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HuD 91gt

Idle vacuum w/ aftermarket cams

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Hello everyone.   I'm curious what everyone is able to get for idle vacuum with their aftermarket cam.  I have a Colt Cams C542.s with the following specifications.

Duration @ .50" 224 I/E or 280 degrees 

valve lift .510

Lobe Seperation 108"

Engine is a L28 w/ about 8.7:1 compression.

The best I can get using Megasquirt fuel injection is around 10"s.   Sometimes jumps into 11" for short periods.  Chickenman and I have been playing with the FI for a while now working out bugs, but my vacuum has always seemed a little suspicious to me.  Either way, I just ordered a fancy schmancy adjustable cam gear to play around a little bit.  What are you guys seeing for readings?

Edited by HuD 91gt

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That's probably all you are going to get with that Camshaft and that low of compression.  I pull about 12 inches with a similar cam but 10.3 CR.

 That Cam really needs 9.5 CR or a bit more to work properly. 

You need to pop over and pick up your USB cable. and so we can finish Road Tuning.. Have a few more Tweaks with AE tuning to try.  

 

Edited by Chickenman

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Well that's good relevant info for sure. I still need to find a few hours to get the thing running again. 

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I don't think the vacuum is that low for that cam or CR as Richard points out. If your having idle issues it probably isn't that . My  490 idles at 750 smoothly . 

What head and had it been shaved ? How old is the cam chain ? You could always adjust the stock cam sprocket advanced . As RacerBrown states the intake valve closing time does wonders , and advancing it might help a little. You don't need a fancy smancy sprocket unless your really going to degree it. 

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It is a P90 head that has been shaved a few times for straightness.  The reason the CR is lower is the CC has had some material removed during the porting. The head has never been shaved to increase CR.

 

Cam was degreed properly with an adjustable cam gear.  Somewhere along the lines in my learning process I dropped the chain tensioner and ended up putting in an OEM cam gear as I ended up dialing it in at one full tooth advanced(Or was it retarded?) from a "stock setup".  Don't ask my why, but now i've lost that adjustable cam gear somewhere in my moves.   This all happened in the first couple hundred KM of the rebuild.   

 

I ordered a new one just to play around.   The slots in the OEM gear are much too large (4 degrees).  Reason I am suspicious, is the "Z man of Washington", (A fairly reputable Z business man in PNW) somewhere online stated no matter what cam you have, you should be seeing 14+" at idle unless it's an all out race cam.  More curiosities sake then anything else.  My car idles pretty decent.

But, as you guys both mentioned.  A lowish CR wouldn't help the situation.  Was more a thread to see what other people are getting.  So far, nothing too extreme.

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Depends on your definition of Race cam I guess. In his books a 270 to 280 duration cam must be a Race Cam. But it will be next to impossible to get 14" with a a Colt 542  and your CR . The 108 Lobe separation is one of the reasons why it makes such good Power... but that will adversely affect idle quality.  The tighter the load separation angle, the more mid-range Torque you will get, but at the expense of idle quality and idle vacuum. You might get 12" with 9.5 compression or higher... but it's doubtful.  That Cam will really come alive with more CR.

You can raise low end advance to help out... but there is a limit to that. I already had it pretty aggressive at 18 BTDC on Tune #3. Then we raised it to 20 during the test run. That was to take care of the Cam overlap . From tune #5 I see you raised it to 21 BTDC. That's getting a bit high... but still workable. I wouldn't go much higher though, or you could run into some issues with knock just of idle transition. 

I used to run a Digressive Timing on my Hillclimb Camaro. 24  Degrees at idle ( 1,000 RPM ) then immediately dropping to about 16 degrees just off idle and then the Curve built normally to 3,000 RPM. Even at 26 BTDC all I ever saw was 12" of Vacuum  on a Chevy 350 with 10.5 CR. Cam was and isky 280 with 224 degrees at  .050" and .485 lift.  LSA of 108 degrees. Which is surprisingly similar specs to a Colt 542 grind.  I ran 1.6 Ratio rocker arms which gave me .517" lift at the Valve. But higher ratio rocker Arms on a SBC have very little effect on the Cam Timing. 

BTW, the idle speed on your car should be around 1,000 RPM with that camshaft. Will help idle vacuum, but more importantly the higher lift and more aggressive profile of the Cam puts more loading at the Cam to rocker interface. And it gets worse with stronger springs..You need more oil from the spray bars or cam oiling holes to keep those surfaces well lubricated.  Helps with Camshaft life. 

 

 

Edited by Chickenman

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Lobe separation  angle greatly affects idle quality. EFI engines often use wider LSA to get a smoother idle. Usually by a couple of degrees. Smaller, higher revving engines will make more Mid-range Torque with tighter LSA's. L-series typically work well with LSA in the 108 to 109 range, with something like a 270 to 280 advertised duration. Race Cams are often ground on  106 LSA .  My Datsun 1200  Race grinds were on a very tight 103 LSA. Meanwhile 700 CI Pro Stock motors run LSA's of around 117 to 118 degrees.

Edited by Chickenman

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Exactly, "race cam" is a bit of flakey term.  Just like Stage 1, Stage 2 etc that we so often hear.  Jibberish.

 

Chickenman, FYI the increase in idle ignition timing in Tune #5 was just a test.   I backed it off as I didn't really see any real benefit, and still wanted some real improvement for a idle "correction" in the lower RPM range.  23 degrees worked, for the correction, but I wasn't seeing any real benefit in idle quality.   20 degrees works well for idle, 22 for the low RPM correction.

 

Anyone else with an aftermarket cam have any vacuum readings at idle?

 

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Lucky to get 10 at idle, lift is somewhere north of 1/2", always battling with idle quality, Mik 44''s or brand new SU's. 

Worth it for the screaming top end, right?

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Your right Cliff - you better let me take care of what cam kit - I'm responsible :)

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6 hours ago, HuD 91gt said:

 

Exactly, "race cam" is a bit of flakey term.  Just like Stage 1, Stage 2 etc that we so often hear.  Jibberish.

 

I can remember the old days when people would say its got a 3/4 race cam.  Still have no idea what that meant. 

Sort of a way I guess to say it had a aftermarket cam. But really didn't tell you anything about the specs.

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Ah... but that lovely Lope from a well Tuned L- 6 with a big(ish )  cam and some decent CR just can't be beat. 

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58 minutes ago, Chickenman said:

Ah... but that lovely Lope from a well Tuned L- 6 with a big(ish )  cam and some decent CR just can't be beat. 

Amen!

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1 hour ago, madkaw said:

Amen!

Your video made me want more out of mine, so thanks but I'd be able to live watching that video and not chirping my Hankooks in 3rd. LOL

Diseazed is my other excuse.

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