What octane

[Honda 250r 001]

ok so ive pretty much come to the conclusion if i hadnt been running aircraft fuel with octane booster in my last 3 motocross races, my 250r would have gone by now and octane booster is expensive so i guess im going to run race fuel. But i dont think i need full blown racing fuel but what octane do i need? My compression is 195 and im going to be running a +4 timing advance. what octane rating do i need?

Thanks!

[thedeatons]

Well... Just to get the thread started, if your compression is higher than 175, then you need more than 91 octane. Perhaps you could try a blend of either 91/100. I have spoken with a few people who have run that.... I am a novice at this question though, I just wanted to get the chatter started.

James

[IcutMetl]

I know the Honda service manual states stock compression limits should be 170 +/- 15psi and recommends premium pump fuel. Premium pump fuel around me is 91-94 octane.

I run a thinner head gasket and have around 195-200psi and run a 50/50 mix of premium and 110 octane. No complaints.

Just my personal experience...

[Honda 250r 001]

Originally posted by IcutMetl
I know the Honda service manual states stock compression limits should be 170 +/- 15psi and recommends premium pump fuel. Premium pump fuel around me is 91-94 octane.

I run a thinner head gasket and have around 195-200psi and run a 50/50 mix of premium and 110 octane. No complaints.

Just my personal experience...

that exactly what i was thinking of doing.

is 110 that expensive? i mean 4-5 bucks a gallon?

[croat1]

race gas is expensive. hell look at cheap pump gas at 3 a gallon.
what are you paying for the aircraft fuel and octane boost per gallon?

[250r grave yard]

you can get away with half/half but i would just run staright race fuel at that comp less hasel with just running straight race fuel you should be able to run a mix or straight without haveing to change much in your settings my 2 cents

[Honda 250r 001]

Originally posted by TL250R
Here in Charlotte, NC:

93 is almost $4 a gallon
100 is almost $6 a gallon
110 leaded is almost $8 a gallon

Sucks spending the money... especially when you go through it so fast.

**** maybe i will just go with the aircraft fuel and octane booster haha. im not sure what it costs but i think racing fuel here is 5-6 bucks a gallon at most.

[IcutMetl]

CAM2 110 octane is $6.00 a gallon here in Sandusky. If we didn't have a race fuel pump so close, I wouldn't even have considered running high compression. Of course; Norwalk (now Summit) Raceway Park is only about 20-25 minutes away.

If I rode my 'wheeler all day every day, the cost might be a different story, but what the heck...if I can gain a few more ponies by running higher compression and throwing a little high test in the mix...crank it up!

[rigger]

Race gas for me is like 8.45 a gallon here. That is a lot for me but it is not like I will be riding every weekend either so it really is not a big deal.

[1promodfan]

I buy 110 in New Bern, NC for 5.99 gallon. I'm like Metl, I don't ride that much, so the cost to me is worth it. But, if I didn't have race fuel 45 min. from me I probably wouldn't run it.

[Aceman]

Originally posted by Honda 250r 001
**** maybe i will just go with the aircraft fuel and octane booster haha. im not sure what it costs but i think racing fuel here is 5-6 bucks a gallon at most.

Research octane booster a little before you start running it all the time. You think race fuel is expensive? Wait till you burn your motor up trying to get by using the wrong fuels...

I spend about 5.50 for 110 leaded right out of the pump. Small price to pay...

[thedeatons]

No thanks... I go through so much gas in the dunes on 4 day trips... I set mine up for low compression on purpose, so I can run 91 octane. So I lose a few hp.... who cares. There is always someone faster than me anyway....

James

[250r grave yard]

i would run a unleaded gas air craft fuel afgas is leaded fuel leaded will leave more ash on your piston

[Honda 250r 001]

Im thinking im just gonna lower the comp with a thicker base gasket next time i come apart with it because i ride ALL the time, after school, on the weekends, snow days, holidays etc. So even pump fuel is expensive.

