engine buildling question

aloha450x · Post by **aloha450x** » March 6th, 2008, 9:30 am

lets say you have a stock 91 motor. and you get a con rod that is 5mm longer and space up the cylinder 5mm. you should have the same hp same torque and same compression right? but it should smooth out the torque curve correct? or am I crazy?

dannygraves · Post by **dannygraves** » March 6th, 2008, 9:42 am

in theory, but where would you get this longer rod?

aloha450x · Post by **aloha450x** » March 6th, 2008, 9:56 am

dunno haven't got that far yet. haha I was thinking that it might cause a little more vibration so a ballanced crank would be needed. to tell for sure a dyno if a 86 motor and a 86 motor with a newer rod and spacer should "key word" prove the theory

dannygraves · Post by **dannygraves** » March 6th, 2008, 10:01 am

I was thinking about dyno tuning my bike after I get the last of my mods in order. MFDB used to work at a shop with a bike dyno. I was planning on switching to the longer rod when I gte ready to do the crank, so maybe I could get you a before and after dyno situation.

britincali · Post by **britincali** » March 6th, 2008, 10:05 am

I wouldnt mind dynoing the nitrous bike just for shits and giggles

aloha450x · Post by **aloha450x** » March 6th, 2008, 10:18 am

sweet!!! in theroy the torque curve should be smoother with the longer rod.

aloha450x · Post by **aloha450x** » March 6th, 2008, 12:24 pm

ok here it is. best rod ratio for a cr500 is 1.75. the 86 with the 139mm rod is 1.754. the later motors rod ratio is 1.8 something. by making the rod length longer you are slowing the piston speed all the while keeping same hp and compression. so a 86 has a harder hit than other years. I can only assume honda changed the rod ratio on the later models to make it slightly easier in the bearings and bottom end. so if you have a 86 and put a 87 and up rod you basicly denutted your motor. but you can take a newer motor put a 86 rod (if you can find one) pull the sleeve out of the cylinder take 5mm off the top and 5mm off the bottom of the cylinder and bam you have a 86 motor. the longer the rod the more vibration you should have so I think a 86 is a lot smoother that 87 and up. so back to the idea of getting a 149mm rod and raising the whole cylinder 5mm would really smooth out the power delivery but cause serious vib issues that could be fixed by counter weights.

aloha450x · Post by **aloha450x** » March 6th, 2008, 12:50 pm

the 86 should rev faster but cant rev as high. am i correct? all in all you would have to have some deep pockets just to smooth out the torque curve like that. custom rob 149mm custom crank because you would have to change the stroke to 85 to get the desired 1.75 rod ratio. would be one bad ass smooth stock motor though.

dannygraves · Post by **dannygraves** » March 6th, 2008, 1:31 pm

well, I guess the bigest advantage to the longer rod is the piston spends more time at tdc and bdc which would leave the ports wide open longer. Remember that this will change the position of the piston at certain crank angles which might change your timing requirements. if on a longer rod motor the piston is at TDC longer, that would have an effect of advancing timing.
the KX500 has a longer rod, I believe its 7mm longer than the 85-86 cr rod. But I think it requires some modification to fit.
honestly, the right porting work, head work and pipe would probably be a better idea to change the motors characteristics than messing with the rod.
you are correct about the '86 motor though, it has a crazy hit, revs quick, but doesn't make much power up high. But I honestly think that has more to do with the timing, pipe and ports. the '85 and '86 ports are really setup for torque and not high reving. If you don't believe me, look at the boost ports on a '90+ or even an '87, then an '85 or '86, the boost ports are TINY and the exhaust is low and tall and narrow. you can tell the later year motors were tuned to rev more. when I did my porting work the first thing I did was cut a crap load of material out of the boost ports and the really helped it rev!

Post by **MojoScojo** » March 6th, 2008, 1:48 pm

Wouldn't this result in a higher crankcase volume and thus a lower primary compression? I would think that would be less than ideal. Jus thinkin..

dannygraves · Post by **dannygraves** » March 6th, 2008, 1:50 pm

MojoScojo wrote:Wouldn't this result in a higher crankcase volume and thus a lower primary compression? I would think that would be less than ideal. Jus thinkin..

redrocket190 · Post by **redrocket190** » March 6th, 2008, 2:54 pm

http://books.google.com/books?id=IC0p88 ... rwn0&hl=en

I knew I had read about this somewhere and it is mentioned in Eric Gorr's maintenance and tuning book related to the YZ250...

aloha450x · Post by **aloha450x** » March 6th, 2008, 4:04 pm

agreed howeve everyone who has a 86 and puts the long rod and spacer in would have that problem. so would it be different doing It with a 87 and up motor?

dannygraves wrote:
MojoScojo wrote:Wouldn't this result in a higher crankcase volume and thus a lower primary compression? I would think that would be less than ideal. Jus thinkin..

dannygraves · Post by **dannygraves** » March 7th, 2008, 7:49 am

same thing, by adding a spacer, you are changing the crankcase volume. In a really built jug, that could be a good thing to give more space to hold a charge if the ports are large enough and positioned to get enough velocity. Your really getting down to the bare bone workings of a 2 stroke motor. on my 86 I created a little bit more crankcase volume to help direct the charge from the re-shaped boost port, but the simple act of just adding a spacer and a longer rod... I don't see it being woirth the cost involved..

aloha450x · Post by **aloha450x** » March 7th, 2008, 8:40 am

so we need a dyno print out to see if my theroy holds up. but again it would not be worth the cost. im a helicopter mech so its in my blood t figure out how things work. shit a month ago I couldn't tell ya how a 2 stroke motor worked. haha

dannygraves · Post by **dannygraves** » March 7th, 2008, 8:46 am

nice!
ya, I'm the same way, 5 generations of engineers (that I know of), so yeah, its in my blood too. I write like shit, I can't draw, I'm dyslexic, so I have trouble reading, but put the most complex engine you can find infront of me and I will be improving it in about 10 minutes. My wife doesn't understand it, but I can already tell my 3 year old has it, he is kick ass with legos and uses logic well. we got him the 5yo+ little plastic cars that come with a wrench when he was 2 and I didn't even have to show him, it was instinctive (he tore them completely apart, lined up all the parts and put them all back together, abd even experimented with swapping parts)... a real trip!

Hes gonna be turning a wrench with his old man in a couple years!

Post by **AlisoBob** » March 7th, 2008, 1:14 pm

aloha450x wrote:lets say you have a stock 91 motor. and you get a con rod that is 5mm longer and space up the cylinder 5mm. you should have the same hp same torque and same compression right? but it should smooth out the torque curve correct? or am I crazy?

Why reinvent the wheel?

Stock CR500's kick ass.

nmdesertrider · Post by **nmdesertrider** » March 7th, 2008, 3:47 pm

If I wanted a friggin' four-stroke I would ride one.. short rod for me!

aloha450x · Post by **aloha450x** » March 7th, 2008, 5:45 pm

lol

im sure they are thats why i bought one. king motor of all time. just killing time while i get my bike together. nothing wrong with thinking how to reinvent the wheel. if everyone thought that way nothing would be improved. just my 2 cents.

bigjay · Post by **bigjay** » March 7th, 2008, 5:56 pm

just get a flywheel weight if you want to smooth it out, hell, ill be pulling the steahly on my bike this weekend and ill sell it cheap

http://www.youtube.com/user/500bigbore · Post by **Roostius_Maximus** » March 7th, 2008, 8:31 pm

get a rod by spec , not by listing. I'll get 1 for u if ur serious

BannedCR500Riders.com

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