Jockey Journal Forum banner
1 - 20 of 20 Posts

·
Registered
Joined
·
36 Posts
Discussion Starter · #1 ·
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
 

·
Registered
Joined
·
706 Posts
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.....
 

·
Registered
Joined
·
117 Posts
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.
 

·
Registered
Joined
·
794 Posts
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.......
 

·
Registered
Joined
·
243 Posts
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
 

·
Registered
Joined
·
194 Posts
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.
 

·
Banned
Joined
·
2,014 Posts
+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??:p
 

·
Registered
Joined
·
1,740 Posts
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
 

·
Registered
Joined
·
457 Posts
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.
 

·
Registered
Joined
·
153 Posts
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
 

·
Registered
Joined
·
706 Posts
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.
 
1 - 20 of 20 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top