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Mods for lower powerband: SR500

Discussion in 'Modifications and Projects' started by mattb, Mar 22, 2009.

  1. I'm making up a bit of a plan of things to do for the over-all project on the SR500, so that if I know what I need I can get the parts (eg cam) over the next 12 months when it comes up cheap on eBay.

    I like the feel that for instance the Royal Enfield Bullet has - a 500cc single that pulls at lower revs both in terms of powerband and at higher speeds. I guess my main thing at the moment is how to get the powerband shift, and I'm interested in any brainstorming over that. Regarding the SR500, which shares essentially the same engine as the SR400, the XT500 and the TT500, there is a lot of knoweldge about hotting the engine up, but not much about cooling it down. People go for the racing thing with more power, whereas I'm going for the chugging old Brit single sort of thing. The one compromise that needs considering however is that I don't want the bike to feel any more laboured on the highway - it already feels too strained at 100kph. The Royal Enfield at the same speed feels out of breathe, but not strained, not like it's gonna blow, so the compromise might be possible.

    In my real lack of knowledge about mechanical mysteries, some of the vague things that have occured to me or that I've thought about are:

    a) Heavier flywheel. Either having my flywheel weighted, or sourcing an aftermarket one from Japan (where the SR rivals the Sportster for aftermarket bits).

    b) Cam grind. Had this mentioned to me, don't know much about it, but sounds like one good and simple (of sorts) mod. I heard mention of a certain Waggots cam (061 whatever that means), but can't find Waggots on the web.

    c)Bigger bore.

    d)Change the sprockets. Stock = 16/44. I'm currently doing 16/41, was using 17/41 for a week but the bike kept dying under load. Maybe some tuning would fix this. However, though this reduces highway rpm, it seems to be at the cost of the chugginess.

    e)Larger rear wheel. It looks like I'll be lacing up a spoked wheel from scratch anyway. I could fix the rear guard to the swing arm instead of the main frame to overcome clearance issues. However, is this any different from the sprocket-change idea, and is there any different effect to just changing sprockets?

    Any thoughts on what I've suggested or other ideas. The plan is not to do all of these, but to consider things that might work and then to decide on the best (and preferable least) combination. The SR pulls happily from 3000rpm onwards, and I sit between 3000 and 4000 all day, though often enough I can coax it to cruise at 2500rpm without snatching and that feels great, that's the sort of thing I want.


  2. You seem to have got the main points covered. These being flywheel weight and camshaft. No point mucking about with the gearing until you've got an engine that makes enough torque at low revs to pull tall gears. Even then, you may find that you're limited by chain snatch.

    Just to clarify, before I get leapt upon for misusing terms, for tha sake of brevity, I'm going to use the word torque here to denote low-rev pull. It's not actually correct but it saves typing.

    Flywheel weight does not, in itself, make torque. What it will do is smooth out the widely spaced pulses provided by a four stroke single turning slowly. It'll help with the chain snatch issue. It will also help with a reliable slow idle. On the downside, it will also give slower acceleration because some of the engine's power will be diverted to spooling up that extra metal. Flywheels can have weight added by either welding on extra counterweights (difficult to do without distorting everything to buggery but it is possible) or by drilling axial (not radial)holes around the perimeter and filling them with lead, tungsten, gold or depleted uranium as might take your fancy. Sounds crude but it's a perfectly valid method. The risk here is that excessive drilling may weaken the flywheels to the extent that they burst under centrifugal effects at high rpm. On the bright side this is something likely to only happen once per engine.

    If you go aftermarket, you have the possibility of making some real gains, although I'd be surprised if you could find heavier aftermarket wheels. The main potential for a gain comes from the possibility of increasing the stroke. I don't know how much room the SR has inside, but I'd guess that, if there is any extra available, someone will offer a set of stroker flywheels to fill it.

