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Discussion Starter · #1 ·
I have a mates 800 motor in pieces at the moment due to a broken timing chain, so while we have it apart we intend on giving it a new lease of life.

12SNipes12 started a topic about using high comp pistons and NOS the other day and it got me thinking.

We are looking at getting some oversized pistons from Wiesco and getting a local engine shop to bore the barrels.

Some of the pistons that we can find state a 12.5:1 comp ratio, where the OE ratio is 10.3:1.

My questions are;

What effect will high comp pistons have on an otherwise stock setup?

Will the engine need tuning?

Will the engine pre-ignite or ping under load?

Will the engine run hotter than they already do?

I've never really had to answer this question before, so I'd appreciate some opinions and some real life experiences.

:beerchug1:
 

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The engine won't detonate under load, as there is no way to adjust timing with anything other than something from monster products.

As I'm sure you know, the HC pistons won't be much good with out supporting mods. Intake / exhaust ect. At that point you would need tuning.
 

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i see my name haha ... you will def need air fuel tuning if your going as high as 12:5:1 ... one other thing you may want to think about is if you run your bike in the winter or in the cold... i dont know where your from but turning your bike over with a 12:5 piston in the winter and take your spray clutch out... because when its cold and trys to turns the high compression pistons dont let the bike turn over easy, i have taken a few spray clutches out in my day... if your going as high as 12:5 like KuntryQuest said you will def need aftermarket exhaust and i would say a new intake wouldnt hurt plus maybe a bored TB... this is y on my buddies bike we decide not too go very much more high compression then stock.... p.s you can run Nos with the stock pistons but it hits harder on a forged aftermarket piston, i would say it may run abit hotter but not much.... depending on what you wanna get out of his bike for HP and wat other mods you plan on doing i would try sticking around 11:5:1 ratio... if you wanna make HP do head work these rotax motors can make alot of HP from the heads http (Link too JE piston 11:5:1 compression stock 91mm bore for an 800, just ordered mine for my buddies 1k for 439 bucks) ---->>> ://www.jepistons.com/Catalogs/Powersports/ATV-UTV/Can%20Am/Outlander_800.aspx
 

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Discussion Starter · #10 ·
Hadn't considered the extra stress that the sprag would be put under.

He's not interested in making any more HP, so I'll keep searching for std comp pistons.

Thanks for all the input, it's helped a lot. :beerchug1:
 

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I would go for it doubt a little 800 would turn over hard. My motor is over 13.5:1 and has never kicked back or turned over slow.
I agree, my 15:1 1026 turns over no problem on 12 volts. Good battery mind you. If your starting/charging system is in good shape you'll spin the heck out of an 800 at 12.5:1
 

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You dont have to worry about sprag or starting issues. Idk how your fuel is over there, but 93 octane in the states is more than sufficient for 12.5:1. I would not do it unless you plan on tuning fuel. More compression = more heat. It wont be significant, maybe not even noticeable, as far as riding comfort. And you certainly wont have to worry about overheating issues. I wrap the headers on stock machines as even they will cause some leg discomfort during summer months. It should be worth a few hp and better throttle response just going to 12.5:1 on stock bore.
 

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It's the 1026 that runs the high compression. Just building the1225 now for an Outlander that was my turbo drag bike. It will be completely uncorked so I'm hoping for 115 rwhp.
 
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