Valve lash and Low Vacuum pressure??? - Team Camaro Tech
Tech 2002 General Tech questions from 2002
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post #1 of 11 (permalink) Old Jun 25th, 02, 03:51 PM Thread Starter
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just finished a head change..Don't have the cam card So set the valve lash to zero+1/4 turn...Fired right up...revs smooth but idle poor..
Cam Lunati 511Lift and [email protected] duration..Had 8" of vacuum before head change now have 5"...
Could incorrect valve lash cause poor vacuum????I don't think its a leak..Checked everywhere??? Thank you

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post #2 of 11 (permalink) Old Jun 25th, 02, 04:38 PM
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Yes it will if they're too tight. Would be worth a look.

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post #3 of 11 (permalink) Old Jun 25th, 02, 04:43 PM Thread Starter
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I am running a hydraulic cam and thought that to loose a lash would give me low vacuum..Hmm...Also would me changing from 76cc heads to 64 cc heads make a difference??

Seems like it runs smoother with more timing 25* base..Seems to be perfect but just seems like to much..Any thought on this..
Thank you

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post #4 of 11 (permalink) Old Jun 26th, 02, 03:30 AM
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Zero lash plus 1/4 to 1/2 turn on the valves with the engine running at the lowest possible idle with hot oil and smoke everywhere.

Set the timing at 36 degrees max with the vacuum disconnected and plugged. Let the initial fall where it will for now. About 12 degrees initial at idle would be nice then add manifold vacuum to the can and reset the idle and mixture. I just went through this but I am not finished tuning my setup. I am using a vacuum can designed for non-emission, low vacuum SHP engines. It is designed for a 65 Corvette 327/365 SHP engine and others so that is not a bad place to start. It was maybe $8.59 at NAPA. You may end up wanting to limit it to 10 degrees according to some tuners. I have not experimented that far yet. The factory can on your distributor will not advance very far with that low manifold vacuum reading. That is why you want to get a new can. THEY REALY DO LIKE ALL THAT ADVANCE AT IDLE.

In the good old days before emissions we used manifold vaccum to the distributor which gave us lots more advance at idle with lower operating temps and everyone was happy. You have to tune the car differently but manifold vacuum is preferred by many people that tune these HP engines that do not face emissions testing. 10 (10-16) degrees vac advance combined with 36 as the sum of initial plus all the centrifugal will be good. Change out the springs to get the ping to go away. That is a good place to start.

-Mark.
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post #5 of 11 (permalink) Old Jun 26th, 02, 06:54 AM
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246 @ 0.050 on a hydraulic cam is pretty stiff duration, combine that with a tight centerlien ( LSA) of 110 or 108 and , yeah, its going to pull 6-8" at idle. What did the cam company tell you???

As Mark says, use full vaccum advance at idle on the timing ( hopefully she'll turnover OK and fire good). It'll smooth you out somewhat. Sounds like the smaller chamber heads are much better suited to that big cam. Big cams need co pression.
post #6 of 11 (permalink) Old Jun 26th, 02, 10:22 AM
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Better flowing heads will drop your vacuum a little and make it idle rougher - larger ports have less velocity at lower engine speeds so you a little more reversion (hot gasses from the combustion chamber go back into the intake through the intake valve). I think higher compression will contribute a little to a rougher idle too - you probably increased your compression a full point with the head swap.
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post #7 of 11 (permalink) Old Jun 26th, 02, 01:54 PM Thread Starter
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Wow..I was hoping that this head change would boost my Vacuum pressure and solve all my idle problems...Didn't work Hah...I love it when a plan just comes apart Seems I go from one can of worms to another

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post #8 of 11 (permalink) Old Jun 26th, 02, 05:47 PM
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Changing to 64cc heads should calm down your cam unless the heads have huge runners. Did you change anything else?

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post #9 of 11 (permalink) Old Jun 26th, 02, 05:52 PM Thread Starter
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Oger.. I believe the trick flows have 195cc intake ports and 75cc exhaust..When you say calm down what do you mean???

Eric68.Why would the better breathing heads lower my vacuum and make my engine run rough??
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[This message has been edited by Badcaiman (edited 06-26-2002).]
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post #10 of 11 (permalink) Old Jun 27th, 02, 05:49 AM
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With more compression there is more cylinder pressure at idle. That will help overcome the bleed off of the radical cam. You have much larger intake runners than a stock head. That is your vacuum loss it will run much better at high RPM but low speed will suffer.

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post #11 of 11 (permalink) Old Jun 27th, 02, 12:08 PM
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When you have a large camshaft you have larger amounts of overlap. Overlap is when the intake valve and exhaust valve are open at the same time. This occurs with a big camshaft at the end of the exhaust stroke and beginning of the intake stroke.

At high RPMs the high velocity exhaust charge exiting through the exhaust port and pipes actually helps pull the fresh intake charge into the cylinder - scavenging. At the same time air velocity through the intake helps push the charge into the cylinder. With very large camshafts some of this charge is actually pulled straight through the cylinder and into the exhaust and wasted.

At lower RPMs there is not enough velocity in the exhaust or intake for scavenging to occur. At idle speeds the opposite of scavenging takes place - reversion. Hot exhaust gasses are forced back into the intake and the incoming intake charge is diluted making the engine idle rough.

Now this is my theory on why large intake ports can contribute to rougher idling . . . Larger runners = lower velocity through the port. Your new head's 195cc port is bigger than the stocker's 160cc. The larger port slows velocity through the port so the incoming charge has less "momentum" to push against those hot gasses trying to enter the intake so you wind up with more reversion and a rougher idle.

I'm not entirely sure why higher compression would hurt or help idle vacuum, but I believe more compression would create greater cylinder pressures - this I believe would tend to exaggerate what the cam is already doing. Better scavenging at high RPM and more reversion at lower RPM. Whenever I increased compression ratio in an engine, the engine always seemed to have a more distinctive "pop" tone in the exhaust at idle, but my theory could be wrong . . .

BTW - I wouldn't be too upset about your idle, I bet it idled pretty rough before the change anyway. Just turn the idle screw up a little and test it out. I bet you'll find you have a TON more power.

Also - you may be very close to needing the small holes drilled in your throttle blades. If you have your idle screw adjusted to high, the throttle blades get cracked open too far and the car starts idling on the main jets instead of the idle circuit.
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