Advance curve on E engine with mod-Solex cam?

mskala · 09-27-2010, 10:41 AM

I recently got a rebuilt long-block upgraded to 2.2 E-spec engine except cams
were T cams. So I got DC-30 (mod-Solex) cams, and got a rebuilt E distributor.

Seeing the chart of all the different advance curves from an earlier post (I'll find and
copy chart in a bit) I'm worried that with the E-curved distributor and solex-style
cam it's going to cause damage. Also, so far it seems setting the timing to 30
deg@6K results in not enough advance at idle speed.

Any proven curve to use with a setup like this?

Thanks,
Mark S

911pcars · 09-27-2010, 12:28 PM

What's the current timing at idle? 30ºBTDC is max for an S engine. I think you're pretty safe going a couple of more degrees added to the bottom end. Spark plug heat range? Not too hot.

Don't let the engine oil temps exceed recommendations (~225ºF), 210ºF should be fine. Assume you have the 11 blade fan, proper pulley size, recommended oil, etc., etc. You may need an external oil cooler to keep things within recommended temps.

Sherwood

mskala · 09-27-2010, 06:22 PM

This is the chart of most of the early engines I saw in earlier threads.

mskala · 09-27-2010, 06:29 PM

Thanks Sherwood but I'm looking something a bit more specific. Basically
avoiding risk of detonation or getting hot from improper timing. I have the
old-fashioned timing light so right now I can't tell you idle advance number.

It's in a 914 so not very easy to play with.

Trackrash · Trackrash's Garage

What's your compression ratio? That has a lot to do with what total advance you want. Higher compression, less total advance. Early Es had a CR of 9.1:1 with a total advance of 30 degrees. The T motor with 8.6:1 CR had a total advance of 35 degrees. This is + - 3 degrees IIRC.
Initial timing on the E should be 5 degrees BTDC.
Are you running carbs or MFI? The only way to fine tune is on the dino. There are some who can re-curve your distributor and maybe get it close if you give them your motor's details.

mskala · 09-27-2010, 08:43 PM

It should be stock 2.2 E at 9.1 to 1 compression. I'm running Webers.
You can see that the E/S curve is way more advance in mid to high RPMs
than the original solex cam 911.

cmcfaul · cmcfaul's Garage

I believe idle timing for the E and S is 5 ATDC

911pcars · 09-29-2010, 11:15 AM

Quote:

Originally Posted by cmcfaul

I believe idle timing for the E and S is 5 ATDC

I believe it's BTDC.

Sherwood

cmcfaul · cmcfaul's Garage

On a 2.4 E and S it's 5 degres ATDC with the vacum retard attached. Might be different with the 2.2 though I thought the engines were basically the same just stroked to 2.4 liters

Chris
73 911 E

911pcars · 09-29-2010, 12:06 PM

Quote:

Originally Posted by cmcfaul

On a 2.4 E and S it's 5 degres ATDC with the vacum retard attached. Might be different with the 2.2 though I thought the engines were basically the same just stroked to 2.4 liters

Chris
73 911 E

On early S engines, there's no vacuum advance or retard. Not sure about Es.

At any rate, vacuum retard only operates at idle. At off-idle, timing reverts to mechanically preset (distributor position) and mechanical advance curve.

Total advance = Ignition timing @ idle (vac. disconnected) + mechanical advance.

Sherwood

304065 · 09-29-2010, 01:04 PM

Vacuum retard didn't come in until the 2,4. Well, the 68 had it for the Smog system but that is a very special case.

Let me first take a moment to reflect on what a handsome person must have authored the graph above. . .

30 degrees at 6,000 rpm may leave you with retard at idle, you probably have a chugging idle at around 500 rpm, right? And with those cams that low an idle can be rough.

So set it to 35 degrees total advance at 6000 rpm, not a half a tick more. That should give you reasonable advance at idle. If idle speed is too fast it's because your advance is kicking in. If you can't get the idle down with the advance below 35 degrees it means the springs and pivots have fatigued and you have to get the distributor rebuilt.

That's really the thing to be afraid of-- if the distributor is worn out, and you set it so it will idle at 900 rpm, then when you rev it to 6,000 RPM you'll end up with 45 degrees total advance and will blow it up.

So, as Sherwood said above, set it up high, don't get anything caught in the moving fan, and then check it down low. If it's within a couple degrees of TDC you will be OK.

The mod solex cams allow better cylinder filling at low RPM, so you will have higher cylinder pressures in the lower rev range. The E distributor curve is probably fine, however, given the intake valve closing point.

