Bonneville America Forums Forums Technical Forums Performance Mods Bore & stroke speedmaster-America

I've been looking for gains on my speedmaster and wanted to ask two questions between the both of them (performance mods)on page #2 of this forum .{Speedmaster}, Bore/Stroke 90x68mm=3.54in.x2.677in{America},Bore/Stroke 86x68mm=3.385in.x2.677in. Q# 1. is 86mm or 90mm the actual Bore size and the 68mm in which the piston travels vertically up and down? [that line doesn't sound right] Q# 2. Compression Ratio 9.2:1 How is that figured ?

1: yes
2: not entirely sure how to explain it, but there is the "swept volume" which is the displacement. That's how much volume the travel of the piston covers. Then there is the volume ABOVE that in the cylinders. The difference in (swept+"above"):"above" is compression ratio. Now if I only knew the real names for those, I think you'd have it So the way they get higher compression is to either make shorter heads so they sit closer to the pistons, or use taller pistons so it shifts that "swept volume" up, thus reducing that "above" colume...

OK I need to go to work now

Thanks. decreasing volume in the "above" area whether it be shorter heads or taller pistons=higher compression. got it. Then would running higher compression evolve more power and torque ? Or a time bomb ready to explode !
Probably running a bigger bore ,and stroke? would do the job right. SMK

This is where I'm not an expert, if you couldn't tell by my 8th grade explanation before... I do know that higher compression usually means a possability of detonation, thus higher octane gas. But yeah, compress the fuel/air more = bigger bang.

Quote:

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thus higher octane gas

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yep, I had to go to high test due to the big bore. I consider it an excellent tradeoff!!

Me Tooooooo!

I'm learning a lot more about these bikes ,more than I thought I ever would. I have petty good comrehension but most of it depleated during the 70's. I might have one cell left,hope it don't split. Actually you explained that in laymens terms;easier to comprehend than a 20 page text on the topic. Thanks til your better paid. SMK
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Well since nobody has corrected me yet, I'm gonna assume I'm right If someone wants to jump in on the higher compression giving more power, that would be cool.

Yes higher compression usually means more power,but you do usually have to run higher octane gas. Unless you consider running water injection. Has anybody tryed this on a Triumph yet.I have done it on higher compression small block Chevys and it does help.Gas keeps going up in price and I may try it. Also I'd like to see Wiesco or some other piston factory make some higher compression pistons for the 865 and 790 engines.It could be a weekend job then and a lot less exspensive. The fact that 790cc and 865cc run so good and better than alot of big bore twins proves you don't have to have a huge engine to run good.

The pistons in the BB kit are higher compression aren't they?
So that in addition to a larger displacement you also get the boost of a higher compression ratio.

There is a difference in the bore because Triumph (to conform to the infinite wisdom of the politicians) began using a bigger, but more mildly tuned version of the twins. I believe that the new version has finally trickled down to the America this year. The bigger engine produces more low speed torque, but the smaller one produces more topend power.
Compression ratio is the volume of the cylinder when the piston is at bottom dead center plus the volume of the combustion chamber divided by the volume of the combustion chamber.
More compression on the same engine with no other changes will normally require higher octane fuel. The semi technical reason is that compressing the fuel/air charge more causes it to heat up more. If it heats up enough, the fuel will ignite before it should. Now, this is time related, (try passing your hand quickly over a burning candle, won't hurt near as much as just holding your hand over the flame) so there are things that can be done. More radical valve timing will tend to reduce the compression at lower speeds, so you don't have any problems. When the engine runs fast enough to get full compression pressure, it is running too fast for the fuel to ignite prematurely. Reducing the mixture burn time by adding another sparkplug or (as Triumph does) set the sparkplug in the center so that the fuel burns halfway across the combustion chamber instead of all the way across as it does with wedge style engines that have the plug at one side of the chamber. Another trick is to richen the mixture a bit or inject water into the cylinder so the engine runs cooler.

Higher compression gives more power the draw back is the more compression you have the higher octane you need.

That is an interesting idea... Higher comp pistons is a much more "bolt on" mod that the BB. No machining... It would be an interesting comparison at any rate.

More compression and a bit of head polishing and some radical valve timing will do a lot for your horsepower without causing you to have to spend a fortune on 120/140 avgas. Of course, if you had maybe 250 cubic inches, you could get an easy breathing 100 HP on the cheapest gas you can find because the only substitute for cubic inches is more cubic inches.

Seems to me like the 904 is really the most economical serious power boost. I mean we were talking about hi-comp pistons. If you're gonna do that, might as well do the head work. How much more would the machining for the cylinders be? Not THAT much more. Problem is, anything above that requires case work, that's where the bucks roll in, right?

correct

Frank

Reworking the cases is easier and shouldn't cost as much as boring the cylinders. The big expense is in casting and sleeving the bigger cylinders. The good news is that the bottom end should be strong enough to deal with it, thanks to having 4 (count 'em, 4!) main bearings.

So I guess the labor involved in getting to the cases to them machine them is a big factor too. A lot of guys will do it themselves, but I'm not about to tackle my very first engine tear down on my own bike by myself!