WR400... what to expect from a 13.5:1 piston?? Both performance and work wise!

Hey everyone,

Looking to majorly overhaul my WR400 this winter. I guess it's not as much of an overhaul, as it is addressing long neglected maintenance on the beast. ALL of the bearings are toast, nothings been replaced on it ever, and it's a constant work in progress. The good news with that though is I'm already really happy with this bike, and it's only going to get better! WOOHOO!

So one thing I'm considering doing is replacing the piston. Might as well just to be sure, I'd hate to have it slip and destroy the engine. Considering it's $150, and not all that impossible to replace, I may as well.

Now I notice pistons for my bike come in two compression settings. The standard 12.5:1, and the high compression 13.5:1.

Were I to switch to 13.5:1, what should I expect in the following areas:

-Will I need to re-jet?

-Will this make the motor less reliable than the standard 12.5:1?

-Will this affect the accelerator pump settings?

-How will this affect kick starting?

-How will this affect power?

-Are there any factors I am not considering?

Thanks in advance to those who share their experience on the topic.

ive only been a WR owner since last saturday, but ive always known that raising compression will make the engine run hotter and youll probably have to run it only on race gas when you get up to 13.5:1. How much is a 5 gallon can of VP now? $50??

Sure it will make more power but you might have to rebuild it more often, but im not sure on these WR engines. My 1989 XT350 has 23,000+ miles on it and ive never cracked it open. It makes a measly 27hp... i think... but its always ran perfect and reliable. I believe it's compression ratio is 9:1.

Your bike has made it this long in stock trim, if you keep it that way it will last long again. But if you want more power, ive always had good results with WebCams camshafts.

ive only been a WR owner since last saturday, but ive always known that raising compression will make the engine run hotter and youll probably have to run it only on race gas when you get up to 13.5:1. How much is a 5 gallon can of VP now? $50??

Nope.

Anything up to and including 13.5/1 is fine for pump 93 octane.

really??? well thats good to know :thumbsup: I stand corrected.

Hey everyone,

Were I to switch to 13.5:1, what should I expect in the following areas:

-Will I need to re-jet? probably not, maybe one step leaner on the pilot

-Will this make the motor less reliable than the standard 12.5:1? No as long as there's no detonation

-Will this affect the accelerator pump settings? no

-How will this affect kick starting? A little harder to kick

-How will this affect power? Good gains on the bottom to mid

-Are there any factors I am not considering?

Thanks in advance to those who share their experience on the topic.

I had a JE 13.5-1 in my 07 for the last season (150hrs) of racing and riding. Good gains (though I did do it the same time I put YZ cams in) 93 and sometimes 91 pump gas no problem. When I pulled my piston this fall the rings and piston still looked new and were well in spec :thumbsup:

ive only been a WR owner since last saturday, but ive always known that raising compression will make the engine run hotter and youll probably have to run it only on race gas when you get up to 13.5:1. How much is a 5 gallon can of VP now? $50??

Sure it will make more power but you might have to rebuild it more often, but im not sure on these WR engines. My 1989 XT350 has 23,000+ miles on it and ive never cracked it open. It makes a measly 27hp... i think... but its always ran perfect and reliable. I believe it's compression ratio is 9:1.

Your bike has made it this long in stock trim, if you keep it that way it will last long again. But if you want more power, ive always had good results with WebCams camshafts.

Aggresive cams are much harder on a motors longetivity than a HC piston.

Starting is a bit harder, rev is a bit slower, power is A LOT better!

Interesting about the slower reving. I wouldn't have expected that! How much harder is kickstarting?

I find the process a breeze right now on my WR400, but I've yet to run it on really really hot days at a crazy pace. It does have hotstart though!

I had a JE 13.5-1 in my 07 for the last season (150hrs) of racing and riding. Good gains (though I did do it the same time I put YZ cams in) 93 and sometimes 91 pump gas no problem. When I pulled my piston this fall the rings and piston still looked new and were well in spec :thumbsup:

Did it require rejetting when going with the new piston?

Did it require rejetting when going with the new piston?

No but read my answers in your quote on my previous post

Oh, right I missed those! Okay awesome. I think I'll go ahead with this!

By harder starting, I just meant you have to kick with more authority as you are compressing more stuff...my 444 still starts first kick, most of the time.

Slower revving is because on the compression stroke, you have more resistance to motion...

Interesting... how does that affect low end log? If there's more resistance to movement, I imagine low end lug would be reduced...

No...low end lug is actually better cuz you're getting more thump from each power stroke.

Ohhhh, okay!

It sounds to me like this is the best way to just add some pep to the bike without altering it's power characteristics too much. Thus far I love it's performance offroad :thumbsup:

You'll love it more with more compression!

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