engine build head scratcher

**monnieh** · 10-09-2009, 11:56 AM

1997 WS6 TRANS AM LT1, 4L60E, 3.73 10 BOLT

ENGINE PARTS BUILD:

SCAT 9000 383 crank, forged i beamrods, forged 11:1 pistons, XR 288 grind on LT1 cam core, stock combustion chamber cc's, ported heads/intake, new lifters/pushrods, new ford red top injectors, aeromotive fuel pressure regulator, long tube pace setter, borla cat back, no cats, 58 mm, electric water pump, 6a-L ignition box, auto, tuned by pcmforless, 97 lt1 f body. Both front 02's are new as is opti.

TROUBLESHOOTING STEPS TAKEN SO FAR:

Car ran great for three days. Then backfire started. Plugs and wires looked fine but we replaced with OTVC Taylor wires and NGK plugs. Back fire got better (not as often) but car is still missing and backfire. No regularity to backfire. We changed the exhaust gasket and plugs and it ran good for 20 min's, and then started backfiring out the tail pipe again. So we changed directions and started thinking compression problem. All cylinders hit 120 psi on one stroke except the number 6 cylinder. We pressurized the cylinder and heard no air out the oil tube, exhaust, or intake; but it almost sounded like air coming straight from underneath the head. BTW at that time the max PSI on #6 was 95, way too low in comparison. So we take the head off and it looks like the metal at bottom of cylinder separated from gasket material (fel pro gasket). We change the gasket after using a square to check for obvious low spots and everything looked good. Put back together and let sit overnight to let all silicone dry. Car ran fine for 5 min and then did it again. A second compression check showed the cylinder would pressurize to 95 psi on first stroke then 120 psi on second, so still not as good as the rest but a big improvement. We take the head back off; inspect block for gouges or anything. There were none as block just had decks planed when building the stroker. Cylinder walls all look perfect and piston travel is perfect as well. Went ahead and took head to the machine shop and had them plane it to clean it up. Just got it back, but they said they removed only a very small amount to get it to clean up and should not have had any problems with it. They also inspected valves and they were fine and the springs checked out as well.

It only backfires once car gets warm seems like but it has to be compression related based on what that #6 is reading like. Any help is appreciated guys. Trying to find solution before putting head back on car.

**jiveass** · 10-09-2009, 12:13 PM

could it be cam related issues?

**bigrondownhiller** · 10-09-2009, 01:54 PM

Is the cylinder leaking down or is the pressure holding? When you torqued the head did you do it to lb/ft or angle? Did you use the original bolts or new ones?

**monnieh** · 10-09-2009, 10:50 PM

It is holding pressure no leak down issue, bolts were torqued using ft lbs according for the two step torque to yeild bolts, and new bolts were used

**JaycenK** · 10-10-2009, 05:53 AM

Everything else run ok before the build?

**JaycenK** · 10-10-2009, 05:54 AM

The head have any cracks? was it checked for that?

**joedude02** · 10-10-2009, 07:24 AM

just an idea since no one else has suggested- what's the possiblity of one of the rings being bad or backwards? i've made that mistake before, but luckily on the spin test before leaving the shop.