Quote:
Originally Posted by Anthony
cool engine build !!!
he stated the engine was built to be a blower motor, with appropriate heads, cam, compression (12.5), and made 790 HP. he thought that if he were to built it to be a NA engine, with ? 16:1 compression, different heads, cam, and dry sump, vacuum pump, bigger throttle body, it would make closer to 900 HP.
when he installed the supercharger, he also installed a dry sump , and ran the boost at 16.5 psi. he never mentioned the difference in air supply temperature.
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In the n/a video he actually did say it was built to be supercharged and at that time it was sporting 12:1 c/r pistons. Again, his ambiguity and decision to avoid answering the c/r question in the second video with the supercharged engine makes what he actually used in the blown engine build difficult to determine, at best. I suspect with a little more time (and vids) the answer will become clear. My bet is still on the reduced compression number.
Ed
p.s. We have often heard the old saw that an engine is just an air pump. In fact that is why cams and head selections carry the significance they do. We are attempting to optimize the pumping efficiency of the engine. the short block is only about reliability.
As luck would have it it takes 10lbs of air to produce 100 horsepower. The interesting thing is it is fairly constant across fuel choices, i.e. gas, methanol, ethanol but not nitro — because nitro is a monopropellant.
Once we achieve a 100% Ve the only way (with out boosting) to increase power is ram tuning of the intake which tends to be rpm specific and not a uniform increase across the operating rpm band. So where does c/r fit into the picture?
Remember when we were kids and got out contraband fireworks for the 4th or July? We all had some with fuses that never lit the firecracker. When we cut them open and ignited the gun powder with a match it just burned with a flash. The event was sort of like the photographers before flashbulbs were invented — just a flash, lots of smoke and light.
Well take that same gunpowder and wrap it tightly in its firecracker paper and external shell and it wouldn't burn it would go bang! Same thing with fuels. If you pour gas on the ground and light it, it burns with a yellow flame but not explosions. Put the same fuel in a hot cylinder, start compressing it and ignite it 30 or 35 degrees before TDC and you have a controlled but very fast burn. The pressure of the burning gases peaks, pressure-wise, somewhere around 7 to 10 degrees or so ATDC, pushing the piston down with a lot of gusto.
If you run too much timing the in cylinder pressure rise is too steep (quick) and the mixture literally explodes at or very near TDC creating the engine killing knocking sound we all recognize and try to avoid. Increasing octane slows down the burn rate and allows a wider window in crank degrees for the burning mixture's pressure to work on the piston.
A Internal Combustion Engineer (ICE) tries to engineer his engine design metrics and ignition curve for the most commonly available fuels to produce the advertised horsepower target. He does this by manipulating the in chamber rate of burn (measured in feet per second).
Once you have fit the completition of the burn event into that 7 to 10 crank degree window ATDC you have not only done all you can for that charge but it has provided all it can for engine power production. If, for example, you were to increase c/r for this event in this engine there would be no increase in power.
Why, you say. Well you have already previously converted all the fuel and air through their normal oxidation (burning) process into pressure energy. There is no more fuel to convert therefore no more power to be made.
Yes but, you say, all we need to do is add more fuel to fix that problem. Well, yes and no. If you just add more fuel the engine will go rich and get lazy. Well what about more air? Sort of late to the dance there also. The engine is already at 100% Ve, there is no more air to be had w/o supercharging.
in general when you play with gasoline your optimum if not maximum c/r is right around 17:1,
if you have the octane to support it. When you have a 12:1 c/r you can safely boost the engine by about ½ BAR (atmosphere). When you multiply 12 by 1.5 BAR (1 BAR is ambient) you get an effective c/r 18:1 — well dang don't that just beat all!
How about my engine 8.8:1 c/r with 2 BAR (atmospheres) of boost and an effective compression ratio of 17.6:1! Well dang! Don't that just beat all, again.
For gasoline and short bursts of performance your threshold max target operating compression is essentially 17 or 18 to 1. Higher and you detonate, lower and you leave power on the table. Other fuels have higher target compression ratios associated with their autoignition and critical points.
The 10 lbs of air per 100 horsepower is an excellent rule of thumb to tell you when you have gotten all there is to get for a particular mass flow of air. At that point fiddling with timing or c/r will bring no benefits to the table. If you want more power you need to process more air — it's really that simple. The rest of the dials will have no effect until you process more air. That means either a bigger engine, higher engine speed or a supercharger.
Ed