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Old 04-11-2006, 05:41 PM   #1
drdeuce
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Default AVCS advance clarification

I'm installing new ported heads with 1mm oversized valves and DPR cams. I'm using a Hydra for tuning.

Phil wanted to know at what degree of advance would there be interference between the intake and exhaust valves. He said that AVCS can advance the intake cams about 15 degrees, so if I install the timing belt but advance the intake cams two teeth while keeping the exhaust cams in normal position, I should be able to see if there is interference by very slowly turning the belt.

The question I have, and even Phil wasn't sure, is to advance the intake cams should they be rotated slightly clockwise (same as the crank turns) or counterclockwise. Phil said he thought clockwise because the crank turns counterclockwise, but that doesn't seem right as I consider the crank to turn clockwise and he said he was unsure himself.

And also, just to confirm, the AVCS does advance the intake during the lower RPM range to increase torque/spool, correct?
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Last edited by drdeuce; 04-11-2006 at 06:01 PM.
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Old 04-12-2006, 01:05 AM   #2
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....I believe this is a 'built motor' forum question
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Old 04-12-2006, 07:53 AM   #3
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In street tuner the Cobb Stg2 map has 25* of advance, and cobb says not to go over ~30*. So I don't know where Phil got the 15* from.

TMS

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Old 04-12-2006, 11:21 AM   #4
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From what I was told by Cobb, the AP/ST AVCS values were computed via camshaft rather than crankshaft values, effectively doubling the net user viewable number. Therefore 15 degrees of crankshaft advance would be equivalent to 30 degrees of cashaft advance. The hydra avcs advance values are computed over crankshaft rather than camshaft and so are the ECUTEK and stock ECU. I know that Cobb discussed making mods in future versions of the ECU to update this; however I'm not sure if the change has been made. It's really not a big deal, as long as the values and their relative effect is understood by the tuner.

My $02.
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Old 04-12-2006, 01:36 PM   #5
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Quote:
Originally Posted by syoung0298
From what I was told by Cobb, the AP/ST AVCS values were computed via camshaft rather than crankshaft values, effectively doubling the net user viewable number. Therefore 15 degrees of crankshaft advance would be equivalent to 30 degrees of cashaft advance. The hydra avcs advance values are computed over crankshaft rather than camshaft and so are the ECUTEK and stock ECU. I know that Cobb discussed making mods in future versions of the ECU to update this; however I'm not sure if the change has been made. It's really not a big deal, as long as the values and their relative effect is understood by the tuner.

My $02.
that would make sense since cam gears are 1/2 the diameter of the crankshaft.
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Old 04-12-2006, 01:50 PM   #6
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Quote:
Originally Posted by Illusive
that would make sense since cam gears are 1/2 the diameter of the crankshaft.
Here I was thinking it was the other way around
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Old 04-12-2006, 01:52 PM   #7
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OK, so to advance timing, the camshaft would be effectively turned counterclockwise (opposite direction of the crankshaft rotation), so that it is at BTDC earlier, correct?

thanks
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Old 04-12-2006, 02:39 PM   #8
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Quote:
Originally Posted by syoung0298
From what I was told by Cobb, the AP/ST AVCS values were computed via camshaft rather than crankshaft values, effectively doubling the net user viewable number. Therefore 15 degrees of crankshaft advance would be equivalent to 30 degrees of cashaft advance. The hydra avcs advance values are computed over crankshaft rather than camshaft and so are the ECUTEK and stock ECU. I know that Cobb discussed making mods in future versions of the ECU to update this; however I'm not sure if the change has been made. It's really not a big deal, as long as the values and their relative effect is understood by the tuner.

My $02.
right on the money
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Old 04-12-2006, 03:20 PM   #9
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Once you have changed the lift and duration, you won't know if you can get interference, but I'd think with degree wheels on both the intake and exhaust cams (no belt, heads off) you should be able to determine how many degrees of intake advance is possible without interference. Mike at Gruppe-S might know the answer to your question as he spent a lot of time with the DPR cams and degree wheels.
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Old 04-12-2006, 05:35 PM   #10
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Why wouldn't I be able to tell? According to Phil, who usually knows what he's talking about, i should test this with the timing belt on and advance the intake sprockets and carefully check for any interference. Are you saying Phil is wrong?
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Old 04-12-2006, 07:12 PM   #11
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there's static interference, and then there is what is safe at 7000 rpm. Domestic folks have a pretty standard Piston-Valve clearance number between .050 and .035 depending on the angle of the valves and other stuff. I dont know how you would perform the old clay test unless you did it for a whole bunch of different AVCS advances, which is why Mike at Gruppe-S probably spent a long time at it.
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Old 04-12-2006, 08:45 PM   #12
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I meant you "don't know" once you move from the stock lift and duration, not "can't tell." I'm saying with a degree wheel you'll know how many degrees you need to keep them apart.

