A client brought us a 66 Bronco w/ a 289 HP/HO engine, after installing a Holly sniper EFI system w/ the hyper spark management system. Client stated "could not get the engine running". He stated he could get it running & idling, but experienced backfires when trying to get the vehicle in gear. Backfiring was not explained in detail. After getting the truck into a bay & starting it, I had noted the engine was extremely hard to start. had to hold throttle to floor to achieve a turn over (doesn't pop off). after a few minutes of running, engine would backfire out of the intake and stall. Talked w/ holly about this system (plug and play.... right.). they had informed me about a sporadic spark issue they've been having w/ there ignition boxes. I mapped ignition timing & saw timing jump to approx 40 degrees btdc every time I got a pop out the intake system. I'm pretty sure that's an issue... but what i'm wondering. While i had the engine running, i saw excessive amounts of fuel vapors coming from the crankcase. billowing... i dropped the oil from the engine, it was extremely diluted and non viscuse. replaced the oil w/ little to no positive gain. ran a compression test and got pretty even numbers. highest 90 psi, lowest 75 psi (dry & cold) i know it has to be warm for an accurate reading. wet i got a positive increase of 10 psi across the board. My question is, does anyone know, or can anyone tell me w/ consistency what a high output 289 ci is supposed to produce during a compression test??

Any info? preferably factory info, not your guess or seat of the pants ideas.. pretty please.

Maybe this will help. From my shop manual that came with may 1967.
When we checked mine a few years ago it was between 90-100 psi on each cylinder.

Firing order, plug wires bad?

I've confirmed the firing order is correct. The Firing order per technical data sheet is for a standard non performance 289. At some point ford put a different crankshaft in their 289 for the high output and/or high performance engine. I've identified the firing order on the intake to be 1-5-4-2-5-6-7-8. Tried that firing order to no affect, the engine wouldn't even try to turn over. 1-3-7-2-6-5-4-8 is how it was brought in, my customer stated he checked several times as well. After some research i had found that is the correct firing order per the high output crankshaft. It has a brand new distributor from Holley (hyper spark management system), with new MSD wires. it has autolite plugs (not the biggest fan), and 4 out of 8 plugs keep carbon fouling. which could be a bad fueling calculation on my part. After talking w/ Holley on this specific system. a sporadic spark condition had been an issue w/ their ignition boxes being timing is solely controlled by this box, no vacuum advance/retard or advancement weights. As i stated before, i watched and witnessed this problem occur w/ the ignition box currently installed in this truck. I had my timing set to about 13 degrees BTDC being im at altitude above 5,000 feet. I can correct this timing through the software on the EFI system. Every time i got a pop out the intake, i saw and witnessed the ignition timing jump to roughly 40 degrees BTDC, then stall. Now to me the intake valve is open during ignition and is backfiring due to the advancement of the ignition system. at a crank speed of 600 to 800 rpm. leaves plenty of real time for my flame front to escape via the intake valve. could be wrong, but this makes the most sense to me. What i'd like some input on is the fuel vapors billowing from the crankcase ventilation system, my low compression numbers. Do i have sticking/ bad rings? I'd like to save my customer some trouble if this engine isn't worthy of the EFI system he installed on the vehicle? being he's going to be deep into this financially. i appreciate the input and don't want to put someone out to be wrong. I've been through the basics and done quite a bit of diligence on my part.

with the engine running & idling (talked w/ Holley again and was informed about a system update that needed to be done, little improvement. it will idle w/out an intake pop), i saw ignition timing right where i set it via the EFI software. meaning, i can command the ECU to set idle ignition timing where ever id like, 5 degrees to 25 degrees not sure why you want to set it that high.. the installation instruction state, holley's distributor needs to be set at 15 degrees BTDC(this is the only mechanical adjustment that can be made for the ignition timing). why they want this, is because it's a base point for the ECU. I command the ECU to run my idle ignition timing at 13 degrees. i run a timing light. And it's exactly where i set it. i can set it to 5 degrees. i can witness the retard in ignition timing and watch it run at 5 degrees. It is 100% controlled by the ECU. I rotated the engine over by hand to 0 degrees TDC. ensured the rotor was pointing at cyl. # 1. pulled the plug on cyl. # 1 and stuck a boar scope into the comb. chamber and saw both valves closed w/ the piston at the top of its stroke. I'd say the likelyhood of it being out 180 degrees is slim. my concern is that this engine is hard to start w/ low compression numbers, extreme amounts of blow by gases and crankcase pressures. meaning im getting blow by past the piston rings. low com numbers could be carbon build up on the valve seats or sticking valves. but i'm a suspect to the billowing fuel vapors coming out of the breather. My original question was, does anyone have factory specs for compression numbers for the high output/ high performance engine, stock from ford.. from what ive seen via my research is 125 PSI to 170 PSI. but that's not a factory number. I've seen they were a 8.5:1 to 9:1 for various engine codes, which tells me it needs to be closer to 125 to 150 PSI. why im realing back is some people were saying it needs to be closer to 150 to 190 PSI. If that were the case, my readings of 75 to 90 PSI indicates its time for a rebuild. or further investigation into why these numbers are so low I.E cyl leak down testing. Running compression test so on and so forth.

Thanks for the replay, it is appreciated!!

My old 289 HP in my old mustang was about 165 PSI as I remember you should see at least 125-150. engine warm all plugs out throttle plate wide open well charged battery. Have you performed a cylinder leak down test?

According to the manual RT9 posted, 130psi @ sea level is minimum compression. If you subtract 15% for 5000 ft ASL, min compression should be 110psi. Sounds like that engine has problems.


However, I don't think the back firing has anything to do with compression. If you are seeing sudden and random changes in ignition timing, there is likely an issue with the ignition trigger. The trigger could be bad, the wiring may need to be better shielded or the gap between the sensor and trigger wheel adjusted.

Can you tell us more about the ignition pickup?

- Is the sensor 2-wire or 3-wire?
- Is the wiring exposed or encased in a shielded conductor?
- If not shielded, is the wiring twisted like this?




From experience with my Megasquirt system, I had a bad ignition pickup and it would cause the computer to register multiple phantom ignition pulses. This made the computer think the engine was spinning faster than actual. (6000 rpm vs 700 rpm actual) The computer reacted by pulsing the injectors a bunch and advancing the time a lot.