MFI Questions following Check Measure Adjust

[304065]

MFI Gurus . . . in an effort to wring every last horsepower out of my 911E, I have been systematically working my way through Check, Measure Adjust . . Air Cleaner Element OK, Leakdown 5% or less each cylinder, all plugs cleaned, gapped and firing, dwell angle 37 degrees. . .

Dwell-I use an MSD 6AL box. Is the dwell angle still important? My understanding is that dwell allows current to energize the coil, before the field COLLAPSES, firing the plug. With an MSD box, it's the reverse-- a capacitor charges, and when the points open, the capacitor fires into the coil, and it's the field FORMING that fires the plug. So dwell isn't that relevant, all you need is a gap in the points.

What is the relationship between points gap and dwell angle? In my experience, the narrower the gap, the longer the dwell period. Or am I in bizzaro-land?

Ignition Timing - Spec for the 1971 911E is 4 ATDC at idle and 29-31 BTDC at 6000 RPM. Am I reading that correctly, that means four degrees of RETARD at 900 +/- 50 RPM and 30 (splitting the difference) of ADVANCE at 6K. My timing light doesn't do retard, but in any event, I was measuring timing at idle straight up, at TDC.

It seems to me that the more important figure is the advance at 6000 RPM, which is the where the engine is 95% of the time on track, so I can ignore the idle retard (understanding that the engine might be harder to start). Yes?

Fuel pressure and flow are perfect, and the injectors fairly fresh. Injection timing was verified- with the "FE" mark on the crank pulley opposite the notch in the fan shroud, the MFI drive pulley's notch was exactly opposite the notch in the pump, at the 12:00 position, verified with an inspection mirror.

Finally, Correlation - I removed the air bleed screws one at a time and cleaned any carbon. Put them all back in and opened them 2.5 turns exactly. Then I disconnected the throttle shafts and tried to loosen the lock nuts for the throttle stops.

How the HELL do you get the lock nuts loose on Cyls 2 and 3? It looks like you'd need a crows-foot wrench or something to get them loose. I'm open to suggestions here. I was able to get a reading of "7" on my synchrometer for all the other stacks, but 2 and 3 only read "5." Gotta adjust them up.

How far open do you want the throttle plates at idle? The "Adjusting MFI" article by Lee Rice suggests that you want them "just off the point where they stick shut." Mine don't stick shut at all, I had them about 1/2 turn of the adjustment screw open from the fully closed position.

The pump control rod was way out of spec, I re-set it to 114mm and put some red paint on the nuts for a witness mark. The throttle control rods were

Once I get the correlation set it's time to use my Gunson Gastester.

Anyone have thoughts on the Dwell, Timing and Correlation questions? THANKS in advance!

[Early_S_Man]

John,

As far as the dwell goes ... your assumptions are true, just as with the OEM CDI! The real reason dwell and pont gap are important ... is so that the points will follow the distributor cam properly up to redline! If the gap and dwell reading is within spec, that fact assures proper point operation up to redline!

As for timing, the '69 - '71 spec book shows 'basic' timing to be TDC. The important spec, since your only advance is centrifugal ... is max advance, 29° - 31° BTDC @ 6000 rpm. That is the only point at which the factory specifies that the running/dynamic timing be checked or adjusted!!! What I always do after dwell and timing are set and verified ... is to check the timing at idle speed of 900 rpm, and make a note of the three readings in my logbook. That way, I have a measured timing value at idle to compare with in the future, along with the dwell and max. advance values.

[Grady Clay]

John,

I will second what Warren posted and add a bit. For your “last little bit” of power at the track, you can up the ignition advance somewhat. I would use 33-34 degrees at 6000 RPM. Some go as high as 36 deg. This is also CR sensitive. To do this you will need to use race fuel – 100+ octane and really work to keep the heads and cylinders cooler. The 1.81:1 fan ratio and the “Rubbermaid Solution” come to mind. Both of these give a net power gain and net increased performance on track on a hot day. What are your typical track conditions (altitude, temperature, humidity)?

