Why so much initial timing?

[danthegeek]

I am kind of curious why my 350 needs so much initial timing. I dont plan on changing it, just wondering. Ive checking top dead center with a piston stop twice. Its dead on.

Initial timing is set at 20 degrees with a total of 38 degrees, all in at 2500. (36 initial and 54 total with the vacuum advance hooked up.) No pinging or chugging under load or cruise. Runs like crap with anyhthing to much lower. Im guessing it is the low compression?

Stock 350.
76cc iron 993 heads giving about 8.5:1 compression
hydraluic cam, 204 duration with .420 lift on the intake side and 214 duration with .443 lift on the exhaust side.
edelbrock performer air gap intake with a 600cfm perfomer carb
MSD distributer.
headers, 2 1/2 exhaust blah blah blah.
Idles great and the plugs are golden brown.

Thanks
-dan

[veno]

a couple of guesses... late model timing mark... on the balancer/ or timing set... some where there is a goof.. or your running Sunoco c16 and need the advance for the reduced burn rate?

[nova64000]

I heard that a low compression motor with a big cam sometimes needs a lot of initial timing...

[danthegeek]

a couple of guesses... late model timing mark... on the balancer/ or timing set... some where there is a goof.. or your running Sunoco c16 and need the advance for the reduced burn rate?

It has a new balancer on it and an adjustable marker and I have verified top dead center. I also measured out the timing marks by dividing the circumference of the balancer and used timing tape next to my marks. As for fuel, i run 89 octane most of the time.

I heard that a low compression motor with a big cam sometimes needs a lot of initial timing...

Yeah, but i wouldnt consider this that big for this motor. It almost sounds stock at an idle.

Thanks for the suggestions. Again, Im just in a curious mood. But after increasing the timing, it runs so much better. I just didnt think it would need this much advance.

-dan

[ALLT4]

The only way to truly know TDC on your balancer is with a piston stop and degree wheel, anything else is just close for reference.

[danthegeek]

I verified using a piston stop but didnt use a degree wheel. Instead, I put a piece of tape on the balancer and turned the motor over until the marker was just right of top dead center on the balancer. then i took the piston stop and tightened it until it hit the piston. I turned the crank over the other direction until the piston hit the piston stop again. Between those two marks is where I marked top dead center. I thought the marker had moved but I verified it by checking again.

I guess this is just what it likes and ill keep it this way until it tells me otherwise.

Thanks again for the input
-dan

[D-Man]

Do you know for a fact that your vacuum advance is working? It almost sounds like it isn't. That sounds like a full mechanical advance set-up with some of the mechanical advance limmited with a larger "stop" bushing.

Are you using the wrong plugs? A non-projected nose plug or a plug without enough reach will cause similar issues (requiring a LOT of advance).

If you are in fact getting THAT much total advance, the only reason for it NOT to ping is if it's going PIG FAT at WOT...

Good luck!!

[tpinovaII]

If it's an older GM distributor (applies to HEI or points), make sure the mechanical advance isn't sticking of hung up. It's a real common problem with a mechanical advance after a distibutor has been run for a number of years.

[danthegeek]

No no. The vacuum advance is working fine. It is at 37-38 degrees total and 19-20 degrees initial with the vacuum advance unhooked. That is what I have been reading as the upper limits of the large chamber heads. The 54 degrees total is with the vacuum advance hooked up (16 degrees). It is a new MSD distributer and it works great. As for the plugs, they look good and are the right reach. I have them gapped now at .045. The distributer only has 18 degrees of mechanical advance because that is the size stop bushing I have in it. I had a 21 degree stop bushing and it ran great. But, it had this stumble from a stop that wouldnt go away. I put in a larger bushing and some lighter springs and its gone. The carb is setup pretty close now, well as close as I could get it without some readings on a dyno. At wot, there isnt any black smoke (from what I have seen and my father as well) or a load up of fuel and my plugs looked good after beating on it a bit today. I also have a rock steady 19 lbs of vacuum that drops to 1-2 when I snap the throttle (Ive checked using two different gauges). There isnt much blow by either, only the usual little bit that you see when taking off the breather when its running. Also, it isn't using a drop of oil and it look almost new when i change it ever few months. I guess I could have a slow burn or need hotter plugs.

Anyways, it is quite a bit snappier then it was and Im very happy about it. I'm not trying to be difficult, Im just curious :D. I know, curiosity killed the cat, but then how would I learn. Besides, all the good info you guys have thrown my way could come in useful for another.

Thanks again, and sorry I'm such a bother. :D
-dan

[D-Man]

What fuel are you running? It sounds like that thing would be happy with the cheapest stuff!!:yes:

[Dan_Lebherz]

What is your idle rpm when you get the 20 degrees of initial?
You need to disconnect and plug the vacuum advance and set your idle to 550 to 600 rpm to set your initial. If you have weak springs on the mechanical advance you may be reading some mechanical already in, so set your idle very low and re-check.

[ALLT4]

Better yet if you suspect some mechanical advance is kicking in wire wrap it so it can't come in at all and recheck initial.

[chevynuts88]

I went thru all that crap! It turned out it was low fuel pressure and not timing at all.

[danthegeek]

Sorry to get back to you guys so late. The phone lines went out along with the DSL. I thought I was going to go crazy without internet.

I tried setting my idle to about 500 rpm and the initial advance is at 19 degrees. It doesn't start advancing until around 1000-1100 rpm. Im running 89 octane gas so i may try 87. I don't want to though because it has ethanol in it around here. My fuel pressure is at 6psi using a regulator.

I did try an experiment yesterday though. I put in a smaller stop bushing with even lighter springs. I was trying to get my timing the same at 2200rpm because that is what the rpm jump to when you mash the throttle to the floor. I think that may be the reason its running so much better, it has more timing at the stall rpm. There didn't seem to be any difference in performance but my idle wasnt as smooth. So i tried putting in heavier springs with the larger (18 degree) bushing and it stumbled again. The timing was about 8 degrees lower at 2200 rpm then before with just the the next set of heavier springs. So, I went back to the original setup. I'm sick of changing stop bushings, they are a pain to get change in the car.:rolleyes:

Do you think domed pistons would cause something like this? I not sure if they are or not because I didnt build the motor. I do know the cranking pressure was at 170-175 for all of the cylinders with 76cc heads.

Thanks again for the help
-dan

[Dan_Lebherz]

If the car runs well and you aren't getting any detonation, I wouldn't worry about what the initial appears to be, but it doesn't seem that you should need that much initial for that engine.

[Dragster66SS]

I have a similar issue with the 372 in my race car. Timing is locked out, but has to be set ~55 degree advanced range as well. Cranks (starts) and runs the best (on track testing) there. I had chalked it up to having short-reach plugs in the aluminum heads (oops)... but purchased/installed a new set of the "correct" long reach plugs and slowed the car down 5 FULL tenths in the 1/8; also did not correct the need for 55 degrees timing.

I put the old "wrong" plugs back in and decided just to run it with all that timing. I've replaced dang near every ignition-related part on the car chasing this.

I'm not convinced myself that it isn't a result of a large dome / small combustion chamber combo.

Good Luck,, and PLEASE update if you stumble upon a solution. I should PM you my cell # just in case you do. LOL.