Minimum Octane for High Compression?

mark1122 wrote:

as for the chart above. i dont know how helpfull it would be in the real world.
u and i run 10.25:1 on 91 octane the chart says 100 ??
i have only heard of 1 guy that says he runs reg gas. have u heard of any??
i have never tryed myself.
before my last build i was getting some ping on 91. so i made some changes and it runs fine on 91 now.

That would be me. I run 87 octane and haven't experienced any pinging, but I also haven't run the bike when it was extremely hot. I will probably up to 91/93 when the heat spikes around here. This is on 1075cc 10.25:1 Wiseco kit with stock head, cams, valve sizes, and carbs and 20W50 Valvoline MC Oil.

Seriously...I haven't seen any problem running the lower octane - no pinging, no nothing. I hate that all our 87/89 octane has ethanol in it, but nothing I can do about it.

DoubleDub wrote:

mark1122 wrote:

as for the chart above. i dont know how helpfull it would be in the real world.
u and i run 10.25:1 on 91 octane the chart says 100 ??
i have only heard of 1 guy that says he runs reg gas. have u heard of any??
i have never tryed myself.
before my last build i was getting some ping on 91. so i made some changes and it runs fine on 91 now.

That would be me. I run 87 octane and haven't experienced any pinging, but I also haven't run the bike when it was extremely hot. I will probably up to 91/93 when the heat spikes around here. This is on 1075cc 10.25:1 Wiseco kit with stock head, cams, valve sizes, and carbs and 20W50 Valvoline MC Oil.

Seriously...I haven't seen any problem running the lower octane - no pinging, no nothing. I hate that all our 87/89 octane has ethanol in it, but nothing I can do about it.

The problem with detonation is it happens long before you hear it. You've been pretty lucky with 87 gas.

I have my own opinion on the subject. I think Mark was pretty close on the issue. It's about cylinder pressure, more then static compression.

If we always talk about the motor being an air pump. The cylinder pressure is determined by the closing point of the Intake Valve. The sooner the Valve closes, the more pressure the cylinder will have.

Stock cams, torque cams & turbo cams have very fast closing points. Thats why some of these cams don't work very well, with high compression pistons & low octane. They have very good torque & performance but octane becomes the real issue. Some will ping like crazy or push in the piston domes, like butter.

Most racing cams have later closing points, that reduces the pressure in the cylinder.(at slower cranking)

It is possible to run a 12:1 piston with a cam that has a very very late closing point. The problem is the motor is no longer streetable.

It is my opinion that you need to adjust your cylinder pressure, to the octane level that is easily avilable to you. I think #220 psi would really be pushing the limit on pump gas.......Fill up your wallet at the same time, just in case.

If it was an air compressor, the intake valve would be equal to the "Pop Off Valve"

Save time, run some of that boot-leg corn fuel that Plummen is selling.........

Well, guess I won't be running 87 anymore...

What symptoms should we look for? Is it true that you kind of hear a "valve rattle", kind of like a diesel sound? I've never been sure what to listen or look for...

its like a knock .usualy in the low to mid rpm's under load.
i guess it may sound a bit like tappets.
have u ever heard this in a car or truck ? it will be similar.
but like Larry said. u may not even hear it, so its safer just to allways run premium.

It's called dynamic compression ratio and it can be calculated. Wallace Racing has an online calculator for that and it's easy to use.

If there's ever a question about serious detonation in an engine, a close inspection of the spark plugs will show tiny balls of aluminum.

www.flowbenchtech.com/porting/CRH/detonation/100_6749.JPG

Look at the photo in the link above. My PC blows it up to he resolution it was taken at. The top of the piston has lots of deposits of aluminum.

Spark plugs showed similar. I monitored the plugs after the build. No problems were seen initially. Then I put that Valero gas in the bike. Had been running Sunoco 94 prior to that.

Ran low on gas....filled with Valero because that's what I had to work with.....15 miles later....BOOM!


Bottom line is really pretty simple. If a few pennies at the pump is the safety factor for the hundreds you spent on a piston kit.......... 'nuff said...

Larry C

Nice pic Larry. it shows the alum deposits very clearly.
i was getting some alum on my spark plugs on my 1135 after i pulled it from the barn(150 to 160 psi). after the rebuild and going to the Dyna2000 the issue cleared up (200psi).

By the way guys. i have a secret that i'll let u all in on.
The cheapest way to get WAY more power and run reg pump gas, is to by a bike that has a ZX on it instead of a KZ. Funny how 1 dif letter can make such a big dif.
:laugh:

My understanding ---

Dynamic compression ratio is what the engine actually sees.

For example, depending on lobe grind and degreeing, an engine with 11:1 pistons may have less actual running compression than an otherwise similar engine with 10:1 pistons.

Largely dependent on when intake valve closes after BDC.

Good Fortune!

79MKII wrote:

On my last build, 1229cc at 10.5:1 c/r, I ran 87 once in a while and really didn't notice much of a difference. I used to think that 10.5:1 was close to stock anyway, but now I realize it's higher, so I always run the highest I can get, usually 91. I didn't see any signs of damage when I took mine apart for the new rebuild. I don't know of anyone running 87 on a regular basis though.

I recently realized and posted in another thread that I haven't checked the ignition timing on my bike. I use the stock electronic pick ups to trigger the msd ignition and don't think they're adjustable. I want to get a timing light and see where I am just to make sure things are where they should be.

Have you found that running slightly retarded helps with the detonation? Where should I set the ignition for best street performance? That's an area I've never really messed with. Maybe the multiple spark discharge helps make up for any ignition issues?

retarding the ignition much leads to overheating

Maybe you guys can help with this question too...

I set my cam timing according to what web cams recommended. What do they base this setting on? Peak hp at upper RPM's? Good mid-range for the street? Anyone know? I never thought of it before but the cam timing would impact the dynamic compression, and therefore the combustion right? Maybe I should re-degree my cams...

Lobe Center: 106° int 108° ex

PLUMMEN wrote:

79MKII wrote:

On my last build...
Have you found that running slightly retarded helps with the detonation? Where should I set the ignition for best street performance? That's an area I've never really messed with. Maybe the multiple spark discharge helps make up for any ignition issues?

retarding the ignition much leads to overheating

Should the timing just be set right on the mark then? I want to verify it with a timing light and adjust if necessary.

Just had a thought - the multiple spark ignition might wreak havoc on the timing light?? Never tried it before...