High MAP reading at Idle????

[MidNite88]

Hey, I'm getting a readying of close to 76 kPa at idle! And kPa rises as I accelerate, which I know is right but, not that high though for gentle acceleration.The engine is a Chevy TPI 350. It has roughly 110-120 psi on all cylinders. No vacuum leaks that I can find. I've sprayed it all down with carb cleaner with no change in idle. The stupid thing wont idle unless I provide it with almost 20 degrees of timing and run it a lil bit rich at like 12.5-13 to 1 AFR. Any help would be awesome!

[Matt Cramer]

Where exactly is the MAP sensor plumbed to on the TPI?

[MidNite88]

I have it plumbed into one of the ports on the back passenger side of the plenum. I've also tried the port on the driver side in the lower intake where the PCV should be pulled from. Both have high readings. Tomorrow the timing cover is getting pulled to see if the engine jumped timing. One of my old friends found out a long time ago that SBC's can run with the cam 180* out but they lack alot of power and use tons of timing. Which is what I'm running into, anything less than 17* of ignition timing and it wants to dies and it doesnt like to idle lower than 1000 rpm unless it has load on it. Oh, and when I flatten out my timing table to say like 15 degrees, the timing jumps back and fourth by about 3 to 4 degrees on the timing light.

[Matt Cramer]

Unless you've got a brand new double roller timing chain, that much timing scatter does not surprise me at all. Timing with a stock HEI distributor is affected heavily by timing chain stretch and cam harmonics on a small block Chevy. You could probably get a bit more power by going to a crank trigger to get better timing.

But that's getting a bit off the subject. Maybe you could post a data log of your MAP readings along with your MSQ? That might shed a bit more light on things.

[MidNite88]

Well, pretty sure we found the culprit, finally pulled the timing cover to discover that someone at sometime tried putting a cam in it and put the cam in 180* out AND it has practically a bicycle chain for a timing chain. Tonight I'm going to go retune it.

[HomeBlown57]

To fix the cam being 180 out you could just rotate the crank 360 degrees. That would correct the 180 degree cam phasing problem.

Doin' my part to help!

[Six_Shooter]

I'm trying to figure out how installing the cam "180* out" can have ANY effect on how a pushrod engine, especially a SBC, will run.

Do this:

Install the cam at your "o*/in phase", now spin the crank 360*, the cam will now be 180* from where it started, continue to turn the crank another 360* and the cam will be another 180* and back at it's original position.

You can also do this:

Install the cam "180* out", then spin the crank 360* and the cam is now at your "0*."

The only effect an improperly time cam will have is if the timing chain has stretched a huge amount, or the cam was just plainly installed without paying attention to the timing marks.

[MidNite88]

No matter how many times you rotate the engine over, the cam will be 180 out. the cam has to be straight up with #1 piston TDC. Hence why you use a degree wheel to set cam timing, you have to be sure #1 is perfectly TDC. If the cam is 180 out, then it changes the firing order completely, then the power pulses land on the crank at the wrong time throwing the whole engine out of balance making less power, hurting bearings, throwing off vacuum signal, no response, low torque output and generaly poor running. Also, if you drop the distributor with a cam 180 out and #1 TDC, it will run but very very poorly. As in the case of this car. Now that the cam is correct, car runs pretty good, but it ran 180 out so long it built carbon up on the intake valves and now it needs a valve job. But the guy who owns the car doesnt want to put out the money for it.

[kjones6039]

No matter how many times you rotate the engine over, the cam will be 180 out. the cam has to be straight up with #1 piston TDC. Hence why you use a degree wheel to set cam timing, you have to be sure #1 is perfectly TDC. If the cam is 180 out, then it changes the firing order completely, then the power pulses land on the crank at the wrong time throwing the whole engine out of balance making less power, hurting bearings, throwing off vacuum signal, no response, low torque output and generaly poor running. Also, if you drop the distributor with a cam 180 out and #1 TDC, it will run but very very poorly. As in the case of this car. Now that the cam is correct, car runs pretty good, but it ran 180 out so long it built carbon up on the intake valves and now it needs a valve job. But the guy who owns the car doesnt want to put out the money for it.

Here I go sticking my big nose in where it's probably not wanted..........

The '180 degree's out' notion is simply not correct. If the cam is installed with both timing marks (cam & crank) aligned with the cam & crank center line then they will be in the proper relationship. How many times you rotate the crankshaft has no bearing on that relationship. That is why the cam and crank sprockets have those timing marks stamped into them.

The use of a degree wheel is generally intended to make fine (and accurate) adjustments to cam timing via offset crank/cam keys and has nothing to do with initial camshaft installation.

Now, having said that, wrong cam timing, (one or maybe 2 teeth off or a stretched chain) certainly will profoundly affect performance! Much more than that and the engine will probably not run at all. (some of the reason I use gears instead of chain)

Nothing is going to change the firing order, period. It has always been, 1-8-4-3-6-5-7-2 for SBC's since 1955.

Matt has it right. Chain slop will most likely cause timing (and performance) problems.

The idea of a camshaft being installed 180* out causing carbon buildup is equally without merit. I have been building engines for ~50 years and have never heard of or seen such a thing.

Ok, I’ll get off my soap box.

Ken

[DonTZ125]

The idea of a camshaft being installed 180* out causing carbon buildup is equally without merit.
Ken

With the engines I'm used to, it causes shrapnel build-up, not carbon. On a non-interference engine, I'd think the unburned fuel would wash away the carbon ...

[drholl]

My guess is he's talking 180* but means 90* out.
Or the timing gear was put on backwards, if there's mark on the other side.
My 2 cents worth.

Good luck
Holl