    I've seen the matter argued back and forth for twenty years and seen theoretical case and theoretical countercase, but, in the real world, stroker engines feel torquier than engines whose capacity has been increased by boring. It's just the way they seem to be. Building a stroker is not necessarily a straightforward task. It may be that the con-rod will need changing to achieve optimum geometry, and it's likely that the barrel will need jacking up to avoid the piston hitting the head, so of course the engine will no longer fit in the frame without modification to the head steady and you'll have to find a longer camchain from somewhere. On the plus side, and OHC design wouldn't require decades of mucking about to get the pushrod/valve/rocker geometry right like a pushrod lump can. If you want to increase capacity to make torque rather than power, upping the stroke is the preferable way to do it, before opening up the bore. Doing both is even more fun, of course :grin: .

    The other factor you (correctly) note is the cam. This probably does more to determine the engine's characteristics than anything else. And, in that sentence, lies the key to how to determine (roughly at least) the camshaft characteristics you need to achieve the engine that you want.

    Basically, find an engine of roughly the same bore and stroke that you like (or think you might like) the power characteristics of. Its valve timing is very likely to be somewhere close to what your engine will need in order to match it. This is mildly complicated by the fact that most (all?) pushrod Brit singles had separate inlet and exhaust camshafts. This doesn't affect what the valves were doing and when, however. It seems a bit approximate (and is), but it's probably the easiest means of determining workable valve timing. Most good cam-grinders should be able to advise too, although it may be hard to find one that does not think you mad for softening your engine.

    If you were to go ahead, I have, in a book, a table of valve timings for what seems to be most JAP engines from the mid-1920s onwards. Such numbers may be of some use to you and I'd be happy to PM you a scan.

    Having done the mechanical mods, you're likely to find that your carburettor is far to big for the low revs you're now using. Low air flows through big carbs give crap fuel metering and vapourisation. So you'll probably need a smaller bore carb fitted and set up.

    The ignition advance curve will almost certainly also be wrong. However, I would regard this as less critical. In my opinion (and it is only an opinion), on a robust, low revving engine, as long as ignition timing is somewhere close to right at idle and doesn't advance so far that your engine pinks itself to slag when climbing hills on hot days, it really doesn't matter that much. In the days of magnetos with manual advance-retard controls, the vast majority of riders would have maybe three timing settings. Full retard for kickstarting without breaking ankles, 3/4 to 7/8 advanced for normal running and maybe 1/2 advance for pulling up hills. Full advance would only be approached when trying to wring out the last smidgin of power when trying hard on a long straight. As the engines in question are what you're trying to emulate, I very much doubt if Yamaha's built in timing curve could do significantly worse.

    Good luck with it.
  3. Exhaust: smaller, longer primary pipes enhance torque.
    Intake: minimise restrictions, smaller carbs inmprove port velocity, improving fuel atomisation and enhancing midrange torque.
    Cams should have little overlap, but keep high lift to improve cylinder filling and volumetric efficiency.

    Regards, Andrew.
  4. Thanks for that guys, both the sort of info I need. Thanks Pat for the detailed analysis, really helpful! The SR has a similar bore and stroke to the Royal Enfield Bullet and, as I'm on a forum for REs, I posted up for this info.

    I'm used anyway to staff in the posh bike shops looking at me like I'm from Mars, and either walking away or explaining things to me like I saw a bike for the first time yesterday! :LOL: (Different story with the guys in the odds & ends shops.)
  5. I've come across some cams specifically for the SR from http://www.megacyclecams.com/ page 59 of their catalogue. Here's two which sound promising from their description, though I'm having trouble reading comprehending the specs (only just started reading up on this stuff).

    The stock SR (TT) cam is:
    Valve lift: .390" .390"
    Duration at .040: 244° 246°
    Lobe centre: 107° 108°
    Intake / exhaust open-close: 15 btc/ 49 abc 51 bbc/ 15 atc

    Cam number: 251-30
    Valve lift: .500" .500"
    Duration at .040": 273° 273°
    Lobe centre: 108.5° 108.5°
    Description and application: Best all around cam for off-road, TT style riding. Must use modified piston and springs.
    Intake / exhaust open-close: 28 btc/65 abc 65 bbc/28 atc
    Valve lash: .005" .005"

    Cam number: 251-40
    Valve lift: 480" 480"
    Duration at .040: 256° 253°
    Lobe centre: 108° 106.5°
    Description and application: OK with stock pistons. Must use 08-0514 or 102-Y springs. Best for low-end, off-road riding on muddy trails. Great torque and throttle response.
    Intake / exhaust open-close:20 btc/56 abc 53 bbc/20 atc
    Valve lash: .005" .005"

    I'll email them and see if they can give me a description of how they might suit my application.

