Bee Jay

I thought I'd ask you C4 guys what the manifold vacuum should be with this cam. I've been trying to get my engine and fuel injection sorted out since I installed this LT4 Hot Cam. The computer kept telling me that my that my manifold absolute pressure at idle was hovering around 60kpa. I put the vacuum gauge on the car, and sure nuff, my idle vacuum is only 12". Isn't that kinda low? It was 15" with the more radical Summit 1105 cam. Do I have a vacuum leak somewhere? I've checked everything, it would have to be at the intake manifold gaskets. I thought my irregularities was because I was using a manifold port on the fuel injection for the MAP sensor and for my headlight vacuum. I have hydraboost, no so power brakes. The vacuum hole in the rpm air gap manifold was directly below the fuel injection, so I originally just plugged it. Last night I bought a brass vacuum fitting and filed by hand untill I could get the throttle body bolted on without interference. I started the car this morning, and still 12" vacuum. Here are the cam and engine specs:
GMMP LT4 Hot Cam 218/228 duration at .050 installed straight up with 1.65 ratio roller rockers and Compcam conversion hydraulic roller lifters.
Engine: Stock L82 block and pistions, 9.0 to 1 compression, AFR 195 65cc heads, headers and 2.5" exhaust, RPM Air Gap intake, Holley 4Di 950 CFM Throttle Body injection. 850 rpm idle.
Bee Jay
Here is the Felpro 1206 glued down

Here is the hole in the intake manifold for vacuum, directly under the throttle body.

Here is the brass manifold fitting I had to file to fit under the fi.

94vettelover2

Call or email GMPP to see what specs should be for your set-up.I don't know the answer maybe someone here will know

rpoL98

I have the LT4 HOT cam in my 5.7L TPI, and it's about 60 kPa at around 700rpm idle, consistently. I didn't know that 60kPa translates to 12"Hg (which is yes, shockingly low). I'm using 1.5 roller rockers, not the 1.6.

I have headers, cat-back Flowmaster, Edelbrock TPI base, SLP runners, aftermarket heads w/ 2.02, 1.60 valves, etc.

been running this combo for years, the MAP has been consistently at 60kPa at idle. I guess it's par for that cam.

I might be a little too big on the injector size, I went with 30# and that probably affects my ability to idle, I suspect it would idle better with, say, 24#, or even the originals. the pulse width duty cycle gets kinda narrow (1.3%-1.7%), and I guess fuel metering at such small injector duty cycles is somewhat inefficient at idle. one of those self-inflicted, "bigger-isn't-always-better" things.

96GS#007

I went and looked at some of my old log files. When I was running the HOT Cam, it used to idle ~50kPa. My 383 (current engine) with a much larger cam idles at ~55kPa.

Bee Jay

I got home today and put the garage tv on the Texas/Bama game, and start tearing the front of the engine apart. Once I get the pulleys, damper, water pump, and timing cover off, I find this:

The UT Quarterback gets knocked out of the game, and I find that the cam gear is off one tooth. How the hell did that happen. I took a picture of the setup just before I buttoned it up, or did I? I do not remember taking the cam gear off for any reason after taking the picture of the timing marks lined up perfectly. Damn.
Well, after reading David Vizard's book, I had regreted not putting the cam in 4 degrees advanced, so I'm doing that now.

I am really bummed right now because of this game and this major screw up on my part. Sorry guys.
Bee Jay

mtwoolford

I notice your throttle body seems on the large side; by way of anology, on a 9.8:1, 350small block with zz1 345 horsepower crate engine cam, my vacum went down a similiar amount when going from a small four barrel carb to a large four barrel carb.

mike100

the cam off a tooth will do all kinds of bad things to the vacuum signal. I did that back in 1988 to a friends car and it barely made 100 hp. It will let it run though.

Bee Jay

I now have 15" vacuum at idle, 150 lbs cranking pressure and the car runs and drives like a raped ape. I have to drop the oil pan and replace the one piece oil pan gasket, but it's all good now. The LT4 hot cam is living up to it's reputation. Test data later.
Bee Jay

Ryan59

Did you advance the cam 4 degrees or run it straight up?

Bee Jay

4 degrees advanced.
Bee Jay