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Here is one dyno shakedown graph. This is with the new short block and valve springs set up a little tighter. At 0.8 bar (12 psi) and 6200 rpm, the valve spring setup change doesn’t enter the equation, so for practical purposes the only change is the short block.
The new short block seems to be running very well so far:
What’s the minimum spark duration for a gasoline engine? Let’s say modest boost of 1bar and not cold but warm charge.
Should I think of the minimum spark duration in terms of degrees or time?
The 1987 928 S4 has a spark duration floored at 30 crankshaft degrees by EZK. The spark duration is whatever it is until the dwell time hits 150 crankshaft degrees. After that as rpms grow, the dwell time gets locked to 150 degrees which in turn gives the spark 30 degrees to burn. As rpms then increase, both the spark duration and dwell time shrink proportionally.
Some sources say that 0.6 milliseconds is the minimum spark duration. However, it’s also clear that a high tumble 4-valve engine at high rpm will not need long to light up the charge. The ignition advance calculations prove that at high rpms the charge doesn’t need anything like 0.6 ms or 30+ degrees to light the charge. We want to increase the engine speed and I’m calculating 0.55 ms at 8000 rpm. I think but don’t know that this should be perfectly fine and enough, all things considered. What do you think?
Then there’s the question of spark energy. If the numbers in this post are correct ( https://rennlist.com/forums/944-turbo-a ... st15838894 ) then there's considerabe headroom with stock 928 coils for higher than stock 6700 rpm redline.
By the numbers in that post, the coil should charge to the 9 A limit in 9 A * 5.85 mH / 13.8 V = 3.8 ms if the system voltage is 13.8 V. This will give spark energy of 237 mJ. If we cut the dwell time to 3.2 ms (8000 rpm in a 928 with the 150 degree maximum dwell ignition computer “EZK” parameter), the peak current drops to 7.55 A and thus the spark energy to 167 mJ. This is a 30% drop, but still far above my guess of the absolute minimum to light up the charge. My guess is that the absolute minimum spark energy for normally aspirated gasoline engine is something like 30 mJ, care to comment?
Another question is the efficiency of the whole circuit. If we input 100 mJ, how much is in losses and how much actually ends up as spark? 50% i.e. 50 mJ per 100 mJ input?
These numbers would be reasonably sensible in light of the 928 stock system's 4.5 ms dwell time at 6000 rpm.
So will this setup run at 8000 rpm without any modifications? Guess there’s only one way to find out.
Working as the lowest level pit crew hand all weekend, this is fun!
Does that mean you're only handing out water bottles?
Well I managed to install the rear axle drive sprocket hub the wrong way around in the hurry, which cost the driver the third prelim heat win, he nursed the car to second place in that heat. This after him breaking the rear axle trying to pass the fastest car here in the second heat for the lead, despite being 14% down on power compared to the fastest car. Swapping that axle is an eight hour job, but three and half men (me being the half) got the car to the starting line in hour and fifteen minutes. Then the sprocket started slipping and he had to settle for second place. Still, starting from the front row tomorrow.
Originally Posted by Strosek Ultra
In Finland they drink Koskenkorva.
Åke
I haven’t even seen beer here all weekend, people are extremely busy. Maybe tonight.
It's just different getting to wear sandals while hoisting heavy car stuff around. I've never been lucky doing that. The agony of de feet and all that. Enjoy the racing.
First time my dad took me to see midget racing. I was about 5 years old. The winner got out of his car; he was hardly a midget!
I used to know all that spark duration and flame-front speed stuff by heart. In the forty-something years since then, it's long gone.
John is taking quick high acceleration runs to map the fuel and ignition to be approximately right enough to be safe for slower dyno acceleration rates. Recall that we are now in a territory that hasn’t been previously mapped, this run terminated at 7200 rpm. And there’s still about a thousand more rpms to map.
This is all done with boost level of about 12.2 psi which makes enough power to be relevant/informative but low enough not to stress the engine too badly.
7750 rpm now and nothing particularly bad has happened:
The car is averaging 600 rwhp in the 5100-7700 rpm interval at low boost, which I guess would make it a pretty fast car even before we turn the boost up.
Dumb question- do you have an oil pressure recording as you get into these high-rpm ranges? I know it's only a few seconds and I'm sure John is monitoring, just curious how steady it is above 7K.
question- do you have an oil pressure recording as you get into these high-rpm ranges? I know it's only a few seconds and I'm sure John is monitoring, just curious how steady it is above 7K.
Not yet as these are just stabs with breaks between them. But we need to hook one channel to oil pressure when we start “holding load” at 8000 rpm.
Making it all work with the stock intake manifold.
As everyone knows at this point, the stock S4 manifold doesn’t really want to make power above 6000 rpm.
I just recently saw some data where someone had dynoed otherwise sensibly tuned 928 GT 5.0 engine (11.5 compression, S3 pistons, ’87 S4 conrods, E85, Colt stage 2 cams, headers, exhaust) and picked up 100+ hp in the 6000-7000 rpm range by swapping the S4 intake to 48mm AT Power individual throttle bodies with trumpets.
(This is John Gill’s normally aspirated race car for those of you wondering.)
So even with valve timing appropriate for higher rpms, the S4 intake manifold reduces the volumetric efficiency of the engine pretty dramatically above 6000 rpm and the penalty gets bigger the higher the rpms. This in normally aspirated use.
Now, since we’ve got turbos, we can literally force feed the engine thru the reluctant intake manifold whatever medicine we think it needs. The question is how much do we need to ramp up the boost at high rpms, starting from 6000, to keep the torque curve about flat?
The engine produces about 19.7 lbf-ft per psia at 6000 rpm, 17.3 lbf-ft per psia at 7000 rpm, and estimated/interpolated 14.3 lbf-ft per psia at 8000 rpm.
To produce a flat torque curve at 500 lbf-ft, the system needs to generate 10.7 psi boost at 6000 rpm and 20.3 psi boost at 8000 rpm.
To produce a flat torque curve at 600 lbf-ft, the system needs to generate 15.7 psi boost at 6000 rpm and 27.3 psi boost at 8000 rpm.
To produce a flat torque curve at 700 lbf-ft, the system needs to generate 20.8 psi boost at 6000 rpm and 34.3 psi boost at 8000 rpm.
The "bad" news is that with the stock intake manifold, the existing data says that the boost ramp up after 6000 rpm needs to be very steep. Much steeper than I expected before running the numbers, we're talking about numbers like 5 psi extra at 7000 and 10 psi extra at 8000. Which instinctively sound like high numbers.
The “good” news is that torque/absolute pressure is almost perfectly linear in rpm up there so a linear correction will work really well and that’s much easier to calibrate than a full rpm-specific boost profile.
Midrange torque consistent with the simple formula
It's kind of dumbfounding how accurate the simple formula is for 6000 rpm and under for this engine: the rear wheel torque in lbf-ft is absolute manifold pressure in psia times 19.7 (lbf-ft/psia). 33 psia gives that 650 lbf-ft that we observe below:
With this new piston dish shape that reduces compression a bit and promotes tumble, there’s no knocks in the mid range rpms with 33 psia / 18.3 psig boost. That bodes well for us being able to run the engine efficiently instead of in “party-mode” and still meet our goals.
08-07-2020, 01:16 AM
Dyno shakedowns continue
