330ex

[400exrider99]

my friend has a 330ex with a 13.5 : 1 comp ratio and can some one tell me what octane of fuel he needs

[rooster300ex]

110 atleast

[zeppelin]

i dont get this stuff with octanes vs comp ratios, my stock yz250f as a 13.5:1 piston and it runs fine on plus (90 octane). i say you should buy some premuim and see if that makes your bike ping or not and then try a higher grade if it doesnt work 'cause 110 octane aint cheap

[2muchquad]

dont get this stuff with octanes vs comp ratios, my stock yz250f as a 13.5:1 piston and it runs fine on plus (90 octane). i say you

exactly,my yzf has a 13.5 to 1 wiseco piston and 93 octane pump is fine.i dont know where these myths come from,maybe people dont realize cam overlap comes into play here:rolleyes:

[dork]

yea, but those bikes are liquid cooled with modern chamber designs with lots of quench areas. if that 300ex has true 13.5:1. then he'll need race gas. try pump gas if you're feeling lucky.

[2muchquad]

air cooled or liquid cooled doesnt matter here,there is still cam overlap regardless of the cooling method,surely one should have known:rolleyes:

[bwamos]

Originally posted by 2muchquad
air cooled or liquid cooled doesnt matter here,there is still cam overlap regardless of the cooling method,surely one should have known:rolleyes:

Air cooled vs. liquid cooled make s huge difference on octane requirements. You can't tell me the temperature of the combustion chamber pre-ignition doesn't make a difference on pre-ignition detonation. Temperature & Pressure are THE factors that cause the sponateous combustion of the air/fuel mixture.

Your "cam overlap" simply affects the final pressure.


A 13.5:1 would probably be ok on 100 octane. But most 110's are leaded which will increase the life of your motor which is why it is prefered by many, while most 100's are not.

[2muchquad]

my 385 runs fine on premium,itsgot a 11.5 to 1 je piston AND i also had .020 taken off the head,so the compression is up there.i think a lot of you guys read too many atv magazines of what fst,sparks etc are doing or saying.while some of us are in the real world doing or experimenting on our own,finding out what works for US,not what a dollar driven company is saying.Now from my experience premium is fine,could be different in another location

[bwamos]

Originally posted by 2muchquad
my 385 runs fine on premium,itsgot a 11.5 to 1 je piston AND i also had .020 taken off the head,so the compression is up there.i think a lot of you guys read too many atv magazines of what fst,sparks etc are doing or saying.while some of us are in the real world doing or experimenting on our own,finding out what works for US,not what a dollar driven company is saying.Now from my experience premium is fine,could be different in another location

11.5:1 with .020 take off of the head is not 13.5:1 You have to increase the dome of the piston to reach that compression.

Using rough math and assuming an 11.5:1 385cc piston (86.5mm) with an unshaved head is 11.5:1 (which it is not, thats why they shave them), you compress 384.7 cm^3 down to 33.4 cm^3 taking off 0.508mm compresses it to 33.1 cm^3 which gives you approx 11.6:1 compression. That's not even close to the pressures you acheive at 13.5:1. Pressure increases exponentially as you increase compression. Kind of like tring to push 2 magnets together with like poles.

I am running with real world experience. I've been running an 11:1 compression 330ex for 5+ years. On a very hot day (100deg plus and high humidity) at the dunes my quad will detonate with 91 octane under high loads. I either have to richen up the mix to cool the cylinder or mix 10-15% race fuel in it to keep it running properly.

Most air cooled can get away with up to 12:1 with a mild cam and porting. But 11:1 is the safe bet.

I also have not boughten an ATV magazine in 4 years. :blah: :D

[2muchquad]

okay bwamos,you got me:D the dunes will put more of a load on a engine than casual riding one would think,so octane requirements would vary.

[dork]

plus, unless you check your deck height and squish clearances, you may well be under that ratio. a lot of pistons based off stock compression heights end up sitting too low in the bore.

also regarding overlap and compression, its the intake valve's closing point and exhaust valve's opening point that has the most effect on dynamic compression. if you move the lobe centers apart(retard the intake, advance the exhaust) overlap decreases along with compression. more overlap does have a natural egr effect though which lowers combustion temps slightly.

[2muchquad]

if you move the lobe centers apart(retard the intake, advance the exhaust) overlap decreases along with compression. more

thats what i was saying earlier,not in those exact words though:)

[bwamos]

I agree, the dynamic compression and tempurature is what matters.

We're just saying that in most ATV motors (low tech), that the safe bet compression for premium pump is 11:1 on air cooled, and 12:1 on liquid cooled.

You can go higher. I've seen 400ex's running at 12:1 on premium. But do the same thing with a different cam, and your screwed. ;) 11:1 in an air cooled is simply the safe bet.

So basically we're all arguing the same thing against each other.. lol.