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Old 04-03-2008, 12:28 PM   #1
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Regrinding a camshaft for FE

In my years of working on cars, when it came time to work on or replace a camshaft, I often would either buy a stock cam or get a torque cam for a truck or a mild performance cam for a sports car. Now with my eye on fuel economy I was wondering how much would it cost to grind a factory cam to better mpg specs. Like the Metro XFI cam, from what I have read it has a little less duration and lift. I never used to think about cam grinding, that was always left to racers, since your removing metal, it seems you could grind what ever you want.

Now on to an even larger experiment, most 4 valve heads were meant for performance, take that same 4 valve head and grind down just one lobe on the intake side. It seems to me that would create a swirl, this should produce better torque, mpg, and a cleaner leaner more turbulent burn.

Honda's VX-HX lean burn system got me to thinking, since it has essentially one intake valve that sits closed until activated at higher rpm, your thoughts?
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Old 04-03-2008, 01:08 PM   #2
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what motor would you be doing this to? i would love to do this and try it out! (no spare money though) i would suggest something like the D15Z1 or D16Z6 try it on a fe and a sport motor to see the effects u know?
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Old 04-03-2008, 01:08 PM   #3
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I'd like to get a 4 valves per cylinder head and split the port, so fuel is on one side and just air on the other, then splay the lobes such that the air side is open longer than the fuel+air side...
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Old 04-03-2008, 01:27 PM   #4
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Now on to an even larger experiment, most 4 valve heads were meant for performance, take that same 4 valve head and grind down just one lobe on the intake side. It seems to me that would create a swirl, this should produce better torque, mpg, and a cleaner leaner more turbulent burn.
I'd be afraid that you'd get uneven distribution and create uneven cylinder pressures, side-loading the pistons and wearing out your cylinders & rings prematurely.

If there is an aftermarket company that makes cams for your car, chances are it's not too difficult for them to order a custom cam. When I buy a cam for my big V8, it takes zero extra effort for them to custom-specify a cam since they keep the blanks in stock then machine them to meet demand.

-BC
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Old 04-03-2008, 03:45 PM   #5
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Quote:
Originally Posted by Scott View Post
Now on to an even larger experiment, most 4 valve heads were meant for performance, take that same 4 valve head and grind down just one lobe on the intake side. It seems to me that would create a swirl, this should produce better torque, mpg, and a cleaner leaner more turbulent burn.
That works, grind one cam lobe to retard the opening of it and have them close together.

I want to get a metro and grind a cam to keep the intake valve open to about 75 deg ABDC and then put a small turbo or supercharger on it. That will effectively reduce the displacement to about .7L so and give a better expansion ratio. The turbo is to make up for the complete loss of power that I'll get from doing that.
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Old 04-03-2008, 04:29 PM   #6
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Now on to an even larger experiment, most 4 valve heads were meant for performance, take that same 4 valve head and grind down just one lobe on the intake side. It seems to me that would create a swirl, this should produce better torque, mpg, and a cleaner leaner more turbulent burn.
Well, it can even be used in a high performance application.

In the mid 90's there were cams floating around in the motorcycle racing industry called "scatter cams" that did exactly what you're talking about.

I think the idea was that it generated better velocity. The engine would have an initial suck of AF through one valve, then the second valve would be added..and that would help produce better acceleration and feel in a road racing application.

I haven't heard of anyone doing it anymore, but everything is also fuel injected too.

Swirl in the combustion chamber and how it "sticks" to possible positions on the cylinder wall can be a very big issue and a black art.

One could simply shim the valves differently to time the individual intake valves a little out of sync, on at tolerance the other with a wider clearance, to experiment.

Exhausts don't seem to need to have this kind of treatment.
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Old 04-04-2008, 05:12 PM   #7
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Scott,
The "off set valve" arrangement you are suggesting is not uncommon in many engines. Grinding one cam lobe will achieve the same effect but it may well take some detailed experimenting to manage the complete mixture rather than having a vortex inside the cylinder which simply centifuges the mix to the outside / walls and leaves a relative vacumm in the centre.

Most car makers aim for tutbulence and use the squish area within the cylinder to get it.

Some worthwhile research by David Vizard has been done on this topic.
You may want to track down some of his books.

Finally work by Ricardo on this same topic established some parameters like gas flow speeds , port angle and squish band ratios which are still viable today despite the time gap from the early 1920's until now.

Cheers , Pete.
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Old 04-04-2008, 05:31 PM   #8
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Remember that asymmetric valve opening/closing is not possible on ALL four valve heads. While Hondas tend to use separate lobes for each valve, other engines (ie Nissan SR20) actuate each pair of intake/exhaust valves with a single cam lobe.
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Old 04-04-2008, 06:48 PM   #9
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That works, grind one cam lobe to retard the opening of it and have them close together.

I want to get a metro and grind a cam to keep the intake valve open to about 75 deg ABDC and then put a small turbo or supercharger on it. That will effectively reduce the displacement to about .7L so and give a better expansion ratio. The turbo is to make up for the complete loss of power that I'll get from doing that.
That's called a Miller Cycle engine. Generally considered to be more efficient, and more complex, than an Otto cycle engine.
Mazda built a "2.3L" Miller-Cycle V6 a few years back.
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