The popping problem: (Please e-mail dzug@delanet.com if you have ANY ideas)

and PLEASE if you see anything in here that could be improved on, explinationwise, tell me as I plan on leaving the progress / final outcome publicly accessable.

After replacing the stock camshaft with a Linati 205/205, rebuilding the oil pump with a Hi-volume kit, and replaceing the timing grars / chain (Ederbrock DR) and lifters(GM) and rods(BGC) I started having some problems with backfire thru both the intake and exhaust. The backfire occurs starting at mid-range RPM's (2000-3500) when I punch it. I can get it to pop at MOST rpm ranges, but its minor at the low range and I really have to punch the throttle quickly to hear little puffs. Theres a slight stumble, then a loud POP from either the intake OR the exhaust... Most of the time its from the exhaust but at lower RPM's it seems to favor the intake. at 2500 - 3500 the pop from the exhaust id VERY loud and if I had mufflers they'd probably blow off. The pop is not a problem at all (doesn't occur) during normal driving, and during hard launches, but the car feels flat. prevously a 12.7 car, i bet it won't pull a 13.9 now.

Heres what I've done so far to try to correct it (thanks to list members for the leads that I followed so far) :

0. Followed proper break-in procedure.

1. re-verified the cam sensor set to EXACTLY 25 degrees ATDC(Compression stroke) and made sure it wasn't 180 degrees out.

2. Made sure the ground wires were clean and tight. Measured resistance between engine (head) and firewall to be very minor.

3. Applied Top Engine Cleaner

4. New plugs (AC R42TS), New coil pak, new wires.

5. Made sure harmonic balancer, which has a damaged keyway on my car, is tight and is NOT moving. I disconnected the serpentine belt and drove a few blocks and sure enough, it still backfires.

6. ohm'd the injectors and checked for leaking injectors.

7. verified the MAF reads up to 255. It BARELY does.

8. verified that TPS is set and transitions smoothly.

9. checked for leaks in vacuum lines especially connected to turbo.

10. Checked the cam grind numbers with list members.. nothing unusual there.

11. Sept 3/98 After a list member questioned me on degreeing-in the camshaft (oops, I just lined the dots up!!) I ROUGHLY checked the degree-in a very UN scientific way.. but without a degree wheel and a height checking guage for the valves I had to do something. I marked TDC (compression) on the top of the crank pully based on the timing indicator being lined up with the TDC mark on the crank, with the engine on TDC compression stroke. I then rotated the crank CCW until the intake valve on #1 was at max lift (eyeball, best guess.. really inaccurate) and marked the top of the pully again. This pully is stock and is roughly 21.5 inches in circumference. My second mark is the position of the crank when the center of the intake lobe has the valve at max lift. This mark was 7.25 inches from the TDC mark. Using percentages (7.25 / 21.5 * 360) we arrive at 121.4 degrees.. a far cry from the listed 106 degrees. Again, a REAL unscientific method, but I now have an inknling that I need to go buy a guage and a degree wheel and do it right.

If the cam IS this far RETARDED, I'll lose bottom end, that makes sense cause I DID lose it BIG. the torque picks up eventually, but only at like 4000 rpm. 2ND gear still rips, and the car shifts higher than it used to, but it seems like its just getting started in the power range when the shift comes.

Heres a chart: assuming I'm retarded by 15 degrees of crank:

Intended numbers / current numbers

in opens -3.5 btdc / 18.5 atdc

in closes 28.5 abdc / 43.5 abdc!

ex opens 36.5 bbdc / 21.5 bbdc

ex closes -11.5 atdc / 3.5 atdc

So geeze, the intake is opening only after a nice puddle of fuel has been sitting on the valve for a while, and BARELY closes before the plug is fired.. sometimes firing at 44 - 49 degrees BTDC! (remember, 90 degrees crank in a stroke, so 43.5 ABDC = 46.5 BTDC!!) POP! right out the intake. So the thing is PUSHING combustable mix out of the intake valve, somewhat.

As for the exhaust, the valve opens later and and is still open AFTER the piston starts downward on the intake stroke. I observed that I was able to develop 21 psi of boost with the cam AFTER its installation, and at the same wastegate rod setting I was only able to get 17 psi BEFORE the new cam. How to explain the backfire thru the exhaust I can't really come up with an explination... maybe an unburnt portion of fuel is hitting the CAT and exploding. dunno.

Next step is to get a degree wheel and a valve height checking guage. To be continued...

12. 9/5/98 So much for the un-scientific method of checking degree-in... The cam is DEAD ON. (162mm / 550mm) * 360 degrees = 106.2 degrees. I didn't check it with the correct instrument, I attatched a pointer to the rocker and put a piece of metal with mm marks on it behind and found the center of the lobe by marking 2 references at the same lift height on either side of the lobe and then marking the center of the two. The stock pully is 550mm around and the distance between the 2 marks was 162mm. The result could not have been any closer, huh.

Went to friday night drags and ran 4 times on a 100 octane chip with a slight modification. I took all the timing values that were in the low-rpm and low load areas in the main spark table that were above 42 degrees and set them to 42 degrees. The popping went away. Its the PLUG thats igniting the explosion that produces the pop, not pre-detonation, thats a good sign. I only had to take 5 or 6 degrees away from a dozen table entries. Thats not the perminant fix for the popping problem I hope, cause my low end was suffering as it was. It makes sense that the new cam may require less timing as the valve is closing later than it used to. What still does not make sense is the fact that when I turned the cam sensor 180 degrees about, the car did not act any differently.

Here is the best run of the night with the new cam: Half tank of 66% 93 and 33% 98 octane, and a 100 octane chip with boost at 19 psi. Little knock on the shifts, hence the lower MPH than I'm used to (normally 105).

With a 1.80 60' BEFORE the cam I ran a 12.80 in 64 degree wether on 94 octane plus booster on the same chip (except the timing I removed in the low range) so I'll be trying to figger out where my 2 tenths went. I'm a little lean at WOT (720mv) so looks like it may be booster pump or adjustable regulator time. I have a 97 SC-Park-ave pump and a 237 reg now. I hate to take away the (mostly) stock underhood appearance with the regulator though.

Things left to do / theories:

1. determine if the minor Synthetic oil film that leaks from the turbocharger into the intercooler / intake may be a factor. This leakage has gotten worse with the new found oil pressure after the hi-volume kit was installed.

2. Determine if insufficient exhaust preload may be a factor.

3. Find out what the stock specs are for the camshaft and determine if I need a new chip to fire the injectors a bit earlier or if I really need to reduce timing in areas due to the new cam or what.