10.5 without twin plugging.

[Jeff Higgins]

Maybe "rotor synchronization" would be a better term, then?

[haycait911]

Originally Posted by 304065

Rotor phasing (not sure why Sherwood suggests that I review that thread, I am one of its authors ) is the use of the centrifugal advance weights to move the rotor tip under the appropriate distributor cap terminal at the time of firing. In older cars where spark timing is established by the movement of the rotor, these things occur together. In newer cars where a crank trigger signals the DME and the DME fires the coil, even though the advance mechanism doesn't determine the timing, it's still present to position the rotor approximately in the right place. The factory felt this was sufficiently important to put it in the workshop manual.

Therefore, locking a 2,4 distributor won't work, because the spark will either jump to the wrong terminal, or have to jump a larger or smaller gap resulting in inconsistent ignition performance, just when you don't want it, at high RPM under load.

The only way you would be in phase with a locked distributor would be if you ran at a constant RPM (like one of those German Air Force stationary 356 engines! )

to say I'm confused would be an understatement

this is from the MSD website.................

The new ignition, Part Number 6530, is based on the same output as the 6AL-2, but rather than rotary dials for rpm adjustments it sports a serial port that connects to a PC. Once you load MSD’s Windows based software on your PC, you’re ready to start mapping and programming. To start, you can get rid of the weights and springs that control the mechanical advance of your distributor. Go ahead and lock it out because you can now create a timing curve that allows you to manipulate the timing down to tenth of a degree increments every 100 rpm. The advantage is precise timing control, with the ability to ramp the timing in or retard it at exactly the rpm that you want it to move. Want more timing out for a start retard? Simply click the mouse and move a couple dots in position. Like a high speed retard? Clickity-click!

what am I missing here?

[304065]

Yep, that's what MSD says.

Here is what Porsche says.

[piggdekk]

Haycait,
If I'm not wrong the MSD unit only allows a very limited amount of timing change. I think maximum 16 degrees or so. For our motors it is out of question since we would need about 30 degrees of range.
luca

[911pcars]

Originally Posted by piggdekk

Haycait,
If I'm not wrong the MSD unit only allows a very limited amount of timing change. I think maximum 16 degrees or so. For our motors it is out of question since we would need about 30 degrees of range.
luca

Need to know if MSD refers to distributor degrees or crank degrees; e.g. 15º of rotor/shaft rotation is 30º of crank rotation. And this doesn't include the initial static timing adjustment which is added to the total advance curve.

S

[Porschist]

Originally Posted by Henry Schmidt

We built this engine last year.
Peanut chamber heads were fabricated form standard 2.2 "T" heads.
The pistons were a set of 8.5:1 Mahle MFI 90mm. (RS)
With .050" deck we produced 11.3:1 compression.

Henry,
Very interesting heads. Are these your product/modified by you?
And how did they perform in the end?

[dipso]

Quick update. I stopped by the mechanics today and we talked a little before he had to rush off.
He said he has the pistons, mentioned something about it being easier to lower a 10.5 than to raise a 9.5.
He said that with all the work done to the heads, and then by the time the cylinder gets spaced out we should be in the 9.8 or 10 range.
After he left I was looking around and think I saw my cams in a box. They had a #296 grind on them, they are webcams.
I think they are a 993SS or 930 SS. I don't know, I just did some research and couldn't find out much more than that.

We had a quick talk about 91 pump gas and 10.5 vs 9.5. He agrees that 10.5 is too much, but swears that with my distributor, cams, msd, carbs, headwork, the reduced 10.5, probably actual 9.8 ish will work great.

We'll see soon.

[Walt Fricke]

Dipso

I hope the "we'll see" means that when all the parts are in, the mechanic will break out his graduated burette and other measuring equipment and actually measure your CR?

One fairly simple method is to trial assemble at least one cylinder (no cam needed), put the piston at TDC exactly, and decant Marvel Mystery Oil or whatever he likes to use for this out of the burette into the spark plug hole. I used a junk spark plug to secure a piece of clear plastic tube sticking up from it so I could accurately account for how much fluid rose above the plug hole and subtract that from the total which went in.

If you know the bore and stroke (and you do), and measure the head volume with the piston at TDC in it with the cylinder base gasket/spacer in place, you can easily calculate the CR. The formula is cylinder displacement plus measured volume divided by measured volume.

I know some of the engine builders won't trust anything but a lot of spearate parts measuring and plastic cover plates with a filling hole and so on, but this seemed repeatable and gave me confidence.

[dipso]

Problem solved! I just ordered a twin plug distributor and a lower plug set up kit.
So much easier than asking all these stupid questions.

[toddu]

Which distributor did you go with?

Todd

[dipso]

Another JB Racing.

Which distributor did you go with?



Todd

[dipso]

Just got back from the mechanic. My cams are in and pistons are in. The cams are webcams Web Cam Inc. - Performance and Racing Camshafts / Porsche 911 SOHC 12v Timing Card

The pistons are 11 to 1 compression JEs.

Twin plugged.



Sounds good.

[911pcars]

Henry,
Curious. Is twin plugging still a requirement with the more compact squish area?

Sherwood

[Henry Schmidt]

Originally Posted by 911pcars

Henry,
Curious. Is twin plugging still a requirement with the more compact squish area?

Sherwood

Twin plugs were required because this engine configuration produced an astounding 11.3:1.

The squish chamber reduces the propensity to detonate but can not overcome low fuel quality. We always build with questionable/ inconsistent fuel quality in mind.
When the green people figure out that old cars can't run on low octane fuel, I predict that fuel octane will plummet.

[Lukesportsman]

I have to admit that I never thought through the rotor phasing thing before. I have a crank fired race engine that I still fire through the locked down distributor and digital 7AL box. Now I'm talking some serious piston speed and pressure with 4.5" stroke and shift points of 7600 w/ 14.5:1. Wonder what is really going on in there??? eeks

You guys create problems (lol) that us mere mortals never thought about

[911pcars]

Originally Posted by Lukesportsman

I have to admit that I never thought through the rotor phasing thing before. I have a crank fired race engine that I still fire through the locked down distributor and digital 7AL box. Now I'm talking some serious piston speed and pressure with 4.5" stroke and shift points of 7600 w/ 14.5:1. Wonder what is really going on in there??? eeks

You guys create problems (lol) that us mere mortals never thought about

Inspect your rotor tip and see how wide it is. 15º of rotation (30º crank) isn't much. With a locked down advance mechanism and at full advance, the rotor tip may not be that far from the plug wire contact to make a difference. Confirm to be sure.

Sherwood

[Steve@Rennsport]

Originally Posted by Lukesportsman

I have to admit that I never thought through the rotor phasing thing before. I have a crank fired race engine that I still fire through the locked down distributor and digital 7AL box. Now I'm talking some serious piston speed and pressure with 4.5" stroke and shift points of 7600 w/ 14.5:1. Wonder what is really going on in there??? eeks

You guys create problems (lol) that us mere mortals never thought about

Hi Luke,

We see rotor phasing quite clearly using our Sun 504 distributor machine and a cut-away distributor cap. Our machine cannot spin it past 4K distributor RPM (8K crank RPM), however its sufficient to see what goes on.

You can do the same thing and see for yourself so you'll know if you have a problem or not.

Remember, these digital ignition boxes were all designed with late-model American distributors in mind with their wide terminal spacing and wide-blade rotors. This is how they alter timing to each cylinder on NASCAR engines.