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Tip: Calculate the BMEP of both engines.

You will need to obtain the peak torque number. If that's not available, you can obtain the torque at the RPM of peak horsepower and recognise that this will give you the BMEP at the peak-power condition, which is not necessarily the absolute highest BMEP.
 

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This is coming from someone who only has a passing knowledge on engine building but I believe part of the issue is the bore/stroke and compression ratio of the engine. As power is typically more efficiently made by increasing engine RPM, and RPMs can be increased safely with lesser reciprocating mass moving a shorter distance, it seems to make sense to me that it is harder to build the engine to be more efficient without changing out to a lighter piston. The higher compression and more aggressive cam profile can only help with more effective use of combustion at the cost of requiring more premium petrol. This seems to be the route taken by GYTR and Spears in their race engine kits. Feel free to educate me if I have made a mistake in my very simplistic explanation :)
R3R6
68.0 mm × 44.1 mm67.0 mm × 42.5 mm
11.2:113.1:1
 

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On that note, has anyone tried the Spears engine kit (high comp pistons and high lift cams)? Curious if it will still run on 92 octane gas considering the R6 has a similar compression ratio.
 

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Watch out when discussing octane ratings. USA and Canada pump octane ratings are (R+M)/2 and are three or four numbers lower for the actual equivalent fuel to the octane ratings used elsewhere in the world. "Regular", non-premium fuel in most of North America, which the R3 is approved to run on, is 87 octane (R+M)/2. The R6 is a premium-fuel-recommended engine, and premium fuel here ranges from 91 to 94 octane (R+M)/2.

I haven't got the Spears pistons in my engine, because the rulebook for the racing series that I want to maintain eligibility for, requires the bottom end to be stock.

I do have experience with one of my other bikes, which is a street bike, but which has a built engine "just because". It is a 2004 ZX10R with Wossner 13.5:1 pistons and a select-fit head gasket (0.1mm thinner than stock) to achieve 0.65mm squish clearance, and Web camshafts (the milder of the available choices, which allows the use of stock valve springs and does not require machining the cylinder head for clearance). It's ... healthy. Part of the tuning involved taking about 3 degrees out of the ignition timing everywhere. It is tuned for, and absolutely requires the use of, PetroCanada 94 octane (R+M)/2 petrol. It detonates audibly and excessively with the ignition timing at the stock setting ... I was lucky not to break stuff before I got that sorted. I've not tried it but it would likely benefit significantly from the use of racing fuel if appropriately re-tuned for it.

In the R3, I would expect that the use of the Spears 13:1 pistons will require the use of premium pump fuel, but it shouldn't need high octane racing fuel. The camshafts probably won't make much difference to the octane requirements.
 

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Watch out when discussing octane ratings. USA and Canada pump octane ratings are (R+M)/2 and are three or four numbers lower for the actual equivalent fuel to the octane ratings used elsewhere in the world. "Regular", non-premium fuel in most of North America, which the R3 is approved to run on, is 87 octane (R+M)/2. The R6 is a premium-fuel-recommended engine, and premium fuel here ranges from 91 to 94 octane (R+M)/2.

I haven't got the Spears pistons in my engine, because the rulebook for the racing series that I want to maintain eligibility for, requires the bottom end to be stock.

I do have experience with one of my other bikes, which is a street bike, but which has a built engine "just because". It is a 2004 ZX10R with Wossner 13.5:1 pistons and a select-fit head gasket (0.1mm thinner than stock) to achieve 0.65mm squish clearance, and Web camshafts (the milder of the available choices, which allows the use of stock valve springs and does not require machining the cylinder head for clearance). It's ... healthy. Part of the tuning involved taking about 3 degrees out of the ignition timing everywhere. It is tuned for, and absolutely requires the use of, PetroCanada 94 octane (R+M)/2 petrol. It detonates audibly and excessively with the ignition timing at the stock setting ... I was lucky not to break stuff before I got that sorted. I've not tried it but it would likely benefit significantly from the use of racing fuel if appropriately re-tuned for it.

In the R3, I would expect that the use of the Spears 13:1 pistons will require the use of premium pump fuel, but it shouldn't need high octane racing fuel. The camshafts probably won't make much difference to the octane requirements.
Thanks for the write up! The reason why I specified cams was because as I understand, the cams can affect dynamic compression which does play a role in engine knock/fuel octane requirements. Anyways, I checked with Spears support and they did recommend a minimum of 93 octane with the right tune.
 
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