IRON WILL

I have been having some timing problems with my car. It is a 355 CI small block. According to the engine builder it is roughly 9.7 :1 , my calculations show 9.5:1. I would thing that when running premium fuel (93) I should be able to run 36 degrees of total timing. At most I can run 30 right now. I have checked all components I can think of. I degreed the cam installed a new distibutor cooler spark plugs. The engine temp stays at 185. I am at a loss. I can barely run any vacuum advance currently also. Would heavier springs on the distributor weights help in this situation. I have one light spring and one medium spring currently on the weights. The weights appear to be standard aftermarket weights(lighter than stock). Any help or advice would be great.

sb69coupe

When you say that you can't run any additional timing, why not? Can you elaborate on the symptoms that you encounter when you try to run 36 total mechanical? At what RPM do you have the total mechanical advance coming in? What happens when you use the vacuum advance? How much vacuum advance are you trying to run?

lars

IRON -
The total timing you run depends a little on the heads and the relationship between the compression ratio and the cam. If you're running fast-burn heads, 30-32 degrees total is actually in the ballpark. If you're running conventional heads, 30 max is a little on the short side and may indicate that the engine wants more cam. Have you checked and verified that the harmonic balancer line really is TDC...? A 355 with a modest cam, conventional heads and 9.5:1 comp ratio should easily be able to take 36 total timing at 2500 rpm without detonation.

IRON WILL

When I checked the balancer was right on. It is an aftermarket fluid balancer and the mark can not move. I have the stock heads 76cc chambers with 1.94 intake valves. The engine pings at WOT if I go above 30 degrees total timing, and that is with 93 octane fuel. It doesn't sound right to me I think I should be able to run 36 degrees. The cam specs are 224 degrees at .050 tappet lift, 112 dergees lobe separation, and .460 lift at the valve. Full advance is achieved by 2500 rpms with the current spring setup on the distributor weights. I don't know what to do, I just changed cams recently and the cam change didn't change anything, this cam is slightly more than what was in the car before. The distributor is a brand new Mallory HEI unit. If I remember correctly I am only running about 5 degrees of vacuum advance any more and i get a ping at part throttle.

any4xx

Is it possible that you are running an extremely lean A/F mixture? Same results, if so.

IRON WILL

I will check the plugs today but I kind of doubt it as the car smells like it is running on the rich side.

checklst

Do you know what distance from the piston top to the flat part of the head is? This number should included the head gasket thickness(and even take into account for rod and piston expansion) Please don't ask how I know this obscure tid bit of info called quench distance because it was quite embarrassing.

It seems the rushing air being squeezed from this part of the head cools hot spots,lowers head temp and helps prevents pre ignition and if this gap is to wide all this rushing air(vortec) is lost!!! I should add this embarassment was on flat top pistons don't know if this hold true for dome setups.

With that being said, I still would do like everyone else suggested look for the simple first, lean jets,bad gas or moisture in gas ect.......and get a second opinion on the timing light!

I can see by lars post he is concerned with timing and cam and although I have only been on this form a couple of week I can see by reading all his past article links that he has more knowledge stuck under his finger nails than most of us(me) will ever know.

Just my .02 worth, and that's probably all it's worth




[Modified by checklst, 4:15 PM 4/29/2004]

Matt Gruber

no harm in bringing the total in a little later with a stiffer spring. sometimes 200-300 rpm makes all the difference. (mine is later as i run 87 octane.)

IRON WILL

Well I am running a steel shim head gasket, .015" thick, the deck height is 9.005", I am running keith black flat top pistons with 7cc valve reliefs. Going by keith blacks website compression calculator i have 9.5:1 as my compression ratio. I have tried using heavier springs on the weights It doesn't seem to be helping that much. All the advance is in by 3000 rpms now. Still can't run more than 30 degrees. I wonder if I am figuring my total advance wrong. Could someone explain the best way to do this. Thanks.


[Modified by IRON WILL, 6:46 AM 4/30/2004]

IRON WILL

TTT

Matt Gruber

add base timing to the weights.
mine is
14+20=34

Boofers

How to set your timing for peak performance by Lars
https://www.corvetteforum.com/techti...=109&TopicID=3
Corvettefaq articles
http://www.corvettefaq.com/c3/distcurve.mht
http://www.thirdgen.org/newdesign/tech/hei.shtml
:cheers:


[Modified by Boofers, 8:28 AM 4/30/2004]

comp

does it ping at F/T when it's cold ? if it does ping it may smell kinda' like it's
rich. i may have missed it , have you tryed running no vac. advance to see
any difference ?? i have a 305 i'm fighting, :smash:

Fevre

One thing you might try is a compression check, if the numbers are on the high side then your dynamic comp might be higher than you think, static compression is not a good measure to use for detonation problems.

IRON WILL

Using the calculators on Keith blacks web site, I get 9.437:1 as my Static Compression ratio, and 8.719:1 as my Dynamic Compression ratio. Right now if I hold the rpms up to 3000 I have 30 degrees of timing total by the timing light. If i rev the engine higher the timing does not increase any more. At 600-700 rpms the initial timing is at 8 degrees. If I hook a vacuum gauge up to the vac advance unit and add vacuum the timing increases from 8 dergees to 16 degrees. To me that means I can acheive 8 degrees of advance through the vacuum advance unit. The way I can figure right now is that I have 14 degrees of mechanical advance. If I add all this together I come up with my total of 30 degrees (8 initial+ 8 Vac + 14 Mech= 30 total.)
I am running premium fuel 93 octane Sunoco. I keep hearing that I should be able to run 36 and yet when I do I ping under a load and I have to decrease my vacuum advance even more, right down to 4degrees or less. I keep feeling that something is wrong. The cam was degreed in so I can't see how that could be a factor. The balance is new and the timing mark was checked. The timing mark is unable to move, it is a fluid filled balancer. I am also running AC R43TS plugs slightly colder that the stock R45TS. I am worried that there is a problem that I am unaware of. i have checked and removed all unnecessary vacuum lines. I hope that someone can eventually shed some light on this for me. Thanks for all the help thus far.

I really am starting to feel like :beatdeadhorse:


[Modified by IRON WILL, 10:25 AM 4/30/2004]

Gordonm

You should be able to run 32 to 36 degrees of mechanical advance. You are way short on mechanical advance. I run 36 total mechanical with another 10 degrees of vacuum for a toatl of 44 to 46 while cruising. When I'm on the throttle I have 36 and no pinging. I do run a long duration cam but my compression is around 10 to 1 with aluminum heads. Something is definitly wrong.

IRON WILL

I wonder if somehow I am figuring my mechanical advance wrong. I know that with the car in park at 3000 RPMs, I can set the timing light at 30degrees and the timing mark on the dampnr will show up at 0 on the timing tab.

Gordonm

Disconnect the vacuum advance when you check the mechanical advance. You might be pulling in some vaccuum advance when you rev up the engine.

IRON WILL

I checked witout the vacuum advance hooked up. I had 30 at 3000 rpms. Does that mean that if the vacuum advance is capable of pulling another 8 that I have a total of 38? I think I am missing something.

Gordonm

Yes it does. Make sure your vacuum cannister is working. If you have a small vacuum pump connect it to the cannister while you have the light hooked up. Put in some vacuum without reving the motor and the timing should change.