Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Technical BBS > 1- Porsche Technical Forums > 911 Engine Rebuilding Forum

Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Moderator
 
304065's Avatar
 
Join Date: Dec 2001
Posts: 9,435
Advance curves for Modified Engine

Ok, after speaking with the experts I will be using a cam grind that is an improved form of the Solex. The cam grinder's recommendation was to begin with an advance profile that is equal to or less than the advance curve of the original Solex engine, due to the the increased cylinder pressures associated with greater valve lift and slightly higher compression than Solex, 9.5 vs. 9.0.

The question then becomes, what advance curve is the best? Obviously the final curve will be dialed in using a dyno. But for starters, here are all the popular advance curves plotted on the same axes. Really puts it in perspective, no?

__________________
'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 03-27-2007, 12:12 PM
  Pelican Parts Technical Article Directory    Reply With Quote #1 (permalink)
Registered User
 
Join Date: Jul 2001
Location: Portland Oregon
Posts: 6,810
Mister C:

One very important variable not included in your question is octane,...

What sort of juice will you be using?
__________________
Steve Weiner
Rennsport Systems
Portland Oregon
(503) 244-0990
porsche@rennsportsystems.com
www.rennsportsystems.com
Old 03-27-2007, 03:59 PM
  Pelican Parts Technical Article Directory    Reply With Quote #2 (permalink)
Moderator
 
304065's Avatar
 
Join Date: Dec 2001
Posts: 9,435
Dr. Ing. Weiner,

No better than 93 R+M/2 I'm afraid. As I'm no longer based on an airport it's become much harder to obtain 100LL moose juice.
__________________
'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 03-27-2007, 06:32 PM
  Pelican Parts Technical Article Directory    Reply With Quote #3 (permalink)
Registered User
 
Join Date: Jul 2001
Location: Portland Oregon
Posts: 6,810
Professor Von Cramer:

You don't wanna be using Sky-Juice anyway; not enough of the RON-type octane and far too much lead,..

For a Solex-cammed 2.0 running on today's swill called motor fuel, I'd be looking at the 2.4 (purple) curve and limiting max advance to 34-35 degrees total all in by 5200-5500 RPM.

Its important to remember that we must tailor these to the worst possible conditions; summertime use in hot weather, knowing full well that the motor can stand more advance when the outside temps are below 50 degrees.


Your obedient servant,
__________________
Steve Weiner
Rennsport Systems
Portland Oregon
(503) 244-0990
porsche@rennsportsystems.com
www.rennsportsystems.com
Old 03-27-2007, 06:58 PM
  Pelican Parts Technical Article Directory    Reply With Quote #4 (permalink)
Moderator
 
304065's Avatar
 
Join Date: Dec 2001
Posts: 9,435
Great and Noble Guru from the Beaver State,

All I need now is somebody with the right equipment and 30 years of experience who is located in the 503 area code to recurve my collection of cast-iron distributors. I have an idea who to call.
__________________
'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 03-28-2007, 05:16 AM
  Pelican Parts Technical Article Directory    Reply With Quote #5 (permalink)
Porsche Junky
 
mede8er's Avatar
 
Join Date: Nov 2004
Location: Salida, CO
Posts: 1,804
Garage
I'm also going with that 2.4 curve on my 3.0 project....
__________________
1986 930 RUF equipped
Old 03-28-2007, 06:22 AM
  Pelican Parts Technical Article Directory    Reply With Quote #6 (permalink)
Registered User
 
jluetjen's Avatar
 
Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,592
Garage
John;
Why not just weld up the dizzy and get a programable ignition controller like the Emerald ECU M3DK. I haven't gone through all of the electronic ignition options lately, but if all that you want to do is program the ignition, it seems to be one of the most economical options. And unlike a mechanical advance on the dizzy, you can actually map this based on temperature and such that will allow you to keep the timing closer to optimum.
__________________
John
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman
Old 03-28-2007, 10:28 AM
  Pelican Parts Technical Article Directory    Reply With Quote #7 (permalink)
Moderator
 
304065's Avatar
 
Join Date: Dec 2001
Posts: 9,435
John,

An excellent point. My concern is that the rotor may not be in the right position for optimum transfer of the spark from the center electrode to the cylinder electrode at the extreme ends of the advance range. I started thinking about this when somebody proposed that they lock out the mechanical advance and use a programmable controller to vary the timing.

