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Detonation faQ Limit On Our Bikes?

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Has anyone had any real experience with detonation and what it took in compression ratio to achieve it on premium pump gas along with which camshaft?

How does any piston manufacturer calculate static compression ratio accurately? Do they tear down a new engine and do the math with deck height, cc's, gasket thickness, ETC or do they enter a fudge factor and serve a piston with a approximation of CR? Does that mean a 14:1 piston could be a 13:1?

Does anyone know the CC's of a JE 13.5:1 piston?

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The math is pretty easy, but if someone is offering a 14:1 piston, you can bet its 14:1 with a stock head and base gasket and a stock deck height and combustion chamber. All that stuff is subject to manufacturing tolerances too. But the advertised ratio is static because you cant figure the cam in there. The greater your overlap number is in the cam usually the more compression you need to run to maintain the bottom power and ultimately make best power up top too.

Now regarding gas....13.5:1 is about all you can get away with on most pump gas premiums. More than that, you really need better fuel to gain any power.

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Not to disagree with what has already been stated here. But from personal experience on occasion we have run straight pump gas with 14:1 in our 2011 450, and 14.5:1 in our 10 250, and can detect no detonation.

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The last JE piston I bought I believe had all specs on the box, I will look. In the mean time you could call them and I am sure they would give you those numbers.

more later.

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I just installed a 13.5:1 in my 08 CRF450 and I can hear a bit of det. in lower rpms when I nail the throttle in some instances..California fuel is great! :busted:

And yes I am running 91 octane, thats all we get here in Nazifornia...

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I have been running a 14:1 CP with no pinging on an 03, stock timing. When I had a Wiesco piston 13.5:1 in it I heard pinging at low RPM or at least I think I did. I then started adding octane booster and never turned back.

I run a 13.5:1 in my 07 and have never heard pinging on pump 93.

Both bikes are running a Cometic gasket.

So I take off the head on my 07 and measure the squish band and it is .060" and think shoot, that's a little much. I buy a stock gasket and measure it to be .015". I measure the Cometic and it is .025". I run the compression ratio calculator and it is 12.88:1.

So I get to thinking about compression ratio which brought me to this post.

I want the squish to be .040". I run the numbers again. 14.17:1 removing .010" from the cylinder.

I ran the numbers based on the piston being 13.5:1 with a stock gasket and my current squish then reverse calculated the CC's of the piston.

I sent an email to JE on the dome volume and have not had a response.

The Cometic gaskets are second to none IMO. However, if I want to achieve the correct squish band I need to use a consistent gasket. I thought about making my own base gasket but a stock one is .015 and it has an oil transfer port. The Cometic gaskets are steel base gasket and hold up well having good compression resistance.

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The camshaft choice greatly influences the octane requirements. That's because a cam with a lot of overlap will reduce cranking pressure, whereas a cam with less overlap will increase cranking pressure.

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The camshaft choice greatly influences the octane requirements. That's because a cam with a lot of overlap will reduce cranking pressure, whereas a cam with less overlap will increase cranking pressure.

Bingo!

However, this does not answer the question of how does the manufacturer determine CR and is it based on stock overlap or just the static compression ratio?

Where would they find the information of squich band to calculate their advertised CR ratio? I don't think they take a stock piston, calculate the CC, get the chamber volume and gasket thicknesses and throw some CC's on top of the piston.

Good piston manufacturers in the performance industry may move the pin location to optimize squish and do the math related to advertised CR. Hell, maybe even the bad ones.

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All the manufacturers have to do is CC a stock head and determine the standard deck height. Their CAD programs can calculate the CC of their piston crown. With those values they can easily calculate the static compression ratio.

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