I mark my crank pulley with some paint in the 28-36 area and scribe little timing marks. I make my #4 plug wire longer at the distributor and my timing light pick-up will fit over both #1 and #4 wires for a much brighter light. I don’t use an adjustable advance timing light because I think it just introduces a potential source of error. I have a sharp piece of welding rod (coat hanger will work) to the outside of the pulley as an accurate pointer.

Having changed the pump rod you will most certainly need to adjust the mixture. Why was the pump rod out of spec? Was it too long or too short?

Be sure to check for vacuum leaks between the throttle body and head. If you think you have a leak, it can also be the throttle shaft(s) or a cracked throttle body casting.

The most critical issue with all the linkage, throttle stops, idle air, etc. is the “just coming off the idle stops” issue. All six must make that transition from perfect idle to equal air flow (and pump rod movement) when just off the stops. If the engine “pops” when just moved off idle, it usually has moved the throttle bodies (some or all) before the pump rod moved, causing a lean cylinder or more. This is where the condition of the throttle bodies is very important.
After setting everything at idle, I check the air flow at a fast idle (1000-1100) using the hand throttle. You want to do this as close to idle as you can. This will give an indication if all six are opening the same.

It is frustrating to have everything proper at idle and all go haywire just off idle. Depending on the situation, it is sometimes best to get everything exactly right just off idle and the make do with the best idle possible, even if it isn’t perfect. These were difficult when new and now after 30+ years … well ….

Are your linkage rods steel or aluminum? The steel ball ends and aluminum rods are notorious for seizing the threads and breaking when you try and unscrew them.

How long has it been since the throttle bodies and stacks have been off the engine? An occasional disassembly, clean and lube of threads and loc-nuts may be worthwhile.

MFI usually runs great at full throttle and high RPM, even with a myriad of things wrong or mal-adjusted. The “Art” is just off idle and part throttle.

Check for just barely full throttle, but not more. You do not want to stress the linkage at full throttle no matter how hard you push on the gas pedal.

Best,
Grady

[304065]

Warren and Grady,

Thank you both for the guidance. Warren, I'll make sure the dwell is in spec. I have found that with a .3mm points gap, the dwell angle is closer to 30 degrees, and to get it up to 38 I had to use a gap of .25mm. I suppose I could have had the feeler gauge cocked a little, opening the points, I'll try it again.

Grady,

Good idea about the 134/84 setup. I might also try some more shims and a smaller belt.

Typical conditions: 1460 to 1575 MSL (toe of the Boot to top of the Esses at Watkins Glen). Historical June high temperatures for Elmira, NY (nearest reporting point) 77.9F. That works out to something like 3300 feet density altitude. What would the reduction in horsepower be, 10% or so?

In terms of total advance, I'm fine with bumping it up. I have very easy access to 100LL Aviation Fuel just across the runway. . . although race gas has different properties. What do you think about using 100LL?

The pump rod was too long, like 120mm. With the pump rod the correct length and the two throttle rods matched at 145mm, the cross-shaft was off its stop and all the throttle plates open with the rods at their original lengths. Some PO had adjusted the pump rod to fit the length of the throttle linkages and not the other way around.

The throttle bodies are in very good shape, I think they were rebuilt not too long in the past. There's no play in the throttle shafts that I can detect, and a blast of B-12 around the base of the stacks doesn't bog the engine, so I think I'm OK. I haven't checked or changed the gaskets between stacks and heads, however. . . might be a good excuse to unbolt and detail the stacks, they are pretty oxidized. Would you soak the stacks in B-12 overnight?

I am listening closely to your guidance on setting the throttle plates. A smooth idle is not terribly important, if I had to make a choice I'd want good off-idle and transition.

One final issue that I am having is that after engine warmup the idle speed increases from 1000 RPM to 2000 RPM. The hoses are intact and there's good hot airflow to the thermostat. 25 pairs of discs with spacers, just like in the other thread. . . seems to me that for the idle to INCREASE the idle mixture has to be set too RICH-- when the rod expands, leaning the mixture out, the engine speed picks up. And yet there are none of the signs of rich mixture, plugs are consistently tan, no smoke from exhaust. The MSD box may be keeping the plugs clear, however.

Any thoughts on why expansion of the thermostat would do the opposite of design?

THANKS guys for your help with this, hope I've made sense.