E: Engine Big Bore Comet

[DucATIRadeon]

I've not heard of a Godet 600 single........Ours are all 500 cc, but 2 different engine bore/stroke combinations used........Stock French engine uses a 92 x 77.2 mm.......whereas our modified engine uses a 86 x 86 bore/stroke.......This second engine was re engineered by Peter Molloy here in Australia a few years ago.......he was a specialized engine builder who came up with this engine spec combo........Both these engines use a one piece all steel crank with 2 piece shell bearing con-rod.......These engines will rev to 8000 rpm plus, the 92 bore engine even higher......of course they all use a high pressure oil pump and the engine casing has provision for the WR Yamaha type pump.......My engine, i have set the rev limit to 7200 on the limiter but seldom take it past 6200 on the track......It has a huge amount of torque, so simply reving it higher is not really necessary...... i doubt any of the French engines has gone near 60 HP.......The design just simply is not that high a spec......Its power comes from big revs and massive duration cam timing.......There is more that can be done, its just time and money...... The French engine is great, but the top end and cam design is too old school i feel.

my bad: Godet 500 Flash claims 60hp.
those BxS combos come near the ABSAF (BSA Goldstar D34) competition engine config, they push out well over 50+hp below 7000rpm.
uses metric bearings and vernier cams but 1:1 interchangeable parts with the original BSA of the '50s.
their full race spec engine with 12.6:1 comp ratio and some goodies makes 7400rpm redline and ample torque to stay ahead of the pack.

coming back to Vincent: I'd've thought there were larger valves for these engines and room for improved air flow in the porting to keep it going with RON98E5 petrol (instead of blown and/or ethanol)?
for my Bullet I've been studying JAWA 2V engines, these do in the region of 60-80hp and upto 13000rpm for bob's sake!! cannot be compared to these English lumps, but still, gives perspective on what can be achieved with modern tech fitted to antiquated (yet advanced in those days) design. speaking of, the Godet 500 Flash is very near the JAWA engine BxS...

have yet to compare a stock Vincent piston with that of a RE 500 (apart from the gudgeon pin dia) perhaps these can fit in a Vin (piston prices in favour of the RE)?

apologies in hijacking this thread.

 

[ericg]

would you happen to know why your Comet isn't going above 4500rpm?
just curious...

The short story was a leaky inlet valve plus a Vincent pattern silencer of unknown origin. Now with a new valve and guide job plus a home made "silencer" she flies!
Not much below 3800 rpm (105 cam) but above this I have to admit that I'm very surprised by the acceleration.
As this engine is still fresh with less than 4000 miles I didn't dare to let it go above 5000 rpm so far but I'm pretty sure it'll easily go well above.
Oh, and the Gold Star RR gearbox is a real pleasure to play with.

 

[greg brillus]

First thing to remember is that the hardest engine to gain more power from is a single, as it has to cop all the mechanical loses within that one piston.......Like any engine the Vincent can be tweeked for more power.......but there are many "Brick walls"........The head design is very shallow in height.......this means the port shape has too sharp an angle change for good gas flow.......The available room within the rocker/valve area is quite restrictive.......Moving the stem collar further up the stem and modifying the lower guide area will give more travel.......The timing chest is very restrictive.......again the available height from the cam spindle centers to the roof of the chest is limiting.......All these issues can be overcome to some extent, but it is a lot of work and takes much experimental work. The cylinder head is the biggest hurdle, as making up a new head is very expensive.......The Horners have done this very well........made the head taller thus allowing better port shapes.......and stood the valves up, thus allowing for a more shallow combustion chamber......this means a lower piston crown, less reciprocating weight, and so on.......The valve train......you can run a cam with a base circle of about 900 and a lift of about 400.......with ratio in the followers and on the rockers you can get good valve lift figures.......(we have 600 at the valves).......But all these parts need to be strong enough to cope.......the lubrication is not ideal.......in an engine with much more power it is going to over heat.........bigger head/barrel fins are needed ( a big downfall on the French engines)......The Horner engine suffer this big time.......overheating.......Running on petrol type fuels you really need an oil jet under the piston crown, not just to cool the piston but lube the small end which seems to be a common issue on these engines as well......One big issue I learnt recently was that running a big bore short stroke engine will result in major overheating issues........apparently, when using a short stroke whereby the piston does not actually come up and out from the crankcase mouth by much, the piston does not cool at all and the engine cooks itself.......It is a common problem on the 95 bore Manx's so I've been told.......This may explain why Peter Molloy here in Australia changed the bore/stroke combo on one of these French engines some years ago, extending the stroke from 75 mm to 86 mm.......But I'm thinking it was to give the engine more torque.......for track racing this drive out of the corners is really what you need........Madness really but all good fun.

 

[Chris Launders]

The short story was a leaky inlet valve plus a Vincent pattern silencer of unknown origin. Now with a new valve and guide job plus an home made "silencer" she flies!
Not much below 3800 rpm (105 cam) but above this I have to admit that I'm very surprised by the acceleration.
As this engine is still fresh with less than 4000 miles I didn't dare to let it go above 5000 rpm so far but I'm pretty sure it'll easily go well above.
Oh, and the Gold Star RR gearbox is a real pleasure to play with.

I had a similar silencer problem with my twin when I got it, my Norvin with a Gold star silencer would hit 100mph up my test track but my "shadowish" would only do 75mph, checked everything, found nothing and eventually swapped the G/S silencer on to it, behold suddenly 95mph, So I fitted a straight through, absorption one.

