need-for-speed

First, let me say how happy I am to have found this section of the Corvetteforum. I've been a member for over 2.5 years. I love wrenching on cars and discussing engines. This section is a gold mine for such information.

I want to replace the OEM HEI distributor on my '79 Camaro. The tach needle goes haywire once I get to about 5,000 rpm and I suspect it's due to misfire. Do you guys agree? They say the OEM is only good for 4500 rpm. I want to stay with an OEM style for reliability and availability of compnents. My engine combo is mild enough that I don't need a multistrike capacitive discharge type unit. I don't want to turn the engine past 5800 rpm. I'm interested in the Petronix Flamethrower. What do you guys think? Is it a good unit? Does it live up to it's claims of 67% more energy or is that just marketing propaganda? :chevy

SWCDuke

Don't throw your money away. The HEI is a terrific ignition system, good to an easy 8000 revs on a V-8. If it was causing a misfire, you'd feel it. The tach bounce is something else.

My Cosworth Vega also suffers from the "bouncing tach syndrome", but it is very intermittent. The AIM shows a "tachometer filter", which is a series capacitor, but my car does not have one. According to the AIM the tach filter is installed regardless of whether the car was factory equipped with a radio. Mine was not, and if you recall, a lot of seventies vintage cars were not ordered with radios because dealers could install better aftermarket radios for the same of less.

Does your Camaro have a tach filter? A properly tuned series capacitor should act as a band pass filter and only allow the basic voltage pulses generated by the pickup coil to pass and filter out higher frequency noise. I believe it's this high frequency noise that causes tach bounce, but it's possible that it is in the tach electronics itself. The problem I have is that it is very intermittent, so it's not a high priority, but if I could find the proper band pass filter, I'd install it and see what happens.

Duke

need-for-speed

Duke,
Yes, my Camaro has the filter. The Camaro originally came with idiot lights but I transplanted the instrument cluster form a '79 Z-28 into it 2 years ago. Thanks to reading an article in Car Craft or Chevy Hi Performance (can't remember which), I knew to grab the filter off of the donor car. I do like the OEM HEI. It's extremely reliable. I just thought I might gain some performance from the "higher voltage output" of the aftermarket distributor. And I thought the tach bounce might be related to misfire but it's possible there is no misfire. In fact, since you mention it, it doesn't feel like it's cutting out. It's entirely possible, as you say, that it's a waste of money. I appreciate your opinion.

p.s. and yes, I do remember those radios of the 70's. My wife is the original owner of the car and it came with the fabulous A/C Delco AM FM radio with the wonderful front/rear speaker system (yes, one of each, not 2). I believe they only charged $400 for it. A bargain at that! :cheers:


[Modified by need-for-speed, 4:38 PM 10/26/2002]

DOCTOR J

Here is a GM dwg of the C4 Tach Filter that someone published a while back:
http://temp.corvetteforum.net/c4/doc...chfilter_s.jpg

Ignitions are not my forte, but I did a literature search on HEI in preparation for a winter project I have in mind. Here are extracts of some of the info found, if you are interested. Note that I have not verified all of this on the bench first-hand, so a grain of salt is suggested and corrections/additions are welcome:

HEI Modules:

1. D. Vizard did some testing on the old-style V8 HEI mechanical advance distribs, and reported no problems with the magnetic pick ups out past 6000 rpm. He did complain about the GM HEI switching modules though, FWIW. I think there were reasons for the complaint:

2. AFAIK, the old 4-pin GM HEI modules were internally current limited to about 5 amps, and had a fixed minimum dwell of some 3+ms. The dwell is probably the source of the mythical "4500 rpm limit" attributed to the stock V8 HEI distribs, at least with the coils of that era. (Note that GM 7-pin modules have a higher current limit & adaptive dwell from the ECM.)

3. 4-pin modules from MSD/Accel/etc have an advertised 7.5 amp current. I believe that is the 'extra energy' they claim over stock GM parts. (They probably also use a smaller min dwell time, but I don't know exactly what.)

4. The current in the switching module affects the energy in the coil primary (to the second power): IIRC, energy in an inductor = 1/2 L*I*I. The extra current from the aftermarket modules by itself won't do anything special for the engine, but it allows faster coil saturation. Directionally, this is better at higher rpms. However, MSD designs have other problems IMO:

5. What I have seen of MSD designs ( ftp://ftp.diy-efi.org/pub/diy_efi/documents/msd6a_02.pdf ) shows that they do NOT have an adaptive mag pick-up 'zero-crossing' feature - they invert the pick-up coil polarity, and fire on a positive going signal when used stand-alone. I think they have to do this for compatability with other systems, but to me it's not the best way to run an ignition.