A couple precedents for my line of thinking, from the Factory: the "advance" mechanism on both the Motronic Carrera and inside the 964 distributor, which does not dictate the timing but it "phases" the rotor so it's in more or less the right position so the spark doesn't have to jump too far.

We know the distributor rotates clockwise when viewed from the pulley end of the engine. The arrow says so. (This point of clarification is for you SC guys!)


Now, if the distributor advances 30 degrees crank (15 degrees in the distributor) and you assume equal sectors (the red and white) where the rotor is alternatively in firing position and in between, with equal-size sectors on either side for insulation, isn't there a risk that the voltage required to jump backwards to the cylinder electrode increases dramatically? I don't doubt that the ignition system can handle it, but can the cap handle the heat?



I'm not saying it won't work, but has anyone run a locked-out distributor with an electronic timing control with good results?
__________________
'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)

Last edited by 304065; 03-28-2007 at 12:10 PM..
Old 03-28-2007, 12:04 PM
  Pelican Parts Technical Article Directory    Reply With Quote #8 (permalink)
Moderator
 
304065's Avatar
 
Join Date: Dec 2001
Posts: 9,435
A fine thread on the subject with some actual measurements from an SC cap.

I get full mechanical advance at 3000rpm?
__________________
'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 03-28-2007, 12:43 PM
  Pelican Parts Technical Article Directory    Reply With Quote #9 (permalink)
Registered User
 
jluetjen's Avatar
 
Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,592
Garage
Just thinking out loud....

1) The distributor has 360 degrees and 6 leads, so that's 60 degrees per lead, but only 30 if you do as you describe and assume an equal arc for insulation purposes.

2) Assuming that you've got 30 degrees at the dizzy, isn't that 60 crank degrees.

3) If this is the case, you should set the dizzy so that with no advance the rotor is pointed at the tail end of the active arc, so let's say 10 crank degrees retarded from straight-up.

4) If you do this, won't you have at least 40 degrees (more like 50 if you don't violate your designated arcs) of advance available before the rotor will be too advanced to allow a clean fire.

Question: How many degrees wide is the rotor tip?
__________________
John
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman
Old 03-28-2007, 01:06 PM
  Pelican Parts Technical Article Directory    Reply With Quote #10 (permalink)
Moderator
 
304065's Avatar
 
Join Date: Dec 2001
Posts: 9,435
1) Yes, and the red sectors are 30 degrees each (there should be 12).

2) Advance/retard is measured at the crank. The distributor turns at half crankshaft speed. Therefore, 30 crank degrees is 15 distributor degrees. We know this looking at the factory advance curve chart, an electronic verison of which I posted above.

3) SC rotor tip 11.5mm wide. Cap contact 5mm wide. Inside diameter of cap 70mm (SC caps are bigger). 70mm diameter = pi * d = 220mm divided by 6 = 36 mm between contacts. So a 5mm cylinder electrode centered in a 36mm space gives 15.5mm on either side. 5mm in 220 is .02273 of the total time 360 is 8.18 degrees. The rotor width is 11.5mm and the clearance from the contact is 1mm, so subtract 2mm in diameter, do the same math and you get 19 degrees for the rotor width.

Summarizing: Contact = 8 degrees, rotor = 19 degrees, sector = 60 degrees wide, with rotor centered. We need a diagram.
__________________
'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Old 03-28-2007, 01:29 PM
  Pelican Parts Technical Article Directory    Reply With Quote #11 (permalink)
Registered User
 
jluetjen's Avatar
 
Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,592
Garage
Here's how I see it... (I tried sketching it on some CAD software that I have, but I'm not good enough to pop out a drawing quickly)

1)The total amount of time that a portion of the rotor will overlap a portion of the cap is: 19 + 8 degrees or 27 degrees of dizzy rotation or 54 degrees of crank rotation.

2) Compared to perfect alignment between the rotor and the cap, the most advanced that you could have the rotor is (9.5 + 4 degrees or) 13.5 dizzy degrees or 27 crank degrees while still having the minimum amount of overlap between the two.

