Hey Rocky,
You could take 2 near identical 95’ GT’s and modify the same as you have, one may run well with no Tune, other may not. As Kurt mentioned, it’ll need a Tune to perform best, but any possible issues first need be identified and repaired prior to Tuning.
Over time I‘ve found associating engine Vacuum numbers with a solid RPM gives more consistent results, I use 1,000RPM’s.
-For modified motors untuned, or with issues it’s more realistic an RPM a finicky motor will tend to agree to run stable at for certain testing, has helped.
Since I do top ends on all Mfg’s frequently, I’ve logged the numbers of most TF parts.
As you’re not new to all this, I’m just trying to establish exactly what the build is, there’s things you’ll already know, I’m just suggesting where I’d start, what direction I’d go...
-Which ProComp head’s are you running?
-And, what’s your Vacuum at 1,000RPM’s?
I’d suggested that possibility of preload being tight on a Cyl, as you mentioned hearing a Tap, and an intake backfire under load. That would be where I’d start, pull the covers and run through the cylinders & move on once you confirm as correct.
Check #1 Piston and Balancer’s lineup @ TDC to rule out any timing issue. Sounds as if it’s OK, but easy to confirm.
Keep lash more on the conservative side, as your chasing this, at 025-.040.(1/2-2/3 turn past zero lash & lock). Norm.is pushrod .020-.060 into the lifer after zero lash.
You won’t always see a slightly tight lash setting affect numbers in a cold compression test, but you’ll feel it engine hot.
No Wideband yet? Have faith in reading your spark plug electrodes for now, they won’t lie.
Motor was running OK before the rebuild, no
Systemic issues, just tired?
I’m uncertain if you’re relying on the stock Oil Pressure Gauge, or aftermarket..It’s absolutely in your best interest & a very well spent 25-30$ if you buy a Mech Gauge to run on a fresh motor..
Still running conventional motor oil for the Ring & bearing break in? Running a High Volume or High Pressure oil Pump of type, both HV/HP...or OE? ARP Pump Drive?
Oil pressure, Cold?..Operating Temp?
Where does the car idle, does it hunt at all?
Is the IACV, TPS set, you adjustEd idle up manually?
I’d be looking at your spark output via spark tester on each cylinder, see how far a gap you can get a spark to jump & see if the others can repeat the same, try it SPOUT in & out, sometimes it has an impact if the PIP’s loose, or on its way out. Usually Injectors are affected first, not always..
Is your Dizzy stabbed correct for proper Injector timing?
When your fresh Engine Mechanical basics are all correct, your new parts functional & become exhausted, keep in mind to view it as you would if troubleshooting any other 86’-95’ Mustang GT, vacuum lines, grounds, Alternator output, aftermarket ignition component avoidance, and all the other basics.
Also consider how the R&R of the engine may wreak havoc on the wiring, connections disturbed and sometimes less than optimal when reterminated, etc.
Build questions, etc.. to get on the same page?
1) Drain your Tank & full up with fresh fuel? If not, how long has the fuel your running been in your tank?
**2) Are you running the Cam’s springs, retainers...did the Shop tear down the heads & check guides, seats, etc, pressure test?
3) Exactly what’s been added to the build, car wasn’t bone stock previously-correct? 3B)Already running a rechipped EEC before the build (you mentioned a deleted Smog item).(?) What injectors, MAF, TB, other Mod’s are you running? Headers? Shorty’s, LT’s?
4) You did verify fuel pressure, also perform a Volume test? You‘ll then know it can keep up during heavy engine demand.
5) Most Smog still intact, are you running an aftermarket exhaust, Cat’s? Mentioned new 02 sensors enroute, improvement?
6) Check all the new Injector connectors you added, add AD grease to improve connections, tested all for correct Voltage at all injectors & presence of pulse.
7) Have you tried Clocking the MAF a bit to see if you can get an improvement in running conditions? Unplug it & the motor should really fall on its face..
8) Been checking the EEC for trouble codes right along?
9) I’d expect the Car to run decent with the parts you’ve mentioned, uncertain what your ProComp head’s have for Chambers, and your resultant C.Ratio. .040 thick Gaskets?
Hopefully, this will get you moving and the questions answered will help to narrow things down.
Best of luck Rocky!
Enjoy the weekend!!
Best! -John
** ProComp head’s, in my own experience, are head’s that should be carefully checked out before using. I’m not saying other’s are exempt from this, the days are over where you can bolt up 95% of new head’s & have certainty they’ll work as desired.