904 Big Bore Pistons - Bonneville America Forums

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We thought we would give you a preview of our 904cc Sports Big Bore Kit pistons for the air cooled twins.

They give a compression ratio of 12:1, have a tight quench, anti-friction coated skirts and have +2mm valve pockets.

The big bore kit should be available in 6-8 weeks.

Can you give us an idea how they would impact the performance of an 865 EFI?
Also, if I already have one of your Stage 1 kits, would a new map/tune be required?

Have to keep in mind 12:1 is pushing it on pump gas.

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Can you give us an idea how they would impact the performance of an 865 EFI?
Also, if I already have one of your Stage 1 kits, would a new map/tune be required?

Testing is being carried out now and dyno charts will be on our site when available. A new tune will be required to go with a Stage 1 kit as the fueling will need to be adjusted as will the ignition timing for the new engine dynamics.

That is awesome!

I have a question regarding big bore kits for our motors that I have been meaning to ask for sometime so nows as good a time as any...Both the 790 and 865 motors have the same compresion, in order to get to 904 the 790 will increase its capacity by 114cc the 865 by 39cc thats a big difference, so which engine winds up with the 12.1 compresion and what does the other one get..hmm

Pete, 790's and 865's both have the same stroke @ 68mm. The only difference between them is the bore.

That is correct Steven, however I'm guessing that while the heads might look the same the combustion chamber on the 865 would be slightly bigger in capacity to achieve a 9.2:1 compresion..because if you bore a 790 out to 865 and use the same combustion capacity you would wind up with a higher compresion, exactually what a big bore kit does..those more mechanically minded than me might want to chip in..

The cylinder heads of the 790 and 865's are the same, so once the cylinders are bored and 904 pistons fitted they both have 12:1 static compression ratio.

The difference will be in the cams used by the 790 and 865 engines. The engines will have a higher dynamic compression ratio if the 865 cams are used due to the earlier closing of the inlet valves and the opposite is true of the 790 cams, i.e lower dynamic compression ratio due to the inlet valve closing later.

In other words you get more from the 790 due to it's cams but if you use 790 cams on the 865 the difference disappears.

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That is correct Steven, however I'm guessing that while the heads might look the same the combustion chamber on the 865 would be slightly bigger in capacity to achieve a 9.2:1 compresion..because if you bore a 790 out to 865 and use the same combustion capacity you would wind up with a higher compresion, exactually what a big bore kit does..those more mechanically minded than me might want to chip in..

There is another variable to maintain the 9.2:1 ratio between the 790 and 865 engines given the fact that the heads are the same, piston dome shape.

The distance from the piston pin the the top of the piston is the compression height. That is different between the 790 and 865 to maintain the same compression ratio. That would be MUCH cheaper to change than changing the combustion chamber on the cylinder head, in the long run.

I have a question. What is a "safe" static compression ratio to run on crap fuel, like 87 octane? Also, would would the "recommended" minimum octane be for these new TTP pistons?

Around 10.5:1

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The distance from the piston pin the the top of the piston is the compression height. That is different between the 790 and 865 to maintain the same compression ratio. That would be MUCH cheaper to change than changing the combustion chamber on the cylinder head, in the long run.

I have a question. What is a "safe" static compression ratio to run on crap fuel, like 87 octane? Also, would would the "recommended" minimum octane be for these new TTP pistons?

depending on where you live and what is available for fuel, this could be a big deal.

having run 12:1's for some time now. i would have to say that 97 octane for regular gas is the minimum for no knocking. I got to this number from playing around with Sunoco 115 octane from a near by track and blending it with lower octane gas. Fun but not a reliable fuel source.
a couple years ago I converted to run on 93 octane E10. that seems to burn slower run the best for pump gas.

Another question for TTP;

What head gasket thickness and bore diameter was the 12:1 measured with? Perhaps a thicker gasket and/or larger bore could help out some of us that have to run cheapy fuel?

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Another question for TTP;

What head gasket thickness and bore diameter was the 12:1 measured with? Perhaps a thicker gasket and/or larger bore could help out some of us that have to run cheapy fuel?

Sorry for the delay in replying.

The head gasket can't be changed, but the base gasket we used is a Triumph OEM gasket which is thinner than the base gasket Cometic supply. So by using the Cometic base gasket you could drop the CR down by around 0.4 to 11:6.

Hi All,

the TTP 904s Big Bore Kit is now available for purchase on our site.

I see that this will be a core exchange. Will you be using the core we send you, or something that you have on hand? The reason I ask, is around 2014 the cylinder styling/design was changed. When I can do a 904, I would like to swap to that new jug, and would be sending in one of those for a core.

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I see that this will be a core exchange. Will you be using the core we send you, or something that you have on hand? The reason I ask, is around 2014 the cylinder styling/design was changed. When I can do a 904, I would like to swap to that new jug, and would be sending in one of those for a core.

Hi Justin,

we have a stock of cylinders ready for core exchange, many are the earlier cylinders, but we do have a couple of pairs of cylinders with the polished fin edges. So we will be exchanging like for like.

Very good point though.

Hi All,

thought you may be interested in the first dyno run for our new [URL="https://www.triumphtwinpower.com/triumph-bonneville-thruxton-scrambler-america-speedmaster-904-big-bore-kit.php"]904s Sports Big Bore Kit[/URL].

On this dyno there are three runs showing, EFI 865 OEM, EFI 865 with TTP Stage 2 Power Induction Kit and 904s with Stage 2 kit.

The bike was an 865 EFI Bonneville fitted with the TTP Stage 2 Power Induction Kit, Tune 12 & short free flowing silencers vs. the same bike with 904s Big Bore Kit installed with 904s Tune 12.

904s St2 vs. 865 St2 - Peak Power +12% (78.76 bhp), Peak Torque +18.1% (61.21 ftlb) & Max Torque Increase +20.5% @ 5,735 rpm.

904s St2 vs. 865 OEM - Peak Power +29.8%. Peak Torque +30.6% & Max Torque Increase 34.3% @ 4,173 rpm.

We're really pleased with the results, not bad for an OEM engine using the 865 cams and 904 big bore. The bike pulls really well through the rev range, which isn't surprising as it has more mid range torque than our carb 904 with Stage 2 kit, 790 cams, less CR and Keihin FCR39's carbs. The carb bike had 4bhp more at the top end due to 790 cams and FCR39's.

Next will be the same EFI configuration, but using the early 790 cams for more top end power........

Good show, Mike.