high compression piston will i overheat?

[ae13291]

hey guys i have pretty much a stock 400ex with a pipe and filter and the sparks key and stage 1 hotcam, i was thinking about high compression piston but whats a safe limit i can go before the air cooled motor starts to overheat? i know it will make it run hotter but whats a safe limit for me to go? thanks guys what compression piston should i get?

[2001trx400ex]

i went with an 11:1 high compression, but as for you i would either sudgest the 11:1 it runs perfect not any hotter really that i know of, or a 12:1 is real common so i would probaby go with that to open it up more with your cam and pipe on it should wake it up once you get the piston.

[slightlybent47]

You will have to run race gas with 12-1 and up. Other wise it will over heat.

[ae13291]

i havent looked at pistons yet but there is only 11:1 and 12:1 there is no medium between?

[slightlybent47]

I run a 416 with a 12-1 here in TX where it gets 100+ in the summer. But I run 110 and 93 half and half now and I haven’t had any issues. Sometimes I run U4.2 strait but only in the winter. I also have a extra large oil tank and I’m going to put a fan on it this year.
I only race so my fuel consumption is not that much. If you stay with 11-1 you’ll be fine with 93 octane. I think you will need to take out the timing key and go back with the stock set up when you put in the high compression.

[ae13291]

what is the stock 400ex compression so i get a idea of how much more 11:1 is?

[Miami_Vice454]

stock is 9.1:1

[ae13291]

wow i think 11:1 will be a big jump and im sure i will be happy with it. thanks guys

[hypersnyper6947]

pretty sure your going to have to remove that sparks key with the high compression piston

[IcutMetl]

11:1 is borderline for use of a 6degree sparks key but I say go for it. I had one in my stock motor and liked it a lot, my riding buddy at the time put an 11:1 Wiseco piston, stock cam, and same 6degree key in his and it woke it up a TON. Ran it on premium pump gas, changed the oil often, rode the piss out of it, and never had a problem. Compression is magic...it builds power everywhere!

[VTredneckgames]

I went 11:1 on my otherwise stock ex. Bored it to 406. Stock exhaust, stock cam. Could definately feel a power increase. And I definately felt a heat increase. It heats up quicker and runs somewhat hotter. It is noticeable to me. I run 93 octane.

I ordered me a spal fan and come summer Im going to get some scoops.

[ae13291]

i was thinking of installing the bigger oil cooler but decided to put it up for sale since i dont need it. but maybe i might need it now lol. usually when i am riding in the woods, when we take a break i just start shoving some snow on the oil cooler fins, but that only in the winter time. i do use a lead additive for my 400ex gas but i dont know if that good enough to prevent detonation

[slightlybent47]

Originally posted by VTredneckgames
I went 11:1 on my otherwise stock ex. Bored it to 406. Stock exhaust, stock cam. Could definately feel a power increase. And I definately felt a heat increase. It heats up quicker and runs somewhat hotter. It is noticeable to me. I run 93 octane.

I ordered me a spal fan and come summer Im going to get some scoops.

That don't sound right, that 406 should not be heating up enough to notice. Check your jetting and or bump up your octaine a few # and see if it don't cool back down.

[Dylan227]

Totallay a je 11.5to 1 or If your gonna be racin get there custom 14 to 1 and then get it worked to a 15.1

[IcutMetl]

Both myself and my friend with the 11:1 Wiseco also noticed a slight decrease in temps when we switched to a synthetic oil like Amsoil for example. Not a lot, but certainly can't hurt. He never had an issue with overheating, and never had to use a fan, larger cooler, or increased capacity tank. I can tell ya this, he rode the piss out of his much harder than I did mine as well.

[VTredneckgames]

Originally posted by slightlybent47
That don't sound right, that 406 should not be heating up enough to notice. Check your jetting and or bump up your octaine a few # and see if it don't cool back down.

really? My current jetting is 1 size larger on pilot and main. Plus always run 93 octane. one problem I have is that I tend to ride slower then most of you. Usually in the mid-high rpm range of 1st and 2nd gear.

Would gladly take suggestions on jetting. Also I know I need to adjust the pilot screw. At idle sometimes it will die without me giving it some gas. Lean? I know lean can cause heat issues.