Fuel/power help

[Mudslinger11]

Ok, so I have the mods in my sig. Right now running the 50/50 mix. I'm am going to remove the head for port polish, kw valves,Web540/539, and a Fci intake. My compression is about 13:1. I'm thinking about putting in the ltz base to lower it to 12.2:1 so I don't need the race fuel. I'm not sure if I want to do that if I'm going to loose to much power? Anyone have any idea what kind of losses I will have?
Also ive had some tell me that it's so close to the line that I might be ok with 93? Should I try that before I tear it down and what would the sighns be if it was too low octane? Thanks

[dtfm33]

I am by no means a fuel expert, but i thought that 93 octane will be enough for a 13.1 setup. CFM or KFXGuy need to chime in on this.

[pimpit_r6]

with a 13.5.1 compression you need minimum 100 octane fuel. But if you use a ltz400 base gasket then you will be ok with 93 octane. I think the minimum is 89 with the ltz base gasket. I dont know how much power you will lose if you switched gasket.

[CFM-Z440]

You might be able to squeak by on premium pump at 13:1 with those cams. You dont need to mix all the way to 50-50, thats for sure.

What is your elevation? That plays into it as well. You can get by with more compression at higher altitudes with the same fuel.


Going to the 3 layer gasket is going to lower you almost a point, to 12.1:1 or so, and you will give up a couple ft lbs of torque pretty much everywhere. It wont be as snappy either.

You might also look into aviation fuel to save some money. 100LL would work nicely. If it were me, I would just reduce your mix more and more, and see if it starts to detonate when you lug the snot out of it.

You will find that lowering your octane closer to what the engine requires will actually gain you some power. Higher octane doesn't make more power. The compression that requires the higher octane is where the power comes from.

[zookie400]

the newer pump gas has 10% ethanol in it, i would try running the 93...i bet it wont detonate.

[Mudslinger11]

I'm at 200ft . As for the aviation fuel, it's not a cost thing, it's the problem of going on a long trail ride and everyone fills up and I can't! I know a bigger tank would help and I could carry it, it's just a pain. I think I'll try the 93. What are the sighns of detanation?

[yOGi420™]

Just get a ims oversized tank and run whatever fuel you want. I like the 100LL personally as a lesser expensive alternative to the 50/50 prem and race option which is what I have been running since I got mine until I switched. Seems to run the same.

[a.graham52]

i run the 100ll as well. smells good and it has no ethinol!

[wheatez]

I think even right now with the hotcams u could get away with running pump, but once u install the webs u have nothing to worry about.

[zookie400]

when your engine sounds like a diesel, its pre-igniting aka detonating or pinging.

its hard to explain in great detail, but if your hearing is decent you should not have a problem picking up the different sound it makes if it starts to ping.

[Mudslinger11]

Ok, cool. I'm going to give it a shot. Would I hear it at idle or higher rpm?

[zookie400]

i have never had a Z ping that i can remember, but i think on these it would come in mid-top under load.

[CFM-Z440]

Originally Posted by wheatez

I think even right now with the hotcams u could get away with running pump, but once u install the webs u have nothing to worry about.

Depends on which ones and their timing. Its not quite that simple.

Originally Posted by zookie400

i have never had a Z ping that i can remember, but i think on these it would come in mid-top under load.

I have ridden a Z 434 that was on the edge, and it was fine when it was up in the high rpm's but when it got lugged down too far, it would start detonating. Low to mid was where it did it.

[kfxguy]

Originally Posted by zookie400

i have never had a Z ping that i can remember, but i think on these it would come in mid-top under load.

ive heard mine before. mike, if you do get the head ported, thew combustion chamber can be opened up a lil bit which will do a couple things: first, as you know it will lower your compression somewhat, but i doubt at an expense that youd feel it. Its better to lose compression this way than to pull the piston farther from the head which reduces squish (a good thing to have) Second, cleaning the chamber up will help resuce hot spots, and sharp edges which promote detonation.

With those larger cams, you are better than smaller cams for sure as it bleeds off cylinder pressure more than smaller cams...which helps too. I dont like running race gas in any of the things i build. I know, i might be leaving a smidgen of power on the table, but id rather have the option of being able to get fuel from anywhere. Thats just me. The honest to goodness easiest thing you could do is get yourself a dyna programmable and retard the timing a few degrees. I remember before i had big ole cams, I was at wiggins doing some sand dragging and (that definetely puts a load on the engine) when the temp rose that day i could hearf it clattering....I got a lil nervous...but i went back to my truck and pulled out my laptop and backed the timing down a few degrees....fixed it and it ran stronger. Keep in mind you dont neccessarily need alot of timing to make good power....it depends alot on the cylider head chamber and piston design....and the z head fares pretty well. I havent really seen a losas in retarding the timing. Now as you guys know, i havent been to the dyno, but i do have a decent real world tuning "procedure"...lol. My buddy has an all out drag YFZ470, and we will make about 5 passes before I make a change. He gets out on me right out the hole and i usually start catching him. So after 5 good runs I know where im at, he is pretty consistant in his driving skillls. Ill go make a change and if im pullinng on him less or he's pulling on me I know i went the wrong way with my tuning. One time we did this I picked up about ten bike lengths because I was going leaner when I should have been going richer. I was racing eveyone with a 170mj that weekend and thought i was spot on, It ended up liking a 210mj!

[smokedya2]

the ltr guys all like to run a 12.8 to 1 piston and run it on 91 pump gas, I know the motors are pretty simular. with some bigger cams, a dyna programable cdi, and 93 pump gas you should be fine at 13 to 1 compression ratio