01 500 compression ?

[powermizer]

What is your thought on max cranking compression for premium on stock ported engine?

[dubious01]

I have taken as much as .040 off the stock head, fattened the main 2-3 sizes, and ran them like that with a pipe, with excellent results.
you will have excellent bottom and mid power.... all on pump fuel

I would want to widen 4mm each side at top outer corners, and raise the exhaust port 3mm, as well as open up the boost ports on the intake for cheap mods.- no prox pistons though with this mod.

This porting will give it more top end, and combined with added compression a nice wide powerband, instead of the peaky stock lightswitch power. I had to go up on the main yet again, and pull a little timing with the exhuast port mods. ... all on pump fuel

over ~170 psi cranking compression, with stock head dome , squish band, and piston shapes required additional octane. how much exactly I am not sure but I went to C12 for good measure.
Thats my experience anyway.

[100hp honda]

throw your harbor freight gauge in the trash:

What about cranking PSI vs Octane requirements?? My opinion is that they are not very closely related. In other words.. knowing what PSI your engine "cranks" out will NOT tell you whether or not you need a higher octane fuel, unless, of course, the PSI number is extremely large (like 200). Then, it may carry more weight and tell you , "yes", you do need some high octane fuel BUT it will NOT tell you which octane you need.

OK, "WHY?"

Let's do some deductive reasoning here:

Take an engine with an exhaust duration of 182 degrees ATDC and install some domes (any domes) that yield.. say 170 PSI cranking compression.


OK, take another engine, exactly the same, except the exhaust duration is at 198 degrees ATDC (ie higher exhaust port) Now, install the SAME domes that the other engines has. Now, the cranking compression reads only 150 PSI.

Hmmm.... so does this engine require a different octane of fuel than the other one? Probably, but not what you think.. It , most likely, requires, a HIGHER octane fuel NOT a lower octane.

I know that the general conception among riders is that the higher the PSI .. the higher the octane needed.. Well, I just told you completely the opposite.
With the general consensus.. it would stand to reason that when ever you raise the exhaust port... you need to run a lower octane fuel because the cranking compression will, indeed, get lower with higher exhaust heights. Does this seem right?? What if you lowered the exhaust height? Would you then need a higher octane fuel because the cranking PSI would surely go UP?


Let's look a few of the determining factors behind the above statements: Please realize that I will be over simplifying the processes but it should get the point across.

1: With an increase in exhaust height or area.. the probability of having more escaped fuel/air charge "stuffed" back into the cylinder just prior to exhaust port closing increases. This will raise your dynamic cylinder pressures and combustion temps.

2: Along those same lines... during the peak scavenging phase the larger exhaust CAN effectively pull more F/A charge in the cylinder and out the exhaust which then leads us back to #1.

3: Crown temps are higher with a raised exhaust. Anytime you have elevated crown temps, you increase your chances for detonation.

IMHO, there is no direct relation between cranking PSI and octane required. I wish there were.

For example.. one of my race engines (snowmobile) cranks only 140 PSI but I know that if I don't run at least 110 octane , the engine will not live for very long.

As we are beginning to realize... these PSI charts really give no relative information unless your engine is stock ported (and even then, I find that they are not useful)

This all relates to information needed when choosing a dome for your engine. I know that when you call up the head shop or go to the counter and ask the parts guy the conversation goes something like this:


YOU:
"So, if I want to stay with premium pump gas, which size domes should I purchase?"

THEM:
"Well, if you want to stay with premium fuel your cranking compression needs to stay below 160 PSI. These XXcc domes will put you right at that correct PSI for premium fuel"


I DO NOT AGREE WITH THIS LINE OF REASONING!


We see that the porting arrangement plays a MAJOR role in the resultant cranking compression. It is NOT only about the head!!!

Then.. You factor in the bore size in the equation and you REALLY can get into a pickle!!

So, you have to be very careful in choosing domes for your engine.

The head (dome) design itself, plays a large role in determining octane requirements of an engine. If the dome is not correct for the engine, you could be forced to run race fuel in order to avoid engine failure. OR on the other extreme, you may THINK your engine requires race fuel, when, in fact, it would live just fine on pump fuel. So, you could be throwing money and convenience right out the window.

[powermizer]

I see compleatly what your saying thats why I asked what you can get away with on stock porting.

I have built many big block chevs and what your saying holds true even on a cam and valve engine.

Like dubious 01

Just hoped to get a simple "I had pinging at 175" or "best at stock"

I was looking for somewhere to start.

Thanks though 100hp

[Roostius_Maximus]

the portwork i did in the cylinder and cases left the port sizes at the sleeve stock and in the stock locations, the exhasut port was stock width with the bridge thinner, opening widened and the port floor corrected. Intake side had a small amount of cleanup on the side ports, gained 1 hp + 7ftlbs 1000rpm earlier and 500rpm peak longer, takes 1 smaller pilot and 2 smaller on the main now, same carb, ignition, reed and pipe. same N casting head, but i changed to a coolhead to see what the hype is, if you work with any calculators to figure out the squish and dome height its almost exact what the coolhead 63cc comes out to, i run pump premium and can drop 1 main forsure when using c12 or a premium/u4.4 50/50 mix. always have used torco oils. I did no piston mods in this engine either. Dyno is posted in the cpi pipe thread.

Stalled the "stock" engine at 8600 on the dyno, ported turned 10000 and i let off. The engine hit 10000rpm in the time it took the "stock" one to hit 8000.
it now has a very linear power no rang and its the most excellent bike i have, yet . i ride it in the trails and hillclimb with it. youtube videos under 500bigbore