< Editorial comment: I note that MSD sells a separate distrib 'zero-crossing' box (MSD-8358) for ~$200. It looks an awful lot like what the LM1815 (Nat Semi) chip does - but I haven't built the circuit yet to verify it. >


An interesting (I think) alternate to the 'aftermarket' 4-pin HEI modules exists, if you are concerned about spark at high rpm:

There is an MSD inductive ignition box (MSD-5900) advertised to draw 7.5 amps. It has a circuit similar to the SGS-Thompson L482 example circuit here: http://eu.st.com/stonline/books/pdf/docs/1339.pdf.

This box moves the coil-switching function outboard of the distrib to a big ST- BU941ZPFI ignition Darlington with its own heatsink, and has some sort of variable-dwell built in from the L482 controller - though I haven't verified its time constant.

Getting the ignition box away from engine heat is generally a good thing. And, if you trigger a 5900 box from the GM module (to use its zero-crossing feature), it looks like you would gain the benefit of higher current switching & variable dwell without relying on the MSD trigger circuit. At least it looks that way on paper. I don't think it would make a big difference, but (with the right coil) it should make more spark energy at higher rpm without using a CD box. Others manufacture external inductive ignition boxes too - but I ran across a schematic of the MSD one, so I know generally what they used for a design and the major parts involved.


Coils:

There are several after-market coils that seem to be oriented toward the higher current capabilitiy of newer ignitions. I note the Jacobs C4 coil has advertised primary characteristics that give a saturation time of 2.4ms (@ 7.5 amps - storing 112 mJ). There are other coils that might be able to throw off more energy within the single-coil V8 time constraint - I haven't looked at them yet.

Coil saturation time is taken from:

t = - LN(1 - (IR/V)) * (L/R)

where t = time
LN = Natural Log
I = current (pick a max value)
R = coil primary resistance
V = Voltage (can vary, especially during starting)
L = coil primary inductance

Other coils have similar properties, and presumably some benefit from better materials of construction over the stock parts.

For further reading there was an SAE paper from GM that dealt with HEI design (SAE 750346 -- HEI-A New Ignition System Through New Technology). I haven't found it - if anybody has a copy I'd like to read it.

There was some discussion of what I'm looking at for a winter project here: http://www.chevytalk.org/forums/Foru...ML/000963.html

HTH, YMMV, etc

SWCDuke

Aftermarket vendors hype big voltage coils. Peak voltage is measured open circuit, but with reasonably serviceable spark plugs, actual system voltage rarely exceeds 10 kV before spark initiation, and then voltage drops signficantly. The HEI willl generate up to 30 kV open circuit, which is more than adequate. So-called 50,000 volt coils are useless.

The most important ignition performance parameter is (per spark) energy, which few vendors state. The HEI generates about 76 millijoules of energy, which is about double the old single point.

Re the tach, remove the tach filter and see if the behavior changes. Also, check the circuit and all connections to the tach, including the body connector on the cowl, which probaby includes the tach circuit, and the tach ground. On older cars, connectors are the most common culprit when electronic systems misbehave.

If all else fails, I would remove the tach and take it to a reputable AC speedometer service station, explain the symptoms, and have them test the tach. The problem could be a flaky component in the tach head electronics.

Duke

need-for-speed

Thanks Dr J. Very infomative post. I prefer to stay away from ignition boxes. I hate to put all of them in one category, but I have seen reliability issues with them.

SWC, thanks for your input. The part about 50 KV coils being useless is good info. I'm leaning towards staying w/ my stock HEI system. I could use the $200 elswhere.......like (2) mag wheels........all I need is 2 more :jester

DOCTOR J

need-for-speed-

You're welcome.

FWIW, the tach dwg shows the component values for the 2-pole filter GM used on the C4 HEI tach input. The resistors & caps needed are probably in stock at Radio Shack. If it doesn't work it's only about $3 and some wire wasted.

