steved69tr6 Posted October 3, 2014 Report Share Posted October 3, 2014 I'm soon to be rebuilding a prefix MG engine fitted to a TR6. Does anyone know what lightweight flywheel this will require? The long back crank or short back crank? Cheers Steve Quote Link to post Share on other sites
Andy Moltu Posted October 3, 2014 Report Share Posted October 3, 2014 I'd wait until you take it apart - you might find the crank is not the original one - they are interchangeable providing the relevant flywheel is used. Quote Link to post Share on other sites
Red 6 Posted October 8, 2014 Report Share Posted October 8, 2014 Fitting a light weight flywheel is probably the best improvement to drivability you can do. I have just sold a brand new racestorations flywheel (also available from Moss) but remember to add in the price of the ringear and having it fitted plus the uprated flywheel to crank bolts (eye wateringly expensive). As an alternative just get your existing flywheel lightened. The last one I did cost about £60 in machining charges and what a difference!! Quote Link to post Share on other sites
Guest ntc Posted October 8, 2014 Report Share Posted October 8, 2014 - 1 waste of time Quote Link to post Share on other sites
peejay4A Posted October 8, 2014 Report Share Posted October 8, 2014 What's the point of a lightweight flywheel on a road car? I've no experience either way but on the web the for and against camps seem divided. For road/track maybe it makes sense I suppose. Quote Link to post Share on other sites
Red 6 Posted October 8, 2014 Report Share Posted October 8, 2014 I am quite surprised that there is even a debate about lightening or fitting a lightweight flywheel. Perhaps it is because the naysayers have never done it, or never driven a car fitted with a light flywheel. In any case it is for the owner to decide. The benefit is that the engine/car becomes very nimble from pull away and low speed performance ie in traffic/town conditions. The 6 flywheel is a big heavy beast, that's how the engines were built in those days. If you take a modern fiesta and compare it with one of the first generation the flywheel is now so much lighter which makes the modern version feel bright and nimble instead of sluggardly. This is because less power is being used to spin up the flywheel. So how lightening a flywheel be seen as anything other than an improvement? I bet someone comes back with tickover. Quote Link to post Share on other sites
Mk1PI Posted October 9, 2014 Report Share Posted October 9, 2014 I think the lightest you need for a road car is the same as a original TR5 / Mk1 2.5PI cast iron one - the late short back cranks had a significantly heavier flywheel so lightening that back to around TR5 weight is desirable if you want the character of a TR5 engine. Good balance is essential - a poorly balanced lightened flywheel is HORRIBLE. Don't recommend anything alloy - My Racetorations one tried to fall off despite 8 bolts loctited - http://3.bp.blogspot.com/-nd2St6DRacw/UUvihrDnYWI/AAAAAAAABV4/-dPWWYJ264g/s1600/030.JPG http://2.bp.blogspot.com/-N1PGvWf7jT4/UUvidi3INuI/AAAAAAAABVo/okrvK3gV3go/s1600/029.JPG Quote Link to post Share on other sites
TRTOM2498PI Posted October 9, 2014 Report Share Posted October 9, 2014 I run a lightweight steel one, not quite as light as Andy's second link, with integral ring gear, and idles perfectly, and is a delight to use in traffic, even coupled to a 3 piece AP Racing clutch. Cheers. Quote Link to post Share on other sites
Fireman049 Posted October 9, 2014 Report Share Posted October 9, 2014 My 3A is fitted with a 4A lightened flywheel and diaphragm clutch and is happy to idle at 500 ~ 600 rpm. Tom. Quote Link to post Share on other sites
Alan Cochrane Posted October 9, 2014 Report Share Posted October 9, 2014 I'm all for the lightened flywheel-but then I have to say that, Tom just sold me his. Alan Quote Link to post Share on other sites
