Need help/advice on what to do about a vacuum issue after JDM cam install

[FEARSNO1]

so tore the motor down and replaced oil pump gears, head gaskets, deleted the EGR (kept the SIP sensor), KB 168 mm Oval TB and put in JDM Super Stock Camshafts.

The Problem:
the car idles great, drives great, and is making power (haven't dynode yet). but my only issue is that now at idle I'm only getting 4in of vacuum below the blower, I've checked with both a mechanical and a electrical gauge that self calibrates as every power cycle, and both show the same. I've contacted JDM, KB, Revan Racing, Jon Lund, and nobody seems to know why it is so low. JDM says it should be 8-12in of vacuum, I have done a compression check and that was fine, I have tested every vacuum line on the motor for leaks and found a couple but made no difference, fuel trims are right where they are supposed to be and show no sign of unmetered air entering the motor. so I have been stumped to say the least. now the tricky issue is that before the blower in the intake elbow I read 12in of vacuum but below the blower its only 4in.(which is where the reading is supposed to be taken from) I also replaced the bypass valve to KB's low vacuum version and have verified that it is opening/closing as required. So I am lost as to why I am having this issue. now admittedly if I didn't have a vacuum gauge on the car I would never suspect that anything was out of the norm!

FYI I even went run a test on my buddies 13 GT500 and between the intake elbow and below the blower his shows about a 2in vacuum loss 21 vs 19.

any insight would be appreciated

 

[19COBRA93]

I'm interested in what you find. I'm having the same issue. I'm running custom Bullet Cams, and I'm also only seeing about 4in of vacuum at idle. Aside from that, everything appears to be operating normally.

 

[1320 Junkie]

Fwiw i see 18-15in/hg with my crower stage 2's.

 

[FEARSNO1]

bump

 

[Coonsnake]

FWIW when i done my L&M motor i was only seeing 3''s on vacuum. After a tune change to Triangle Speed shop i'm seeing 7''s. Probably in the tune

 

[19COBRA93]

I can see that. The cams are going to make it run rich at idle, which would normally drag the idle down. So the PCM is going to open up the tbody a bit to bring the idle back up, which will reduce the vacuum.

I'll have my tuner dig into mine and see if that helps.

 

[Catmonkey]

Regarding the boost-by-pass valve, you say you verified it's opening and closing, but can you determine what is it doing at idle? If it's closed, that may account for your difference. If your tune will let you increase idle speed, you might take those readings at higher idle speeds (say 50 rpm increments) and see what happens.

 

[19COBRA93]

Increasing idle speed will more likely drop vacuum even lower.

 

[fullboogie]

Gotta wonder if the cams were installed retarded - that will lower vacuum readings.

 

[69b302]

You can try and back your idle timing down a bit. This helped a lot for the stage III comp cams I had. Stock was 15, the car liked 7 at idle. I believe most idle control on our cars is via timing and not the TB.

 

[FEARSNO1]

Thanks for the input guys, at least I'm not the only one to answer a couple questions, I installed the cams straight-up as JDM instructed, Lund does the tuning so I have no way to change the idle timing.

Anyone have a suggestion on why I have such a big difference in vacuum readings before and after the blower? 12 vs 4

 

[Sielmo]

Just did the install myself to factory vice degreeing the cams. Added L&M low vacuum booster and have surge issues. Discussed with JDM and my tune idle was bumped to 850rpm. Also adjusted the TB idle adjustment screw 3/4 turn to the right. Still have the surging throttle decay, sometimes as much as 500 to 750 rpm, usually while coasting . Any suggestions?

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[genesmob]

Mine with ss cams has about 5in vacuum at idle. Quickly increases
to 12 -14in with slight increase in throttle. The decel surge on mine was cured by increasing the values of "idle dashpot decay" (hptuners) This change also makes the idle come
down quicker when revving it or decelerating which I like.

 

[FEARSNO1]

i never really got a solution since I started this thread, I just lived with it, and it performed great up until I cracked a sleeve late last year, but that was unrelated, LOL. but like i had said before, if i didn't have a boost/vacuum gauge on my car I would have never known the difference.

 

[ibleedblue65]

Just did the install myself to factory vice degreeing the cams. Added L&M low vacuum booster and have surge issues. Discussed with JDM and my tune idle was bumped to 850rpm. Also adjusted the TB idle adjustment screw 3/4 turn to the right. Still have the surging throttle decay, sometimes as much as 500 to 750 rpm, usually while coasting . Any suggestions?

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You're not supposed to adjust that screw.

Sounds like you need a proper custom dyno tune.

 

[Sielmo]

You're not supposed to adjust that screw.

Sounds like you need a proper custom dyno tune.

I'd agree as everything I've read..but... it was recommended by them to adjust that screw; 1/4 increments, max one turn to the right as needed. They also mentioned to plug the air scoop that leads to my air filter. (Hmm) As my idle surge occurs when on the move (clutch in when coming to a stop) and the disruptive air flow through this vent may be confusing the MAF. I received a new tune for these cams although it was mentioned that it really only adjust my idle up from 750 to 850rpms...as they state, maybe they aren't telling me everything but also looking for solutions.

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[Sielmo]

FWIW when i done my L&M motor i was only seeing 3''s on vacuum. After a tune change to Triangle Speed shop i'm seeing 7''s. Probably in the tune

Same here..JDM tune, 9.5hg. Lund tune, 6.5hg. All runs well

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[GT500RedStripe]

A closed bypass will burn up your KB rotors. I learned that the hard way after cams. KB makes a low vacuum bypass just for this application. Get it fast! I'm running 4 pounds of vacuum and no problem with the KB low vac bypass.

 

[biminiLX]

As mentioned above, you need a low-vacuum bypass for the blower when running cams. Now as to why the vacuum is so low, not exactly sure but my tuner currently has my idle set to 950-1050rpm with JDM cams.
We are just starting fine tuning.
-J

 

[genesmob]

With large throttle bodies, it sometimes becomes necessary to adjust the screw on the tb. Don't just guess at it. View a live monitor and go up or down a percent at a time with the key on/motor off. If the car has a hanging idle, surge on decal a percent or two less can make all the difference in the world. If the idle drops too bad on decal or wants to stumble from too low of rpm then recovers, adding a percent or two can cure it sometimes. Ive used this method on a twin 67mm, an FRPP mono and a whipple 170mm elliptical and I have Jdm ss cams. More tuning is still required sometimes but this is makes life much easier when tuning. As these TBs get bigger, a percentage of opening is a lot larger airflow difference than the stock TB.
A standard setting on them isn't always ideal.

As far as idle vacuum, one way to increase it is to add idle timing which will reduce throttle opening to hit target idle. This will increase idle vacuum for a given idle rpm.