[Semper Fi 71]

My timing issue has been ongoing. I just ordered timing tape and an advance springs and weights kit. I have a 350 with edelbrock intake, and performer 600 carb. No cam "yet". So I have heard mixed things. Should I be atdc or btdc? The car gets real quiet at 18* atdc but as I come back down to closer to btdc it gets louder. If someone could help me or give me a few timing scenarios it would be great. I already read ignition 101 and I still need some help with this. My cars been barely running for months now and it's painful. Please help me

 

[MEJ1990TM]

Under no circumstances should your timing be ATDC. With what I assume to be the stock cam I'd set the total timing for somewhere around 36* BTDC. Depending on your advance works you will be somewhere at 16-20* at an idle. I almost don't think it would even run that far ATDC. Are you sure that it's actually after top dead center?

When we first started the '70 for break in the timing was quite retarded. At around 2800 RPM it was just firing at top dead center. The headers had started to glow red. Hit it with the light and discovered how far off the timing was. We advanced it to get it close to where it should be. Even so when trying to idle it down it just wouldn't stay running below 1200. Hit it with the light again to find since the mechanical advance was backing out it was approaching TDC again. My point in that is with how badly this engine was running just slightly before top dead center I'm not so sure your timing is that far after top dead center.

Regardless, 36* is a good starting point from what I have gathered here on TC.

 

[anychevy]

My timing issue has been ongoing. I just ordered timing tape and an advance springs and weights kit. I have a 350 with edelbrock intake, and performer 600 carb. No cam "yet". So I have heard mixed things. Should I be atdc or btdc? The car gets real quiet at 18* atdc but as I come back down to closer to btdc it gets louder. If someone could help me or give me a few timing scenarios it would be great. I already read ignition 101 and I still need some help with this. My cars been barely running for months now and it's painful. Please help me

This is a general guide for any aftermarket cam in most SB or BB engines.
But even with a stock cam your engine will idle better with more than the "recommended" 6-8○ of initial advance.

Use the centre of the timing tab as your reference point, mark it with something so you can see it easily with the timing light, make sure #1 cyl is at TDC
of it's comression stroke and the timing mark on the ballancer lines up with it.
Stick the timing tape on with the zero over the timing mark and set your initial timing (@ idle) around 16 -18○ BTDC
With the vacuum advance unplugged, set the total around 36 -38○ BTDC all in around 2700 - 2800 RPM
You want a nice even curve from around 800 rpm to 2800 rpm
Different cams and different compression ratio's will need different variations on this, but this should get it running o.k.

If it pings with the vacuum advance connected, you may need to limit it's travel, some are adjustable, some not.
Some distributors have more than 20○ mechanical advance built in, that's where the bob weight and spring kits come in.
I used an ACCEL last time with good results.
Hope that helps

 

[mr]

My timing issue has been ongoing.. Should I be atdc or btdc?

I read ignition 101 and I still need some help ..... Please help me

from the Evans Tuning Tech forum.... Combustion, Timing, Cylinder pressure and Knock
http://forums.evans-tuning.com/view...pressure&sid=a7c4e4f180f821fd973c1b37f98cba5f
posted by imprezor » July 13th, 2005, 8:46 pm

Nice thread. Now here is what I know.

Ignition timing (or spark timing) depends on a bunch of factors. To stay close to the thread subject, I'm gonna talk about a few here.

First of all we're gonna talk a little about PISTON SPEED. As cylinder pressure is a function of piston position (volume) and spark plug timing (and many other things we're not considering right now),
you'll need to combine both to get the maximum cylinder pressure at aprox. 14º ATDC.

Remember that in piston/cylinder engines, the linear movement of the engine is converted to a rotational movement. That's what the rod/crankshaft combination does, so to move the crank you need to torque it, and to torque it you need to apply a force to the crank's arm.

Why 14° ATDC? Why that number?
The torque the crankshaft "feels" depends on the crank's arm length (that's why people stroke engines, stroke = 2*crank's arm length) and the force applied to it, and of course, the angle between the rod and the crank's arm. Cyl. pressure decreases fast as the piston goes down ATDC (so the force applied on the piston decreases too), and the angle betwen rod and crank arm increases. Max torque would be around 90º
(since RxF = F.R.Sin O where O is that angle, and Sin O = 1 when O = 90º)
but at that point cyl. pressure is really low so there's practically no force making work (giving energy to the system). Then, you need the better combination between maximun cylinder pressure (which is forced to be close to TDC) and the widest angle at which you can really get to that pressure.
So 14º aprox. is the point where the relation between cyl. pressure and that O angle is best.
You can get really high pressures at that volume (remember a crank angle means a volume displaced between the piston and the combustion chamber) and a not too good angle for the torque, but is the best you can get!

