Looking for some general "engine characteristics" advice, please. But the responses here might in some way benefit anyone building/rebuilding other engines (that's the excuse, anyway).

Have recently bought a Triumph TR25W Trophy (£1500). These 250cc singles, as most will know, are pretty much the same as BSA B25s. The TR25W has a high-compression engine, and it's fairly harsh (heavy kicking to start, and a near hammer blow tick over).

Today at Kempton Park I was riding a BSA C15T, which is a very similar 250cc single, and this bike was very mellow - as you might expect. Easy starting. Gentle. Progressive.

I'd like to get the same characteristics with the Triumph, so the obvious thing to do is fit a low compression piston. But can anyone explain exactly what will change; i.e. starting, top speed. carburetion, and general pick-up through the gears?

The thing is, the BSA C15T felt fairly brisk throughout the gears and picked up evenly. The TR25W pulls away in first okay. But by 2nd gear, it needs a lot more throttle to get it moving - as opposed to rolling on in a linear way. That's where the harshness comes in.

So is this likely to be simply a gearing issue? Or does the high compression piston effectively act as a significant brake unless plenty of extra fuel is introduced? That seems probable.

Can anyone explain these fundamentals and advise on the pros and cons of switch high to low comp?

Incidentally, people keep telling me these TR25Ws are good for as much as 80mph. But I recently took this one on a 40-50 mile jaunt, and 55mph-65mph was where it all pegged out. And at that point, the vibes are a pain.

email (option): dannydefazio@sumpmagazine.com

compared to a BSA Starfire which a TRW 25 is, the starfire has a 9-1 piston as std and the trw 8-1, so it should be ok unless someone has fitted high comp piston..I used to own both bikes, the TRW would top out at about 65 and the starfire about 85mph at 8500 rpm!

Hi Danny..The same basic principles..and characteristics, apply to most British bike engines....
They were initially designed at a time when power output wasn't the only thing people thought about and when fuel economy, long term reliability and reasonable tyre and chain wear etc. were all factors that meant something...

As a consequence most ordinary road engines had low compressions, conservative port and cam specifications and a moderate performance that resulted from these specifications...These original designs were then 'stretched' and 'stretched' some more to extract more and more power, with inevitable consequences...

Low compressions pistons weigh less and therefore create far less inertia loads at speed than heavier high compression ones, so out of balance forces are much smaller. Soft cam profiles required less valve spring pressure to keep everything operating correctly and were mechanically quieter in operation, lower engine revs also kept inertia figures far lower...Smaller ports gave better gas speed at low revs and so aided starting and low speed running. (though with a consequent reduction in their ultimate flow potential at the upper end).

The result in the case of both singles and twins was that the engines produced less power than later versions but vibrated less, were more economical, easier to start and generally were much more 'sweet'...Just the way their designers intended them to be....

Just compare the 'turbine like' plunger A10 motor with the later and much harsher Rocket Gold Star or a Thunderbird with a Bonneville....
Or more to the point compare a C15 Star from 1959 to a 1972 250 Gold Star...The former was 'sweet as a nut' the latter, the same engine in a grossly over tuned form, 'rattled like a bag of spanners'..(from new)..
If you really want to experience the effect at its greatest...try a BSA B50MX..
Assuming you can get it started at all that will make your eyeballs rotate and your fillings drop out!!

Your TR25 seems to have less performance than it should to my mind...That could be related to the engine specs/settings, state of wear or incorrect gearing....If the gearing is substantially lower than standard it will run out of steam early at fairly high revs...(or there is a possibility the standard gearing may be a little lower than ideal to suit the different demands of the American Market..)

If you want to make it more pleasant to ride fit standard road gearing if it doesn't have it.(or raise it if it is too low as standard)..Fit a low compression C15 piston and go for the cams from the softest version of the later B25 motors (Fleetstar?)...or if they will fit, C15 cams. Also reduce the carb size to around an 1" and set up to suit...Ian

Okay, thanks to Baza57 for the reply, and thanks to Ian.

But Ian, can you be more specific about this? What I'm asking is whether the engine braking effect of higher compression pistons can give the effect of "poor" gearing, or "inappropriate" gearing. In other words, if there is a lot of engine braking, surely it would need a bigger handful of throttle. As I said, it pulls away eagerly enough in 1st, but by 2nd gear it seems to me that it wants a lot more effort to shift it.

I suspect (but haven't checked) that the bike is on fairly standard gearing. All I really want to do is fit a lower compression piston and see if that ameliorates the problem. But I'd be concerned that if other engine factors aren't changed (such as valve spring tension or carburetion or the cam profile) that would minimise the benefit.

I really don't want to spend much money on this. It's already soaked up a few bob and a fair amount of time. I just want to make it gentler to ride at minimal cost (i.e. by perhaps finding a low compression piston or kicking it or threatening it or something).

And please note two points. 1. The bike isn't particularly quick of the mark. 2. It doesn't much like hills.

email (option): dannydefazio@sumpmagazine.com

['And please note two points. 1. The bike isn't particularly quick of the mark. 2. It doesn't much like hills']...

If that is the case then low gearing isn't the issue..rather a lack of power to pull the gearing you have and/or gearing that is too high...

Higher cylinder compression does not work in quite the way you imagine as an 'engine brake' though it does contribute in a small way to the effect when the throttle is suddenly closed...

When this occurs a partial vacuum is induced in the inlet tract...The engine is still turning (and 'pumping') but now has to work against this additional high load....

The load is caused by the fact the engine is trying to draw the same amount of air through a now restricted port. This requires more force, absorbs power and slows the engine down.
In engines with high compression ratios there is also a higher compression loading but this is present at all times..not just during sudden closing of the throttle...
It is, however, a higher load than would be present in an engine with a lower compression ratio and in that sense plays a (minor)part in the process...

The braking effect is temporary, disappearing as the engine slows to an appropriate speed to match mixture demand to port capacity.. and is not present at all with an open, or opening throttle...

The high compression load in this instance is more than offset by the greater power produced as a result of it...Ian

Okay, thanks for that, Ian. I'll mull it over.

email (option): dannydefazio@sumpmagazine.com