I recently installed a new Comp Cam hydraulic roller cam #08-305-8 and a Holley Stealth Ram. I have brake issues around idle speeds. It seems even with an idle of 800rpm in drive Im only able to pull 8". Its fairly steady 8" so Im fairly sure theres no leaks. I also disconnected all the vacuum lines that come off the plenum and there was no change. The car is still not tuned but Im not sure if I should try to fix with tuning or address before tuning. The car is also running 1.6 Crane gold roller rockers. The only other things I can think of are maybe 1) the cam is off screwing up valve timing
2) maybe the springs cant handle it
((shop said they are good for 0.550" lift))
Anyone reccomend where to start or if Comp Cams has a tech support? Thanks
Kris

 

I'd start by checking your timing. Retarded timing can cause low vacuum. Make sure to disconnect the timing wire from the ECM before you set your base timing.

Then I'd double check the valve adjustment. If you're hanging one or two off the seat even slighly it'll have a rough idle and low vacuum.

Beyond that I'd wonder about basics like plug wires, wiring, vacuum leaks, etc.

 

Plugs, o2 sensor and ignition wires are brand spanking new...

Timing is set at 8*

Would it hold a steady 8"s with a vacuum leak? I though a leak would make it bounce like crazy.

I think your idea about valve adjustment may be it. I talked to a few techs at work, one ex-GM tech who spent time with a race team says hes pretty sure there is a problem with the way the rockers are set. Unfortunatly Im not gonna have time for 2 weeks to touch it but Im gonna redo all the rockers at my buddies dyno shop and have him double check my procedures and the new adjustments. I really appreciate the reply. Anyone else wanna give me some ideas? I guess its pretty much narrowed down to valve timing/cam timing.

 

A vacuum leak at the plenum won't typically cause a fluctuation, just generally low vacuum. A runner-base gasket leaking could cause a fluctuation, depending on how bad it is. An over-tightened or loose rocker will cause fluctuation, but if they're all too tight it COULD give you a steady low reading (I'm betting against it but anything's possible.)

I'd be looking for leaks again. That's the most likely culprit. Failing that, spark and valve timing are going to be the most likely causes. Did you degree the cam or just put it in and line up the timing marks? I've heard of cam's coming from the factory with the sprocket flange a couple of degrees off...that could easily cause your problem. You might also check to make sure the TDC mark on your balancer is correct. If the balancer has slipped then your base timing will be retarded, even though it reads good.

 

Hmmmm. I thought vacuum was vacuum. I figured if there was a leak at the base it would affect vacuum everywhere. Well you learn something new everyday. I did check the base and plenum. I checked the base using a stethoscope from a tech at work. I checked all bolt holes....I checked all along the base where it meets the heads....checked where the plenum meets the base..... checked tb to plenum.... all this with the ports on the plenum for the vacuum lines capped. I unplugged the timing interferance wire to the distributor and confirmed timing. I measured vacuum using a port that I hooked a gauge to.... 8"...plugged vacuum lines in 1 by 1 with no change in vacuum. I didnt degree the cam but simply installed with the cam sproket "O" at 6 o'clock and the cranks "O" at 12 o'clock. I guess if all else fails I will degree the cam. Ill definetly will double check the vacuum leak thing again since thats really easy. Im gonna try to email Comp Cams to see what they say since this cam was made for a TPI. Thanks for the input


Kris

 

It's probably the cam and the lift on the lobes (holds valve open longer), less vacuum. Call the tech guy he can tell you for sure. You may have to install a vacuum pump. It's not uncommon with high perfromance cams.

 

I dunno....that cam only has a 220/230 duration at .050" with a 114* LSA.....the cam in my 91 is 235/235 on 112* and produces PLENTY of vacuum for the brake booster. I have slightly less lift but much more duration and I pull about 16" at a 750rpm idle (dumping gas out the tailpipes thanks to the overlap.)

It's not THAT aggressive a grind. EFI-specific cams generally aren't.....it's to keep the ECM happy.

 

try running more initial timing and see if it improves. right now i'm running a 230/230 duration cam, .480/.480 lift and pulling 9 inches at 900RPM and 15 inches at 1500 RPM. so brakes are kinda iffy as well. I kept advancing initial timing until vacuum improved. now i think i get 13 inches or so at idle but it's on a 108 LSA and a carb'd car.

 

Hmmm. Ill try that. I just reset the preload onmy roller rockers. Comp Cams said that if there is too much preload it will cause low vacuum in the plenum. I will bump my timing though.
Ive spoken to a few very very knowledgeable guys with years of prom tuning and third gen stuff and they are saying the same thing you guys are which is reassuring. They tell me that cam will make at least 13"s. Since this cam is for a TPI Im assuming there should not be any vacuum issues. Ill bump my timing to say ...10* and use a feeler gauge on the throttle body to hold the idle at like 850rpm and read it. Basically Im ready to learn prom tuning and have all the equipment to do it I just need to resolve this issue first. Thanks guys. I really appreciate the info. Ill let you know what I find out after this weekend.
Kris

 

You know I have a very similar cam from comp cams also and it does the same thing; 8" of vaccum at idle.The car drives fine(except when I'm at low rpm's brakes also) and my gauge doesn't fluxtiate either. I would be interested to know what you find out because no one seems to know anything here in austin except "that you have a vaccum leak".

Brad

 

LOL. Thats what Im being told. Everyone says vaccum leak ....but I still had low vaccum on my Edelbrock TPI with this cam. I searched for leaks everywhere. I remove all vaccum lines and plugged the ports...used a stetoscope to listen for leaks...used propane to see if it revs... Ive just reset the lash on the rockers so Ill try that and then up timing. The car ran good on the TPI intake ...runs rough now but really needs custom tuning. The only issues I have really is with the breaks at idle. Ill let you know what I find...if I find it!!! But if you figure it out can you let me know. Thanks
Kris