It seems that my wideband is registering in a rich 12.8 idle, great cruising AF, and I have not got into WOT. But according to my o2 MV's bank 1 is .200-.400 and Bank 2's are .50-.130. My values for my PE vs RPM table are -19.1

Here is my datamaster log, please feel free to download it, and give me pointers. Thank You!

http://home.comcast.net/~deenasty83/turbo.uni

 

Ask Bret how much vacuum a 383 should be pulling with that cam. Forget about the PE business and figure that out first.

 

Brad how did you figure out how much vaccum it was pulling? By the Kpa?

 

What you do is take the barometric pressure outside and subract from that the reading you get from the MAP sensor. Im more familiar with inHG vs KPA so I set datamaster to inHG but its preset to KPA. Anyway with your motor it was only pulling 7-8 inHG and honestly a cam that size in a 383 Id think would pull twice that. Maybe the tight lobe seperation is causing it but I havent seen that same thing with my friends car who also has a similar Bauer cam.

Who set the motor up? Was the cam degreed?

 

Brad, the car idled at 65-67 kpa with the previous 950 desired idle. After we lowered the idle to 850, she now idles at 74-76 Kpa, could this tell us that we lost 10kpa through 100rpm, if we raise the idle to 1000, we would gain 20 kpa? Cam was degreed, and it went in fine..

 

Well obvious answer is yes raising the idle will raise the vacuum. However, I do see a problem with that low of vacuum even at 950 rpm... its still around 9-10 inHG. You want to shoot for mid to low 50 KPA for the best idle. Ill have to do some digging but I have a chart that listed idle vacuum in a 350 with various cams and going off memory something gm874 range and larger still pulled 12 inHG at 850 rpm.

 

how would it affect stuff being that its in a 383?

 

Alright I found some logs from my friends 355 LT1 with a Bret Bauer cam with the greatest difference being slightly less duration. At 800 rpm his motor was pulling 14-15 inHG which is more than my smaller cam. So Bret knows how to cut a well performing cam even at idle.. its something else causing the low readings.

 

what should I check for? Readjust the valves? PCV is new? How could I tell if the manifold is not sealing right with the head? Because the heads were milled and we used a thinner head gasket, and to get teh intake to seal I slotted the bolt holes. Maybe its not sealing 100% on the head?

 

Im sure youve heard people say a cam will act smaller in a 383 than in a 350. Thats because the way a motor breaths or works is due in large part to the opening and closing events of the valves, controlled by your cam. The extra stroke of a 383 allows for MORE duration before the same undesirable(desirable to some) effects like a choppy idle are present. Its hard to explain without a model but the basic idea is a 383 minimizes the effects of a cam relative to a 350.

 

I understand what your saying. At this point im going to shoot the idle to 1000 and see what kind of vacuum it pulls there..

 

Judging by some values in your logs specifically the IAC readings I would say without knowing a great deal about your setup like what throttlebody you are running that you are dealing with a vacuum leak... somewhere.

Typically a heads/cam 383 with a stock TB and no reworking of the IAC whatsoever would see IAC readings topping 140 or so at idle.. yours are 20. Cracking the TB blades will affect those readings though it doesnt appear you have done that and so might an aftermarket TB.

 

Its a BBK 58mm TB. I have never had to hunt down a vacuum leak, so I guess Im gonna have my hands full. I mean the throttle body blades are cracked open, but barely anything. Therefore would that mean, that the vacuum leak is somwhere after the TB?

 

Yeah my guess is the intake didnt seal or something of that nature. Vacuum leaks can be easy to find if you have the right tools and are willing to spend a little time. The quick and dirty way is to get a can of carb cleaner. Start your motor and let it idle as you spray carb cleaner around the intake. If the idle picks up at any point that means its sucking in carb clear where the leak is. Its a proven method but doesnt always work.

Next method takes more time but you get a better idea if there are any leaks especially for areas the carb cleaner method cant test such as inside the lifter valley. You have to get yourself a vacuum guage and a pump. Then basically youll be sealing off the intake so you first remove the TB... all the vacuum lines and I believe loosen all the intake valves to seal off the cylinders. Next cap all the vacuum fittings off with plugs and tape off the TB port. Leave 1 vacuum port open to attach your pump and guage. Then the idea here is to put a negative pressure on the intake with your pump and monitor it to see if the pressure returns indicating a leak.

 

ohhh God... So on the second test what do you mean by "negative pressure?"