Compression ??? (too high)

[ant_mb]

Hey guys,

I rebuilt my motor top to bottom last April-May. I did a compression test after rebuild and it was at about 155-160 cold. I thought that was a little high, but also thought my redesigned head had something to do with it.

I've checked 3 times since then, and it has only increased. It's holding strong at 165 today!

My question is this. Is that too high for pump gas? This is the only blaster I've ridden, and I didn't ride it before I tore it a part so I have no reference for performance. It seems to run great, but who knows?

 

[brandoz28]

do you get speckles on the spark plug porcelain? do you get detonation? does it overheat easily?


if the answer is no, run it! guys have run up to 170psi on pump fuel

 

[slickerthanyou]

Hey bro,
I have my +3 ken Oconner motor at 165lbs, an run 93, no problems. You should be all good!

 

[ant_mb]

do you get speckles on the spark plug porcelain? do you get detonation? does it overheat easily?


if the answer is no, run it! guys have run up to 170psi on pump fuel

I had detonation when I was running +4 timing, but none since I returned it to stock. No heating or plug issues.

 

[ant_mb]

I'm also using the short spark plug adapter because of clearance. If I were to put the long adapter like the plug it it would only increase the compression amount.

 

[Whitty21]

run the highest octane pump gas u can, any higher then 170 is usually when race fuel comes in, what plug are u using?

 

[brandoz28]

you may want to go to a br9es plug if you're worried about it. the colder plug will be a little more forgiving

 

[ant_mb]

you may want to go to a br9es plug if you're worried about it. the colder plug will be a little more forgiving

I'm running the BR8EIX

 

[Whitty21]

I'm running the BR8EIX

like brando said, the BR9 is gunna compensate for the higher heat created by more compression, if u really want to get educated gve ken oconnor a call, or even better the builder that did your head

 

[brandoz28]

if you're not running into any issues i would leave it alone. you just won't be able to advance the timing without putting higher octane fuel in it

 

[0_o]

run the highest octane pump gas u can, any higher then 170 is usually when race fuel comes in, what plug are u using?

can you use octane booster?

plus give me some compression i only have 30 psi! X(

 

[brandoz28]

octane booster won't do much of anything for you. you need real high octane fuel

 

[sicivicdude]

Most octane boosters don't actually boost the octane... They clean the fuel system allowing the fuel to fully combust raising the "effective" octane rating for the fuel system.

You may want to go with the 9 heat range plug. It would help if you ever have a lean event (low fuel sloshes around runs you without gas for a split second)

 

[ant_mb]

like brando said, the BR9 is gunna compensate for the higher heat created by more compression, if u really want to get educated gve ken oconnor a call, or even better the builder that did your head

My cylinder is 80 over now, when-if she blows I'll be giving ken a call for a portrd and fresh bore head. I'm hoping to get several years out of this one though.

 

[0_o]

Most octane boosters don't actually boost the octane... They clean the fuel system allowing the fuel to fully combust raising the "effective" octane rating for the fuel system.

You may want to go with the 9 heat range plug. It would help if you ever have a lean event (low fuel sloshes around runs you without gas for a split second)

what if you use jet fuel? i read somewhere a skyline had like 900hp and it ran on jetfuel

 

[brandoz28]

please.....................do some research.


probably not jet fuel, but AV (aviation) fuel. i've never had good luck with it, and its a naturally dry burning fuel (you have to add a top end lube in most automotive instances, imagine how your 2 stroke will like that)


the only reason to run high octane fuel is if you have problems with heat, or detonation. otherwise you don't need the colder burning fuel

 

[sicivicdude]

Avgas is probably what you're talking about. That's what brandoz is talking about too(real high octane fuel).

Jet A cannot be run in a gasoline engine. It's akin to kerosene and won't burn in a blaster engine.

Avgas, or 100LL as it's proper name, is high octane gasoline. It's also usually leaded fuel so you would want to use as little as possible to keep detonation at bay.

 

[sicivicdude]

Avgas can also refer to 110LL (depending on the airport). If you are lucky enough to have a local strip which supports vintage WWII aircraft they may actually have the 110 octane fuel on hand. Most of the old radial direct injection engines needed the 110 octane with a top end lube so a few airports still have it.

Most aviation engines manufactured today are designed for 100 octane unleaded fuel. They run on leaded fuel but don't require it.

You can also source racing fuel (if you have the money, need the increased octane, and don't have an airport nearby) from VP or sunoco. They both sell it by the barrel and some sunoco stores have a high octane pump off to the side still.

 

[YFS101]

dont run a shorter plug..... there is no clearance issues even on a rechambered head.

With many of my head we simply leveled the uneven oe squish andthen simply milled the head .050" and had aprox 160-163 psi. Its works out to about a 17.7cc head.
With this set up we were able to jet it to 1100 degrees and still use a BR8ES plug on 92 octane. No need for race fuel.

if you dont level the squish you can mill .035"

 

[sheeblast]

Avgas, or 100LL as it's proper name, is high octane gasoline. It's also usually leaded fuel so you would want to use as little as possible to keep detonation at bay.

why would you want to use as little as possible?