another octane question

[mdono10375]

Hi all, I know cylinder head design plays a role in this as well as compression and jetting, so... I have a 2002 400EX, with an 11:1 forged piston, and a stage 1 hotcam. should i be able to run regular hi test pump gas, or is race fuel required? I have a stock exhaust too (to keep the neighbors at bay). I know GSXR's have like 13:1 compression and run pump fuel, so i was hoping I would be okay with pump fuel as well. Thanks

[Bolinracing]

Your fine on pump gas. I would however stay at the high end though. 91-93 ish

[zrpilot]

Originally posted by mdono10375
Hi all, I know cylinder head design plays a role in this as well as compression and jetting, so... I have a 2002 400EX, with an 11:1 forged piston, and a stage 1 hotcam. should i be able to run regular hi test pump gas, or is race fuel required? I have a stock exhaust too (to keep the neighbors at bay). I know GSXR's have like 13:1 compression and run pump fuel, so i was hoping I would be okay with pump fuel as well. Thanks

Octane requirement is a tough one to predict. I have an 11:1 running the Honda HRC cam and mine rattles (detos) after it is good and heat soaked if I'm running 93 or less. But running 110 it is fine.

[GPracer2500]

Similar experience here---->10.8:1 JE, old HC stg 2, 91 pump (no 93 here), and it would deto under full load once it got fully hot. I was good with VP110 and pump mixed 50/50.

Everytime I see someone say "oh yeah--11:1 on a 400EX, you'll be fine on premium pump gas", I want speak up and mention that's not always the case. But I get tired of going against the grain on this common question. I don't want to appear to be on a crusade or anything. Since zrpilot posted I decided to go ahead and share too.

[wilkin250r]

There are many complex interactions going on inside your engine, it's not just about compression and octane. I won't go in depth with all the details, but there are some designs that are just less-prone to detonation, allowing you to push the envelope further.

This explains why some engines can push 13:1 compression on pump fuel, but other engines will detonate at anything higher than 11:1. Different designs and different parameters equate to different performances.

[mdono10375]

i know some engines will detonate if too lean, thereby requiring a fatter main jet... I had a 10.8:1 that detonated on 94, but it was way too lean. i switched to 11:1, before fixing that issue. like i said, the cam, high compression mixed with the odd stock exhaust, may or may not help the situation too. i dunno. race fuel is scarce around my area. VP is almost cost prohibitive as well. can one get VP delivered from some place???

[SpeedyWho]

I would say or best bet is just to try it with the best pump gas around and see what happens.
I bought Torco100 for my 450r with 12:1 stock compression, it doesnt seem to run as well as shell 93.
Different engines like different things. It gets cold in Pa you might run different now than summer time.
Good luck

[250r4life]

i too love how people like to tell others on here "oh yeah thats fine," especially when they dont know what theyre talking about...

when youre talking about something that could very well destroy an engine, and cost somebody a lot of money in repairs, you dang sure better know what youre talking about when you tell them "oh yeah youre fine..."


the one guy said "i would stay in the higher 91-93 range..." i dont know why you would ever run less than that in ANY quad...

whats stock 400ex compression like 9:1 or 9.5:1? it cant be in the 8's

[mdono10375]

Stock compression ratio is either 9.5:1 or 9.8:1 with a cast piston.

[GPracer2500]

Originally posted by FoxHondaRider
whats stock 400ex compression like 9:1 or 9.5:1? it cant be in the 8's

It's 9.1:1.

[400exrider707]

Originally posted by 250r4life



the one guy said "i would stay in the higher 91-93 range..." i dont know why you would ever run less than that in ANY quad...

because if you run more than needed you're just wasting money IMO. Straight octane doesn't do much of anything for you, a differently blended fuel is a different case.

I ran 87 octane a few times on my 11:1 wiseco 400ex, running cometics extra thin head gasket, full Honda OEM base gasket, and a stage 1 hotcam. Ran fine on 87 with no deto, but I only did it a few times, other than that ran 91 no problems. Though like others said many things will play a role in this.

[maxfior]

Correct me if I'm wrong but doesn't the octane rating say how much fuel can be compressed before it will spontaneously ignite? So, when you are running a high compression engine, you'll need better gas so that the fuel doesn't ignite by compression alone (aka, no spark) and that is what would cause a knock. At least, that's why I usually run 91+ on most sport quads and dirt bikes, since they are usually built with 9.1+ compression. Just my $0.02 though... ;)

Max

[Big - D Racing]

my personal rule 10:1 to 11:1 I run 50/50 mix of VP C-12 and premium pump, above 11:1 I run the VP straight. I hear lots of people say 11:1 is safe with pump, I don't risk it.