A10 Plunger Engine - Cam & Comp Ratio

[MidlandsA10]

Dear All

With the expectation of being crucified for once-again reviving this topic - I would like some opinion's over optimum compression ratio and cam set-up for an A10 plunger engine.

The engine itself is being completely overhauled, with a large journal crank, thick flange barrels but retaining the iron head (I have an alloy head A10RR so wanted something a little different).

I was planning on using a 356 camshaft - given that it has the iron head, what is the highest (safe) compression ratio you'd suggest using?

Many thanks

Tom

[RDfella]

On an iron head, generally 7.5:1 or 8:1 max. Cam? I have a 357 in mine as I like a bit of get up and go. After all, it's hardly a race cam. Some would prefer the 356 for a smoother tickover and better mpg. All depends on your riding style.

[muskrat]

G'day Tom.
I'm with RD on that. 357 cam as I too like a bit of get up and go.
My A7 plunger rebuild is getting new 8:1 and 357.
Cheers

[Rocket Racer]

no cam tunnel clearance issues with a 356 and in a road rocket still good for 40hp. the extra 3hp for a 357 super rocket is all over 6000rpm. my preference is a 356 unless i’m going to exceed 6k, which on my sidecar i do continuously. a 357 will happily run to 7500, quite unnecessary on a plunger twin unless you’re Muskrat

[Rocket Racer]

if a 650 i would fit flat top pistons, i’ve heard good things about IMD

[Greybeard]

I put a 356* cam in my standard comp Plungie. It seemed to make a small improvement to performance but I do not push the engine long and hard. Also, the exhaust note is crisper. I love it.

*Thanks to Andrew at Priory Magnetos 

[Black Sheep]

A 356 cam. No point in the 357 as, has already been mentioned, no benefit below about 5,000 rpm. 

[RDfella]

I don't understand the comments suggesting a 357 doesn't do anything under 5,000. With c. 290* dwell and 88* overlap it's not a race cam. Hardly even 'half race'. A race cam would be expected to come in around 4,000 but it would have 100* overlap and 320* dwell (I have 'full race' cams in my B31 and power comes in strong around 3,500). 
If indeed people are experiencing no benefit starting in the 2,500 range, then there's summat wrong. Probably the exhaust not suiting the cam or an induction issue. The 357 already has around 3* advance, but another 3* would bring the power band lower if that's what's wanted. Would need a stepped key, as there's no vernier availability.
Having said that, I find it strange the 357 inlet has virtually the same dwell as the 356 - they just advanced it by 6*. I'd have preferred the inlet to stay open longer, bringing the dwell to over 290*. They did, however, get the exhaust lead spot on, something rarely found on a performance cam, where most grinders seem to think the answer for a sports cam to take standard timing and add an extra 5* all round - and another 5* for a race cam. It don't work that way. The beauty of twin cams is that you can juggle things (eg advance the exhaust) to find the optimum. Sadly we can't on the A series.

[muskrat]

I tend to agree with RD.
The medium comp A7 plunger (with A7SS head) starts coming on the 357 cam @ 2500-3000 (no tacho). The highly strung Cafe A10 (BVSR, 10.5 comp, twin 30mm carbs) starts working @ 4000-4500 and drops off @ 7500 
Cheers

[Rocket Racer]

When Tom asks what combination of cams etc to fit to a "plunger A10" the initial two responses were immediately 357.

I sought to balance that with my experience from early swing arm cases (54 & 55 season respectively) that as standard could not accept a 357 without opening out the cam tunnel (admittedly not a significant change) to allow the cam to rotate due to the larger base circle.
Secondly that the earlier 356 sports cam (which will fit straight in) delivers almost the same power (40hp in a stock RR motor) so the difference is largely academic.

So I double down on my suggestion that a 356 is a good option and would go so far as to say if I owned a plunger a10 (which I don't) unless the cam was stuffed, I would not bother changing it from a 334 (if that was fitted). I enjoy pedaling along on cooking motors. There is nothing wrong with low comp and cooking cams.  A stock plunger flash was tested in the day doing 99mph which is plenty fast enough on a plunger, whereas the super flash with higher state of tune was seen as being a bit marginal as a package.

Unless I am looking for some specific outcome such as detuning or uptuning, I would keep it as the gods of BSA intended. It appears Tom already has an alloy head go-fast motor so he doesnt appear to need the plunger tricked up. My experience of plunger bikes (owning a cooking ZB for decades and having ridden a flash and a star twin a number of times), is they are delightful. A stock cooking ZB33 just wont exceed 5500rpm, you can run it on full throttle for hours (and I have) and it will go up hill and down dale like a clydesdale, it only has around 21-23hp I recall. It'll hold open road speeds and manage close to 80mph. 

Sure a 357 is a nice cam and if the best spare cam in my spares was a 357 I would probably fit it, but my alloy headed 54 RR is retaining its stock 356, TT carb and stock valves (I had the choice). 
Your new LJ crank is a good investment, it does give you options while retaining a strong engine. I dont know your riding style so the definition of an optimum set up is difficult, but either the 334 (a nice gentle cam perfect for rallies and touring), or the 356 (fitted in the star twin) are nice options. Sure fit a 357 but that wouldnt be my first goto choice for a 650 plunger motor although like many things you will find many points of view. Neither Muskrat or RD fella like to hang around and if you want acceleration a 357 will improve it.

I recently saw a video on a closed NZ BSAOC FB page of a swing arm super rocket in the NZ countryside. What struck me was it was geared for 100kph at 3000. The rider never exceeded around 3500 accelerating through the gears to get to that speed. The cam in that case was largely irrelevant. 

I recently discovered that there are several (at least two) slightly different castings used for the swing arm cases, they are different around the oil pressure valve area, I am less familiar with the plunger castings so whether that as std they will accept the larger base circle later cam without modification I don't know. perhaps some might.

[Rocket Racer]

https://www.a7a10.net/forum/index.php?topic=12729.0

found this on an excellent forum
If I read this correctly on those three A10's with 334, 356 and 357 respectively,
at 3000 : the humble cooking 334 was pulling 2-3hp more than the sports cams 18hp vs 16 and 15
at 3500 : the 334 and 356 were close to parity and the 357 only 1hp less. 20hp vs 20 and 19
at 4000 : all three were pretty much tied with 20.7, 21 and 21.

[muskrat]

G'day RR.
The 357 fitted into my plunger no problem. It doesn't start working hard till 4-4500. At the 60mph speed limit it's just below this so a back shift gives the kick to get around tin tops.
Cheers

[Jules]

do all these comments apply equally to the iron head s/a engine too??

[Rocket Racer]

do all these comments apply equally to the iron head s/a engine too??

I think you'll find all the cams fit straight into the later s/a engines (58 on?). I'm not sure of the casting date change, but the earlier casting used in 54/55 has not taken a 357 in either of my engines, but can readily be modified to take one if required.

[JulianS]

I fitted a 357 into swing arm cases CA10 1848 (March 1954)  with no mods or issues so I dont think there is a hard and fast rule for swinging arm at least.