How much cranking compression loss with high lift cam?

[AncientGeek]

How much compression loss can I expect with a high lift cam during cranking? I fitted a genuine euro high lift cam on my 1500 during a head refurb and can't get it started. During running basic checks I checked the compression and I'm only getting about 700KPa which seems very low to me. I'm just cranking with the plugs out. I never checked the compression before but it started easily and ran fine. I've rechecked the tappet clearances and they are fine.

 

[Dan Sarandrea (Phila)]

Lift is one thing, duration and lobe timing is another, although they all are pieces of the puzzle.

It's much more likely that the problem is with putting it back together wrong, or that a component happened to fail in the process.

How about some pix of your crank and cam timing marks, dist rotor orientation, and comparison on crank pulley marks to flywheel and bell housing marks.

 

[Dr.Jeff]

That's about 100 psi, which is very low. However it should still be enough to at least run (but it won't be strong). I'd be inclined to recheck your cam timing after assembling everything. Same for ignition timing. This is assuming you have verified there is a spark and the fuel system is functioning correctly.

 

[carl]

Both my Xs came with high performance cams and on both I removed them and installed stock cams. When doing a traditional compression check those high performance cams knocked off about 10-20 psi. Your euro cam should not knock back the stock compression enough to keep it from starting. I agree with the others, you might have gotten the cam in off a few teeth. Also, on both motors I needed some hefty ignition advance to get the car to start and idle, around 20-30 degrees at idle rather than the usual 10.

 

[AncientGeek]

Yes it does look like I have something wrong but baffled as to what. I have spark and fuel. Pictures are difficult because of the locations. With the cam pulley on the mark the flywheel is at tdc (PO installed the flywheel wrong but made new marks which I verified with a dial gauge when I had the head off). In this position rotor points to cyl 4. Also at this position valves on cyl 4 are fully closed judging by the lobes. The cam pulley is a VAS ali one but I verified the the locator is the same as the original.

 

[dllubin]

Yes it does look like I have something wrong but baffled as to what. I have spark and fuel. Pictures are difficult because of the locations. With the cam pulley on the mark the flywheel is at tdc (PO installed the flywheel wrong but made new marks which I verified with a dial gauge when I had the head off). In this position rotor points to cyl 4. Also at this position valves on cyl 4 are fully closed judging by the lobes. The cam pulley is a VAS ali one but I verified the the locator is the same as the original.

In addition to pointing the rotor at cylinder 4, check the continuity of the points to ground and make sure they just open at whatever your specified timing at idle is (somewhere between 0 and 10 degrees usually). Just pointing the rotor can leave the timing way off. I'd recommend setting the engine to whatever timing mark you want, and then very slowly turning the distributor clockwise until the points just open.

 

[kmead]

Is that where the Ducelier has #4? On the Bosch it’s around 90* - 120* clockwise from there. Different cap construction in all likelihood.


The question is timing.

The last person with this same problem had an issue with the points and condenser. If you have spark, then it would seem you have it at the wrong time.

 

[EricH]

Also at this position valves on cyl 4 are fully closed judging by the lobes.

If you're looking at the lobes through the oil filler, you're looking at #1 - #4 is the one at the flywheel end of the motor. If you have the cam cover off so you can see all four sets, you're good.

 

[Dr.Jeff]

Yes it does look like I have something wrong but baffled as to what.

It might help to review the step by step guide for replacing the timing belt in the Wiki (linked below). It covers the setting of cam timing and ignition timing, which are the most likely targets here. Sometimes following a written guide as you recheck everything sheds light on an obvious thing that was inadvertently overlooked.
http://xwebforums.com/wiki/index.php?title=StaticTiming

 

[AncientGeek]

Well it started. Easily.
I was static timing the ignition with a multimeter and due to my extreme stupidity I ended up timing it on the closing of points, not the opening!
This is the Marelli distributor and what I really don't understand about it is that when the points open you can not position the rotor on #4. The way I've got it now is that the rotor has already mostly gone past the position when the points open. It seems to work though.
I did try positioning it ahead of the contact in the cap and got some spectacular backfires which caused my son to come running out the house.

Thanks everyone for the excellent advice.

 

[lookforjoe]

Good to hear you got it resolved. Did you check the timing with a gun after?

This thread reminded me that I cannot recall what Fiat's rationale for setting timing off #4 vs. #1 was. That procedure hasn't applied to my setup for some years now, since I switched to ignition systems used by Volvo in the 80's-90's (my current setup uses a crank sensor, so no setting at all).

 

[Dr.Jeff]

Excellent news. Yes, what Huss said - now that it is running, reset your timing with a timing light on the crank timing plate (main pulley).

 

[Jefco]

When you set timing statically, the rotor will come out a little behind because the advance weights have been taken out of the setting; when running, the weights will advance the rotor to align with the contact in the cap.

 

[Ryan59]

Just to clerify cam timing. Years ago racers settled on using lobe centerlines to quickly share cam timing info. Narrow intake lobe center numbers of 106, 108 etc. These are crankshaft degree numbers. Have higher cylinder pressures than 112 or 114. SOHC are limited compared too DOHC where lobe separation can be adjusted separately. A single cam is fixed as far as lobe separation so basically moving the cam is advancing it or retarding it. Advancing a too large cam with low vacuum will, in most circumstances, make it milder, raise vacuum and lower the HEF highest efficiency point. For the most power off a corner, advancing the cam generally helps. Not all cams respond to advancing. If a cam is ground for low-end power it won’t improve by advancing the timing to a 108 degree intake lobe centerline. However, Racing cams with long durations, do.

 

[Dan Sarandrea (Phila)]

Good to hear you got it resolved. Did you check the timing with a gun after?

This thread reminded me that I cannot recall what Fiat's rationale for setting timing off #4 vs. #1 was. That procedure hasn't applied to my setup for some years now, since I switched to ignition systems used by Volvo in the 80's-90's (my current setup uses a crank sensor, so no setting at all).

I have to think that if your setup had a cam-end distributor, everything needed would be right there in front of you---adjust the distributor with one hand while the other holds the timing light on the flywheel dot and bell housing hash marks. Best I could come up with!