[Bigblockdude]

Looking for info on how much compression one can run on these old things (Triumph/BSA) before the use of pump premium is no longer acceptable.
I have the means to correctly cc the chambers and piston domes. Also access to a mill should I have to taylor the piston dome to what compression I want to run.

I'm assembling an S&S 113 now for a bike, these run 10-10.5 to 1. The chambers do have a quench area though and flat tops. I know the hemi chambers, such as Triumph/BSA can be less forgiving.

Any real experienced input would be appreciated.

thanks

 

[Bigblockdude]

bump

 

[swooshdave]

Triumph and BSA have hemi chambers? I think you are mistaking them for Nortons.

 

[Bigblockdude]

Ok, basically a hemi.

 

[Glider]

The best thing you can do to prevent the detonation associated with high compression on a Triumph is to drill and tap the head for center plugs. My 750 Triumph runs very small, 10mm plugs. The bike has about 10.5:1 compression and runs fine on Regular (with ethanol). By the way, ethanol is a detonation suppressor.

If you're going to run the stock plug location you'll probably find that you need to run higher octane fuel to prevent detonation, or retard the timing a bit (or both).

You don't mention what you plan on doing with your engine, but if it's going to be a street bike I'd really recommend that you run stock compression or at most 1 point higher. If you want more power, fit a 750 kit. A blueprinted and balanced 750 Triumph with mild cams, R followers and a touch more compression will run very strongly and will pull some very tall gearing while still giving impressive acceleration.

High compression gets old fast on a street Triumph.....

 

[Bigblockdude]

Just curious, I have a set of 10 to 1 pistons for an A10. Also a set of lower compression ones. I will cc them both to get actual.
That type of info is what I am looking for. Thanks

 

[Hillbilly Deluxe]

I ran a 61 pre unit Bonnie with 13 to 1 pistons for a few years,it had other work too though.Stronger rods,gas flowed,bigger carbs,almost everything in the motor was uprated.It was 25 or so years ago so the details escape me,but i know it was a bastard to start with me being just over 10 1/2 stone at the time.The Bonnie was a monster,but the stock T110 i had at the same time was a sweeter ride.

 

[scottto93]

On my '68 I am running a big bore kit (750cc) with so called 9.5:1 pistons.....upon cc'ing the setup, i had well under 9:1....running a .006" base gasket and a thinner than standard head gasket brought it a tad above 9:1...had to cut pistons cuz i am running mid-high cams and had some interferrence between pistons/valves..not sure i would want to run higher compression on a kickstart with no compression release....unless you don't mind walking funny.......

 

[Jack Adams]

Bigblockdude,the general opinion is about9:1 on a stock motor,BUT and it is a big but,If you do some things to toughen the motor against detonation,like cam timing(overlap),smooth any sharp edges, etc. you can go higher.I run a 803 cc triumph stroker on the street on 93 octane pump gas and 10.5 JE pistons(c.c.ed out to 10.1:1),with NO detonation. And you are right about Triumph and BSA having hemi heads,it seems those Norton guys always think Nortons are "special". Jack

 

[swooshdave]

And you are right about Triumph and BSA having hemi heads,it seems those Norton guys always think Nortons are "special". Jack

Depending on which definition you use, for example a central spark plug, would mean none of the bikes have hemis. But the term probably was just to differentiate them from the flat or wedge heads of the time.

 

[Old Codger]

Hello JJers,
In my Triumphs and BSAs I try to use 9 to 1 or 9.5 to 1 compression ratios. I then take the time to fine tune the engine. I set the timing, valves and the air/fuel mixture as close as possible to factory specs. I usually run mufflers, stock or shorties. The next step is to pick out the fuel that I am going to use, for grins let say 89 octane. Now I ride and tune. Ride and listen to the bike. Am I getting a ping or spark knock under load? If I am I can go to a higher octane grade of fuel and or retard the timing. Most times I just go to the higher grade and that fixes the concern.

 

[Jack Adams]

swooshdave, I use the only real definition of a hemi head, that is a head with a HEMISPHERICAL combustion chamber. Where the spark plug is located is not part of the term. Jack

 

[Rubone]

+1 with Jack,
Plug location has zip to do with combustion chamber shape. It has to do with combustion efficiency, but that is a whole different subject.
And compression ratio does not affect the type of fuel nearly as much as timing and burn efficiency does. Dual plug it and run 10-1 on pump gas, no big deal!
Robbie
And what's a Norton??

 

[DarkGoat]

He's that guy on the "Honeymooners"

 

[DarkGoat]

I built an A65 with 11:1 years back with little problem just back down the timing a bit. I still have a set of these piston sitting on my shelf reserved for next chopper hotrod build. Boy did that little bike sound great and it was screamer. There wasn't much in this area that could touch it.

One of these days...bang zoom right to the moon!

 

[bobscogin]

Looking for info on how much compression one can run on these old things (Triumph/BSA) before the use of pump premium is no longer acceptable.

For regular street use, limiting the compression to around 9:1 will allow for some variations in the quality of available fuel, and some timing and jetting errors. You could probably bump that to 10:1 if everything is kept spot on, but 9:1 gives you a lot more "safety factor".

Bob

 

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[DarkGoat]

different cam overlaps can bleed compression. A better measure than static compression ratio is dynamic cylinder pressure test. Use a compression tester to get a PSI rating that will help determine what kind of octane to run.

 

[mblab]

I'm kind of surprised that nobody has mentioned anything about keeping the RPM's up. Detonation is far more likely to happen in a low RPM/high load situation, even with low compression.
Mike

 

[Glider]

That's how Romero won Daytona on the 750 triple when all the other factory bikes blew up due to detonation: he kept the revs up.