XDi ignition systems users
 

Would like to pick your brains a bit. I am using the XDi system on my twin plug 3.3SS engine. As a starting point, I have the 4 timing dials set at:
1) 10 degrees
2) 24 degrees
3) -2 degrees
4) 5,000 rpms

With #1 being the initial timing, #2 is max advance, #3 (adjustable) backs off the timing -2 degrees on top end and #4 is the max rpm cutout setting. Compression is 10:1 and cams are the 964 units. Any real world experience/suggestions on the timing?

Thanks,

That will be fine.

5k RPM Max Cutout?
 

If Steve says the settings are fine - I'm sure they are fine.

Meanwhile I'm planning to go with XDi single plug on my 3.2L rebuild. Appreciate if you could explain the 5k RPM cut out. Is that the RPM limiter (doesn't let you throttle up higher than 5K?).

If so, why such a low RPM cutout (I planned on limiting mine to ~6,500 RPM with only bottom end mod being ARP rod bolts).

Thanks, been following your rebuild thread - great stuff.

Best of luck,

Gordo

I'm also doing this with single plug on stock 3.2. Will also try the vacuum (pressure) sensor piece. I'm thinking 36 BTDC total advance at 3K RPM.

RPM Limit
 

I've got it set at 5k just for the break in period. After that, it goes to 7K! The upper limit on the unit is 15K!

Single vs Dual Plug timing......
 

Sounds like your numbers will be correct for a single plug engine. One of the advantages of having the Dual Plugs is that the max timing advance with dual plugs is less (22 - 24 degrees). This supposedly will let the engine run cooler and be less sensitive to knock. One reason why I decided to go with the XDi system is that the timing is just about infinitely adjustable by simply turning the 4 knobs. One thing that I did when installing the timing sensor on my engine was to set the gap and then put a small hose clamp on the (round) sensor. That way, I can remove and re-install it without having to measure the gap which is somewhat difficult to do with the engine in the chassis. Good luck with your project.

Make sure to take note of the relative position of the crank sensor to the toothed wheel. If the crank sensor is not at the trailing edge of the 11th tooth, you need to adjust your base/dial settings accordingly. Being off by one tooth = 6 degrees!!! (360/60 teeth = 6 degrees per tooth). Don't assume the crank sensor bracket automatically "zeroes" you at TDC.

Even if you happened to be spot on with your correction relative to tooth #11, you MUST check your timing with a voltmeter or timing light. DO NOT rely on the dials to be accurate.

A handy tool to use to set up your ignition is this trigger simulator

Fifty Eight Tooth (60-2) Wheel Simulator - Electromotive Engine Controls

It fires the ignition w/out having to run the engine. In the case of the simulator, you need to confirm the actual timing setting by reading it with a digital voltmeter

Checking timing
 

Yes, something else to do before driving the car. I think the battery in my digital volt/ohm meter is dead so will have to get a replacement battery. This is on my short list of TTD. When I was assembling the engine, I set it all up very carefully and counted the teeth on the trigger wheel. I believe that it is "right on" the correct tooth.

XDi has knock control, too. Works well with 993 knock bridges.

XDi Timing Check
 

Got new batteries for the digital VOM. Did a timing check on the XDi system. I must have gotten it put together correctly as the VOM matched the reading on the dials
+- 1 degree! One more "thing to do" off the check list!

Must have XDi2 (squared) to get knock control. Basic XDi not capable of knock unless piggybacked with another controller

I ran my 2.7 race motor (46s, big cams, nominal 10.3/1 Mahle race pistons, crankfire twin plug) with:
0 static
12 at 1,000 for 12
14 at 3,000 for 26
2 at 8,000 for 28.

Worked fine.

Once you have your motor broken in, take the car to a chassis dyno and get a torque curve. Use the torque curve and your transmission gearing (you don't need tire diameter or R&P for this, but can add them in) to develop "thrust" curves at 500 rpm intervals for each gear. Where these curves cross (all will except first for stock type gearing) is where you should be upshifting, as the delivered rear wheel thrust will be equal in the "old" gear and the new, higher, gear. Those are the optimum shift points. You can do this with a pencil and graph paper, or make a spread sheet so you can easily see how changing gears or torque curves will affect things, etc.

