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Timing and Advance Curve for 2.5 Short Stroke

A long while ago, I posted in the engine rebuild forum in the interest of establishing what advance curve I should seek for the motor I was building. Here is the thread:
Ignition Timing for 2.5SS?

There was a lot of good information volunteered there, and lots of useful graphs of the various factory options. I wonder, however, if we really got to the bottom of the issue. My motor is a 2.5 short stroke, with measured compression of 9.7:1, S ports, E cams, and MFI. My distributor is a stock 2.4E with no vacuum being supplied to the retard diaphragm. The car seems to run pretty well (I'm still breaking it in), but it does take more effort to start from cold than it should. Timing is currently set at 2 degrees ATDC. This should result in a total advance of 28 degrees at 6000 RPM.

What is the consequence of the configuration of my motor? It has the bore of a 2.7, but the stroke of a 2.0/2.2. There is a lot of compression, and I use 92 octane gas. Should I try advancing it a bit? Should the distributor be recurved?

Thanks for any insight!
-Scott
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Old 05-13-2008, 11:56 AM
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I'm interested in the consequences of bore vs. stroke for ignition timing. As my motor is an unusual spec, I'm concerned that the information I have seen in many other threads concerning ignition timing may not be right for my application.
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Old 05-14-2008, 06:54 AM
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Scott, I would think that with your comp. of 9.7 having the distributor recurved to something like a total of 26 degrees at 6,000 RPM and about 2 degrees BTDC at idle should work fine IMHO. The degree or two of advance BTDC will help on the cold starts. After that a good Dyno will show you what is really going on with the timing and you can recurve from that test result also. I'm using an SC dist. with the Andial conversion for my twin plug 2.7 that has a comp. of 10.5. The curve on this dist. is 2 degrees at 1500 RPM and up to 24 degrees at 5700 RPM. I'm setting up a time with Jeff Gamroth @ Rothsport Racing this week for dyno time and will know what timing is best for my 2.7 short stroke. Hope this helps.
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Old 05-14-2008, 09:13 AM
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Mark-
Thanks for the response. I'd be interested to hear where you land after your visit to Rothsport. My motor is single plug, so I imagine I will need more advance than with you twin plug motor?
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Old 05-14-2008, 09:49 AM
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Scott,

JMHO of course, but your configuration may not need anything drastically different given the modest change in bore size, same cams, and small increase in compression. Stroke doesn't play a role here.

The main concerns are compression ratio, octane availability, and camshaft profile.

I think your timing is quite retarded for your configuration and thats why its hard to start. This also makes for hotter cylinder head temps, too.

Look for 32-35 deg total @6K and let the idle timing land where it will. Assuming that your distributor is operating properly, you'll likely be around 5-10 deg BTDC and that will improve both starting and a better idle.
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Old 05-14-2008, 09:59 AM
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Steve-
Thanks for the info. That is what I expected, but I sure wanted to be conservative. The recent thread concerning Jeff Higgins' motor made me pause to think. I'll bump it up a little and see how it does. Better starting and idle, and reduced cylinder head temperatures all sound like good things!
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Scott, Steve knows what's best. I'm just a little gun shy with today's fuel octane. The 71 S back in the day's of higher octane ran at 30 degrees @ 6K so with today's octane and E cams, and your higher compression I was thinking a little more on the safe side. 32 - 35 degrees OK. But what do I know. I'm just an old surfer beach bum.
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Old 05-14-2008, 11:35 AM
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Scott,

Steve and I are almost always on the same page. With pump premium (~92 octane?) fuel, I’m not comfortable with 35 at 6000 rpm, 9.7:1 CR and E cams.

I just left Steve a voicemail and will talk with him in the next few days.


What needs to be discussed here is the relationship between spark timing and many other factors. On the ignition side there are; type of CDI, the capacity of the high voltage system to see very high voltages and currents, spark plug type and gap, single or twin ignition and more.

On the engine side are: the bore size, combustion chamber shape (piston & head combined), mechanical compression ratio, cams etc. (dynamic compression ratio) and more.

On the driving side are: fuel octane, temperature, part throttle operation, various rpm, transitions, idle & just off idle, starting from cold, starting from stop and more.

This is a VERY complex subject.

A distributor advance curve is a compromise between competing requirements. This is why Porsche now uses computer controlled fuel and spark management. We need to pick the best compromise with our antique mechanical systems.

The good news is that it is doable.

An important technique here is having access to very high octane race fuel (~112 octane). If you temporarily raise the octane and performance changes, something else needs to change (advance, etc.)

Best,
Grady
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Old 05-14-2008, 04:26 PM
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Grady-
Thanks for your thoughts. I'll fill in a few blanks: Ignition is single plug, with an early, blue anodized Permatune CDI box. Spark plugs are NGK BP8ES. Gap is .63 mm. Fuel is 92 octane, with 10% ethanol per Oregon state law. The car will mostly see use in the 50-80 degree range.
As an interim adjustment, would you agree with Steve in that my current setting of 2 degrees ATDC @ 900 RPM is too retarded? If I bump it up to 32 degrees BTDC @ 6000 RPM are you comfortable (+-2 degrees BTDC @ 900 RPM)?

Last edited by Scott Clarke; 05-15-2008 at 06:44 AM..
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Quote:
Originally Posted by Grady Clay View Post

. On the ignition side there are; type of CDI,


old Smokey Yunick text where he says a good ignition spark should have long duration and high amps.

he also stated that manufactures are reluctant to sell a product that may potentially be fatal.
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Old 05-15-2008, 07:28 AM
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Scott,

One of the interesting features of this is the function of spark timing for idle stabilization and taking off from a stop.

I’ll be the first to admit that I paid way too little attention to this 40 years ago. We tend to think of the ignition advancing from some number at idle, through various rates-of-change to some fixed maximum. At idle and below there are important functions.

There is the situation at idle where the engine misses once, the idle drops and the system slightly advances the timing to bring the idle back to norm. This same feature is involved with advancing the timing when you start from a stop and the idle goes low and/or the vacuum under the throttle changes.

The vacuum advance & retard system lends itself to much tinkering to help our 911s be more ‘drivable’.

YES! For best performance there is an ‘ideal’ advance curve for full throttle performance. Our typical driving isn’t that. This means there is always a compromise depending on the various driving circumstances. When Porsche built our cars new, they had to deal with all these issues, including cost and eventually emissions. Even our carbureted & MFI race cars have to start, run at ‘idle’ and take off from a standing start. Can you imagine a 917 or 911R stalling on the start of the LeMans 24 hour?

I can tell first hand that our old Porsche race engines perform better at part (30%) throttle than full throttle when ‘off cam’ and during transition of power. Part of that is ignition timing. This is why Porsche uses ‘drive-by-wire’ to control throttle position, spark timing, mixture, turbo vanes and more.

For this discussion we need to address all the issues that CAN be done with our old mechanical systems. This is also a good basis for understanding how to apply computer based electronic systems.

Best,
Grady
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it seems that the basic issue is generating piston power at the most efficient deg of stroke. Power comes from combustion.

In an ideal world you program a computer info derived from monitoring combustion with optimum timing adjustments or have sensors of some sort doing the adjusting.

Mechanical centrifugal advance is a compromise.


as a side note I couldn't detect a noticable EGT difference when setting initial advance at 5 or 10deg BTDC. [2.7 Euro/RS/Carrera dizzy 10 BTDC = 35 total] without touching slightly rich carb settings. So far I can't detect these engines to be timing sensitive in creating power. Maybe if the carbs were leaned a bit?
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