Hot Rods BBC timing question...

Timing the 454 in Locomotion, unsure as of now on CR and cam lift but it's a pretty serious motor. Right now it's set to 35 degrees @ 3000rpm. It fires up and it quick on the throttle, no stumbles. Only thing is, and this may be perfectly fine for all I know, but if you have it at 3 grand or better and drop it right off quick, it will backfire through the headers a little as it winds down. Sends some flames out the open headers. Any thoughts? Timing off or too much fuel? Thanks,
-Dean

 

35 degrees @ 3000rpm should be a safe setup. Is it a street car or drag only deal? Pump gas, race fuel? If it's a strip only car, feed it a fresh set of plugs before the first run, then do a plug reading after the pass to determine where the fuel is.

 

If you are running 105 octange or high 38-42 degrees would be fine also. Just really need to run it and check the plugs to see where your at also get the motor warmed up and see how it starts then if it starts hard take some timing out of it

 

fuel can be a factor but is the curve in the distr set up for it. i usually set up a msd distributor for 39 total degress and all in at 2000 rpm , with no vacum advance

 

what heads does it have? Newer chamber designs can make power with less timing.

Old BBC heads seem to run great with lots of advance.

 

Squirrel is right on here. My old 12 to 1 iron head 468 Et'd best at 46 degrees total. My 14 to 1 489 w/ aluminum edelbrock heads dyno'd best at 36 degrees total. Keep in mind that about the only thing these 2 combinatons shared is the intake, carb, ignition and headers.
There are way to many variables to give you a rule of thumb timing setting. The only way to know for sure is to put it on a dyno that has good knock sensors, that and to have a dyno man and engine tuner that know what they are doing.

Back to your original question, my gut is that you are loading the cylinders with raw fuel with the high vacuum that is created by closing the throttle at 3000 rpm. It is also possible that you have an exhaust leak. You will get all sorts of flame and loud noises from the pipes with even a small leak in the exhaust. Especially if the leak is close to or at the collector.

 

A small leak in the exhaust will get you all sorts of poppin and bangin on the overrun. Ask any old Harley rider.

 

I should add that you can get to a safe total advance by reading the spark plugs discoloration or "burn back" on the ground electrode, it should be about half way from the tip to the base. Hi exhaust temps (glowing headers) is usually a sign that the timing is not advanced far enough, yet a vacuum leak can cause it as well. For a street engine one method I have used to establish a baseline is to use a vacuum guage. Advance the timing until you get the highest vacuum reading and the retard the timing to get the vacuum reading to drop about 2 HG, then drive the car listening for ping/detonation. Retard the timing a couple of degrees at a time until the ping goes away, then maybe one more degree to be safe.

As always a detailed build sheet on engine would give the engine gurus here a better shot of helping you establish a baseline.