Here is some further info - I'll dump it here, in case anybody needs a short summary of ingniton/coil info in the future:


The HEI info posted above didn't speak to secondary voltage in the coil, but if anyone is interested there is a depiction of the general case here:
http://www.mr2.com/TEXT/DavidKucharczyk/spark.jpg

A short discussion of voltage & field density is here:
http://www.mr2.com/TEXT/DavidKucharczyk/ignition.html

A short discussion of arc time and current is here:
http://www.daytona-twintec.com/tech_ignition.html


Secondary voltage (referencing the 5900 example circuit) is induced by the collapse of the field in the coil primary. That voltage follows something like V= L* dI/dT; and is multiplied by the turns ratio of the secondary/primary windings. AFAIK the stock HEI coils used about 85:1. The energy transferred to the secondary depends on how the coil was made and the properties of the core.

The Megasquirt guys posted their simulation of fixed dwell time vs primary current & secondary voltage here:
http://www.bgsoflex.com/igncoil.html


Voltage claims for aftermarket coils may be an indication of their insulation breakdown voltage; or possibly a reference to their initial voltage when used with a CD box. (From the MSD6 paper, note that the coil primary in a CD application is fired with -400 to -500 v from the cap. With a 100:1 coil that puts the secondary in the 40-50 kv neighborhood.) Only the insulation breakdown v seems to have much relevance to the ignition system.

Spark energy stored in an inductive coil was given in the earlier equation; for a CD box the spark energy is from the capacitor in the form energy = 1/2 C*V*V. I haven't found much in the way of measured data, but apparently the Kettering type ignition only gets 1% or so of the primary energy out to the plug gap where it is used.

BTW, the Motorola controller used in the original GM 4-pin modules is shown here:
http://esw.eee.hku.hk/Motorola/Volta...c3334rev0f.pdf

The current limit & coil saturation (variable dwell) along with the 'zero-crossing' circuit are on pp 1&2. You can see quite plainly where that circuit begins to drop coil current above 3000 rpm - which is lower than I remembered.

HTH

jackson

Doctor J / HTH:
First, I'm no BSEE, not an elecronics tech nor TV repairman ... couldn't do figures with e or LN if my life depended on it. But I try to read & understand what I can ... thanks, I appreciate your posts such as these. So I read the pdf download of Motorola description of MC3334 HEI circuit. You refer to this controller as what was used in original GM 4-pin modules. However, on page 3 ... under "circuit description" ... first sentence reads "The MC3334 high energy ignition circuit was designed to serve aftermarket Delco five-terminal ignition applications."

Obviously, I'm not equipped to engage you in an argument about how circuits work ... & I'm not doing it here. While I plead ignorance, I do have a question. Is the MC3334 the same controller found in both 4-pin & 5-pin GM modules ... or does a module having 4 "pins" actually have 5 "terminals" ... please clarify? :confused:

kaiser

The back of the GM module is the heatsink/ground connection, AFAIK. That is the fifth terminal.

HTH is 'hope this helps' - it's a TLA (three letter acronym).

INMYBLOOD

I have a tach problem too. We have a 70 Camaro and the tach reads high. The error must be progressive or increase as RPM goes up. It is an Accel HEI hooked up to the stock tach. It seems about right at idle but when you cruise lets say at 50mph its 3,000 with only 3.73 gears. that's wrong. I have also seen it hit or almost hit the 7,000 mark which I think is hard to believe with a non-solid cam and performer manifold. Is there a way to calibrate this tach???

jackson

Thanks ... I'd considered the back as a fifth ... but didn't know. Also as far as I know = AFAIK? I guess.

jackson

Doctor J:
You post ... I read & appreciate! I was able to interpret the Motorola doc & convert Hz to rpm (8 segments, cam-dist runs 1/2 of crank) for coil current & coil on-time ... also see how specs indicate a drop much earlier than oft-reported 4500rpm. Doc, I didn't mean to imply I thought you'd sought turmoil ... on the contrary, I believe you post to share the info you've picked up & thoughts thereon. But if I do disagree, be assured it'll be friendly in nature & not personal. Again, thanks for your posts!

DOCTOR J

jackson- glad you enjoyed reading it.

Re the Motorola circuit: I think you're on the right track - but to restate:

At 600 rpm a V8 only needs to fire a plug every 25ms; at 3000 it's every 5ms; at 6000 it needs to fire every 2.5ms. Hence the problem of single-coil saturation (coil energy) vs. time at high rpm.

Here are some additional thoughts, if you are interested:

As near as I can tell, the stock HEI coils were about 1.5 ohm(?) & 8 mH on the primary with 85:1 windings. (Measuring actual coil inductance, resistance, & capacitance outside of a lab is not doable - you have to disassemble the thing to get meaningful readings.)

