Minimum Octane for High Compression?

Now all we need to do is solve what the correct Dynamic compression ratio is, as per the OP? That would answer what the max. cylinder pressure would be, for pump gas. Our best answer at this point, would be Mark with #200 psi. The mystery continues.

just buy a 55 gallon drum of 114 and mix your own special blend! :laugh:

kzz1p wrote:

Now all we need to do is solve what the correct Dynamic compression ratio is, as per the OP? That would answer what the max. cylinder pressure would be, for pump gas. Our best answer at this point, would be Mark with #200 psi. The mystery continues.

I am curious about this as well. Also, as heat increases, wouldn't the pressure increase as well? If so, wouldn't we need to find the dynamic pressure at a certain operating temperature? How about atmospheric forces? Or am I just on a tangent?

Anyways...I am curious about what dynamic psi the various fuels are recommended for.

I am curious to know the dynamic compression ratio of a stock Z1?

use 4.647 for the rod www.wallaceracing.com/dynamic-cr.php

kzz1p wrote:

I am curious to know the dynamic compression ratio of a stock Z1?

very little,look in dictionary under will run on pond water! :laugh:

PLUMMEN wrote:

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

Retarding much? It isn't how much or a number that ones concerned with. Timing needs to be backed off as much as is needed so the motor runs proper. If you have a 1015cc kit for example, you may need to back the timing down alot. If this is not done the motor will vibrate and the higher you rev it, the greater the stress on it. I would'nt want people thinking DON'T back the timing down it will burn up. Then detonate the hell out of it cuz plummen said not to do that. Or did he say not too back it down too much? Am I clear here?? My 1015cc is backed wayy down, if it isn't it has power to pull old teeth from bony jawas, but I can't stand the fact that it's done making power so early. It could be just another wives tail. Take everything with a grain, look how far off the octane numbers are in the info given. Human instinct can be dead on. What are the numbers for that?

larrycavan wrote:

use 4.647 for the rod www.wallaceracing.com/dynamic-cr.php

This calculator is a simple version, if you know the givens. What is the event timing of a stock Z, at running clearance. We know it is 30/70, 70/30 at check clearance. That makes a big differance!

I'm just GUESSING somewhere between 34-38 degrees int. closing, I'd round it off to about 36 degrees int. closing?????


@ 36 DEGREES intake closing.

7.96:1 dynamic compression ratio.
157.94 psi cranking compression.

What I found interesting was a 1015cc motor with 10.25:1 compression ratio & with an event timing at 36 degrees running clearance(stock or race cams)

9.58 dynamic compression ratio.
200 psi cranking.

That might just be a good street motor, for pump gas and an experienced tuner!.........With the event timing a little later, you could lower the cranking psi more.

If you do not have years of tuning experience, I would keep the dynamic compression ratio & cranking compression lower. I would give myself some room for error.....Mistakes can hurt!

Just my opinion.

nads.com wrote:

PLUMMEN wrote:

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

Retarding much? It isn't how much or a number that ones concerned with. Timing needs to be backed off as much as is needed so the motor runs proper. If you have a 1015cc kit for example, you may need to back the timing down alot. If this is not done the motor will vibrate and the higher you rev it, the greater the stress on it. I would'nt want people thinking DON'T back the timing down it will burn up. Then detonate the hell out of it cuz plummen said not to do that. Or did he say not too back it down too much? Am I clear here?? My 1015cc is backed wayy down, if it isn't it has power to pull old teeth from bony jawas, but I can't stand the fact that it's done making power so early. It could be just another wives tail. Take everything with a grain, look how far off the octane numbers are in the info given. Human instinct can be dead on. What are the numbers for that?

i said dont back it down too much or it will cause overheating