Discussion
Sorry, it’s me again. Still stuck in hospital so brain is running riot.
Since I’ve had the Cerb and been reading on here about ‘Wasted Spark’ something has been burrowing deep in the back there. I’ve just read, in an article on ECU signals, the definition that ‘wasted spark’ is all four plugs on the same bank firing simultaneously.
Well this can’t be true can it ?
If piston 1 is on compression and coming up to ignition then one of the other pistons on that bank is completing the input cycle which would mean that, at the completion of that stroke, the whole input charge would ignite at the bottom of the stroke and you would have 180° pre-ignition.
The fact is, this would have already happened for cylinder 1 and there wouldn’t be a charge to ignite.
I hope that someone can follow my thoughts and explain my mistake/stupidity
Ta !
Mr. Dullo
Since I’ve had the Cerb and been reading on here about ‘Wasted Spark’ something has been burrowing deep in the back there. I’ve just read, in an article on ECU signals, the definition that ‘wasted spark’ is all four plugs on the same bank firing simultaneously.
Well this can’t be true can it ?
If piston 1 is on compression and coming up to ignition then one of the other pistons on that bank is completing the input cycle which would mean that, at the completion of that stroke, the whole input charge would ignite at the bottom of the stroke and you would have 180° pre-ignition.
The fact is, this would have already happened for cylinder 1 and there wouldn’t be a charge to ignite.
I hope that someone can follow my thoughts and explain my mistake/stupidity
Ta !
Mr. Dullo
on most engines, tdc events happen in pairs. it's harmless to fire a spark plug between exhaust and inlet strokes so coils can be driven in pairs and ties to the cylinder which share tdc.
On most vee engines, these are across the two banks, but on most vee engines, the vee angle is a direct function of 360 and the number of cylinders. V8s are usually 90, V6s are usually 60, V12s can be either.
The ajp8 runs each bank like a 4 cylinder and includes two wasted spark pairs on the same bank. That the timing between the two banks is literally odd firing is probably to give it a nice sound even though it has a flat plane crank.
On most vee engines, these are across the two banks, but on most vee engines, the vee angle is a direct function of 360 and the number of cylinders. V8s are usually 90, V6s are usually 60, V12s can be either.
The ajp8 runs each bank like a 4 cylinder and includes two wasted spark pairs on the same bank. That the timing between the two banks is literally odd firing is probably to give it a nice sound even though it has a flat plane crank.
tofts said:
It woulld fire wasted spark on 2 cylinders on one bank, and 2 on the other bank. As above, the second plug on the first bank will be on the exhaust stroke and the other two on the other banks will be somewhere in between, 75 degrees early/late.
do you want to re-think that ?It is.
And in a 90 degree V8 with a cross plane crank, the piston pairs that hit TDC at the same time will be formed by one from each bank.
If the engine has a flat plane crank, then the TDCs are shared by pairs on the SAME bank. This is still true if the Vee angle isn't 90 degrees thereby creating an odd-fire engine, like the AJP8.
I can't think of a good reason for the 75 degree angle other than to make it sound nicer. Anyone got any better theories?
And in a 90 degree V8 with a cross plane crank, the piston pairs that hit TDC at the same time will be formed by one from each bank.
If the engine has a flat plane crank, then the TDCs are shared by pairs on the SAME bank. This is still true if the Vee angle isn't 90 degrees thereby creating an odd-fire engine, like the AJP8.
I can't think of a good reason for the 75 degree angle other than to make it sound nicer. Anyone got any better theories?
Dear Guys,
Thank you very much for all your replies, especially to nonsso_scerbera whose link put me on to Exhaust Power Valves and Heimholtz Resonance.
For the last 40-odd years, I have been under the wrong assumptions of valve overlap.
Thanks, once again
ETA
A decent 'EXUP' system for a Cerb would net some interesting power gains.
Anyone got a pencil sharpener ?
Thank you very much for all your replies, especially to nonsso_scerbera whose link put me on to Exhaust Power Valves and Heimholtz Resonance.
For the last 40-odd years, I have been under the wrong assumptions of valve overlap.
Thanks, once again
ETA
A decent 'EXUP' system for a Cerb would net some interesting power gains.
Anyone got a pencil sharpener ?
Edited by Mr Cerbera on Monday 18th May 16:46
Gassing Station | Cerbera | Top of Page | What's New | My Stuff