My 250cc B25 Engine is currently with Roger Taylor (RTS Engineering) to try and resolve the breaking pushrod caps issue.
While its being worked on there is an option to increase the capacity by about 10% with a new liner, piston etc.
According to Roger this makes a very noticeable differance to the feel of the engine - not so much dropping down
a gear on inclines and headwinds etc.
To me this seems an attractive proposition but I'd like to know if there are any downsides to this aside from
the purist's position of it not being standard. The bike isn't standard anyway as its been rebuilt in a "baby goldie style".
Roger isn't trying to sell this to me in any way, quite the opposite really as he's totally overloaded with work.
Any thoughts gentlemen?
Hi,
I would think that it would come down to cost. A 273cc instead of a 249. If your cylinder needs a new liner and you get this 10% advantage then go for it. If it just to get a 10% increase and cost is not as problem then go for it.
The original big bore A65 gave a similar increase in capacity and a very noticeable difference in power.
Dave.
Roger has done an engine rebuild for me in the past.
His work is excellent and what he doesn't know about BSA unit singles is not worth knowing.
I would takes any advice he gives as good advice.
I was trying to work out why the B25 ended up with a plain big end bearing given that all the other unit singles used a roller bearing, even the later C15 which preceded the B25. I had a quick look through Rupert Ratio but couldn't find any comment about this aspect of the design.
The earlier C15's did have a problem with the durability of the plain big ends which is why BSA switched to roller bearings which are stronger and not so reliant on good oil pressure and filtering of the oil. So my only concern with increasing the torque provided by the B25 engine by increasing its capacity would be whether the plain big end bearing would be up to the job and whether this mod will shorten its life :o
Hi,
I am sure that I read somewhere that the configuration of the bore and stroke is that of a 500cc Triumph and that the conrod is the same. So perhaps to be able to use components common across the BSA group of companies.
DFave.
I assumed it was all about saving cost rather than driven by engineering excellence and was probably down to Edward Turner's doing just before he retired in 1967 from the BSA Board! Re-use of a proven Triumph conrod makes sense and it was also likely that manufacturing a one piece crankshaft was a lot cheaper in terms of machining effort than for a built up crankshaft required for a roller big end?
Hi, what is the model year of your B25? IMHO it is of utmost importance to supervise the oil pressure in these engines (well, for all plain bearings engines) whether tuned or not. The later models are possible to fit with an oil pressure gauge and also have a strenghtened conrod. Another useful addition is an oil filter. Cheers
Thanks everyone.
Dave - I'm cheered by your "big bore A65 ... a very noticeable difference in power." comment.
spaceman - I too had wondered about longevity but its not as though I'm going to go full bore for any great length of time and, as Roger suggests, I wont need to drop down a gear so often then I'd have hoped that lower revs would more than compensate for the other.
LightningAndStarfire - Its a 69 bike but the engine is based on BSA a prototype crankcase for the 71 production run. It also has the latest stronger conrod and an external oil filter + warning light but not gauge.
One thing that has just occured to me while typing is "won't the compression ratio be up by 10% ish?
Your base is good, would be very interesting to know what kind of oil pressure you see if you proceed with a gauge.
I think the midel year -71 also had a cast iron oil pump, if you have that you are extra lucky.
Yep I believe it has the 3 stud cast iron one.
Hi,
I would say that the compression remains the same. Although you are compressing a bit more volume it will only compress by the same amount as determined by the piston. The lower compression Fleetstar is apparently a much nicer engine due to the lower compression but lowering the compression would probably negate the extra power from the 10% capacity increase.
I could never decide between swings or roundabouts.
Dave.
Rupert Ratio has some info about big bores and compression ratios in his book about the Bsa unit singles. A very valuable book in general.
Often the cylinder is given a small adjustment to blend with the larger bore.
LightningAndStarfire - By cylinder did you really mean the cylinder head?
Dave B. If the cylinder head is unchanged and the swept volume is increased surely that extra volume has to be squeezed into the same space (cylinder head volume) doesn't it?
Hi,
Yes, thinking about it, the compression would be up but I am not sure if it translates that 10% increase in swept volume relates to a full 10% increase in compression. I am trying to picture this in my head but if the head remains unaltered there will be clearance around the top of the piston to the head where it overlaps the cylinder so taking some of the volume. If this does result in a slight overlap of the head to the barrel then you may have a squish affect with a resulting increase in combustion efficiency. There must be an engineer out there who can explain this.
Dave.
As I see it. The original capacity is 247cc and the compression ratio is 9.5:1. The combustion chamber volume = swept volume (247cc) / compression ratio = 26cc.
A 10% overbore takes the swept volume to 271.7cc , so the new compression ratio is 271.7 / 26 = 10.45:1.
I'm sure someone will correct me if I'm wrong but it can't be far out.
Yes, should have written cylinder head. Sorry.
Please google "formula for compression ratio" .
Results seem relevant.
Quote from: Derek996 on 11 Mar, 2022, 19:46
As I see it. The original capacity is 247cc and the compression ratio is 9.5:1. The combustion chamber volume = swept volume (247cc) / compression ratio = 26cc.
A 10% overbore takes the swept volume to 271.7cc , so the new compression ratio is 271.7 / 26 = 10.45:1.
I'm sure someone will correct me if I'm wrong but it can't be far out.
The above formula is not quite right as compression ratio = (combustion chamber vol + swept vol) / combustion chamber vol. This gives a combustion chamber vol of 29 cc.
The difficulty in predicting the CR of the enlarged bore is that the piston shape may have changed and therefore the combustion chamber volume may be different. In fact, it's possible that the new piston shape may have been designed to give the same CR - without specific design info on the new piston, it's difficult to know :o
Hi,
I worked out that the 'extra clearance space' under any cylinder head would be less than 1cc so probably of little consequence.
Dave.
Quote from: spaceman on 11 Mar, 2022, 21:18
Quote from: Derek996 on 11 Mar, 2022, 19:46
As I see it. The original capacity is 247cc and the compression ratio is 9.5:1. The combustion chamber volume = swept volume (247cc) / compression ratio = 26cc.
A 10% overbore takes the swept volume to 271.7cc , so the new compression ratio is 271.7 / 26 = 10.45:1.
I'm sure someone will correct me if I'm wrong but it can't be far out.
The above formula is not quite right as compression ratio = (combustion chamber vol + swept vol) / combustion chamber vol. This gives a combustion chamber vol of 29 cc.
The difficulty in predicting the CR of the enlarged bore is that the piston shape may have changed and therefore the combustion chamber volume may be different. In fact, it's possible that the new piston shape may have been designed to give the same CR - without specific design info on the new piston, it's difficult to know :o
On reflection I see that you are right. I just did it of the top of my head without fully thinking it out. I also assumed no change to the piston crown which would make a compression change by itself.