[Diy_efi] 928/749

Fri Aug 17 00:07:06 UTC 2007

Hi Kerry,

Nice thing about the '749 is that it properly handles spark and fuel 
under boost conditions so no rising rate fuel pressure or spark retard 
box is necessary.  Looks like my problem is just weak spark as pointed 
out by several here.  I found yet another coil in the cellar and got it 
to run this afternoon.  I bought a new coil but have not had a chance to 
put it in yet. 

Thanks for the reply,  and thanks to everyone else who chimed in,  too.

Best,

Bill

Kerry Smith wrote:
> Hi guys.  I'm a street rodder who just wandered into this forum 
> because I built my own EFI.  On street rods, which are mostly 8 
> cylinder Chevy, Ford or Chrysler engines, we usually use an 
> aftermarket ignition system consisting of a control module, coil and 
> distributor, from MSD, Mallory or other.  This gives the hot spark 
> needed to fire high compression or supercharged engines reliably.  
> Just something to consider.  My MSD setup also has an adjustable boost 
> retard feature, to avoid detonation at high boost.
>
> Kerry Smith
> ----- Original Message ----- From: "Torbj?rn Forsman" 
> <torbjorn.forsman at gengas.nu>
> To: <diy_efi at diy-efi.org>
> Sent: Thursday, August 16, 2007 3:45 PM
> Subject: Re: [Diy_efi] 928/749
>
>
>> A correctly built ignition coil will never - even at open circuit - 
>> give so high voltage that it could cause a full internal insulation 
>> breakdown. However, so-called partial discharges might occur.
>> Many interesting things can be learnt by studying the ignition 
>> voltage by oscilloscope. Of course, either a proper high voltage 
>> probe or a capacitive clamp on the ignition cable is needed.
>>
>> But another problem is that the primary peak voltage also rises too 
>> high during no load conditions and this can damage the switch 
>> transistor in the ignition module. This is particularly true for GM 
>> HEI ignition modules that do not have any overvoltage protection for 
>> that transistor. Those systems rely on the fact that their original 
>> ignition coils never give more than 350-400 V across the primary. But 
>> almost all other ignition modules (except suspect aftermarket parts) 
>> either have a zener diode chain from collector to base of the switch 
>> transistor, or a switch transistor with zener diodes incorporated on 
>> the chip.
>>
>> Best regards
>>
>> Torbj?rn Forsman
>>
>> Kevin Murray wrote:
>>> Hi Bill, I understand your frustration as I've been there once 
>>> before. In my case it turned out to be a bad ignition coil. Firing 
>>> the plugs in open air is much easier than when under compression. 
>>> When the impedance at the plug is too high (in the cylinder under 
>>> compression) the coil energy will discharge internally. To make 
>>> things worse, this situation wears the coil out faster.
>>> I'll admit I'm not familiar with Porche ignition systems but perhaps 
>>> this will give you some direction.
>>>
>>> Kevin Murray
>>>
>>> ----- Original Message ----- From: "Bill Shaw" <b.shaw at comcast.net>
>>> To: <diy_efi at diy-efi.org>
>>> Sent: Wednesday, August 15, 2007 11:23 PM
>>> Subject: [Diy_efi] 928/749
>>>
>>>
>>> I've been chasing a long series of problems with my 928 since I
>>> installed the new motor this spring .  Right now it won't start,  it's
>>> having some weird electrical problem.
>>>
>>> A little background...
>>> I ran it in a similar configuration for 3 years... Porsche 4.7L v8,
>>> Vortech supercharger, GM ignition module, sensors, throttle body, and
>>> 749 ecu.  Ran great,  had loads of fun flogging the crap out of it on
>>> all the road courses in the northeast.  Last fall it ate 2 spark plug
>>> tips and scored 2 cylinders at Lime Rock.  New motor was in the 
>>> works so
>>> I finished it up and got it in the car.  The new one is a 5.0L Porsche
>>> v8, '87 big dish pistons,  16 valve ported & extrude honed heads all
>>> adding up to about 8.5:1 cr,   bigger cams, bigger throttle body, 
>>> bigger
>>> exhaust.  It ran pretty good for about 2 days,  good enough to set me
>>> back in my seat and put a grin on my face,  then it all went to he!! 
>>> again.
>>>
>>> I've swapped ECUs, ignition modules, plugs, cap & rotor, been through
>>> all the plug wires with an ohmmeter, wrung out the wiring harness from
>>> the sensors to the ECU.  I'm getting fuel,  I can hear all the 
>>> injectors
>>> clicking,  and have 43 lbs of fuel pressure.   The REF signal from the
>>> distributor makes it through the ignition module to the ecu and the ecu
>>> is putting out EST pulses, verified by o'scope.  #1 is firing at TDC
>>> with bypass unplugged.  Everything is great,  it just won't start.
>>> Tonight we checked the cam timing and retensioned the timing belt.  
>>> I've
>>> been through all the basics I can think of, all is good but no fire in
>>> the hole. I've reverted to my last known good bin while doing all this
>>> testing.
>>>
>>> The only weird thing I can put my finger on is with spark.  I move my
>>> inductive timing light from wire to wire checking for spark on all 8.
>>> I'll find one not firing (it changes some times),  pull the wire off 
>>> the
>>> spark plug,  insert another spark plug in the wire and ground the 
>>> tip on
>>> the block and it will fire,  the timing light will light up.  Move the
>>> wire back to the plug in the block and it won't fire.  Pull the plug
>>> from the block,  put it in the plug wire and ground the electrode on 
>>> the
>>> block and it will fire again.  WTF???  This is driving me nuts.  I've
>>> pulled one plug wire off 3 times to check resistance, wiggled both ends
>>> and the whole wire looking for an intermittent break,  everything is
>>> great,  3.1k ohms per spec.
>>>
>>> I'm running out of ideas.  Anyone have a thought??
>>>
>>> Thanks,
>>>
>>> Bill
>>> 928s 5.0L Vortech/749
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>>
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