZ is one I need for sure. Thanks

OK, your point and question is a valid one. Certainly one we have considered for several months now., It is no secret that the front side cutting of a trans for an override transitions the transmission from near ALL radial loading, to quite a bit more axial load. That is something the geometries of that trans were never meant to do. That is putting some serious load on the forks, drum, slide pins, etc. I am just going to see where this thread goes and go from there.... B

Yep, thats a hole..... I suspect octane as well on this one. Keep in mind that sometimes 87 octane comes out of the handle, even when 91 is selected. With 21cc, +4*, +4mm, and cold temp, you are at the edge there. Keep in mind that cold air is dense air, this raising the dynamic compression ratio over what it might be at say 80*F. I think you would be fine just going back to a 50/50 blend. MIght take a quick peek at that main jet to make sure nothing else is going on but I think you are all over it. B

Not sure if I agree with creating a port from the aux port to the bore but to each their own. Those cylinder looks really good though. Jim does nice work. Brandon

Thanks. Same statement as above applies. If you have a minute that is. Please feel free to email any pics to our website email. Thanks

Tru dat dawg...

Basically a pic of the shifter shaft area between the shifter and the engine case. Need to know how much, if any, shaft is there exposed. IIRC, the lever nearly touches the case. thanks Tyler