13.5:1 Back to stock?

6 replies to this topic
  • tpastrana

Posted September 20, 2010 - 11:54 PM


Found a guy with a jug and cylinder local that's parting out his 05 bought the bike from dealer couple of months ago and hasn't rode much. Cylinder is in very good condition, piston too. Do I need to Rejet? I'll put this in and put some hours on the piston, and then upgrade to a 13.5...again. Grey would probably know this what are the hp differences between the stock and 13.5? What high comp piston would you prefer or would you prefer stock ratio? JE, CP, Wiseco Pro X? Ive used Wiseco in all of my 2 and four strokes just because everyone else uses them, but input from TT members!!

  • rcdude33

Posted September 21, 2010 - 05:02 AM


I think both are good but I thought cp pistons are a little better and I saw they offer pistons that can go up to 14:1 which us high but I think they are both pretty good

  • grayracer513

Posted September 21, 2010 - 07:18 AM


My preference for my own use is OEM pistons at the stock CR. If I were building a 13+:1 engine, I like CP's.

Higher compression increases the strength of the vacuum signal through the carb on the intake stroke, and as a result, it can sometimes make the bike a bit richer on the pilot and the base of the needle, but doesn't necessarily need to be rejetted in many cases.

The piston can be good for one or two HP.

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  • brentn

Posted September 21, 2010 - 08:11 AM


I think the big question I have that is relevant to this thread is if it's really worth it?

Can you notice the difference?
I ask because 100 octane + fuel is expensive to come by here. I think for my trail riding needs I'd stick to stock and save myself a crapload of money on fuel.

  • grayracer513

Posted September 21, 2010 - 09:02 AM


Fuel with a higher octane than pump premium is rarely needed above anything more than 1000 feet above sea level with 13.5:1, but it can be. You would never need a true 100 octane, though. That's just not required.

High compression usually yields the biggest gains at low-mid to mid range, tapering off some as you the engine reaches peak speeds. It can help an engine run more sharply and responsively at lower speeds, and can make a big difference if you live above 5000 feet.

You have to evaluate the bang for the buck with anything, of course, but in combination with a good 4 stroke racing fuel you could get 4 HP total out of the two.

  • brentn

Posted September 21, 2010 - 11:17 AM


Nice, that's a decent amount.

I mentioned 100 octane cause that's what the GYTR 13.5:1 piston says that you need to run "minimum". Maybe they are refering to the research octane number and not anti knock index?

In reality would 94 be decent enough?
The only gas around here with 94 octane rating has up to 10% ethanol... Would that work?

  • tpastrana

Posted September 21, 2010 - 01:20 PM


Here in Weston were 4750 ft above sea level, so I don't know, I was leaning towards the cp, but when you run race fuel or c12, my guess is best bang for buck is stay at stock. $170-230 Aftermarket, or $100 Stock OEM piston, 70-130 dollars for 1-2 hp :banghead::ride:

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