  6. I'm not a specialist, but an eyeball perusal of those figures suggests that neither of those cams will shift peak power down to Bullet rpms.

    If possible, try and get a dyno curve out of the manufacturers for each cam to get some idea of what they actually do and whether they'll achieve what you want.

    When I get home, I'll try and remember to go and look up the figures I've got for old JAP singles. I may have some BSA figures knocking about somewhere too.
  7. I would never touch a flywheel that wasnt CAST or billeted. ModifieD? fcuk that. have you SEEN what flywheels do when they explode? Cars are bad enough, straight through gearbox casing, floor pans, and anything else in the way. Bike? hell no.
  8. how do u make a flywheel billeted?
  9. Modified bike flywheels are much less likely to burst than car flywheels. Being (approximately) half the size, the centrifugal forces on any part of the rim (the most highly stressed part) are half those on an equivalent car flywheel at the same rpm. Having rather different duties to a car flywheel, they tend to be beefier too, for reasons only peripherally related to strength, (like the need to provide a decent amount of seating area for the crankpin) and so have a much more substantial safety factor built in. As the purpose of the exercise here is to reduce the rpm at which the engine operates, which will also reduce the forces acting, I'd be pretty confident that careful work need not increase the possibility of a blow up.

    If the worst were to happen, the small diameter of the flywheels again works in our favour. At equal rpm to our hypothetical bursting car engine, the rim of the flywheel will be travelling at half the speed of the larger unit. This means that any shrapnel will be travelling half as fast and carrying only one quarter of the kinetic energy (which is what does the damage and allows a car flywheel to cut the vehicle and driver in half if it lets go). I wouldn't expect to be able to use much of the bike engine again, but I wouldn't be expecting much in the way of fast moving debris getting out either.

    If anything does escape with sufficient remaining velocity, on a single cylinder, conventionally framed bike, the rider is still shielded by frame members, the top end of the engine and the seat pan.

    There is pretty much no way the rider is going to get hit by anything moving fast enough to hurt.

    This is borne out by the fact that, in drag racing and LSR attempts, there is no requirement for a scattershield on bikes, whereas the rule is rigorously enforced in car classes.

    My opinion is also coloured by the fact that I've witnessed some quite spectacular engine blow-ups, including one in which a flailing con-rod effectively sawed an engine in two, and the net result of most of them, for the rider, was oily boots and a severe haemorrage of the wallet. Worst case was when engine debris got into the gearbox and locked the back wheel. Nasty but generally survivable and very rare.

    Folk have been modding bikes in crazier ways for over a hundred years now. Most of them have survived to die in their beds.
  10. for your case with flywheels. Yes, true, at the same rpm. not sure how high singles go, but most cars don come near matching bikes standard rpm. a flywheel blowing on a car might happen around 8-9 thou rpm, where a blowing flywheel on a bike would be closer to 15-20 thou.

    secondly, in terms of stroking, would reducing the conrod lengths be a viable way of doing this (assuming there is room at the bottom of the piston housing?

    would increasing compression ratio also help to your advantage?

    Have you considered supercharging?
  11. I'g guess redline on an SR500 to be around the 6500-7000 mark, with plenty of indication from the engine that it's not happy well before that. Comparable to cars.

    A shorter rod on a stroker isn't a good idea. You want to minimise the angle the rod swings through and a shorter rod increases it. Harry Ricardo, who, literally, wrote the book on how internal combustion engines work, found a rod length/stroke ratio above which bearing loads and side loading on the piston increase exponentially. I can't remember what the number is off the top of my head though and Google isn't being much help.
  12. Yeah, about 6500 it's crapping itself.

    This is an interesting thread...
  13. Well, I've just dug up my table of JAP valve timings and I'm astonished to find that, on the figures I've got in front of me, the standard SR cam is actually softer than what JAP were sticking in their 1947 500 OHV singles.

