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Old 01-16-2007, 08:20 PM   #11
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I also found a post that confirmed I may have to watch the ignition timing after the swap.
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Old 01-16-2007, 08:43 PM   #12
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I would set the timing back to 6 degrees advanced like the factory calls for. I don't think you will have any problems with it knocking unless you have really bad gas or something. A knock sensor out of something from a junkyard is pretty cheap but if you have good hearing then you don't really need it.

Also I got better mileage out of my xfi when I have the cam advanced to 11 degrees
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Old 01-16-2007, 09:14 PM   #13
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Do you have to run higher octane with the cam advanced 11 degrees?
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Old 01-16-2007, 09:42 PM   #14
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Nope I am still running 87 with the cam at 11 degrees and .040 shaved off the head so I am at 10.5:1 compression ratio. I am only running 4 degrees of ignition advance but the car seems happy with this combo. I just need to get it to run for 3 weeks or so straight without breaking down to get a good mileage reading.

I would put the cam in straight up with the ignition at 6 degrees and get some time on it before changing the cam/ignition timing just so you get a feel for the mileage with the new cam before changing more variables.
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Old 01-17-2007, 08:56 AM   #15
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Quote:
Originally Posted by MetroMPG View Post
Apparently not. My understanding: the different cam profile - lower lift & duration - raises the cylinder pressure and shifts the torque peak downwards, so less throttle is required at low RPM. It's a mechanical thing.

It lowers the XFi's HP rating from 55 to 49, but the torque value is unchanged (the RPM at which the max occurs is lower). (EDIT: The cam alone may not be the HP limiting factor - it could also be ECU.)

A number of people have done similar swaps including GeoMetry on this forum,
I sense a bit of "misunderstanding" here....
A lower lift and/or duration cam profile (opening the "doors" to gas flow later, closing them earlier, and holding them open for a shorter time) is going to raise the BMEP? ...and shifts the torque peak downward? How so? If gasoline is the source of power (pressure), then how is less of it gonna make more?
Torque is what moves the vehicle. Power is the rate of producing this torque.How is less of it going to move it better? Easier? More efficiently? Are you aware that HP = torque (by definition) at 5252 RPM? The two are tied together so tightly that "nothing moves without them". Check your physics book!
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Old 01-17-2007, 09:42 AM   #16
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MetroMPG: I would go ahead and make the cam swap. I would also set your timing up to match whatever the Xfi specification is, certainly to start out with.

The cam basically controls when the valves open and close, relative to the crankshaft and pistons. It won't change the actual compression ratio, but it can change the effective ratio. Additionally, the cam lobes can be ground to have different ratios of opening and closing, how long the valves are open/closed and when they are open and closed. Depending on the engine rpm, they are optimized for a specific rpm, but are generalized at the same time, to allow the engine to operate over a wider rpm range.

It is possible to get cam's set up that are optimized for lower rpm, but they typically run more poorly at higher rpm's. When Honda introduced their VTEC technology, it basically allowed them to have two different cams in the engine, one optimized more towards the low end, one towards the high end.

I would doubt it you will need to change the ecu. To a large extent because of your driving style, I expect the engine is generally in closed loop, so that it is monitoring your O2 and other factors, to control the engine parameters. In open loop mode, the setups in the ecu are probablly already close enough, for what your asking it to do.

I've thought about pulling a camshaft off of a 91-94 CX and putting it into my 89 Wagon, towards the end of improving the fuel economy. However, it's rather far down my list of things to do and preceded by issues of more a more pressing nature, which seems to be the way things go, more often than not.

Make the change and then let us know how it goes, in comparison to your previous mileage.
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Old 01-17-2007, 09:55 AM   #17
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I'll be the first to admit I'm not a cam expert. You can see I've mostly quoted things I've read elsewhere.
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Old 01-17-2007, 10:01 AM   #18
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It won't change the actual compression ratio, but it can change the effective ratio.
Yes, I had read this also. Which addresses Ted's question, "If gasoline is the source of power (pressure), then how is less of it gonna make more?"

Quote:
It is possible to get cam's set up that are optimized for lower rpm, but they typically run more poorly at higher rpm's.
That's true.

It's mentioned in the source I linked in the previous post. The shorter duration on that Olds cam meant a higher lift rate was used, which resulted in a lower red line (the valves would float at a lower RPM because of the higher lift rate).

The same is probably true of the XFi cam - and perhaps that's another difference in the XFi ECU: a lower rev limit.
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Old 01-17-2007, 01:47 PM   #19
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One of the items which is on my list of I'd like to do, is to get the HF transmission I have, rebuilt, so I can get it into the Wagon. After I do that, my rpm should come down substantially and then I may try to make a cam swap to more adapt to that change.

In your car, I believe you already made that change?
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Old 01-17-2007, 03:27 PM   #20
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Yes, I've swapped transmissions and have a much taller final drive. So you're right: the cam swap and lowered torque peak should match the engine better to the gearing I have.
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