REAL 80+ CC Happy Times!

Discussion in 'Performance Mods' started by andyinchville1, Jul 12, 2008.

  1. HI All,

    I was just looking at an old partially disassembled engine today and was thinking of a low $$ way to increase displacement and power...

    Recasting an entire cylinder would probably be expensive so I thought to myself....what about just making a spacer that fits on the bottom of the cylinder jug and make a new connecting rod (of course slightly longer studs would be needed too).....the longer stroke will add displacement and many additional cc would 1/8 more give you?....How about 1/4 inch more?...Of course hopefully there will be enough clearance inside the crankcase?

    I know connecting rods can be made of aluminum so machining a rod shouldn't be too difficult....of course the balance issue comes into play BUT since we are using the same piston maybe it won't be too far off?

    Minor porting for the additional displacement may be necessary but then again maybe not.....same fuel / air mix with a longer stroke should yoiels tons of torque...

    So what do you think? Seems like it wouldn't be too expensive to get more from our engines!


  2. datz510

    datz510 Member

    You'd need to change the crank to change the throw of the rod as well to get the displacement change.

    If you cant change the crank, then you have to increase the piston diameter to up the displacement.
  3. Ozi

    Ozi Member

    datz is right,

    might aswell put the spacer uptop it'll still do the same - stock rod length - lower compression, increase the rod length - return it to normal cc

    You could have a 1 foot long spacer and a 1 foot long rod, it will still be original cc.

    A stroker kit involves crank and rods.

    Nothing wrong with thinking about how things work, every day since I first started
    my bike and It roared into life, Ive been thinking about how to Improve it.

  4. Even if you can stroke another 1/4 inch both up and down your max RPM would go down because your piston is working harder. Vibrations would probably go up as well unless you can balance it all.
    A good example of the RPM thing is crotch rocket engines. They can go 10,000 RPM because they have very short strokes.
    In contrast on a Harley Davidson they pretty max out around 6,000 RPM because they have a longer stroke.
    But they also have more bottom end power.
    So a stroked out Happy Time would work but you may need to get that tooth count lower in back.. unless you don't mind burnouts.
  5. augidog

    augidog Banned

    as datz sed...without a crank mod, there's no way to get more stroke. and re: larger piston...i'd be wary of adding any more mass to the already-"heavy side" of the crank unless i was gonna re-work it.
    Last edited: Jul 13, 2008
  6. HI Guys,

    Hmmmm...interesting thoughts....I was looking at the 383 Chevy where they took a 400 crank on a stock 350 block to get the increased displacement.....So I guess simply increasing the the cylinder height and rod length won't do anything Hmmmm.....I seem to recall something though about rod length affecting the characteristics of the engine but I think then one would have to alter the piston too (I think it was raising the location where the rod joins the piston and having a longer rod (or maybe vise versa) and that somehow altered the torque output or power of the engine.....without need of crank alterations or cylinder alterations....But one would then have to mess with a rod and piston which I would think would be easier and cheaper than messing with the crank itself....(making a crank sounds more expensive)...In keeping with the "cheap" theme I am wondering if it would be possible to cut and weld the crank assuming good welds?...Of course maybe heat treating is needed too then....Hmmm


  7. srdavo

    srdavo Active Member

    I just found this post & it seems to fit here too.
  8. azbill

    azbill Active Member

    reliability is the key to motoredbiking success...either individually or as a group
  9. speedrider

    speedrider New Member

    has anyone tried just boring it to get the extra power everyone seems to be looking for?
  10. HI,

    I think boring the thin walls of the cylinder may ruin it....not to mention having to replate the cylinder wall and having to make a custom piston....I though simply adding a spacer would be the solution but appears I didn't figure on the crankshaft needing a longer throw to boot...

    FWIW - I contacted a company in Ohio to see what the would charge to make a custom crank for our engines.....the answer was $4000!! ....And that was EACH for a production run of 15 Cranks....I would guess there there wouldn't be too many takers here? ;-)

    I'm still looking! I love to tinker tho....

  11. speedrider

    speedrider New Member

    we wouldn't have to have a piston custom made, just find one that is bigger and fits, i did this on a golf cart. but on the cylinder, finding one with a thicker wall would be a good thing
  12. Yeah Andy about rod length, the rod ratio determines the piston dwell at TDC, will slow piston velocity slightly, will also slightly delay peak piston velocity and will reduce piston acceleration/deceleration.

    If the crank rotates 10 degrees, then a very short rod will be at a greater angle than if the rod is very long. since the rod doesn't change length then some of the pistons downwards motion will be a result of this phenomenon, so the longer the rod, the longer the "dwell" and the longer the dwell the more complete combustion, higher cylinder pressure and more torque, the rod length is limited by block deck height, piston height and so on.