The crank turns clockwise when looking at the enginen from the front of the car and the opposite when you have the clutch off, looking toward the front. I'd have to look up the camshafts and the path of the timing belt.
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Old 04-12-2006, 09:06 PM   #13
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The crank and cams all turn clockwise. The intake cam is advanced by moving it further clockwise relative to the exhaust cam.
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Old 04-13-2006, 12:21 AM   #14
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Quote:
Originally Posted by syoung0298
From what I was told by Cobb, the AP/ST AVCS values were computed via camshaft rather than crankshaft values, effectively doubling the net user viewable number. Therefore 15 degrees of crankshaft advance would be equivalent to 30 degrees of cashaft advance. The hydra avcs advance values are computed over crankshaft rather than camshaft and so are the ECUTEK and stock ECU. I know that Cobb discussed making mods in future versions of the ECU to update this; however I'm not sure if the change has been made. It's really not a big deal, as long as the values and their relative effect is understood by the tuner.

My $02.
This confuses me. There are 24 teeth on the crank and 48 teeth on the cam sprocket. So if I advanced the crank one tooth (15 degrees) the Camshaft only turns half as much and would only be advanced 7.5 degrees.

How does 15 degrees of crankshaft advance equal 30 degrees of camshaft advance. Seems to me that it would be the other way around. What am I missing here?
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Old 04-13-2006, 12:55 AM   #15
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According to this document: http://www.drivesubaru.com/Win05_WhatsInside.htm

AVCS can advance the "CRANKSHAFT TIMING" as much as 35 degrees. This is slightly more than two teeth on the crank pulley (30 degrees). And since the camshaft pulleys move 1/2 the speed of the crank, 35 degrees of CRANKSHAFT advance would equate to about 17.5 degrees of CAMSHAFT advance.

Therefore, if cobb says it can advance it 30 degrees, this must be based on Crankshaft advance. This would also indicate that Phils statement about 15 degrees is correct if it applies to Camshaft rotation.

So based on this, the above info appears wrong to me. The Cobb must be referring to Crankshaft advance, whereas the Hydra is based on Camshaft advance.
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Old 04-13-2006, 08:22 AM   #16
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When I had a chance to talk to my tech what Bboy said is correct, rotation is clockwise and rotating the cam sprocket clockwise advances the cam. I should have remembered this since I watched while this was done This is why I tune the engines and don't build them

How the AVCS "target" is determined is irrelevant in this case but for the record the Hydra’s targets are also in “crank degrees” but the values used are on a different scale than 0-30 degrees. Two teeth on the cam sprocket is approximately what the AVCS system is capable of moving the camshaft and is absolutely the easiest way to check clearance. There are more precise, more difficult, more expensive, and more time consuming ways to determine interference.

This isn’t rocket science. If you want to know exactly how many degrees one tooth of the cams sprocket is then count the number of teeth and then divide 360 by that number. 360/48 = 7.5 degrees per tooth on the cam sprocket. On the crank sprocket there are 24 teeth so 360/24 = 15 degrees per tooth.

This will conclude our math lesson for the day

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Old 04-13-2006, 12:44 PM   #17
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Thanks Phil, this confirms what i thought was true. Basically advancing the intake sprockets two teeth forward (15 degrees) would equate to 30 degrees of crankshaft timimg advance.
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Old 04-13-2006, 05:50 PM   #18
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The crank rotates twice for every 4-stroke engine cycle, so the cam degrees are half the crank's because of the exhaust stroke. In a 2-stroke each cycle corresponds to crank turn.

Using the belt and the teeth is practical for a go or no-go assessment.

Nice to know the number of teeth, I've never counted them. Good luck!!
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