I had been shifting at 8,000 rpm. The motor mechanically will take this well enough - may shorten valve spring life, but then overrevs which don't bend any valves can also shorten spring life. But with a torque curve I found 7,600 was the place to shift. Sort of win/win, doing it right and gaining acceleration while reducing strain.

Do you think the 964 crank will produce a torque curve low enough that your optimum is below 7,000?

Unless you are using stock rod bolts, I'd think you might up your limit if you autocross or do DEs and the optimum is over 7.

Fred - looked at your main posting on this motor. CIS intake. Ah, maybe 7,000 is as high as you would ever need to go.

Does XDI (I used the original Electromotive crankfire system, and now have the Tec3R, but don't have XDI memorized) support shift lights? Soft rev limiting? Lights are neat to have, especially if for whatever reason your tach quits working occasionally when on a track. No real problem in the shorter bits where you can just upshift by where you always do and not bother with looking at anything other than the track, but those upshifts on long and somewhat featureless straights (think backstraight at Sebring) work better when you have an idea of engine speed.

XDi .............
 

The XDi does use soft rev limiting, but not a shift light. It also can use/support an external rev limiter and a couple of other features related to boost that I did not need. The basic wiring for the XDi is pretty simple, with getting the two small harnesses that feed the DFUs plugged into the master harness being the most difficult part. Other than that, there are 4 wires plus a ground wire that need to be connected. The tach feed wire from the XDi plugged directly into the wire to the tach without any adapters. The only complication that I had was due to using the CIS system with the DFUs mounted on the shelf behind it which hides the right side DFU. When lifting the engine into place, I got it within about 6" of being in and then plugged in the bottom spark plug wires. If necessary, I can pull the CIS boot and see to work on the plug wires. Thanks for your thoughts on this "issue" with my new engine!

Help with XDi...Just installed my system...Fires right up but I have to shut it off because it goes wide open(didn't hit the rev limiter). Seems like a vacuum problem. I used golf tees in the two small lines from the old distributer. Anyone have a similar problem???

Timing
 

Is your system single or twin plug? What do you have the timing dials set at? Also, check to make certain the plate that lifts with an open throttle is not stuck open. I used the 10/24/-2 settings (twin plug) and my engine ran quite well. Also, check to make certain that your sensor is reading the proper tooth on the crankshaft degree wheel.

Thanks...I used the settings that Clewett Eng recommended. It.s a single plug. .050 air gap, Initiall timing 12 deg, 3000rs 21 deg, 5000 red line till break in. Degree wheel is pre set for 911 by the pin. Thanks I think the plate might be the culprit might be the plate.

XDi
 

Be sure to check your timing event with a timing light. and if possible watch the timing while doing a dyno pull to be sure.
Mike Bruns

Sticking throttle pedal
 

Ever since I lowered my motor to put on the Clewett crank pulley I have a sticking throttle pedal. Could I have damaged the linkage when I lowered the motor?

If you lowered the engine a large distance, sure you could have bent the throttle rod that comes out of the car chassis.

Take off the throttle rod that connects to the bell crank on the side of the transmission. It's a ball socket that you just pop off by leveraging with a 10mm open end wrench. Once disconnected, pull on the rod that goes into the car and see if you feel any hang-ups

Walt,

Will you please explain further the thrust curve procedure you mentioned? I always assumed (possibly erroneously) that the horsepower peak is used to determine the up-shift point. Shift 500 RPM higher, so that the RPM drop in the next gear is only 500 RPM (more or less) below the horsepower peak. That way you are maximizing the area under the HP curve, no?

Dave

Yup, you were in error. At least insofar as you were using a rule of thumb - go 500 over HP peak, and shift. Though sometimes that works out pretty close. But sometimes it does not.

If you multiply torque by a gear set and then by the R&P, you will get what you might call rear wheel thrust. You can also factor in the rolling radius (or the diameter, or the circumference) of the tire if you want exact MPH values, but in terms of rpm and shifting it doesn't make any difference.

