Advanced the timing in my comet weird exaust

So i advanced the timing to 8 degrees but then did it by ear to until it sounded about right. But when i checked the exaust i felt no pressure but did hear noise. But once i retarded it back a little i could feel the pressure out of the exaust again. What is this? Is it bad or good or normal?

 

Dunno never heard of such...

I generally set timing to at least 10-12* reguardless, if it pings then it's time to rework the dist timing curve...

 

Thanks!! but its just strange that when i advance it more than it is right now there is no pressure out of exaust the car still runs fine but it just seems weird.

 

If running the stock distributor, the advance springs are a mix of one medium one and a heavy one. Finding another medium to replace the heavy one is typically a great (inexpensive) mod. Really adds to the driveablility.

Warning ... don't go out and buy two light springs ... strictly drag racing aftermarket stuff.

 

Pretty busy but this thread caught my eye and brain enough to quickly reply. Think about the affect that ignition timing has on the cylinder pressure that's generated in the combustion chamber and it makes perfect sense at to why the pressure and sound "changes". You are manipulating the cylinder pressures by moving the ignition point around.

Manifold vacuum at various rpm's is the ultimate rule for the best running most powerful engine. You'll never fully optimize the ignition without recalibrating the fuel curve too. Each system affects the other to the point of playing ping pong to really dial things in. When you give the engine the higher initial advance required to make more cylinder pressure at idle and light cruise speeds, thereby increasing manifold vacuum and making the engine more responsive at the pedal, the idle and transition circuits WILL go lean on you. Get greedy and you'll also need to replace main jets too. Both the ignition curve and fuel curve must be tailored to work together if you want the last little bit the engine has to offer. If you want to get a "crisper" exhaust note(denoting more cylinder pressure)?.. run the vacuum advance hose to a full manifold vacuum source to bump up timing at idle and light cruise speeds. Motor will make more torque, almost always a good thing, and become more fuel efficient too.

And btw, never set ignition timing "by ear" for any car you're going to rev up and play around with. Fine for getting things ballparked at idle and winging the throttle with the engine unloaded for cool sounds. Unfortunately that can and usually does get you into trouble real quick when your ears are flooded with exhaust and road noise and you can't hear the preignition/detonation events occurring during playtime where cast pistons and head gaskets get the P beat out of em'. And your spark plugs will "go away" really fast to further compound into even more problems. ALWAYS use a timing light and manifold vacuum gauge to know where you are and what the engine likes best.

 

I have tried hooking my hose to full vacuum but did not run well at all at idle in gear not sure why, I have a fairly radical cam and at 12 degrees maybe that is too much timing

 

Did you make any other changes at all? Adjust/fine tune the front AND rear transition slot exposures, float levels, and mixture screws?

Not trying to be demeaning or dismissive.. but you should seriously read up on internal combustion dynamics. Bigger cams set up the need for MORE ignition advance.. not less. Has to do with the weaker induction pulse and lazier combustion chamber filling at very low rpms that more duration(performance cam) causes.

I can downright guarantee that this motor.. almost any motor.. will run better at idle and part throttle(this has nothing to do with peak power @WOT) with timing locked out at 36° than it will @12°. You might think your motor runs good now but would be amazed at how much better it runs with even more ignition lead. But to get it right.. you need to recalibrate the fuel curve to match. Otherwise it will lean out and you'll be going nowhere fast with sky high combustion temp spikes that can burn spark plug ground straps completely off and even crack piston ring lands.

As far as the swapping of the vac advance hoses goes.. all you're doing is allowing the supplemental advance to come in earlier the the ported source would allow. Otherwise the ignition curves are identical above about 2,200 rpm for either configuration. Maximize manifold vacuum and any motor will run better with less pedal required. More torque is always a good thing.

 

without knowing if your timing marks are 100% accurate and you say you have a radical cam.. one of the old school tried and true ways is to get car idling at a good rpm, hook up a vaccum gauge to manifold vacuum, advance timing until max vacuum is reached (adjust carb so it stays under 1k rpm while doing this, closer to 800), then back off 2 in lbs. that should get you damn close for a good base. drive car. listen for detonation, back off until it goes away, shouldn't be much. best way is to use an afr and egt and plug chops, but the vacuum method is old as the hills but works. reset idle speed if nec, done.

who cares what the exhaust does at idle? doesn't matter.