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Old 03-02-2009, 08:31 AM
How fast is fast enough?
 
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cam advance/retard question

I'm in the process of putting an engine together with a 274h06 cam. Its 274/274 218/218 106LSA 102 ICL .480/.480 lift (with 1.6 rockers) My question is, because of the ground in advance and reatively tight LSA would it be possible to gain vacuum by retarding the cam to a 106 ICL? I know this is normally not true, but my thinking was that it would retard the IVO just enogh to actaully gain vacuum, even though it'll also retard the EVC and the IVC am I just over thinking this one?

I'm just trying to get around 10" at 800 RPM.

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Old 03-02-2009, 09:01 AM
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Moving the cam around won't change the idle vaccuum.
If anything, runing the cam a bit advanced will maximize idle vacuum because the intake valve closes sooner creating more engine torque output at idle, which allows the smallest possible carb throttle opening at idle for a given idle rpm.

Optimizing the idle spark timing, will have much more effect.
The high cam overlap allows/requires increased idle spark timing.

"Hi bleed" down lifters will increase idle vacuum.

1.6 ratio rockers may reduce idle vacuum slightly.
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Old 03-02-2009, 09:21 AM
How fast is fast enough?
 
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I know about the rockers and the lifters, but those are two things that I'm not looking to change. I was hoping that some tuning may help. I know that retarding the cam usually doesn't help vacuum, but I didn't know if the narrow LSA would possibly change that. I guess I'll just have to live with whatever it comes out to. I've ehard of others with this cam saying it idles at about 800RPM, which is what I'm really trying to get. I don't want a fast idle.
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Old 03-02-2009, 11:13 AM
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cam

Advancing the came is the direction to go to gain vacuum. But it reallly don't make a bunch of difference. like if you have 11.5 hg now and you advance the cam 4 or 6 deg it might be 11.75hg.


With that cam ,you should have no issues getting the Vacuum you are after, unless it's a 8:1 static compression engine , or something like that.

Remember, like was stated before, Ignition timing will make a bigger impact then anything.


Keith
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Old 03-02-2009, 11:35 AM
How fast is fast enough?
 
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thanks for the help Kstar. I apprecaite your reassurance on the vacuum. This is a narrow LSA and even with the small duration it had me a little concerned about the idle. My compression is around 9.2:1.
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Old 03-02-2009, 01:55 PM
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AHHH its a baby cam. Not to worry.
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Old 03-02-2009, 02:19 PM
How fast is fast enough?
 
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Its a lot different from most of my builds, that's for sure! Usually I'm running the biggest cam I can live with, so the idle issues still haunt me.
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Old 03-02-2009, 02:28 PM
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Remember, it's not a retarded question. The only retarded question is the one you don't ask.
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Old 03-02-2009, 03:24 PM
How fast is fast enough?
 
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Or is it a question of an advanced level?
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