So make a graph (nowadays, make a spread sheet) with column A RPM at, say, 500 rpm intervals. B is the torque at those RPMs from your dyno run. RWTorque is just fine, no need to wonder about accounting for transmission loss. C is the torque multiplication due to the gears and R&P, and that is thrust in 1st gear. D is the speed in 1st. E and F are torque and speed in 2d, etc..

For a 5 speed, you can plot the thrust columns as a series of curves, with thrust as the vertical and speed as the horizontal. Each plotted point represents an RPM. Their basic shape is that they start at a lower thrust, reach a peak as rpms climb, and then drop. 1st gear is the highest curve, and is at the left of the graph. Each succeeding curve starts lower but at a higher speed, climbs, and drops.

Where the curves cross is the shift point from the lower to the higher gear. This is the point where the torque (aka the acceleration) from the falling lower gear is the same as the rising higher gear. That is the area under the set of curves you are maximizing.

You can guess this pretty closely by knowing what rpm is indicated on each side of both curves at the crossing point. Or you could use a finer graded input table between those RPMs. Or assume the lines to be straight between any adjacent pair of RPMs, and do some algebra.

So peak HP isn't directly a factor here. Gearing affects the shift RPM. For instance, typically your curves won't cross between 1st and 2d, because 1st is so low that you reach your rev limit (however you have defined that) before you get to a point where it will cross the 2d gear curve. The remaining shift points will vary some depending on the gear sets.

But most stock 915 gear boxes will have the shift points pretty close to the same RPM. For the torque curve I got long ago for my 3.0 SC, I shift at 6,250. Optimum didn't vary by more than 100 rpm either way as I recall. A gear box built the way Bruce Anderson suggested in his book - equal MPH gain in each gear - works out to a fairly constant shift RPM even as the RPM drops between gears keep falling as you go up through the gears.

I'll see if I can find a visual.

Yup, you were in error. At least insofar as you were using a rule of thumb - go 500 over HP peak, and shift. Though sometimes that works out pretty close. But sometimes it does not.

If you multiply torque by a gear set and then by the R&P, you will get what you might call rear wheel thrust. You can also factor in the rolling radius (or the diameter, or the circumference) of the tire if you want exact MPH values, but in terms of rpm and shifting it doesn't make any difference.

So make a graph (nowadays, make a spread sheet) with column A RPM at, say, 500 rpm intervals. B is the torque at those RPMs from your dyno run. RWTorque is just fine, no need to wonder about accounting for transmission loss. C is the torque multiplication due to the gears and R&P, and that is thrust in 1st gear. D is the speed in 1st. E and F are torque and speed in 2d, etc..

For a 5 speed, you can plot the thrust columns as a series of curves, with thrust as the vertical and speed as the horizontal. Each plotted point represents an RPM. Their basic shape is that they start at a lower thrust, reach a peak as rpms climb, and then drop. 1st gear is the highest curve, and is at the left of the graph. Each succeeding curve starts lower but at a higher speed, climbs, and drops.

Where the curves cross is the shift point from the lower to the higher gear. This is the point where the torque (aka the acceleration) from the falling lower gear is the same as the rising higher gear. That is the area under the set of curves you are maximizing.

You can guess this pretty closely by knowing what rpm is indicated on each side of both curves at the crossing point. Or you could use a finer graded input table between those RPMs. Or assume the lines to be straight between any adjacent pair of RPMs, and do some algebra.

So peak HP isn't directly a factor here. Gearing affects the shift RPM. For instance, sometimes your curves won't cross between 1st and 2d, because 1st is so low that you reach your rev limit (however you have defined that) before you get to a point where it will cross the 2d gear curve. The remaining shift points will vary some depending on the gear sets.

But most stock 915 gear boxes will have the shift points pretty close to the same RPM. For the torque curve I got long ago for my 3.0 SC, I shift at 6,250. I could shift at 6,500 into 2d, then 6250 into 3d, and about 6,000 into 4th and 5th, but that's too much to remember and I don't like to rely on shift lights. A gear box built the way Bruce Anderson suggested in his book - equal MPH gain in each gear - works out to a fairly constant shift RPM even as the RPM drops between gears keep falling as you go up through the gears.

I'll see if I can find a visual.

