Help me with my synchrometer maths
Discussion
My synchrometer has a scale from 1 to 30 kg/h (I assume air at ambient).
a) 1kg of air is 856lit.
b) 4.5lit engine / 8 cylinders = 0.563lit per cylinder
c) Induction stroke once every two revolutions
d) Thus, average 0.28lit per revolution
e) Thus, 1,000rpm = 280lit per minute
f) Thus, 16,800 lit per hour
g) 16,800 / 856 = 19.63kg air per hour
Thus, each cylinder should be flowing roughly 20kg of air per hour.
If this is the case, then the synchrometer on general sale won't be that accurate as the scale seems to be logarithmic with the accuracy rapidly diminishing over 10kg per hour.
I think my synchrometer is optimised for .250lit or smaller cylinders.
Have I got my maths wrong or is there another design of synchrometer available?
a) 1kg of air is 856lit.
b) 4.5lit engine / 8 cylinders = 0.563lit per cylinder
c) Induction stroke once every two revolutions
d) Thus, average 0.28lit per revolution
e) Thus, 1,000rpm = 280lit per minute
f) Thus, 16,800 lit per hour
g) 16,800 / 856 = 19.63kg air per hour
Thus, each cylinder should be flowing roughly 20kg of air per hour.
If this is the case, then the synchrometer on general sale won't be that accurate as the scale seems to be logarithmic with the accuracy rapidly diminishing over 10kg per hour.
I think my synchrometer is optimised for .250lit or smaller cylinders.
Have I got my maths wrong or is there another design of synchrometer available?
Read up on volumetric efficiency. It only sucks in 19kg/hr/pot if the engine is 100% efficient at filling its cylinders to atmospheric pressure on each induction cycle, IE throttles wide open and no losses. The syncrometer is designed to measure tiny flows around idle when the throttles are 99% closed, hence it's logarithm scale biased toward tiny flows of air. But the maths looks spot on for a cerbera doing the mulsanne straight.
I take the hoses of the inlet manifolds, place the synchrometer in one of the ports. Check the reading. Repeat for the opposite bank. Use the adjustable throttle link until both banks match. There is a proper sequence regarding which bank to start with and in relation to also adjusting the throttle pots.
I could be wrong but I seem to recall taking the throttle cable off, disconnecting the link and winding the throttle stop out.
Then I started the engine and held a slow tickover using the throttle stop on the offside bank. Then I reconnected the link and used the synchrometer to match the air flows. This raised the tickover which I reduced by winding the throttle stop back out.
After all this I noticed I was getting slightly different flow readings from different cylinders on the same bank which rendered the excercise useless.
Does anyone know why I would be getting different readings on the same bank other than worn throttles.
I could be wrong but I seem to recall taking the throttle cable off, disconnecting the link and winding the throttle stop out.
Then I started the engine and held a slow tickover using the throttle stop on the offside bank. Then I reconnected the link and used the synchrometer to match the air flows. This raised the tickover which I reduced by winding the throttle stop back out.
After all this I noticed I was getting slightly different flow readings from different cylinders on the same bank which rendered the excercise useless.
Does anyone know why I would be getting different readings on the same bank other than worn throttles.
If the object of the exercise is to balance the banks via the link rod then you could take the average of the readings for each bank and balance those (there's not much else you can adjust). As soon as the throttles are opened a bit, the differences between intakes on a bank become less significant.
"Does anyone know why I would be getting different readings on the same bank other than worn throttles."
There is always some small variation due to mechanical tolerances, for example the throttle return springs are at one end of a rather long shaft which over its life develops some twisting. Also minor variations in the quality of the inlet valve sealing, cam lifts etc. As rule of thumb the variation should be no more than plus or minus 5-6% between any two on the same bank as this would represent an air fuel ratio variation of maybe 14.2 on the richest and 15.3 on the leanest and that could cause emmissions problems as the lambda struggles to find an adaptive figure to keep things on the right side of the MOT requirements.
The tiny variations become inconsequential once the the throttle is cracked open a little in normal driving though. Dodgy throttle bodies with a little air leaking around the shaft seals are an MOT and idle problem not really a performance problem, unless they are really, really screwed. Balance between the two banks is more critical. You don't want one bank running a very respectable idle and part load mixture and the other bank 2AFRs leaner with a glowing exhaust manifold and overheating head. This is especially important on a cerbera as the MOT requires that both pipes are tested as it has separate exhausts per bank with no link. If either fails then the car fails. The best way to check is not axctually with a syncrometer but to look at the adaptives around idle with the diag software.
There is always some small variation due to mechanical tolerances, for example the throttle return springs are at one end of a rather long shaft which over its life develops some twisting. Also minor variations in the quality of the inlet valve sealing, cam lifts etc. As rule of thumb the variation should be no more than plus or minus 5-6% between any two on the same bank as this would represent an air fuel ratio variation of maybe 14.2 on the richest and 15.3 on the leanest and that could cause emmissions problems as the lambda struggles to find an adaptive figure to keep things on the right side of the MOT requirements.
The tiny variations become inconsequential once the the throttle is cracked open a little in normal driving though. Dodgy throttle bodies with a little air leaking around the shaft seals are an MOT and idle problem not really a performance problem, unless they are really, really screwed. Balance between the two banks is more critical. You don't want one bank running a very respectable idle and part load mixture and the other bank 2AFRs leaner with a glowing exhaust manifold and overheating head. This is especially important on a cerbera as the MOT requires that both pipes are tested as it has separate exhausts per bank with no link. If either fails then the car fails. The best way to check is not axctually with a syncrometer but to look at the adaptives around idle with the diag software.
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