Compression ratio change.

ozymax · Feb 12, 2015

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:

Gene P · Feb 12, 2015

Higher compression slugs will force you to run a higher octane fuel .
That much I know , not sure about the other stuff.

ozymax · Feb 12, 2015

We all pretty much run 95 octane fuel in our bikes round here.

We can get 98 at the pump, but it's not so plentiful.

So what do you call "higher octane"?

Gene P · Feb 12, 2015

Wow!
High for us is 93.
10.5:1 engines are OK with 87.

ozymax · Feb 12, 2015

Our standard fuel in Oz is 91.

I have used 91, 95 and 98 in my bikes over the years and found 95 to be the best. 98 showed no appreciable advantage in power or economy, but I definatly notice a change if I go back to 91. It's just not as crisp.

lunatic · Feb 13, 2015

Research or motor octane?

KuntryQuestTSi · Feb 13, 2015

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.

12SNipes12 · Feb 13, 2015

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

12SNipes12 · Feb 13, 2015

sorry thats SPRAG clutch not spray haha

ozymax · Feb 13, 2015

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:

lunatic · Feb 14, 2015

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.

climbin · Mar 13, 2015

lunatic said:
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

DSCZ71 · Mar 13, 2015

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.

12SNipes12 · Mar 13, 2015

climbin said:
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

you running 15:1 on like a can am outlander or renegade ? didnt even know they made that high of comp pistons for these bikes

climbin · Mar 13, 2015

12SNipes12 said:
you running 15:1 on like a can am outlander or renegade ? didnt even know they made that high of comp pistons for these bikes

Stroker motor and thin gaskets

12SNipes12 · Mar 13, 2015

is that the 1225 outlander you have in your ride thing? whats it running for RWHP?

climbin · Mar 14, 2015

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.

12SNipes12 · Mar 14, 2015

wow neat .... not a drag guy myself but i love the sound of them numbers in the HP department lol that would hold a diff together for about 5 minutes with big mud tires haha

lunatic · Mar 14, 2015

http://www.pencilgeek.org/2009/05/octane-rating-conversions.html

lunatic · Mar 14, 2015

As you can see in the link we have different octane numbers from country to country.

Compression ratio change.

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