SBC engine that requires loads of idle timing, what's going

General Chat About Engine Build

Moderator: phpBB2 - Administrators

Post Reply
sidecar
Top Dog
Top Dog
Posts: 2399
Joined: Sun Feb 04, 2007 9:52 pm

SBC engine that requires loads of idle timing, what's going

Post by sidecar »

Hi Guys

I’ve been working on a SBC engine trying to get the timing setup and I’ve come across something interesting. The first thing that my mate and I did was check that the TDC mark lines up at true TDC, which they did. The next thing we did was to find out what the current setup was and this is where things go a bit odd. The idle timing was up at 26 degrees (vac not connected), the all-in was 44 at 3000RPM. Now ‘everyone’ states that the idle timing on a SBC is around 12-18 and the all in is around 34 so the measured figures are miles off he accepted norm. The engine cranks over OK and the tickover is smooth, in fact as soon as the timing is retarded at all the engine goes rough. It only pulls around 10-11 inches on idle and figure drops as soon as the timing is backed off. I don’t know the spec of this engine which doesn't help much but I'm beginning to suspect that it has a lot of cam overlap and therefore a low cylinder pressure on idle. (Maybe the CR is low). The timing figures were cross checked with a dial back gun and timing marks on the damper wheel. (The gun then set to zero dial back when using the damper marks).

I’ve done some googling and came across a thread where someone else has posted up a question because his engine also required a lot of initial timing, the first 3 pages are just rubbish but then someone (Bad Banana) did actually post up that he has seen this many times before…


http://www.jalopyjournal.com/forum/thre ... 643/page-1


I was looking at ways of trying to get the timing backwards the more ‘normal’ figures whilst still retaining the high tickover advance by connecting the vac canister to the non-timed port, the canister adds 12 degrees which means that the static timing could be set to 14 degrees but the actual idle figure would be 26. The all in would then be 14 + 18, i.e. 32. (18 being what the bob weights can currently add). I’m now not sure that this is the right way to go because 32 may not actually be enough all-in for this engine. I guess a dyno run to work out the required all in is the correct thing to do here.
Anyway has anyone come across this sort of situation before? I don't believe that the engine was detonating even with 44 all- in but it is hard to tell with sidepipes and I don't think that the "advance until is detonates then back it off" method is a good method of setting up the timing anyway!


Cheers,


Pete



Post Reply

Return to “Engines Area”