Peter Cobbold Posted October 9, 2014 Report Share Posted October 9, 2014 (edited) Triumph will have designed the flywheel's mass as part of crankshaft torsional vibration control. The mass of he flywheel acts as a sort of anchor ensuring that torsional 'twisting' of the crank moves forward to be taken out by the damper on its nose. Lightening the flywheel will upset torsional damping, and the damper my no longer be effective as Triumph intended. This diagram gives the idea: http://www.bhjdynamics.com/images/WP/DWP01_illo01_500.gif More here on light fwheels and torsion:http://www.bhjdynamics.com/index.php?main_page=page&id=4 "" Other Rotating Group Parts Heavy vs. Light Flywheels — Light flywheels lower the whole system inertia, move the high torsion stress point a little farther forward (away from the flywheel), raise the first mode frequency a bit, and sometimes allow a little smaller damper. They also allow more crank lugging motion which usually shows up as gear rattle in the transmission. (This is called "rigid body motion" in the torsion trade since the crank is just seeing RPM variation without twist) Transmission Gear Rattle — This is mostly caused by low frequency torque pulses ("rigid body motion") in the crank at lower speeds when flywheel energy is low, although this motion usually persists out as far as 2000-3000 RPM. Aluminum flywheels make it worse for the obvious reason. These frequencies are way lower than the damper is designed for and the damper has virtually no effect on the gear rattle problem. Dual mass flywheels go after this problem. "" So the light flywheel moves the highest torsional stress point forward- so it may no longer be where Triumph intended, which is presumably within the rear main bearing. I have no idea what the effect a light fwheel will have on crank longevity, altough I'm pretty sure it wont improve it!! Peter Edited October 9, 2014 by Peter Cobbold Quote Link to post Share on other sites
Mk1PI Posted October 9, 2014 Report Share Posted October 9, 2014 Not convinced that fitting the vastly heavier Mk2 / TR6 flywheel was engineering excellence by Triumph. They went from a nice compromise with the TR5 and then added about 10lbs ( with exactly the same front damper) for the TR6 Didn't kas kastner reckon that the lighter the flywheel the better , theory being something that weighed nothing couldn't fall off. Quote Link to post Share on other sites
dblenk Posted October 10, 2014 Report Share Posted October 10, 2014 (edited) l fitted a lightened flywheel as part of my fast road project got everything balanced in clouding the con rods revs very freely and gets stright up to red line when pushed Sounds great David Edited October 10, 2014 by dblenk Quote Link to post Share on other sites
Guest ntc Posted October 10, 2014 Report Share Posted October 10, 2014 Most would not tell the difference until it cracks or chops your leg off and you lose torque . Quote Link to post Share on other sites
dblenk Posted October 10, 2014 Report Share Posted October 10, 2014 Roughly how much torque do you lose More torque gives quicker acceleration ??? So would it be better to fit the tr5 flywheel got it in the garage Cheers david Quote Link to post Share on other sites
Guest ntc Posted October 10, 2014 Report Share Posted October 10, 2014 The 5 flywheel will not fit your 6 David, torque = more pulling power Quote Link to post Share on other sites
Stanpartmanpartwolf Posted October 11, 2014 Report Share Posted October 11, 2014 Someone like to explain how a light flywheel "loses torque"???? Back to school, folks... All of the foregoing is basically immaterial- any 4-bolted flywheel on the 2.5 will detach at 7200rpm. An 8-bolter will last somewhat longer before it lets go, but by then the cam drive & distributor dogs will have departed. As long as the engine is kept out of the deadly third torsional at the aforementioned rpm, the only flywheel issues will result from good old Haynes, which lists the bolt torque at 46lbft (the TR2-4A setting), rather than the correct 95lbft. Never use Loctite, always thread lubricant. Other points to consider: It is unwise to take an 8.5" clutch into the danger zone, even with a proper crank damper, as on occasion the pressure plate will move off-centre in the cover, causing vibration that will break the crank within an hour. This is another reason we use 7.25" clutches on race engines- the pressure plate cannot stray. Gerhard Berger-style early downshifts will also cause trouble by "buzzing" the engine; though the harmonic is less vicious off-power, it's still there. End of lecture.....just use what suits you & bear in mind the 6-cylinder crank and clutch assembly carry plenty of momentum in their own right; a light flywheel barely affects drivability, but aids acceleration. The effect obviously decreases up through the gears. Finally, do not fret about catastrophic detachment. There is a 1" thick first motion shaft through the centre of the clutch & flywheel into the crankshaft. Emotive talk of "lost feet" stems entirely from foolhardy lightening of stock clutch covers & flywheels, which can burst with devastating effects. Anyone remember a certain Mustang at Mallory Park ten or so years ago..? Cheers SPMPW Quote Link to post Share on other sites