Now, as the flame front speed is almost always the same (there are small variations because the mixture conditions are fully able to change) you need to adjust spark timing so the time it gets the flame front to increase the cyl. pressure to its maximum value is almost the exact time it takes for the piston to get to 14º ATDC. BUT, that time the piston takes to 14º ATDC depends on the piston speed, which is a wave whose amplitud depends directly on the engine RPM. So as you increase the RPM level, you need to advance timing so you give the flame front the necesary time to increase the cyl. pressure to its max. precisely at 14° ATDC.

Now, after you have the goal of getting max. cyl. pressure to 14º ATDC, (and, on a dyno, you notice you got it right 'cause it's the max. torque output you can take of your engine, I think) you'll go through all the problems talked about below like fuel octane, of compression ratio, camshaft profile and timing, etc.

The reason why higher compression ratios are a lot more difficult to tune is because the graffic Jeff showed below depends directly to it. The safety range Jeff talked about, means that it's you who gets to actually "control" the cyl. pressure increase with more/less boost, plus, that natural increase (the natural pressure increase in the engine without boost) will be a lot smoother with a low comp. ratio, so you have a lot more "room" to move in. What I mean is that, with a low compression ratio, the maximun cylinder pressure will depend more on spark timing and less on RPM, 'cause when you lower the compression ratio, you lower the speed of natural pressure increase the engine has when the piston moves up. And that's why I said up here "it's you who controls the cyl. pressure increase".


Well that's like about it, pardon me if it isn't well written my english is not perfect as I'm from Venezuela.

If the cylinder pressure is at it's maximum at precisely 14° ATDC, you need to advance timing to BTDC so you give the flame front the necesary time to increase the cylinder pressure to its max. precisely at 14° ATDC.

A really low compression engine, needs more lead time to complete the timing cycle at 14* ATDC.
In my low compression 454, the timing is set @ 40* BTDC for best performance, @ 36*, it just feels too lazy.

It's not impossible, your engine will feel at least 50 horsepower + stronger, with the correct timing curve !!!

 

[Semper Fi 71]

ok awesome info guys thank you. do you think being so far atdc would cause the motor to surge or sputter when slowly increasing rpm? Once i get above 3k or 3500, it starts smoothing out. again, thank you so much

 

[Calculated Risk]

what balancer is on the engine? if it's an old stocker it is very possible that the outer ring may have moved. you may need to find true TDC and verify the timing mark is acurate. Additionally if you find the outer ring has moved, you will want to buy a good stock replacement balancer which is not real expensive.

 

[Semper Fi 71]

Ok rookie mistake. I forgot which side was btdc. So I'm gonna try why u guys told me

 

[BillyGman]

what balancer is on the engine? if it's an old stocker it is very possible that the outer ring may have moved. you may need to find true TDC and verify the timing mark is accurate. Additionally if you find the outer ring has moved, you will want to buy a good stock replacement balancer which is not real expensive.

x2 what Rick said. That has happened to me in the past where the engine was running, and I put the timing light on it, but it was reading somewhere like 90 degrees before TDC! I knew that there was no way the engine would even be running with the timing that far off, so I shut it down and examined the balancer, and lo and behold, it was a stock type two piece deal that had come apart and the part that has the timing marks on it was spun around. I'm pretty sure that at that point, the balancer was no longer doing it's job of dampening the vibration 'harmonics" of the crankshaft either (which isn't good for the engine to say the least).

Another thing to make note of is if you have an MSD ignition box, that can cause your timing light to do some crazy things if your light isn't one of those which is MSD compatible. Don't ask me how, but I had a Craftsman timing light that worked fine on my other cars, but when I put it on my Chevelle, it was reading about 90 degrees off, and there again, I knew that there was no way that the engine would be running if that were the case. But come to find out, it was actually reading another cylinder instead of number one cylinder, because it isn't one of the lights that is compatible with the MSD box I have. So I just went to Summit racing, and bought one that was listed on that website as "MSD compatible" (which it tells you in the product description of each light they sell whether it is or isn't) and that new light works fine.