DDK - Die Deutschen Klassiker

Home Built By Jeff wrote:This week I look into the issues I had with Harry 911 on the Dyno and attempt to diagnose them.

https://youtu.be/la6loJBzjqI

Interesting vid. Might be a stupid question, but have you made sure the spark plug wires are connected to the right coil packs / plugs ? I run a clewett twin plug system and each coil pack runs multiple cylinders rather than both plugs on a single cylinder

Eg :

Yes, they are definitely connect to the correct plugs, otherwise I would have a miss.

Home Built By Jeff wrote:Yes, they are definitely connect to the correct plugs, otherwise I would have a miss.

Not necessarily when running both banks as you could still have one plug firing correctly ... but when you,disconnected either the upper bank or lower bank then you’d feel a difference or miss then ...

I’m not sure how your system is set up though ?

I went through in the video every plug individually and they were all firing. There isn't a miss. I wish it were that simple

No problem as it’s most likely something else .... but maybe just another thing to double check. I might not have explained my thoughts properly though .... It’s not they won’t be firing , it’s that some might be firing in the incorrect firing order ... which wouldn’t show with both banks connected because at least one plug would be firing at the correct time , but when you pull the fuse to,disconnect either the upper or lower bank, there is a change in running so possibly maybe one plug not firing in the correct firing order ?

Can't you enable electrically the top plugs so it is a '6' plug engine and get that lot all firing in the right conventional order at the right time to TDC, then isolate that lot and repeat for the 'lower' 6 and check the firing order/time is right again to the conventional order of a normal 6 plug 911 engine?

Obviously then connect both top and bottom so the order is the same and the timing is correct (I am assuming there is a delay in timing between the 2 plugs in each cylinder, might not be so)

Maybe I'm being too simplistic.

hot66 wrote:No problem as it’s most likely something else .... but maybe just another thing to double check. I might not have explained my thoughts properly though .... It’s not they won’t be firing , it’s that some might be firing in the incorrect firing order ... which wouldn’t show with both banks connected because at least one plug would be firing at the correct time , but when you pull the fuse to,disconnect either the upper or lower bank, there is a change in running so possibly maybe one plug not firing in the correct firing order ?

If they fire in the incorrect order, those cylinders would not actually run at all, so if I had 2 cylinders swapped it would be firing on the wrong stroke which means that it would only run on 4 cylinders. That is not the case.

I am now aware that the engine revs should drop slightly when running on just the top or bottom plugs as the twin plug burns the fuel faster which essentially advances the timing. Remove one bank of plugs and it is like I have retarded the timing, hence the drop in revs.

They would run if one plug was firing correctly but the second isn’t ,,,, Mine had two plugs wires swapped around once by a mechanic .... 4 cylinders were fine with both plugs in each cylinder firing at the same time .... 2 cylinders had one plug each firing at the right time, but both had their second plug firing at a different time in the stroke. You can see how This happens if you look at the instructions I posted earlier for the firing order..

Eg two coil packs fire on one cylinder so coil pack A1 fires a plug in cylinder 1 and in cylinder 4 :

Coil pack A1 ... fires cylinder 1 top plug
Coil pack A2.... fires plug cylinder 1 lower plug

Coil pack A1 ... fires cylinder 4 top plug
Coil pack B2 .... fires plug cylinder 4 lower plug

Notice it also states do not wire one coil pack to both plugs on cylinder 1 as it would place the load of 2 plugs under compression on one coil pack and a severe performance problem would result.

Not trying to push this but think our descriptions are crossed wires slightly so kind of hoping you see where I’m coming from. We are currently in lockdown in the uk so have nothing better to do than think about tech problems like this , wether we’re right or wrong is another matter...

I understand what you are saying, and as you saw in the video, the engine would run without missing on ONLY the top coils running OR the bottom coils running. So only one plug per cylinder firing and it wasn't missing. Hence there is no possibility that the plugs are switched. There is also no chance that there are 5 top plugs and one bottom on the same circuit, as they have a completely different harness.

Will be interesting to see what’s causing the problem

Jeff... did you get to the bottom of this problem ?

hot66 wrote:Jeff... did you get to the bottom of this problem ?

I am pretty sure now that the headers are too small at 1.5". I have a 1 5/8 on it's way

In this video I give you an update on Harry's progress over the last few months and build yet another exhaust system, to hopefully this time sort out my power restriction.

https://youtu.be/-oeEAj_VQio