max PSI on pump gas

[44oEX]

I was wandering how much PSI 91 octane fuel can take before pre-ignition, or spark knocking?

I know 12:1-12.5:1 is the maximum recommended, but thats only the static pressure, and the importante numbers are the dynamic pressure witch is affected by different cam, valves....

What I want to know is the max PSI (cylinder pressure ) I can safely run.

any help would be appretiated

thanks

[TRXRacer1]

Originally posted by 44oEX
I was wandering how much PSI 91 octane fuel can take before pre-ignition, or spark knocking?

I know 12:1-12.5:1 is the maximum recommended, but thats only the static pressure, and the importante numbers are the dynamic pressure witch is affected by different cam, valves....

What I want to know is the max PSI (cylinder pressure ) I can safely run.

any help would be appretiated

thanks It's a combo of cylinder temps and pressure but more a question of cylinder temps. 93 may be fine in a 12.5:1 liquid cooled bike but it's not for an air cooled.

[John451]

So would a High comp 11:1 piston run well with regular gas? Or would I get alot of pinging or pre ignition and false spark?

[John451]

That would be with a 300ex.

[44oEX]

yes a 11:1 in a 300ex will be just fine, just run the good grade of pump fuel.

yeah i'll probably go for a 12.5:1 I would still like to get a better idea of max psi, cause a web 208 has good duration, and overlap, so it would be able to run higher. then say a stock cam

[John451]

Originally posted by 44oEX
yes a 11:1 in a 300ex will be just fine, just run the good grade of pump fuel.

yeah i'll probably go for a 12.5:1 I would still like to get a better idea of max psi, cause a web 208 has good duration, and overlap, so it would be able to run higher. then say a stock cam

What is the difference between the 12.5:1 and the 11:1? I know that it means one compresses more than the other, but which one does which? And does a different cam need to be used for those? Thanks! And sorry for askin about this in ur forum!

[400exchick]

the compression number (11:1), means that the volume of air in your cylinder when the piston is all the way down, gets compressed to 1/11th of it's volume when the piston is all the way to the top.

Say the volume of air when the piston is down is 100 cubic centimetre (cc) and the volume when the piston is up is 10cc, well that would give you a compression of 100/10=10, so 10:1 compression.

The higher the number 11, 12, 12.5...the smaller volume you will have and the more the gas will be concentrated, so you will make more power....to keep it simple, that basicly what it is.

in a 300ex, you can run a 11:1 with a stock cam, but I wounld't go much higher then that. unless you have more engine mods and plan to run race gas.

EDIT: this was Conrad (44oEX) using 400exchicks comp.

[John451]

So what's the highest piston compression? If i were to run a 12.5:1 compression without any mods i would problably have alot of pinging and maybe even pre ignition? Because from what i understand, the higher the octane (or the better fuel, cant remember if super has a highr or lower octane) the more it can be compressed. Would the cam actually help to fix that?

[Leadfootracing1]

Octain is basically a rating on the resistance of the gas to ignite, the higher the octain raiting, the more compresion you can run without worriing about detonation.
You can get away with 12:1 in an air cooled motor if you dont plan on running it too long and lugging it around. I'd stay at the 11:1 mark for any air cooled quad, that way it gives the ability to run premium pump gas and stay safe, you can have some mud on the motor and not have to worry about it.
And no, fuel additives wont help you in this situation either, the qualty of the gas is where its all at. And some places its harder to get race gas, if you travel a lot, so yet another reason to stay in pump gas range.....

[John451]

Why would it matter if I had mud on the engine? I don't usually worry about that anyways. Should I?

[John451]

Why would it matter if I had mud on the engine? I don't usually worry about that anyways. Should I?

[speedjunkie13]

Originally posted by John451
Why would it matter if I had mud on the engine? I don't usually worry about that anyways. Should I?

Mud on the engine could be bad if there is enough to effect heat transfer to the air, which could cause an overheating issue. and if you are are already close to the borderline of a safe compression ratio some detonation could start occurring with that bit of added heat.