3) So you set up the distributor initially, you will want it to be retarded 27 crank degrees from straight on alignment between the rotor and the cap. From this point, you use the electronic timing computer to advance the timing up to 54 crank degrees as needed from there.
__________________
John
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman

Last edited by jluetjen; 03-28-2007 at 03:51 PM..
Old 03-28-2007, 03:06 PM
  Pelican Parts Technical Article Directory    Reply With Quote #12 (permalink)
Registered User
 
Join Date: Jul 2000
Location: So. Calif.
Posts: 18,048
Good insight here with the rotor angle. I think you'll find that 15 dist. advance (30 crank) represents a relatively small arc of the rotor relative to the spark plug wire spigots in the cap. Typically, the ignition system is most taxed (spark requirements are the highest) when the engine is under a load, and that usually happens at WOT and at relatively low-medium engine speeds.

I just want to add that whatever distributor advance is decided, that total must be added to the initial static timing (at 0 rpm or idle).

For example:
If dist. advance total is 30 and static timing is 5BTDC, total is 35BTDC, etc.

If dist. advance is 35 and static timing is 5ATDC, then the total is 30 BTDC.

Sherwood
Old 03-28-2007, 03:19 PM
  Pelican Parts Technical Article Directory    Reply With Quote #13 (permalink)
Registered User
 
Join Date: Mar 2006
Posts: 1,107
John wrote,

3) So you set up the distributor initially, you will want it to be retarded 27 crank degrees from straight on alignment between the rotor and the cap. From this point, you use the electronic timing computer to advance the timing up to 54 crank degrees as needed from there.

John,

A timing computer or ECU cannot see into the future and anticipate the trigger signal to "advance the timing". Programmable ignitions can only delay the ignition firing from the trigger signal, essentially lowering the amount of retard as rpm increases. Most ECU's have you set up the ignition trigger signal outside the full advance range, and your program reduces the amount of retard to "advance the timing". On a Haltech system you set up the distributor to trigger between 60-90 BTDC, then use a timing synch setting to verify 10 BTDC on the engine data page is actually 10 BTDC with a timing light. You check rotor phasing with a cut up cap at the center of your advance range. If you have a problem, you can adjust it by altering the trigger setting or the point at which the rotor is locked.
__________________
Paul
Old 03-29-2007, 05:18 AM
  Pelican Parts Technical Article Directory    Reply With Quote #14 (permalink)
Registered User
 
jluetjen's Avatar
 
Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,592
Garage
Paul;
You're talking about the trigger signal, I was only talking about the rotor and cap alignment. I guess I wasn't clear, but I would assume that you've got a timing wheel mounted somewhere to drive the ECU. I've heard of some ignition systems that do use the distributor signal for timing, but with 6 clock signals per rev versus a timing wheel which will have something like 32 or 64, there's no question regarding the loss of accuracy resulting from using the dizzy for a clock signal.

But if you were, then I guess you'd just reverse what I suggested and set the dizzy at something like 27 degrees advanced, and then retard the signal as needed.
__________________
John
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman
Old 03-29-2007, 05:59 AM
  Pelican Parts Technical Article Directory    Reply With Quote #15 (permalink)
Registered User
 
Join Date: Mar 2006
Posts: 1,107
No, I am talking about firing the ignition. No timing computer or ECU can operate with the set up you suggested. The distributor must be mechanically set beyond your max advance for the computer to control the firing of the ignition at the right time.

The Emerald M3DK is a distributor triggered system. There is nothing wrong with a distributor signal for an ECU ignition, it is routinely used for 200 hp/lt Honda engines. If you are going to the bother of fitting a crank trigger, you should go to a waste spark or COP set up and forget the distributor. As an alternative to recurving a worn out mechanical advance, a distributor based ECU is an definite improvement and the rotor phasing problems are largely imaginary with timing between 10 and 35 BTDC.
__________________
Paul
Old 03-29-2007, 06:54 AM
  Pelican Parts Technical Article Directory    Reply With Quote #16 (permalink)
Registered User
 
jluetjen's Avatar
 
Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,592
Garage
I stand corrected.
__________________
John
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman
Old 03-29-2007, 09:13 AM
  Pelican Parts Technical Article Directory    Reply With Quote #17 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 

Tags
901/05


All times are GMT -8. The time now is 04:09 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2016 Pelican Parts - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.