Spark advance forged piston

Hi, I changed the piston on an ATV 700cc single cylinder. Oem he has a compression ratio of 10.8:1 and now 12:1. how much should I reduce the Spark advance for the first start? -3 degree? And in what range? How much at idle speed and how much at wot and high rpm?

thanks and best regards

Robin

"The engine will tell you".

Seriously, depending on the fuel you're using, the OEM advance programming and it's 'safety factor', and many other factors such as any intake or exhaust modifications, you may be perfectly OK, or you may need to retard it at some points.

You may be better to check the relevant forums specific to that model ATV, as there should be people who've done this before

Hi Gord, thanks for the reply. Yes, I know the question is difficult to answer. The piston is a custom-made product, I didn't find any information from other people. The Spark advance is very advanced. That's why I ask...what would you do? I would reduce in mid and hi range -3 degrees but what do I do at low range? -1 or also -3?

You would normally be expected to pull more at lower rpm, but, again, it's going to depend...

Exactly what is the ATV and engine?

That's pretty high for a relatively large cylinder capacity, what fuel are you intending to use, and what was the OEM minimum recommendation? Does it have knock retard build into it, do you have a set of head-phones for detecting knock? Is it air or liquid cooled - I assume the latter?

You may even find that detonation isn't the only issue, you may find pre-ignition is also occuring.

While there are many variables and we can't possibly answer this optimally, or guarantee success:

You likely need no changes before first start. I'd get it running, get idle adjusted.

Then if timing was previously optimized, perhaps remove 2 deg advance at low load ramping 5 deg less advance at high load as a starting point then retune from there starting by testing and retuning light load. See what that needed compared to the last setup.

Then carefully get into mid loads and retune them.

At that point you'll have a better idea of what high load areas might need before you get there, allowing you to keep the lack of optimization to a minimum in the area where you can most quickly cause harm.

And to be clear I would not just make these base changes and send it. I'd work my way through it carefully, starting, then idle, then light load, only adding more load once all that's optimized again and I have an idea what the piston change has impacted and what the engine now needs, based on dyno testing.

Hi, thanks for the answers! It is again an Arctic Cat Alterra with 700H1 engine. The manual says 87 US Octane. I think it is to be compared with 91 EU Octane. I always drive 98 EU Octane which can be compared to 94 US Octane. So much higher than the OEM spec says. Unfortunately, it doesn't have a knock sensor. Yes, I have headphones for noise but unfortunately no dyno. It is liquid cooled and the intake and exhaust system is OEM spec. I have now taken the oem Spark advance map and reduced it by 3 degrees. When I accelerate from idle speed, then I briefly hear a slight knock. The engine then sounds soft and harmonious. It now has a little bit more vibrations like oem but I think this is the lighter piston because it no longer fits to the counterweight shaft.