 

[DucATIRadeon]

I had a similar silencer problem with my twin when I got it, my Norvin with a Gold star silencer would hit 100mph up my test track but my "shadowish" would only do 75mph, checked everything, found nothing and eventually swapped the G/S silencer on to it, behold suddenly 95mph, So I fitted a straight through, absorption one.

that's interesting too, the exhaust:
on these they are 42mm diameter, whilst on the Enfield they're 44.5mm.
my current exhaust is an original bullet bazooka modified internally to straight perforated with fibreglass wrapped around to make absorption. admittedly over the past 19 years the fibreglass has probably blown out and makes more noise than speed to be pleasant...

the Vin original exhaust is also kinda absorption with smaller diameter and does make excessive noise...
id' ve thought the Vin exhausts would be same size for the better heat and exhaust escape, perhaps they've chosen this size to give resistance which increases torque somewhat?

 

[DucATIRadeon]

First thing to remember is that the hardest engine to gain more power from is a single, as it has to cop all the mechanical loses within that one piston.......Like any engine the Vincent can be tweeked for more power.......but there are many "Brick walls"........The head design is very shallow in height.......this means the port shape has too sharp an angle change for good gas flow.......The available room within the rocker/valve area is quite restrictive.......Moving the stem collar further up the stem and modifying the lower guide area will give more travel.......The timing chest is very restrictive.......again the available height from the cam spindle centers to the roof of the chest is limiting.......All these issues can be overcome to some extent, but it is a lot of work and takes much experimental work. The cylinder head is the biggest hurdle, as making up a new head is very expensive.......The Horners have done this very well........made the head taller thus allowing better port shapes.......and stood the valves up, thus allowing for a more shallow combustion chamber......this means a lower piston crown, less reciprocating weight, and so on.......The valve train......you can run a cam with a base circle of about 900 and a lift of about 400.......with ratio in the followers and on the rockers you can get good valve lift figures.......(we have 600 at the valves).......But all these parts need to be strong enough to cope.......the lubrication is not ideal.......in an engine with much more power it is going to over heat.........bigger head/barrel fins are needed ( a big downfall on the French engines)......The Horner engine suffer this big time.......overheating.......Running on petrol type fuels you really need an oil jet under the piston crown, not just to cool the piston but lube the small end which seems to be a common issue on these engines as well......One big issue I learnt recently was that running a big bore short stroke engine will result in major overheating issues........apparently, when using a short stroke whereby the piston does not actually come up and out from the crankcase mouth by much, the piston does not cool at all and the engine cooks itself.......It is a common problem on the 95 bore Manx's so I've been told.......This may explain why Peter Molloy here in Australia changed the bore/stroke combo on one of these French engines some years ago, extending the stroke from 75 mm to 86 mm.......But I'm thinking it was to give the engine more torque.......for track racing this drive out of the corners is really what you need........Madness really but all good fun.

very interesting approaches there. the overheating does sound predictable given the cylinder layout, the rear nr1 being the first to get buggered?
been doing this long have you?

 

[nobby]

My single 665 race engine (92 x 100) last run on the dyno put out just over 50 RWHP That was with the old piston that had lowish comp ratio.......after that we installed a hand finished piston somewhere above 15:1 (on methanol ), then I set about fine tuning the fueling to lean it off and the power difference was noticeable.......No racing this year so far, as loads of time spent making up roller followers and special one off cam to suit, plus all the time spent getting the replica Grey Flash ready for Goodwood. I am hopeful to see figures upward of 60 but this could be dreaming a bit, but it all looks promising so far. A good road Comet with a 600 kit, Mk 2 cam and 36 mm Mikuni or Dellorto will see around 35 on a good day. It really transforms the bikes, and I tell folk that if the factory could have made them like this, they probably wouldn't have sold as many twins.

Years ago I bought a 500 engine from aussie Alec Frederick Corner. It was build by him with a prototype Terry Prince top end kit. It made 43 horse on the dyno. I still miss this engine, had to sell everything some 10 years ago. Now looking for a replacement...

​

 

[greg brillus]

The TP kits have not been available for some time, but they are going to be available again sometime soon apparently......A chap in Florida USA is going to continue making and supplying them at some point.......most all the parts/castings are still to be done here in Australia. The TP kits are good basis for building a stronger engine, but there is still much that could be done if you really want to make these things fly........To answer the question from post 36, I have been playing/working on these bikes since 2007, full time since 2015.......no where near as much time as many folk past and present.......racing since 2011 but not really seriously, I don't really have that sort of time.......But learnt enough from folk on here and several others you never hear from........You have to go past the "Norm" if you expect to get good gains from one of these engines.......and no not much help from the likes of Ken Horner.......he has no interest in helping us mere mortal Vincent folk......A good friend and I decided to do things on our own.......Mine a single and his a twin.......Of course mine is the guinea pig, but if a single could put out good power, then how would that translate to a twin with double the capacity.........

 

[DucATIRadeon]

from what I've gained in knowledge from the Ducati bevel and pantah engines, and compare that to English singles: the cylinder head (i.e. cam timing/duration/lift and valve sizes etc) is the most crucial part in the whole plot, its where the most is to be gained.
as said with my Bullet: larger valves and ported head, but still std cam timing and lift, and 32mm dellorto it goes 5000rpm/100mph. earlier with the std valves it could barely go above 4600rpm, it was literally asthmatic! ported the inlet to the manifold, can always go larger than 32 but in my eyes no gains.
32mm is already a substantial throat, its larger than a tuned Ducati 900 belt and bevel. largest throats I saw were on a KTM 690SMCR (701cc), bloody massive that is, almost fits a beer can!

 

[rapide049]

Need to find John Treases notes ,The bike Ian Boyd ( Feather Bed Frame ) owns still has a better lap time than the Godet , at PI