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#509973 10/11/13 5:52 pm
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a65t 1969/70 engine can anyone tell me how much differance is made by running 7:1 comp pistons to 10:1? I belive the engine has been built with low comp pistons,is it worth putting higher comp pistons in? will it improve performance that much :

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Real world performance will change a little, because high comp pistons bring you more vibrations.
Cost of gas will rise too.
I have low comp engine in my A65 and it performs very nicely, however with some help smile.
If you can tame the vibrations this could be different story.

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Higher compression ratio pistons go with longer duration cams. I would think if you used the 10:1 pistons in a motor with a cam designed for 7:1 pistons you might get uncontrollable detonation.

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You know Larry, all the 69 / 70 BSAs had the same "sport" camshaft.
So you are choosing compression ratio to work with this camshaft.
I would never use 10:1 ratio with today gas anyway, but 9:1 is still quite safe.
However, as I already wrote to Allan - high compression is overrated smile.

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If you went from 7:1 up to 9:1,you'd get about 11% more torque at any rpm.
Vibration won't change if the piston weight is the same.

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Thank goodness, someone's posted a sensible answer. Thanks Pete. Because of the torque increase you'll find your changing up sooner and improving mpg.

Regardless of what pistons you run you will get detonation, my old 7.5:1 would detonate something chronic. On my 9.2:1's I rounded all the corners off and it doesn't pink for anything.

There is a lot to be said for high compression, especially as BSA balanced the cranks with HC pistons, after trying both, I would never go back to LC pistons.

I would recommend only using premium grade fuels and not over retarding the engine.

2c

Sorry Adam, 7.5:1 are good pistons, just not for me or what I want to do with the bike.

If we put these questions forward, it might help choose.

1) do you have a dodgy leg? Low comp is easy to start!

2) are you going to be doing big mileages?

3) what sort of speeds do you expect to ride at? 50-60 mph maximum, use LC, if your looking for the Ton, and want better acceleration, fit the HC pistons.


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10:1 is generally regarded as racing equipment on old bikes.

7:1 will probably make your bike noticeably slow.

I see that most Triumph 650 pistons on sale are nominally 9:1. Is it the same for BSA?


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I see, have to make the video showing my bike in action.
Will try to do it today.
For now, how many of you guys using freeways regularly holding steady 120 km/h with the rest of the cars ???
I don't see anybody else here in Toronto, but perhaps I'm just mistaken ?

PS. It will be done Monday - less traffic during Thanksgiving and my kids will help with filming smile.
I came from big japs to BSA, don't think it's so slow anymore.

Last edited by Adam M.; 10/12/13 12:19 pm.
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Ok thanks for all your answers,think I will le.ave well alone,till rebore is required

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I`m running 10,5:1 forged piston in my A10. Was a little bit sensitive at first, needed to replace the advance spring in the magneto, had gotten way to soft. A new one cured the pinging at light acceleration and low rpm`s.

Other than that I`ve had zero problems. You can also raise the needle a notch or two.

I`m running the super rocket cam in mine. The healthiest stock bsa cam. Gotta love the torque from long stroke high compression motor. Going from 7:1 to 10:1 would be very noticable at all rpm`s, give extra torque and hp and better mpg. More bang with the air and fuel supplied. I`m all for high compression, 10:1 isn`t crazy high either, nice on the street.


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As promised my BSA in action smile.
Quite difficult to show something in city traffic, but we tried.
Once again this is 7.5 : 1 comp ratio engine, highway speed was 110 going to 120 kph.

http://www.youtube.com/watch?v=H3dE--TGR_A

http://www.youtube.com/watch?v=_UEJ-4I_QnQ

http://www.youtube.com/watch?v=FCICSVgp_lQ

http://www.youtube.com/watch?v=KAqKSHMVFrg

http://www.youtube.com/watch?v=7JyLuS-n3G0

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the way to film a bike in traffic
speeds in excess of 70 miles/ hour

http://www.youtube.com/watch?v=E4aw4xBOG6o

regards A

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I thought you used MPH in USA? so your running between 68 and 75 mph. Still your video is not to be sniffed at, it runs and sounds very nice but your biggest benefit on there will be the small port head, the more you can draw into the cylinder the more compression you will make - the small port will do that. At a guess you'll probably make similar power as a OIF lightning with 9:1


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Higher compression is always synonymous with higher horsepower and torque. The full benefits can only be arrived at through a higher lift and duration camshaft as well. There are lots of trade offs in the bargain. Complex engine design deals with these and many other factors. In the days of high octane leaded fuels high compression was the easy way to reach for more horsepower and better performance. Todays crappy ethonaol laced concoctions prevent too high of compression, as predetonation is a real problem. I have 10.5 to 1 pistons in my Spitfire and I have to add octane boosters to control the pinging. I will also on occasion spring for aviation fuel. The thing is thrilling to ride when it has a tankful of 100+ octane. I also try to ride it on cool days. High compression also equals more heat.
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Allan, I live near Toronto, Ontario - we don't have miles but km.
It was the first try by my daughter to film motorcycle in traffic, so the end product isn't very high quality, but the next would be better.

Yes, I planned to build a stronger engine with higher compression, oil conversion, even bought a donor engine but finally decided against it and moved to that :

[Linked Image]

Last edited by Adam M.; 10/15/13 8:52 pm.
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Originally Posted by lemans
the way to film a bike in traffic
speeds in excess of 70 miles/ hour

http://www.youtube.com/watch?v=E4aw4xBOG6o

regards A


This video deserves it's own thread ! ... it made my day

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OMG Ollie Free reincarnated as a camel jockey


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Originally Posted by quinten
Originally Posted by lemans
the way to film a bike in traffic
speeds in excess of 70 miles/ hour

http://www.youtube.com/watch?v=E4aw4xBOG6o

regards A


This video deserves it's own thread ! ... it made my day


shocked that's mad! Impressive but mad - not even a helmet !!!

Adam, sorry I took the assumption from looking at your location that you were in the US, Toronto - now that's one I understand grin

I'll try and do a video of mine when it's running again. I did one when it was on low compression but the video shows speedo images that I don't want broadcasting


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Once 6 months from the date of recording is up the Police can do nothing about what is shows, and as long as your reg is no shown then they can't pick on you either.

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Hmm interesting.


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Originally Posted By: lemans

the way to film a bike in traffic
speeds in excess of 70 miles/ hour

http://www.youtube.com/watch?v=E4aw4xBOG6o


I love the bit where the organ donor nearly rear ends a car mind you lane splitting 2 trucks is pretty .......err


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