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Discussion Starter #1
Hey guys let me pick your brains
I just rebuilt a sbc 350 for my 87 suburban and now I have backfiring under a load from the exhaust
I left it tbi but upgraded internal components but I don't have specs on cam it's supposed to be a 327 with 350 hp set up any advice will be good
 

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Administrator
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Welcome....First thing I would suspect is that an exhaust valve is set a bit too tight and staying open a bit.
 

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True Hotrodder
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The above and usual suspects - timing, plug wires, carbon tracking, too lean.
 

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We will the specs of what you put in the engine. TBI has very limited flexibility to understand changes that effect mass flow through the engine. It runs on a set of factory fixed ratios programmed into a 3D map of throttle position measured by the Throttle Position Sensor (TPS), to manifold vacuum measured by the Manifold Absolute Pressure (MAP) sensor, to RPM. The result is trimmed by the O2 sensor for engine wear.

The relationship of the three major sensors was used to calculate an address for each factory tested crankshaft load condition which is a measure of the mixture flow through the engine in that moment. The address is used by the computer to look up fuel and ignition timing for that the integration of the 3 major sensors says lives there.

The problem with making modifications to these engines is now the relationship of these measured functions is different than what is programmed, so the integral produced in the computer leads to the wrong address for the actual load condition on the crankshaft.

So changes that are taken for granted that can be done on a carbureted engine of tweaking the jets and resetting the timing become a several hundred computer chip reprogramming exercise for EFI engines, the TBI being the lesser expensive of these exercises.

So simple errors of assembly or tune a problem of any engine build get added to the issues of reprogramming the computer for changes to displacement, cam event timing and lift, headers, dual exhaust, different heads, changed compression ratio are all things that singularly or in concert can have the computer generating addresses for the wrong answer to the engine loading or making up addresses that don’t exist.

The O2 sensor drives both a trimming function to clean up an addresses act a little, it may cut the lawn but it‘s not going to remodel the house. If the O2 sees consistent errors that are not very large it can over seeing the error and fixing it a multitude of time can put new instructions at that address, but this is a slow function that has to repeat through time and is wiped out if power to the computer is lost. The computer is on all the time even when the keys are in your pocket. Power loss happens if it is disconnected from its connector or the electrical system is disconnected from the battery.

So in the end excepting an error of assembly this problem on an EFI engine can result from making modifications that are too far away from the base program for the computer to fix, thus we need to know the details of you changes to the engine in order to offer any assisstance.

Bogie
 

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Google "Tunnel Ram 406"
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Make sure the timming mark is at tdc. You may be too advanced on the timming.
 
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