Yes a cam can help lower the dynamic compression ratio depending on the grind of the cam. There are many things to consider when you are talking about compression. Generally with premium fuel (normally 92 or 93) you do not want to exceed an 11:1 static compression ratio piston with an air cooled engine.

[John451]

ohh, I see, so what's a good cam for a 11:1 if i ran 93 super? And would I need new valves or rockers? it's a 1 cylinder 4 valve. Don't believe i have any mods in the engine. maybe a little bore. and would that need a different camchain or the little sprocket on the end of the cam? and what would the price of the piston, rings, cam, and whatever else i would need cost? thanks.

[44oEX]

You won't need anythign else to run a 11:1 on super in a 300ex. A cam will help the power, but you won't have any detonation in your case, so your ok. Any cam will give you more power then the stock on does, it all depends on your budget as to what you should get.

The cheaper cams, are drop in cams, basicly, take the old one out, put the new one in, set the valve clearance to spec and your good to go.

The better cams, as not just drop on, they need stiffer springs, shorthen valve guide ( normaly put in by amachine shop ), hard weld rockers, titanum retainers....so basicly they give more power, but cost alot more to run. Unless your planning on doing some serious competing with your 300ex, I would suggest a good drop in cam...something like the HOTCAMS. You won't need to change the sprocket or the chain, unless they are worn out.

[TRXRacer1]

If you just ran moto's with it and kept it clean you'd be fine but try taking it out on some real trails, add a dash of mud, a pinch of gnarly terrain and a whole lot of throttle and see what happens. You could easily make a 300ex ping at 11:1 with a cam on pump gas!

[John451]

Ok, so a hotcam will add a bit of power without having to go through the valves other than adjusting them? Would it be a noticable difference in power? Or worth the $120?

[TRXRacer1]

Originally posted by John451
Ok, so a hotcam will add a bit of power without having to go through the valves other than adjusting them? Would it be a noticable difference in power? Or worth the $120? The hotcam is totally worth the $120. It's not that the cam is cheap or anything it's just not as radical as others. Unless you're going big bore, race porting and a bigger carb I wouldn't bother spending money on any other brand of cam. You just won't fully utilize something that needs new springs and shortened valves.

[John451]

ok, so what stage cam would be the best? I could only find stage 1's but i couldnt find the 2's. What's the difference?

[TRXRacer1]

Originally posted by John451
ok, so what stage cam would be the best? I could only find stage 1's but i couldnt find the 2's. What's the difference? There's a bit of some internet myth on the subject so best thing you can do is click on the dyno links at hotcams web page to see what the cam is actually going to do. The stage 1 cam is the only one offered for the 300ex but as you can see on the dyno it's a really nice power increase all the way from the lo-mid to the top end.

[John451]

Ok, and ima go right ahead and asume that it's hotcams.com. But that explains why i couldn't find anything but stage 1's.

[John451]

lol, jk. that's not it. just went there and lol wrong type of cam. and 10x the hotness. does anyone know the real hot CAMSHAFT website? lol.

[TRXRacer1]

LOL, oh the adventures of the internet!

http://www.hotcamsinc.com/

[John451]

So i read that you have to use the parts from the origional decompression lever; however, mine doesnt have a decompression lever.... or does it... Idk I looked on both sides of the engine, saw no lever or cables near the cam. Guess someone took it out. is that a problem? I don't wana sound like a noob; but, what does a decompression lever do exactly? How and when is it used? I've heard it called a hotstart too. I'm new to bigger 4 stroke engines. Is it bad to use it if it's not needed? Is it something I would want back?

[TRXRacer1]

That cam is used in the 250x also and that does have a decompression mechanism. They were probably being very general and covering both machines. What it does on the 250x is opens the exhaust valve just a little, lowering compression, making kick starting it easier and safer. You do not have these parts so it is not a concern.

[John451]

Ok, that's exactly what i thought it was. I found where it should be and it's not there. but I don't have a kickstart so i don't really need to use it. My xr100 could defiantly use that lol. Little thing always gives kikback and whacks my leg. I swear it knows when i'm not paying attention for the recoil.