Looking for constructive thoughts on problem. Not my car with the problem but very similar cars. Here is the specs and situation. Just to clarify this is a late model lt1, roller cam, non-interferance engine. Thanks guys-

SCAT 9000 383 crank, forged i beamrods, forged 11:1 pistons, XR 288 grind on LT1 cam core, stock cumbustion chamber cc's(58cc), 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.

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, 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 staright from underneath the head. BTW at that time the max PSI on #6 was 95, way too low in comparrison. 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 silcone 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 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.

Is #6 still low after milling the head?

Afterfire is usually the result of poor combustion, rich mixture, sticking valves or late ignition. All of these contribute to the exhaust opening with the burn still ongoing or excess fuel is pooling in exhaust system where it is being ignited. Sometimes super lean mixtures will also do this as they burn slowly, but they also will cause the engine to pop back thru the intake. But where a port injected engine is concerned, you will not get fire out of the throttle body the way you do with a carb or TBI since there no mixture in the manifold to burn. But if you listen carefully you can hear the port pop into the intake plenum if its running lean. Try a stethoscope on #6s intake when it afterfire into the exhaust. The symptom of being fine when cold and afterfiring when hot could be a symptom of going lean as when cold the computer is running an enrichment cycle (choke) which goes away as the motor get up to operating temp. Given there are 8 independent injectors, you could be having trouble with just one.

Bogie

Are the valve lifting ok on that or all cylinder?

I hadn't thought about the tune as it was tuned by a reputable company that we have not had issue with before. At one point there was backfire in the intake but not anymore so I did not list that as a symptom. After we changed header gasket and intake gasket, that did not occur anymore.

As far as for current cylinder PSI on #6, don't know. Hesitant to just reinstall head when not certain what initial problem was.

I agree that usually after fire is timing related, but am miffed by it only occuring after a few minutes of run time. Our scanner does not register air to fuel ratio so I can't tell if it is leaning out like you said. I will say that We were standing behind the car this last time and noticed that immedialtey following the back fire, the exhaust flow thru the tail pipes changed. It was a steady flow and then after backfire, it was inconsistant(colder day so you could see the exhaust). If that helps diagnose anything.

flat cam lobe on #6

Luck - Valve movements look good and even across all cylinders, put I did think about the possibility that maybe one of the new lifters (hydro roller lifter) is bad.

454 - It is extremely rare in hydraullic roller cam situations, which these cars are equipped with from factory and stated at top of thread, to have a flat lobe like in a flat tappet cam. Add to the fact that this is a new cam, even less likely. But I do appreciate the thoughts, just not very likely in this situation. I have done a fair amount of buisness with this grinder and never have his cams failed on me.