cam upgrade! But to low vacuum? experts only!

[rps13drift]

So i have been battleing this odd vacuum problem i have been having for a few months now. At idle around 850 rpms my car is only pulling 7-8 inHG wich to me seems realy low to me. I have installed hks 264 in/ex step 2 cams and the thicker apexi 1.1mm headgasket the only 2 compression altering mods. The car runs and idles spot on and it pulls realy hard aswell when in boost. I have tried everything to diagnose the problem. I did boost leak tests like 20 times already! I have checked mechanical and ignition timing like 30 times. before i put the motor in my car after doing all my new mods i did a leakdown test with great results.
Like i said though, the car is running realy good my vaccuum numbers just seem off compared to other peoples rides with similar mods. If any body could give me some useful information about this i would greatly appreciate it!

-mike-

[SoguRacing]

Well, I dont know what is normal but as you increase valve overlap you're going to decrease you vacuum at idle. If everything else on your car is fine I wouldn't worry about it as the reason you installed cams is to have more power at top end. Cams with above stock duration and lift will cause lower vacuum readings in lower rpm because you are sacrifcing the low end intake "ram effect" for upper rpm gains. "ram effect" being the velocity of the intake air through the runners as it is sucked into the cylinders. look up "ram effect"

[S14DB]

The ram effect really only happens in jet engines. The rest is marketing.


With high duration/overlap cams you will experience some loss of compression and vacuum as air escapes out the exhaust side. Positive side is that you get some more exhaust scavenging at higher rpm.

264's should not be abnormally bad in the idle/low rpm department. Did you degree your cams when you installed?

[rps13drift]

The ram effect really only happens in jet engines. The rest is marketing.


With high duration/overlap cams you will experience some loss of compression and vacuum as air escapes out the exhaust side. Positive side is that you get some more exhaust scavenging at higher rpm.

264's should not be abnormally bad in the idle/low rpm department. Did you degree your cams when you installed?

Absolutely! I went over the cam marking like a hawk as per enthalpys website. If i was a tooth off wouldnt the car idle/run like shit?

[Def]

My HKS Step1 264's pull about -12 to -13 in-Hg at 900-950 RPM idle. It'd probably pull a bit less at a lower idle.

[rps13drift]

My HKS Step1 264's pull about -12 to -13 in-Hg at 900-950 RPM idle. It'd probably pull a bit less at a lower idle.

Interesting! i am going to try and bump up my idle!

Any body else have some input?

[Def]

Also - isn't 1.1mm the stock HG thickness, or is it 0.9mm? If it's thicker, it will change cam timing slightly due to changing the crank to cam gear distance.

[SoguRacing]

stock HG is .8mm thick. 1.1 is the next bump up to compensate for decked heads and blocks.
Valve Timing & Engine Performance, blow down overlap and ram effect via camshaft (http://www.auto-ware.com/combust_bytes/valvetiming.html)

scroll down to "ram effect"

[rps13drift]

stock HG is .8mm thick. 1.1 is the next bump up to compensate for decked heads and blocks.
Valve Timing & Engine Performance, blow down overlap and ram effect via camshaft (http://www.auto-ware.com/combust_bytes/valvetiming.html)

scroll down to "ram effect"

Very awesome point! I never got my head or my block decked! Could this plus the cams realy be giving me such low vacuum?

[SoguRacing]

I'm giving an educated answer based on what was stated in the first post. I do know that "bigger" cams do lower your vacuum at idle but to what degree i don't know and that a thicker headgasket based on the decking that was done, can lower your compression.

[S14DB]

Absolutely! I went over the cam marking like a hawk as per enthalpys website. If i was a tooth off wouldnt the car idle/run like shit?

If you were stock. When you deck the head/block or in your case change the HG thickness. You need to get a degree wheel and adjustable cam gears and dial them in.

Also, going to a thick HG lowered your compression ratio.