    The nearest equivalent you've got listed, in terms of both duration (the number of degrees of crankshaft rotation the valves are open) and overlap (the number of degrees of crankshaft rotation during which both inlet and exhaust valves are open) is the 251-40.

    However, I would add two qualifiers. Firstly, I'm not sure of the bore and stroke that JAP were using in their 500s. Secondly, the numbers I have simply state "Inlet/Exhaust Opens/Closes" without giving any indication of whether that refers to the valve just cracking off its seat or whether it's at a certain amount of lift as is noted with the modern cams. If it's the former, it could easily account for several degrees of crank rotation, making the JAP cam significantly softer than it appears when the numbers are read in modern terms.

    Another quickly available reference I've got immediately to hand are the figures for a Peugeot 504 I once owned. I know it's a car and probably has a completely different combustion chamber shape to most bike engines, but, nonetheless, it can be regarded as four 500(ish) cc singles bolted together on a common crankshaft. Bore is 88 mm, stroke is 81 mm.

    The cam timing figures indicate an Inlet duration of 215.5 deg and an Exhaust duration of 225.5 deg, with an overlap of 36 deg. These are measured on a comparable basis to the modern cams you list.

    Those are very short duration numbers compared to the bike cams, and the overlap is also measurably shorter than any of them.

    Lo and behold, the ol' Pug developed its maximum power at 5600 rpm. Sadly I haven't got a figure for the torque peak but I can say, from driving the thing, that it pulled like a tractor all the way from tickover upwards.

    I'm not suggesting that you have a cam grinder copy a Pug cam, but it might be worth looking at car type timing numbers, seeing as you want your engine to behave itself at car type RPMs. Ford OHC 2 litre Pinto timing and engine characteristics would be interesting, as would Holden Red 186 figures.

    Anyway, having probably raised more questions than I've answered, I'll go away and keep digging through my (less extensive than it once was) library.
  14. I tired to contact megacyclecams for dyno charts for their cams but, would you believe it...they don't seem to have an email address!

    The SR bore is 87mm and stroke 84mm. The Royal Enfield Bullet is bore 84 mm & Stroke 90 mm. I was told the SR is over-square compared to the Bullet negating any helpful comparison there. The BSA Gold Star is the same bore and stroke as the SR, which no doubt gives secret joy to many SR owners, but that's not exactly the sort of bike I'm trying to emulate (mind you I have no idea from what rpm it pulls well).

    I hope I'm not doing my engine harm, that I'm not lugging too much, but I'm getting adept as smooth running at 2500rpm - at the skill of feeling when the bike can do it and how to get the bike to do it. It feels fantastic! I'll contact Waggots in Sydney (no email or website! What is it with cam grinders?!) when I'm closer to having some dosh to have this done, or find a used cam and need to decide whether to buy it or not, and see if I can get some details of what they can do. I don't know if they start with a new cam or if I can send one to them.
  15. Most engines will tell you when they're unhappy. If you can ride smoothly at 2500 rpm, it's probably not unhappy.

    Camwise, it's worth remembering that the BSA Gold Star came in a great many differing forms. I know everyone restores them in sexy but impractical Clubman trim because that's where the dollars are and it makes small boys ask "Wot'll it do, mister?", not to mention being what the replica parts manufacturers cater for, but the Goldie could also be had with sensible gear ratios and touring cams to improve its road manners or allow it to be used as a dirt bike. I'm pretty sure it shared bore and stroke numbers with its cooking cousin, the B33 too. I know the B33 engined special I rode at Collie had a Gold Star piston in it so it must be close.

    Remember: Heavy flywheels make smoothness. Soft cam makes torque.

    I'm also assuming here that you realise that you'll sacrifice some power by going down this route. For example, the Pug I mentioned earlier was making about 25 hp per cylinder. Or, coincidentally enough, about what a 500 Enfield with a reasonably free breathing carb and pipe will do.