Here is a 2.7 race motor with a tall (60 mph) 1st gear. The dyno run went up to 8,000 rpm for data gathering. I plotted in three different 5th gears. This technique was found in an article in Pano taken from some factory race literature, and reproduced in one of the Upfixins. Lots of work with the hand calculator to create a table and then plot it in. You can see the typical Weber carbureted engine torque dip in the 4-5500 rpm range. And why I was losing out shifting at 8,000 anywhere except from 1st to 2d (and that turned out to be something only useful on a slow paced race start). Except for that, 7,600 is pretty close for all the other shifts.

http://forums.pelicanparts.com/uploa...1380855236.jpg

Here is letting a spread sheet do the work for a stock 3.0 with stock gears. The stock gears aren't quite as evenly spaced as the race box for the 2.7.

http://forums.pelicanparts.com/uploa...1380855488.jpg
http://forums.pelicanparts.com/uploa...1380855551.jpg
http://forums.pelicanparts.com/uploa...1380855605.jpg

David Ferguson created the basic spread sheet maybe 15 years ago for a 901 transmission, hence the table of gears. But you don't need that, you just need to know what your gears are by tooth count, or as a ratio based on that. My friend John Mackin refined this spread sheet using spread sheet voodoo (or knowledge gained from actually taking a class in this stuff) and had the spread sheet calculate the curve crossings. But it is easy enough to do by inspection.

Walt,

Thank you very much for the detailed explanation. But I don't understand why torque is being factored instead of horsepower. The peak of the torque curve (the point of maximum engine efficiency) is lower than the horsepower peak but the car still accelerates past the torque peak because more work is being generated by the engine. Acceleration only begins to drop off after the horsepower peak is passed.

Dave

Dave - I don't think it is correct to say that acceleration only begins to fall off after the HP peak is passed. And even if it does, I'm pretty sure that is because of the shape of the torque curve.

My understanding is that torque is what governs acceleration. But it is easy for many of us lacking a suitable engineering scientific understanding (like me) to get confused.

Why I think you are wrong about thinking that HP is what should govern the shift point involves a couple of things. One is that, when long ago I had data, I could see the acceleration number in Gs before and after an upshift. The book that came with the G Analyst noted that the proper upshift point was where the before Gs and the after Gs matched. It was pretty easy to see on the data display if you shifted too early or too late.

The second is that not only my friend Dave Ferguson (an engineer), but also Porsche's racing department, used torque in quantifying optimum shift points.

Gearing affects the torque applied at the rear wheels, where it counts. Gearing doesn't affect the HP a motor produces. RPM does, but that is because of the shape of the torque curve. And I think that, knowing the drag coefficient (for air mostly, but also for other drag components) for a car, you can specify the horsepower needed to attain a given top speed. It takes power to push the air aside and overcome various frictions. That's not dependent on gearing directly, it just is a factor in selecting a gear which will have you at the HP peak at your top speed drag limit, if doing Bonnevile stuff. Not the same as accelerating.

I remember reading Hot Rod magazine in the '50s as a kid, where there were discussions about "overpeaking" the HP max. There was agreement that your shift point would be up there. But if it was about connecting the scientific dots, that part didn't sink in. A rule of thumb is easier to remember. And for the engines most of us drive on the track, it is generally going to be true that the optimum upshift point is going to be above the HP peak. But it is the torque curve which makes that so.

I had an engine which I ran up to 8,200 on the dyno. Didn't want to run it up higher. The graph of the torque curve there showed that the optimum shift points were above 8,200. Don't know just where, of course. So I shifted at 8,200. Optimum is over 600 rpm over HP peak. But street motors aren't like that.

High idle with Xdii
 

After installing my Xdi, I had problems with high idle. I greased the old bell crank and added a new return spring. Idle is MUCH better but still too high Air correction screw does not help.

Check for a vacuum leak by removing the oil cap while the engine is running. If the idle drops then your engine probably does not have an air leak. However, if the air bleed screw is completely closed, something is wrong. Check the idle timing advance knob. I have mine set at about 9-10 degrees. Adding advance with this knob will cause the idle to increase. Also, if your (CIS?) mixture is set too rich, it will cause a high idle. Good luck!