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Reading Georges post on the wide torque curve got me thinking. There seems to be some misconceptions on timing and the ability to run high compression motors on pump gas. There are many things that affect the anti-detonation properties of any motor. Port velocity, squish, chamber size/shape, plug placement, and bore size are some of the most important. Squish alone can make a motor with 11 to 1 compression have better anti-detonation properties than the same motor with 10 to 1 and poor squish. We can alter most of these with cylinder head design and gasket thickness. What they are trying to promote with these is a faster burn speed. A faster burn is much more resistant to detonation. Chamber turbulence is a MUST. The old cleveland heads have large valves for lots of airflow but do not have the best chamber design. Some are better than others but regardless there is lots of room for improvement. With bigger valves and larger runners the air entering the combustion chamber (particularly at low rpm's has a poor swirl because of low velocity air. Newer designs utilize smaller runners, more central placement of spark plug for better flame front propagation and specifically designed quench pads to aid in keeping the air turbulent as the piston comes up. With this significantly faster burn speed peak cylinder pressure happens sooner in the cycle. It has been found that peak cylinder pressure should happen around 15 degrees ATDC for peak torque production. If our burn is happening faster and peak pressure is happening sooner we have to reduce timing to make best power. We tune A LOT of modified muscle cars and have found that we end up pulling timing not because the cars are detonating but because they are plain and simply over timed. We have seen power gains in excess of 20HP by LOWERING timing by several degrees. Aluminum heads also have a higher detonation threshold because of their ability to shed heat faster. Remember, more timing does not always = more power.
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We have seen power gains in excess of 20HP by LOWERING timing by several degrees. Aluminum heads also have a higher detonation threshold because of their ability to shed heat faster. Remember, more timing does not always = more power.

That's right. BTW, my book deals with optimizing the timing, including effects like the one mentioned... Cool
I would think piston design also plays into the effectiveness of a chambers design given the pistons opposing surface and its relation to quench and ability to create turbulence in the chamber. If an engine builder (not assembler) takes into account the design of the chamber in relation to the opposing surface of the piston being used, they would have a better idea of how to build in a quicker burn.

Since we are on the subject of naturally aspirated pump gas, lets see if we can bring up some pump gas cleveland headed examples.

This has probably been posted before, but they used yates intake modified for an injection system with procomp modified heads (yeah i know), and I would be interested to know what the volume was on the intake. my understanding is FPS reworks the whole head including chamber. While they do have a methanol injection setup on it, its still running on 91 octane with 11:6 compression. I'm not endorsing a knockoff, just interested in how they worked the head for this displacement motor. They didn't say what ratio of water/meth though or when the methanol is being triggered.


i went with my girlfriend to help her buy a mazda3 with their skyactiv direct injection engine which injects fuel directly into the cylinder. No dribbling of fuel here, and they are running 14:0 on euro pump gas vs 12:1 in the US on 87 octane. The other hidden advantage is they can pull/add timing on the fly with the ECU of course.

I know this is stating the obvious, but it seems largely the secret to a carbureted car running more compression is by matching a head to a displacement capable of holding a velocity with a given cross section and throat size so that the air moving fast enough to keep the fuel in suspension vs part of it dribbling down the runner into the chamber creating inconsistent burn. You open the valve up and you have a puddle waiting. you put too big of a head on an engine and you get puddling at low rpm, you put too small of a head on an engine you won't know if its a dud until you compare it to a similar cubic inch motor with a bigger head. Then add in all the other variables.

I assume this is the basis for injection systems on street cars for emissions, because you are placing the point of injection/atomization closer to the valve/chamber and directing it as well vs a carb, you are pretty much spraying and praying hoping to get even distribution through to each runner, no pun intended Wink same reason low rpm improves going from downleg to annular boosters. better fluid atomization.
Yes, piston design is very important in the combination and most often overlooked as it is too easy to buy "off the shelf" pistons. In a perfect world the piston would be matched to a specific head. Quench pad matching is very important especially on dished pistons. On flat tops its not as much of an issue as the entire piston is on the same level as the quench pads. For domed pistons matching the piston to the head can help flame propagation through the combustion chamber. Big domes affect burn speed because the flame front has to travel around these raised areas.

Fuel injection is certainly leaps and bounds better than carburetion for atomization. With the modern injection systems we can dictate not only how much fuel we are injecting but also at what point in the engines cycle. At low rpm's and idle we can start the injection when the intake valve is open and the port velocity is highest. The makes a very noticeable increase in idle quality as well as gereral driveabilty. At high rpm's when we are running higher duty cycles and cannot fit the injection event into ideal times it is less important because port velocity is so high.
Direct injection goes even farther above this. At this point we are injecting super high pressure, cold, highly atomized fuel directly into the combustion chamber exactly when we need it. This allows previously unheard of compression ratios that are able to run on pump gas. You would be amazed at the boost these engines can run in stock form. The drawback being increased maintenance due to fouled intake valves. In a nutshell technology has advanced to the point where 11 to 1 is on todays motors is no longer "High Compression".
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