You can plug those values into the B&G ignition simulation and see what happens to spark energy as rpms go up - remembering that the simulator only has one fixed dwell angle, like an old-fashioned points distrib. (Also remember that an early HEI is current-limited to 5 amps, so you can ignore readings higher than that.) The result doesn't look too good - that's why GM improves the systems in later years.

A newer design single-coil system can use a higher current limit and a coil with lower primary resistance/inductance (with variable dwell) to even out the secondary voltage/energy over the rpm range. (Run a coil with 0.5 ohm, 4 mH, and a 7.5 amp limit thru the same simulation and note the difference.) The improved design just means you don't burn up the coil at low rpm, nor run out of juice at high rpm, and still show high energy values. That seems to be the direction of newer GM single coil ignition.

Aftermarket ignition boxes & coils try to adapt the newer technology to old engines. However I note there is precious little design info available from box & coil vendors (and not much reason to believe what is published), so how it actually works is a crap-shoot.


Also note that by the time you get to the LS1, GM scraps the whole design problem above, installs eight coils, and puts the distrib in a museum. AFAIK the LS1 coils have current limiting built in, and (as I read the info) do their own dwell calc as each plug fires.

The next-generation ignition is supposed to have per cylinder ion-sensing on board, detect mis-fires, & refire the plug in real time - Saab has published some papers on this IIRC. FYI, there are some Delco-SAE papers for download here (recent stuff, not just ignition related) that speak to future electronics:
http://www.delphi.com/news/techpapers/2001/

DrJ

Edit: Oops - I deleted this by mistake:

Re tachs in old Camaros - I dunno. I've never taken one apart.
(The only tachs I've built for myself were datalogging units - if you are interested in those, they were described here:
http://forums.corvetteforum.com/zero...960101#3960101 )



[Modified by DOCTOR J, 9:42 AM 11/3/2002]

HighHopes85

A 4-pin module definitely has 4 pins. A 7 pin has seven. The grounding pad/heatsink is not considered as a pin in these systems.

Basically, there are 2 different styles of "ignition module under cap" modules GM used, and they include the large spade-terminal ones like the early HEI's and the large coil-in-cap HEI units. They are non-weatherpacked becasue the wire harness actually goes into the distributor through a grommet. GM realized this might cause a problem and supposedly went to screw-terminals on the cap for this reason on the Vette. Supposedly less prone to spontaneous explosions from vapor, if you believe Nader.

The second style is the module found in the small diameter distributors with remote coil. It is a weatherproof version, because the the module itself actually completes the base of the distrubutor cap.

Both different styles of the GM modules in discussion serve the same purpose though...to make us wonder if they are a cause for misfires in upper RPMs ;) :D


[Modified by HighHopes85, 6:13 PM 11/2/2002]

DOCTOR J

HEI Coils - evaluation of published data:

To clean up my desk I collected my notes on 8 coils that might be useful in an HEI inductive ignition (single-coil), and stuck them in a spreadsheet. A jpg of the result is here:

http://temp.corvetteforum.net/c4/doc..._comparo_2.jpg

Note that I used the best info I could find for the original GM coil-in-cap; I have no specs for the LT4 coil but expect it is an improvement over the old-style. Everytihing else is from manufacturer's specs.

Mallory & Accel publish NO specs that I could find, so they are not represented. Jacobs advertising makes unusual use of electrical engineering language so I take their specs with a grain of salt. MSD is technically competent in their writing - but I have NOT measured any of the values claimed by anyone, so use your own judgement.

The coils were compared via the simplified equations previously given for stored energy & saturation time, just to put them on a common basis. I used 13v and 7.5 amp feed (except for the stock design).

Conclusions of spec evaluation (which claim no connection to reality whatsoever):

The stock coil does a pretty good job as designed, but dumps energy above ~4000 rpm.
The C4 coil gives the most constant power up to 6000 rpm (if the specs are real) with a HIGH CURRENT feed.
The Jacobs can-type or the Blaster SS look like a reasonable choice for an external coil, especially if you are using a GM HEI module.
The MSD coil-in-cap has a good time constant but low power overall. Might work better in CD, or with 8+ amps.

These were considered ONLY from the view of inductive ignition. CD ignition is a whole different animal - coils there do not store energy, so a good CD coil will want properties totally different from the inductive kind.

Your mileage may vary. If you have better info/results I'd be interested in seeing them.

DrJ



[Modified by DOCTOR J, 12:25 PM 11/7/2002]