[250r grave yard]

Originally posted by Honda 250r 001
Im thinking im just gonna lower the comp with a thicker base gasket next time i come apart with it because i ride ALL the time, after school, on the weekends, snow days, holidays etc. So even pump fuel is expensive. i wouldnt do that it willl mess up your port timing if you rasie the base of the cylinder try getting your head reworked or run a cr head gasket along with the regular trx head gasket i reuse my trx head gasket a few times before i throw them a can of black spray paint makes them reseal would not try it on anything over 200 psi tho

[Honda 250r 001]

Originally posted by 250r grave yard
i wouldnt do that it willl mess up your port timing if you rasie the base of the cylinder try getting your head reworked or run a cr head gasket along with the regular trx head gasket i reuse my trx head gasket a few times before i throw them a can of black spray paint makes them reseal would not try it on anything over 200 psi tho

ive got a 310. and im already running a pump gas dome, i will only move the cylinder up .10

[thedeatons]

Very true what was said above. Use a thicker HEAD gasket, then check compression. Tell us what it is. You should be shooting for the 170-175 range to safely run 91 pump gas. I am at 173 and my 3 month old 91 pump gas goes through it well.

Here is what you should coat all the gaskets with, recommended by Neil and all the others on 250rdotnet:

Threebond 1211 Silicone Gasket

http://www.bikebandit.com/product/8886?WT.mc_id=1484958&CAWELAID=338430045

James

[thedeatons]

Pump gas dome or not, 195psi is way too high for pump gas. You have to raise the HEAD, not the cylinder. The compression is at the top, which is why you add a thicker head gasket. Raising the cylinder will accomplish nothing for you...

James

[thedeatons]

Does that make sense?

James

[Honda 250r 001]

Originally posted by thedeatons
Pump gas dome or not, 195psi is way too high for pump gas. You have to raise the HEAD, not the cylinder. The compression is at the top, which is why you add a thicker head gasket. Raising the cylinder will accomplish nothing for you...

James

actually it does lower the compression but its not much, im shooting for 185 and im running o rings so i cant just put a thicker base gasket on.

[Honda 250r 001]

Originally posted by Honda 250r 001
actually it does lower the compression but its not much, im shooting for 185 and im running o rings so i cant just put a thicker head gasket on.

[thedeatons]

I just reread my email and noticed it was confusing... It will lower compression, but like someone else said it will change your effective port height (port timing). I wouldn't mess with that.

If you are running an o-ring head from Pro-Designs you might try giving Tom (owner) a call and asking whether you can add a head gasket in there... If not then get a different dome (~$45).

My opinion (and Tom's because he designed my dome for 91) is that is still too high.... Tom said 175 is safe, just like 32:1 premix. Your call whether you want to run on the edge or not, I prefer to build motors once, in safe mode.

(You have to account for not getting exactly 91 octane every time, because it is pump gas, so you run compression a bit lower to be safe with bad/old gas)

James

[troybilt]

Hey I'm new to the race gas/fuel thing: Dan turned me on to VP110 and apparantly we have a larger distributor of VP race fuels in town, 5 minutes from my house... yeah... anyway, they have like 15 different options recommended for race atv's, wonder which to I get? Stick with VP110, go with U4.4, etc...

I should prefice this with, my motor is running 185-190 psi...

The ones listed and available to me are: (I had no idea there was this many) On the website, the one recommended for 2 strokes is MRX01 @ 98 octane. Does not leave deposits...

Leaded/Oxygenated:
U4.4
MRX01
MR12

Leaded/Non-Oxygenated:
VP110
C12

Unleaded/Oxygenated:
VP100
Motorsport 109
M8-1

Leaded:
VP Red

Leaded/Oxygenated:
U4E
MR11
VP113
C10

Unleaded/Oxygenated:
Motorsport 101
Motorsport 103
Motorsport109E