What is TDC?

Andries

Andries

Member
I've got the following problem/question which is rather vague. I've got the heads off and turned the engine until cylinder number 1 was at it's highest point. When reaching the highest point in it's cylce, the reading on the crankshaft pulley is 6 degrees before TDC. When I turn it further the cylinder remains for a small amount of time at it's highest point before travelling down again. But what is exactly the top dead center for the igntion timing? When it reaches the highest point or when it's at the highest point? I could do the math on it and work out many degrees it seems to stay at it highest point, but I rather won't! :D So what should I do in regards to timing it? Should I advance it more or just wait until I've got the engine running and go for the trial and error method?

This is especially important as I'm having the heads skimmed, valves reseated and installing the RPI system next week. I want to get it spot on because otherwise the engine is flawless. It has had either long drives and regular oil and filter changes or is a very low milage engine.

Kind regards,
Andries
 
Its always difficult to judge exactly where TDC is by looking at the piston whilst at TDC. (does that make sense?)

Turn the engine so the piston is coming up and stop it when it's 10mm from the top. Use a straight edge across the top of the cylinder and a spacer of some description.

Then turn the engine the other way and do the same. Again stopping it 10mm from the top.

Mark the crankshaft pulley at each of these points and TDC is mid way between.

You don't have to use 10mm, as long as the measurement is the same both times.

Richard
 
Hi Andries,

True TDC is the exact middle between the two points when the piston is at the top of its travel. If your crankshaft pulley shows 6 degrees BTDC when the #1 piston is actually at TDC, then your pulley markings are out by 6 degrees. So it you wish to set it at 6 degrees BTDC when the engine is running, you'll need to set it to 12 degrees BTDC, which will allow for the 6 degree error.

Ron.
 
It does make sense Richard! I will try your method this afternoon, see if I can get a final reading on TDC. It does make me wonder about some problems I've been having in the past. I have always figured that the marks on the crankshaft pulley were always in line with the crankshaft and thus, in line with TDC. Isn't it splined or anything?

I kind of reached the 12 degrees BTDC by myself, but now it leads me to the following question when you're here anyway. So Ron, you're running a 4.6 which, I assume, is suitable for low octane fuel.. (98, V-power etc.). Now, I don't know the compression ratio of 4.6's but as mine a 3.9 (38D) with a low compression I think I can advance the ignition timing slightly more than the indicated 6 degrees even on low octane. My Alfa is advanced 9 degrees and it's got more oempf!

Thank your for your awnsers!

Andries
 
Hi Andries,

I run 12 degrees of advance with my 4.6, so you could comfortably run 12 to 14 degrees of advance with your 3.9. What you do have to be careful of is that the total advance for your engine doesn't exceed 34 degrees if at all possible. The more initial advance you run, the better the engine will perform and the lower the fuel consumption will be.

Ron.
 
I would go with Richard's "halfway" method as tried and tested way of setting a cylinder to TDC. IIRC, can you lock the flywheel in TDC using the little hole in the flywheel housing and the locking pin that should be bolted next to the hole? At least I think that's how it works on my 4cyl? Are V8s the same?

Can recall quite a few folk on the forum saying that P6 pulley timing marks have a reputation for being inaccurate, so measuring it yourself is maybe a good idea?
 
It worked, altough it's not recommended to use something over 6 mm's thick underneath the ruler. The distances between the marks get so big that it's very difficult to determine the halfway point and thus, TDC. Turned out it wasn't 6 degrees off but only 3. So the run 6 degrees of advance and need to set it to 9 etc. I will work my way up from there to find a suitable setting, I would like to run 95 octance because it's about 8 cents a litre cheaper at Shell.

JVY, I won't know. I can't see anything on the front side of the pulley due to my bigger rad and puller fans. Will check my dad's car. As stated, it's not entirely accurate but he, could be worse! :D

Very much for your help!
Andries
 
Be warned that 95 octane fuel isn't just different by the octane rating. It contains ethanol which most 97/98 octane fuel doesn't. I've tried running my v8 on 95 octane the last couple of tanks with composite gaskets and it just doesn't run as nicely. Back to the good stuff next time.
 
I think the V-power, excellium -types have the same ethanol content? Problem is, I only know of one(!) station that actually sells 98 octane fuel here. On this subject, I spoke to a taxidriver in Montreal,who owned a muscle car (can't recall which) and he drives down the local airfield once in a while to get a couple of drums of fuel used in Cessna's and other small aircraft.. He made his own 110!

Is ethanol not dangerous for fuel lines etc? I heard something about clashing with rubber parts. There was a website in the Netherlands last year were you could see if your modern car was compatible.
 
I believe aed cars can have issues and the o ring on the reserve tap. All bits available so on that front no problem.
 
There may be issues with the higher E values of the alloy bits dissolving if they were made from
low quality metal. I have heard of engines in Brazil needing lots of bits replaced when they went
to high E levels.
 
unstable load said:
There may be issues with the higher E values of the alloy bits dissolving if they were made from
low quality metal. I have heard of engines in Brazil needing lots of bits replaced when they went
to high E levels.
Click to expand...

And now I'm worrying about my Weber carb.. Anyway, I've heard of a place 50km from here which sells 102 octane! :shock: Must go there!
 
Be careful that it doesn't contain ethanol also Andries. There was a 100 or 102 octane fuel available if I recall correctly from Shell a couple of years ago here in Australia. It contained ethanol unfortunately.

Ron.
 
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