    Hence 6 inch rods in chevs, a standard 350 with 5.7 rod and 3.48 stroke has a rod ratio of 1.64:1 and a rod angularity of 17.774°, which is not real good, change to 6 inch rod and you get RR of 1.72 and RA of 16.858°. which is better, but you have to change the pin height so the piston doesn't stick out the deck.

    The only problem trying to get more capacity out of the HT is you don't use all the crank stroke available to start with, usable stroke is from the top of the exhaust port to the top of the barrel, it doesn't start compression till the ports are closed, lowering the roof of the exhaust port would gain you some capacity but also retards exhaust timing so there would be no gain.

    The only real option is cylinder size, I have done some searching and there is heaps of lawnmowers, brush cutters etc with 49, 50, 51 ,52 mm cylinders, eg husqvarna has a 50mm piston with rings for 60 bucks US.

    But there's the issue of cylinder thickness and also pin height.
    Last edited by a moderator: Dec 18, 2015
  13. HI Phil,

    Thanks for the great info....with all this new info...maybe the cheapest and best option
    would be to find an existing cylinder head and piston that will for the HT block and machine a rod to work?.... Of course finding a head to work as a direct replacement would probably be a little much to ask would probably require some tinkering... Well the search is on! ;-)

  14. my advice would be to contact a machine shop with a CNC machine and get a quote on a cylinder. after that hit the internet and local bike shops and get a good idea on the 2 stroke aftermarket, i know there are still alot of fanboys out there building 2 stroke racing bikes. and see what you can get piston wise, make sure to get measurements on wrist pin sizes and what not. shimming the cylinder will just lower your compression and the flip side fly cutting the head will increase your compression, so your idea will not yield any more CC's but has potential to boost compression. you could shim your cylinder and find a little turbo :)
  15. JemmaUK

    JemmaUK Guest

    umm.. why are we looking at more swept volume?

    Minx engine - 1592cc - 7.1:1 compression - 59hp
    Sceptre engine - 1592cc - 9.5:1 compression - 85hp

    That is an identical motor - the only changes is one has a high compression head and the other doesnt - everything else is the same.

    We dont need a bigger engine with all the fitment problems that would entail - what we need is simply a cylinder/piston etc configured for a compression ratio of about 9:1 instead of 6.5/7:1.

    If I take my tanaka motor @ 32cc on 9:1 compression it produces 1.6 hp peak output. Double the displacement to 64cc and you would be looking at 3.2 hp and all that would require is a compression increase on the already available HTs.

    Re the idiot rider problem - you will always have them - just like you have the people who ride home in the dark with no lights and then wonder why they get hit by cars. I dont think it will make much difference if you put them on a 2.2hp HT or a 220hp superbike - they're still gonna have the idiot gene *sigh*

    Jemma xx
  16. Zev0

    Zev0 Member

    Amen to that Jemma.
  17. biketec

    biketec Member

    I agree

    Is all you would need to do to make more power out of any engine is deck the lid, and if you want to go further start with polishing ( really just debur the ports ) mirror them if you want. Then when your comfortable try to raise and lower ports, maybe open them up a bit, the better she breathes the better she goes. Look into a carb at a bike shop make some jets get a new cdi that will burn holes in your piston look into lubes IDK sky's the limit but theres no real need for such radical mods.Not to be a whiner but my bikes do just fine I love every last one no matter what they do granted powers fun but its a bike not a GSX R but I do hear ya the more the merrier leave the ccs to the factory and leave the hp tunes and tweeks to us the users.Good luck in your quest !
  18. dave1490

    dave1490 Guest

    a longer rod would allow higher rpm,s aswell,because of less back and forth dont see many{if any} 383 small block chev,s made with 400 rod,s.also you have grind down the oil pan rail,s to clear the rod bolt,s.with the longer arm.i milled 50 thou off my head{$60 at a shop}and it realy helped.sell milled heads.but i understand what you mean{thier no substute for cubic inches}.it,s the basis and all other mod,s just emphsize it.
    Last edited by a moderator: Aug 18, 2008
  19. no, a longer rod will only decrease the deck height and increase the compression, but with a 2 stroke in turn changes the 2 stroke port duration. so to keep everything with the engine the same and to avoid problems with the fuel you would have to shim the bottom of the cylinder which would negate the effects of the new rod.

    and you do realize the 383 stroker kits come with a crank as well.
  20. mnbiker

    mnbiker Guest

    After some thinking, the orignal posters idea would work, it would bring the piston up further createing a smaller expansion chamber thus creating more compresssion/power. However, increacing rod length is not really a good way to go about it, your better off using liquid gasket sealer for the head gasket and milling the head down some.