Hey guys,
IÂ’m hearing popping/backfire only under hard acceleration, usually starts after 3000 rpm, and only hearing it from the passenger side pipe. At each pop I can feel an extremely brief loss of power, similar to a misfire. Could this loss of power be due to expansion upon detonation in the exhaust which is causing exhaust valves to open during power stroke? I donÂ’t believe itÂ’s missing at low rpm, as it actually runs very smoothly at idle and lower speeds. Have driven the car since new, now has 8,000 miles and first time IÂ’ve experienced this. Please chime in.
Some more info on the car:
427W
Holley 750 carb 4 barrel
Champion spark plugs
MSD distributor
3/8” fuel line, mechanical pump
Running 91 pump gas with octane booster
Could this loss of power be due to expansion upon detonation in the exhaust which is causing exhaust valves to open during power stroke?
No.
"Popping" on acceleration *usually* indicates "lean", and if it's only one side, and "new"....
I'd take the primary metering block out of it, hose it down with carb cleaner, blow it out "thoroughly" with compressed air, and see if that changes anything or cures it.
If not, proceed to primary + secondary metering blocks, and carb body with the same process described above.
I agree it could be the carb but I would pull the plugs on that passenger side to see if it’s running lean or if it’s one plug misfiring. I seen cracked plugs run fine at idle but misfire under load. Spark Wires I assume are not frayed and in good shape.
If plugs and the carb are good, I’d check the coil next. If that was good....distributor issues, valve or push rod issues...I think some more info the engine would be helpful.
Good luck
Orrrrrr check you header on the passenger side and make sure it hasn’t loosened up and is letting air into the exhaust system.....
Originally Posted by wareaglescott
I was thinking possible exhaust leak also since only coming from one side.
Popping due to an exhaust leak will only occur upon deceleration, not under load. Since the OP reports being able to feel it as well as hear it I'm also thinking a lean misfire and agree to pull the plugs and see what they're showing.
I've seen this before, here's something to consider:
My guess is you have one or more cylinders on the right bank that are misfiring that creates a loss of power you can feel in your butt dyno. The popping noise is an air/fuel charge (misfire = unburned fuel in exhaust) ignited in the collector. Note that two things that place a heavier load on an ignition system is cylinder pressure and RPM. Cylinder pressure increases as you open the throttle and RPM follows so it is more likely to see a misfire from a compromised ignition system when the engine is under load at WOT and higher RPM.
A misfire can be caused by many things so you need to take a systematic approach to troubleshooting.
Dart Little M 406" SBC 800 HP N/A & 1,100 HP on nitrous, 2-spd Powerglide with trans brake, 6,000 RPM stall converter, narrowed Moser 88 3.90:1 spool with 35-spline gun-drilled axles & Torino bearings, custom parallel four-link, custom tube chassis & roll cage NHRA certified for 8.5-sec (only two FFR Hot Rods have this cert).
8000 miles, new problem, only one side so maybe only 1 cylinder... I'd pull the plugs. Plug breakdown or gaps that are too big will breakdown under higher rpm and higher cylinder pressures. When I used to test aviation plugs, it was amazing how little of an increased gap would create a complete shutdown of the plug at higher rpm / cylinder pressures.
Definitely read your plugs. Pay attention when you pull the wires off, one may not be on tight. Also make sure you don't have one that got burnt. Then the other things that were mentioned.
Here is one that got me. I started getting misfires over 5000rpm with about 4000 miles on the car. It felt like the rev limiter was kicking in. The distributor cap contacts were badly corroded. I live at the beach, but it looked more like I live in the ocean.
Today I pulled the spark plugs and found each of them pretty badly fouled. I’m going to replace all of them and see if this does it. Be back soon...
Also, this is a Champion RC12YC plug. I tried doing some research and found 20 ft lbs to be the torque. Sound right? I was also going to use anti seize as it was clearly put on by the engine builder.
If you're going to use anti-seize be careful not to get it on the ground strap or electrode portion of the plug. Also, be advised that torque specs can change significantly when you lube the threads and if you use a dry torque value on lubed threads you can easily tighten the plug too much and damage the plug or heads or both. I'm a firm believer in apply a torque wrench to critical fasteners or in some cases, measure actual stretch. However, I never torque spark plugs. When installing new plugs tighten them until the crush washer is flattened and just a tad more to snug them down. I've never had one blow out.
Dart Little M 406" SBC 800 HP N/A & 1,100 HP on nitrous, 2-spd Powerglide with trans brake, 6,000 RPM stall converter, narrowed Moser 88 3.90:1 spool with 35-spline gun-drilled axles & Torino bearings, custom parallel four-link, custom tube chassis & roll cage NHRA certified for 8.5-sec (only two FFR Hot Rods have this cert).
If you're going to use anti-seize be careful not to get it on the ground strap or electrode portion of the plug. Also, be advised that torque specs can change significantly when you lube the threads and if you use a dry torque value on lubed threads you can easily tighten the plug too much and damage the plug or heads or both. I'm a firm believer in apply a torque wrench to critical fasteners or in some cases, measure actual stretch. However, I never torque spark plugs. When installing new plugs tighten them until the crush washer is flattened and just a tad more to snug them down. I've never had one blow out.
I agree. As you tighten the plug you can feel when it starts to crush the washer. That crushing action takes a certain amount of increased effort which stays the same for maybe one to 1.5 turns. At the point where the washer is fully crushed, the effort to keep turning suddenly increases. At that point stop turning, you are done.
FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.