Tom Boyd Posted October 11, 2014 Report Share Posted October 11, 2014 ^^^^^^^^ that is a great reply and from someone who knows ^^^** Quote Link to post Share on other sites
Guest ntc Posted October 11, 2014 Report Share Posted October 11, 2014 http://en.wikipedia.org/wiki/Flywheel Quote Link to post Share on other sites
Guest ntc Posted October 11, 2014 Report Share Posted October 11, 2014 ^^^^^^^^ that is a great reply and from someone who knows ^^^** So Triumph got it wrong ? boll!!!!! Quote Link to post Share on other sites
Peter Cobbold Posted October 11, 2014 Report Share Posted October 11, 2014 ^^^^^^^^ that is a great reply and from someone who knows ^^^** I read it as a warning not to even think of going anywhere near 7200rpm. Quote Link to post Share on other sites
Alec Pringle Posted October 11, 2014 Report Share Posted October 11, 2014 (edited) Flywheel weight will influence driving characteristics to a certain extent, but the flywheel is only an effective store of torque, it's in itself neither adding nor subtracting to engine torque. In broad terms less flywheel weight equates to nimbler acceleration, whereas a heavier flywheel may reduce gear changing - the difference isn't huge, but it is noticeable on minor country roads, the lighter flywheel requires more cog swapping to keep up speed through the bends and rises. I'm not convinced either lighter or heavier is better in itself . . . . it does rather depend on the driver's preference. Personally I like the lugging, slogging ability of a longish stroke Triumph, whereas an oversquare Ford is blessed with a snappier lump - both have their virtues. I do recall a conversation with a Triumph engineer long ago, during which he explained that the increasing flywheel weight of the 6-pot was largely a result of sales and marketing perceiving an ageing profile for the customer base, and adjusting the driving character accordingly. That makes sense, I'd suggest. Cheers Alec Edited October 11, 2014 by Alec Pringle Quote Link to post Share on other sites
RobinTR6 Posted October 11, 2014 Report Share Posted October 11, 2014 Alec, Thanks for the note of our passing years. I'll have to agree tho, the 2000 Alfa GTV 1970 that I recently had was quick off the mark and gutsy as long as you got her up on the revs, I have to admit that the long torque Triumph 6 pot is more enduring in terms of enjoyment and practicality. Those triumph boys didn't do a bad job, actually a very good one... Quote Link to post Share on other sites
Red 6 Posted October 13, 2014 Report Share Posted October 13, 2014 Why would you worry about going to 7200 RPM? The thread is about lightened flywheels, the benefit is at low revs, ie pulling away, when you want to go to high revs ie 7200 and above (and below) the engine internals need to be optimized for the purpose as mentioned by stanpart........, which was a good response. Now I wonder how fast 7200 Rpm would be in overdrive top and a 3.45:1 diff? My race engine was very happy at 8K rpm but I never managed to find the conditions to pull that in top gear (I couldn't find a steep enough hill with a long enough straight!!! :-). Quote Link to post Share on other sites
Peter Cobbold Posted October 13, 2014 Report Share Posted October 13, 2014 Tuning for power is all about getting more air into the cylinders. I regard a long-stroke engine as a pretty poor air pump. So I fit a device designed to pump 50% more air mass into a standard, totally unmodified, engine operating within normal rpm. The load on the crank and rods are raised maybe 10%. Whereas revving from 5000 to 7200 doubles rod tensile loading. Supercharging doesnt need the flywheel lightening, nor trick cams, expensive rods, flowed head etc etc. That's precisely why supercharging is not popular - its cheap in comparison, and suppliers' profits would suffer. We wont agree on this. The engines have totally different feel. I prefer to drive torque, you prefer revs. Peter Quote Link to post Share on other sites
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