I thought 7.0--7.5 dynamic c.r. was ideal for a street engine.

 

From what I read, it's the little edges, ridges, bumps, casting flash etc. of a non-polished chamber that can create hot spots, like a fire poker tip, in a high performance application. These can ignite the fuel before the spark goes off. By polishing the chambers, you remove this possibility.

It'd be fun to implement something like this. I agree, it'd be nice to just be able to rework some of this stuff, but redesigning the head for better flow is a little out of my reach. (It's too bad that classic inlines never got their aluminum head finished).

Granted, I'd rather just play it safe and keep it nice and streetable.

 

That's kinda what I'm reading.
7.0 - 7.5 for 87 octane.
7.5 - 8.0 for 89 octane.
8.0 - 8.5 for 91 octane.

Not sure how accurate that is, but it seems like a good ballpark figure.

https://www.rbracing-rsr.com/comprAdvHD.htm

This calculator, which uses the straight up numbers off of Comp's site (the 56° for the Comp 260), also calculates my Dynamic CR at 7.6. However, at 2500 feet elevation, where I'm at, it says I'm at 7.2. Maybe I'll just have to put a little mid-grade in when I visit my wife's parents in California. =P

Otherwise, I only go up in elevation. At 9000 feet (about the highest I've been with it) is down to around 5.6.

 

Great! Which piston/head combo is that using? And I'm assuming it is with the Comp 260 cam? (I don't know how they call that cam a 260, when other cams called 264, 263, actually have less duration! lol.) It is a monster sleeper cam. My only criticism of that cam is that it doesn't lope at idle!! 'Now how can you be cool when you cruise the doughnut shop if the engine doesn't lope a bit? ha ha.'

 

That's with 9:1 compression. The combo of parts doesn't seem to matter in the slightest when it comes to dynamic compression ratio.

From what I'm finding, it's based off of:

Static compression ratio
Connecting rod length
Cam valve closing angle

That's seems like that's it. The combination of parts to get the compression ratio appears to be irrelevant.

I've heard the 260 is a good cam. Just a nice, all around, does everything well. Not the extra torque and economy of the 252, but still torquey and economic. Not the high end power of the 268, but still extra high end power.

But yeah... smooth idle. heh.

It would be pretty cool to go with the 268 and actually get some lope. People know you're serious then.

 

lol! Wait a minute. "The combination of parts to get the compression ratio appears to be irrelevant."

The DCR is based on, or calculated with, the SCR, so therefore the SCR is the foundation used to determine the DCR, and the parts used to create whatever SCR are all players in that formula. If that is so, then the parts combo used to get the static cr are all factors in determining the DCR, and anything but irrelevant. Change any of those and the SCR changes, hence the dcr changes, right?

The grind of the cam determines when the intake valve is seated in relation to where the piston is on the compression stroke. Using that info the DCR is calculated.

 

Oh, no no. I only meant that the particular combination of parts that you used to achieve your desired SCR is irrelevant to the DCR.

If you used a 74cc head with 19cc pistons to get 9:1, or if you used 70cc heads with 23cc pistons to get 9:1, it doesn't matter.

That was all I was getting at. Nothing more.

Once your SCR is calculated, you use that number with the rod length, stroke, and cam timing to get the DCR.

Connecting rods/stroke matter since a longer rod with a shorter piston height moves the piston through the cylinder at a different rate of speed than a shorter rod with a taller piston height.

Granted, if you start changing your connecting rod length and engine stroke, then that's a different story.

 

Oh, phew! lol I thought maybe I had misunderstood DCR completely.

 

I switched from 110 RON to 112 with good results.

 

I'm not sure what this is. Can you elaborate?

 

Lol, okay, nevermind. Research Octane Number.

112 octane is a touch higher than I want to go.

What compression are you running out of curiosity?

 

13.4:1 on the S/P altered.

 

I was wondering about something concerning your cam. I know comp calls it a 260* duration cam, and that is the number you're using when calculating the DCR, but their 260 actually has more duration than the Howard's cam I was looking at, and Howard's calls theirs a 263*. So, with that in mind, how can your equation to find DCR be accurate? Maybe you'd be better off use the a 264 duration # to calculate the DCR?

Maybe that was why discrepency between your final number of degrees and that from comp?

 

To my knowledge, there is no such term. You're thinking of pre-ignition, which is what it sounds like: the fuel/air mixture is lit off way early, before the arc-over of the sparkplug. Yes, this is usually caused by something glowing-hot, and yes, I've also heard that those bumps and edges can do this, and I used to accept it, long ago. Now, from decades of welding, I doubt the likelihood (though I suppose it occasionally happens) of the edge of a piece of the combustion chamber getting hot enough to glow; the casting is too much of a heat-sink, and it's water-cooled. I'd think it much more likely that a flake of carbon, the sort of thing often seen when you tear an old engine down, starts glowing and lights off the F/A mixture.

To the extent that polishing the head surfaces and edges might tend to lessen carbon build-up, that could reduce the chances of getting into pre-ignition.

What you're trying to avoid when figuring how high you can safely go with compression is detonation. The mixture gets squeezed so tight, and therefore gets so hot, that when when it lights off it burns far faster than the normal flame-speed, almost an explosion. The mixture goes off so fast that it hammers the piston/rod/bearing and head, so hard it can shatter the sparkplug ceramic or blast a big hole in the top of the piston. In fact, if you tear down a bad engine and find a hole in a piston, if the edges of the hole look melted, that was pre-ignition, but if the edges are sharp, that was the hammering of detonation. I happen to have, as show-'n-tells, two Datsun L-16 pistons with holes, and they happen each to be perfect textbook examples of pre-ignition and detonation holes!! Just to confuse things, either condition can get the other one going.

(There must be a hundred internet articles explaining pre-ignition and detonation. Be skeptical of the guys who describe detonation as a consequence of "colliding flame-fronts," oft-repeated, but it's bull).

 

Thank you for that explanation, Smitty! Yes, I was blending the two terms. Seems both in their name and description.

That makes a lot of sense on the pre-ignition and detonation and clears a lot of things up.

Seems it's detonation that I'm trying not to encroach upon, which porting and polishing isn't going to do anything about.