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Old 04-26-2008, 11:28 PM   #1
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Default Cam Timing And Forced Induction

I brought up cam timing a while back on FI motors and was basically told I was wrong and that our cams are rather ideal for FI. Consider the following.

Here's a graph I hand copied out of Corky Bell's book Supercharged. When I say copied, it's ***** on accurate, to scale, etc. It shows cylinder pressure vs. crank angle. I do believe his graph is inaccurate. It shows cylinder pressure for a naturally aspirated engine going to zero around 105* after TDC. There's no way. Just think about it. Everything I've ever read, and everyone I ever spoke with that was knowledgeable on this subject will tell you the exhaust valve opens while there is still pressure in the cylinder. Granted his graph was meant to illustrate the difference in cylinder pressure's for a NA and FI motor. The bottom line after peak pressure is his line. The one above it is the one I believe is more accurate. I highlited it but it didn't show up when I scanned it.

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If there's still pressure in the cylinder why would you open the exhaust valves, letting it out? Surely it's doing work, pushing the pistons, and making HP. While that is true, the engine has to expel the exhaust gases on the exhaust stroke. This takes power for the piston to push all of the exhaust out. Welcome to pumping losses. Usually the exhaust valve is timed to open such that pressure in the cylinder is minimal and doing little work. When the exhaust valve opens, exhaust gases still under some pressure poor out of the engine, which reduces pumping losses. The exhaust valve timing is modeled around striking a balance between these two such to maximize HP. Our exhaust valve opens at 127* after TDC (ATDC).

Let's assume an arbitrary value for cylinder pressure when the exhaust valve opens. Say 100PSI. Mazda engineers determined opening the exhaust valve at 127* ATDC resulted in maximum power. Now let's turbo charge this motor. As Bell suggest, and many others, cylinder pressure is up later in the power stroke due to the increased amount of oxygen burning. As his graph suggest, a FI motor will have more cylinder pressure at any given crank angle over a NA motor, as well as carry cylinder pressure further into the power stroke.

If this is so, then it sure sounds like our exhaust valves are opening too soon. A lot too soon. I am going to assume our motors are at the arbitrary value of 100PSI cylinder pressure when the exhaust valve opens. Now that this motor is FI, we have more than 100PSI in the cylinder when the exhaust valve opens. More like 300-400 PSI. Looks like ideally we would hold our exhaust valve closed a bit further through the power stroke and capitalize on the the higher cylinder pressures that are present later in the power stroke between 127* to say 140* ATDC.

Thoughts?
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Old 04-27-2008, 05:08 AM   #2
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Also, Bell say's that power is the area under the curve. This isn't true either.

Power is more like the pressure at any given moment in time, times the sin of the angle because at TDC, your not inducing any torque into the crnakshaft, hence cylinder pressure x sin(0)=0. At 90* all the force from the piston is going into the crankshaft, hence cylinder pressure x sin(90)= cylinder pressure. Of course, then one could take into consideration the connecting rod angle too at various angles too.

Perhaps something like the intergral from 0 to pie of f(x)sin(theta)

Where 0-pie is the crank angle from TDC to BDC, f(x) is your function cylinder pressure, and theta is your angle in crankshaft degrees.

Of course that's still oversimplified because it neglects the constantly changing con rod angle. Looks like the real formula is quite complex, but it's not simply area under the curve. One could easily double the area at or near TDC and not get much more power.
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Old 04-27-2008, 06:46 PM   #3
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Ok, this isn't about cam timing exactly so much as it is about the graph I posted. Oh well, nobody responded to any of that anyway so new topic.

By looking at the graph, one can see that adding 1 bar of boost increases peak cylinder pressure by about 30%. That puts 30% more load on the connecting rod. Peak cylinder pressure occurs around 25*-30* ATDC. It's at this moment when the con rod and pistons and everything are under maximum strain.

Now, what if we wanted to keep our stock engine? Let's say our new goal is NOT to make 350whp as efficiently as possible, IE with the least amount of boost and fine tuning, but rather to do it the most reliable way possible with regards to the stock engine.

Retarding ignition timing would decrease peak cylinder pressure. That's good for the pistons, con rods, etc. However, it increases EGT's. That's hard on the exhaust valves.

One could probably put their car on a dyno and fine tune their fuel and spark maps for maximum HP at say 20 PSI with a T3/T4 and reach 350whp. However, this motor is tuned on the verge of knock.

Would it not be better to tune the same motor for say 22 or 23 PSI and tune timing a few degrees more conservatively? In my mind, it seems you would be building a more reliable engine by simply running more boost and keeping the timing conservative. This does a couple things. One it shifts peak cylinder pressure further away from TDC. As you move away from TDC, peak pressures drop. To me it seems like running more boost and less timing would actually make the same 350whp while having lower peak cylinder pressures. However, this would be at the expense of increased EGT's.

I'm not sure if the increased EGT's would be significant or detrimental to the engine. Perhaps the WI would keep EGT's in check.
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Old 04-27-2008, 06:53 PM   #4
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well, how does LPP fit into this, and I assume that unless the exhaust valves are open at the lpp, then you still get the same cylinder pressures.
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