    Seeing as 25 hp should give you about 140 km/h with typical naked bike aerodynamics, and big torque will allow you to get off the line quite hard if you need to keep up with traffic, I'd say that it would be, in the words of Messrs Rolls and Royce, "sufficient".
  16. Google tells me the B33 is bore and stroke 85x88mm. Maybe I should check out of Goldie / BSA forums. I wonder what they pull from?

    I often have the engine idling rather high, because of the propensity to to stall, which isn't constant but because it happens in traffic that I've usually just filtered to the front of, always chooses that moment just as the lights turn green, and involves up to three desperates kicks in such circumstances, I feel the need to keep the risk at bay. So I go for about 1500rpm as opposed to the 1100 recommended, and in reality, with the engine idling differently throughout the day, she'll do 1500 sometimes, and push up toward 2000 at others, which is not great, but which I ignore because she's only idling for a minute at the lights. My point about this is that I'm happy with this scenario and so I guess the heavy flywheel - which would stabilise the idle making it much less likely to stall - isn't such an issue to me. It'd be be nice because I like showing off the spaced beautiful beats of the single, but I can happily live without it.

    Yeah, I've figured in the loss of power. As long as it's not dramatic, as long as I'm no worse than a Royal Enfield Bullet, I'll be happy. Riding wise I take pleasure in just cruising along, changing gear as little as possible, I'm generally not fazed if I'm stuck behind a slow coach - oftentimes I like it. The SR has been relegated to weekend duties, and I rely on the beast of a GR650 to get me home alive on the commute. Interestingly, mentioning the GR, I gave QuaterWit a display fo its powers the other night. It idles at 1100rpm, and from there you can move off and cruise along...without touching the throttle! Not at any point, you just slowly release the clutch and the bike slowly pulls away, then cruises along at an alright pace, and it does all of this very smoothly. It has a dual-mass flywheel that nobody ever bothered to use again, and yet which is compact and bullet-proof, one of the great ignored innovations.
  17. Well, at Collie, first gear had absented itself and so I was launching in second (and, on a couple of occasions, third) without complaint from the bike or embarrassing slowness of any kind. That's with a clutch that i didn't dare slip more than the very bare minimum as well. Basically it seemed to pull smoothly from idle upwards.
  18. Bang an XT head on it had smaller valves than a SR.

    wack a 2 mm aluminum decompressor plate between two head gaskets drop the compession

    two easy fixes to get the power shifted lower

    more complex

    extend the inlet manifold to 4 to 6 inches and bang on a 20 to 24 mm dia carb

    remove the alternator cover and see if you can add a machined ring over the top or the rotor

    dont get to fussed with the cam at the moment

    by the way bore and stroke on an enfield is not what makes them plonkers the problems ae as follows

    small carbs they ie built for economy not speed

    #$%^ up heads try sticking a finger down a port its all looks not action in there. If you look at the crappy casting on the outside its twice as bad on the inside

    Valve angles it may look like a hemi head but thats where the comparison ends they are so splayed (included angle) that any over lap sends it straight from the inlet out the exhaust.

    bottom line they would not will not hold together if any performance mods are undertaken without changing the parts totally.

    any body that builds a motorcycle in the twentieth century that still believes piston operated oil pumps have a place all i can say is :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL: :LOL:

    good luck with it
  19. Could you adapt the dual mass flywheel from the GR to the SR?
  20. I've often thought what a joy it would be to have that flywheel in the SR, but here's where I stand mechanically: when I got the SR two years ago it was a big deal for me to work out how to fit indicators on to it. Everything I know has been learnt on this and friend's bikes since then - i.e. I don't know a lot; and my workshop for the bike consists of a K-mart tool box full of, albeit good, tools, and the footpath in front of my house is where I work.

    I reckon that sort of task would range between being impossible to, more likely, being a major(!) headache for a professional skilled in such things with a proper workshop in which to do it. And if they were charging somebody to do it, I'm guessing it would be so much that I ought to just go buy an old Enfield.

    Brucey, I read a pretty knowing guy on the SR Forum once comment that his XT pulled from lower than his SR, ie 2500rpm, and when people said, 'But they're the same' he answered that he knew that, and that all his friends insisted on the same thing, but his experience was simply that it was capable of doing so. Maybe this valve difference makes that difference?