Tyson Schmidt · 09-29-2010, 02:03 PM

Quote:

Originally Posted by 304065

The mod solex cams allow better cylinder filling at low RPM, so you will have higher cylinder pressures in the lower rev range. The E distributor curve is probably fine, however, given the intake valve closing point.

I'm not sure I agree with this part of your post.

The mod-Solex will have a wider lobe center and an intake valve that is closing later than the E cams. Therefore the dynamic compression ratio will be lower. So these cams will like a more aggressive advance curve to fill in the bottom end.

304065 · 09-29-2010, 03:18 PM

Quote:

Originally Posted by Tyson Schmidt

I'm not sure I agree with this part of your post.

The mod-Solex will have a wider lobe center and an intake valve that is closing later than the E cams. Therefore the dynamic compression ratio will be lower. So these cams will like a more aggressive advance curve to fill in the bottom end.

I consider it an honor, and I'm being completely serious, that you are actually READING that part. . . been a long time since we slammed beers in NYC with James Shira. . . but I digress. . .

I thought that Mod Solex had the same LC as the E? Camgrinder's web site shows a 102 lobe center for the Mod-Solex and a 102 for the E. I'm assuming that the ramp is quicker to get the Mod-Solex valve open faster than the E cams, and total lift is higher at .455" (gotta get John to go metric one of these days) vs. the .405" total lift of the E cam.

I hear you about the intake valve closing point, you're right. If the intake closes later then dynamic compression will be lower and cylinder pressure will be lower, so you can use more advance.

What we really need is for somebody with a cam doctor to put all the old Porsche cams on it and publish the data, then we can run them through a dynamic compression spreadsheet (Kenik built one that was pretty cool) to figure things out.

Hope you are well, I saw Jack's post about how you helped him put in the engine, I still think moving the engine forward two inches in the car was a feat yet to be widely duplicated. . . some might say. . . EVIL?

mskala · 09-29-2010, 05:40 PM

Thanks for the tech info guys, the internal part of internal combustion is not my area.

My E distributor is fresh from rebuild, but I of course haven't plotted it out myself (yet).
The engine does have difficulty getting off idle if I set it to 30 degrees. So I'll probably
fix it so the total advance is lower so I can avoid turning it to >30 degrees.

911pcars · 09-30-2010, 10:38 AM

Quote:

Originally Posted by mskala

Thanks for the tech info guys, the internal part of internal combustion is not my area.

My E distributor is fresh from rebuild, but I of course haven't plotted it out myself (yet).
The engine does have difficulty getting off idle if I set it to 30 degrees. So I'll probably
fix it so the total advance is lower so I can avoid turning it to >30 degrees.

Perhaps you're chasing the symptom incorrectly. Does the car act more normal if the ignition is adjusted differently..... or no difference? Perhaps carb settings (e.g. accelerator pump, etc.)

Sherwood

mskala · 09-30-2010, 04:42 PM

Quote:

Originally Posted by 911pcars

Perhaps you're chasing the symptom incorrectly. Does the car act more normal if the ignition is adjusted differently..... or no difference? Perhaps carb settings (e.g. accelerator pump, etc.)

Sherwood

I think it's unlikely to be carbs as they worked fine recently on my last engine and I know how to adjust them. Things just work better down low if advanced past the appropriate setting. Below 3K it's lumpy at low throttle, idle is too slow, and it's too finicky getting going from a stop smoothly.

911pcars · 09-30-2010, 05:32 PM

Quote:

Originally Posted by mskala

I think it's unlikely to be carbs as they worked fine recently on my last engine and I know how to adjust them. Things just work better down low if advanced past the appropriate setting. Below 3K it's lumpy at low throttle, idle is too slow, and it's too finicky getting going from a stop smoothly.

Okay. So you have a confirmed fix for this, but don't want to exceed the max. ign. timing.

What initial advance setting seems to work well? For example, if it's 5ºBTDC, then make sure the total advance doesn't exceed X degrees. With a stock engine, I would guestimate that at approx. 32º BTDC given "normal" operating and ambient temperatures, sea level altitude, proper mixture and spark plug heat range, etc.

However, if total advance exceeds the "safe" max. (you don't want unaudible detonation at high rpms), limit the total mechanical advance setting of the distributor while keeping the initial ign. timing for the desired engine response. If there's no one around you able to do this, I believe Steve Weiner has the dist. machine and knowledge to set this up for you.

The most through method of determining the ideal ign. advance throughout the rpm range is to adjust the timing through the rpm range on a chassis dynamometer, then record max. torque/HP for each tested rpm (e.g in increments of 500 rpm) all the way to an rpm beyond distributor advance (around 6K rpm). From this plotted curve, you can choose the needed combination of centrifugal weights, shape, spring tension and the mechanical stop that closely matches the curve.

Sherwood