# intake reservoir to smooth idle with hot cam?



## ziddey (Apr 16, 2006)

Would adding an auxiliary container (like a vacuum reservoir) to the intake tract help to smooth out the idle when a performance cam is used?
I've only got a 268* hydro cam with cis-e, but even with new motor mounts and snub mount, it shakes pretty bad at idle. Bringing the idle to 1200 only helps a little. It's not until around 1500rpm that it's completely smoothed out.
I figured a 268* cam would be smoother than this. Ignition timing is at 12*btdc. Cam, intermediate, and crank are in time.
New stock rubber mounts.

I seem to recall some cars having such a reservoir in the intake, and am hoping this is what it's for.
And if so, what would be a good way to implement this? Single port? Port after the metering plate and port before the tb? I deleted the idle boost valve, so I've got a port right before the TB I can use to hook up a container.


_Modified by ziddey at 12:19 AM 3-3-2010_


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## WackyWabbitRacer (Apr 24, 2001)

*Re: intake reservoir to smooth idle with hot cam? (ziddey)*

From my experience, a high performance camshaft and CIS equals a rough engine idle speed.
CIS hates radical cams because they cause valve overlap. There is a small period of time when both exhaust and intake valves are open at the same time. This situation sends a pulse of air back through the intake which disrupts the movement of the venturi plate in the fuel distributor. This movement therefore disrupts fuel flow to the engine, resulting in overall poor idle speed operation.
Cheers, WWR.


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## ziddey (Apr 16, 2006)

*Re: intake reservoir to smooth idle with hot cam? (WackyWabbitRacer)*

Exactly. That's why I'm wondering if an added "storage" container between the plate and tb would help to smooth the pulse out.
I've seen such a chamber in some other cars and always wondered what it did. I'm hoping that's a partial cure for this problem, no?


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## Southcross (Jul 2, 2004)

*FV-QR*

would like to see this.....
having a "storage area" (I know there has to be a better term for it) would cause a HUGE lag in throttle response.... when you pop the throttle, the engine would have to suck all that air out of the vessel until sufficient vacuum was produced to lift the plate... the entire time the engine would be running lean.
solution to super hot cams... idle at like 2000-2500


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## ny_fam (Apr 3, 2004)

*Re: FV-QR (Southcross)*

Sounds like a resonator chamber, Helmholtz is what you need to lookup, shouldn't be all that hard to try out.
You could also just put a washer on the air flow plate bolt of the flow meter, Little added weight will help.
You could also experiment with different diameter zip tubes(between CIS meter and TB) maintaining a little higher air velocity into the plenum may reduce the reversion.
Though if you go to small it will hurt top end power.
How about moving that cam gear a little to see if that makes a difference.


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## ps2375 (Aug 13, 2003)

*Re: FV-QR (ny_fam)*

From my understanding, those extra tubes and what not on those intake are for sound resonance. In other words to quiet the intakes. Most people don't want to hear what their car is doing.


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## sandrunner (Apr 26, 2003)

*Re: intake reservoir to smooth idle with hot cam? (ziddey)*

268 or 286 cam? I don't see the need for an air resivoir if your only running a 268. Well tuned CIS gave me a factory smooth idle at 700rpm with 268 cams in my stock JH, overbored JH, 3A, and 3A with massive head work.
I didn't start having a lopey idle till I went to a 272 and it got unbearable with a 286.


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## ziddey (Apr 16, 2006)

*Re: intake reservoir to smooth idle with hot cam? (sandrunner)*

268 hydro


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## sandrunner (Apr 26, 2003)

*Re: intake reservoir to smooth idle with hot cam? (ziddey)*

I'd highly recommend checking out our your CIS and ignition system for potential problem areas that could cause a rough idle (uneven injector flow, poor connections, vacume leaks, etc)


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## antichristonwheels (Jun 14, 2001)

*Re: intake reservoir to smooth idle with hot cam? (ziddey)*

You are not idling rough from lack of vacuum. It's like WWR said. This is typical with CIS and bigger carms. You need to tune it pretty fat at idle. Run the timing your gas will allow and drive it. Your car should already have a vaccum resevoir in the fenderwell. The vaccum resevoir is for your power brakes, cruise control or other vaccum operated stuff.


_Modified by antichristonwheels at 10:48 AM 3-14-2010_


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## Southcross (Jul 2, 2004)

*Re: intake reservoir to smooth idle with hot cam? (antichristonwheels)*


_Quote, originally posted by *antichristonwheels* »_ The vaccum resevoir is for your power brakes, cruise control or other vaccum operated stuff.
correct... the primary use of the vacuum reservour on the Mk1 was specifically for the Factory AC equipped cars as their HVAC system actuators were all vacuum controled


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## deer_eggs (Oct 4, 2006)

*Re: intake reservoir to smooth idle with hot cam? (Southcross)*

I'm not sure how relevant this is, but I'm running a TT 288 cam on CIS-Lambda in my '86 Cabby. 
I noticed it to idle & run a little smoother at low rpm's, when I unplugged the WUR - basically lowering the control pressure. 
I figured it would have done just the opposite, making it easier for the plate to move up & down with the pulsations... so as of now, I just leave it unplugged, and have absolutely no problems with cold, or warm starts.







It may be something else for you to try. It's not exactly solving the problem of the plate moving during overlap, but it seems to help with idle & driveability.


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## stick90 (Feb 28, 2008)

*Re: intake reservoir to smooth idle with hot cam? (WackyWabbitRacer)*


_Quote, originally posted by *WackyWabbitRacer* »_From my experience, a high performance camshaft and CIS equals a rough engine idle speed.
CIS hates radical cams because they cause valve overlap. There is a small period of time when both exhaust and intake valves are open at the same time. This situation sends a pulse of air back through the intake which disrupts the movement of the venturi plate in the fuel distributor. This movement therefore disrupts fuel flow to the engine, resulting in overall poor idle speed operation.
Cheers, WWR.

Yep. I have this problem with a TT288 It only wants to idle at 4-500...until you press the skinny pedal to the floor


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## John Milner (Jul 20, 2005)

*Re: intake reservoir to smooth idle with hot cam? (stick90)*

For interest sake I ran before the turbo setup, a TT276 & really didn't have much drama's other than the normal idle you'd get from a performance cam, but nothing crazy. I set it to idle at approx 950-1000rpm. It was a pig though in 4th gear at 60km/h cruising. It would buck & fart about & I'll say a ***** in peak hour traffic. 
I wouldn't call the 276 radical though. More the 288.

I also had a Volvo 240 airflow meter & dizzy head, which the air flap is alittle bigger, thicker & probably alittle heavier than the stock GTI K-jet

In my turbo setup at the moment I'm running a Autotech 270 which is silky smooth with a good power band. I'd also ran this cam in NA form & quite liked it. 
I could cruise along in 5th at 50km/h & it was smooth & could pull away quite easily.


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