Change in ignition timing with cam

[curbsquirrel]

hey people, i've been lookin all around for some info on this, maybe someone here can give me some more insight as to what ignition timing i should have my engine set at with the cam i have.

motor is L26, 10.5:1 CR (crane reccomends at least 10:1) dished pistons, early SU's, ztherapy SM needles, everything else stock.

i've set it to 5-6 degrees initial timing, with carbs synched, it revs pretty nice. just want to make sure (with out a dyno) that i'm squeezing out all the power i can with this cam setup at this timing setting. maybe someone could share with me how i'd figure out how much advance i'd need?

 

cam specs

Overlap of 47.00 degrees and has in Intake Duration of 262.00 degrees. The Exhaust Duration is 272.00 degrees. The Inlet Cam has an Installed Centerline of 109.00 degrees ATDC. The exhaust cam has an Installed Centerline of 111.00 degrees BTDC.

set intake valves to .15mm and .2mm exhaust , hot

 

timing @.254mm

22 IO btdc

60 IC abdc

67 EO bbdc

25 EC atdc

 

thanks!

[MikeBZ]

5-6 inital seems a little low to me. Use a dial back timing light to figure out your total ignition advance. 34-38 degrees total is a good ball park. But really, detonation determines how far you can go.

 

I searched "total ignition advance" and found some great topics.

 

Here is some great info from BRAAP

Not to burst any Bubbles here, (I made a funny huh?), that is not a Typo, (“trippintl0â€, you were correct about our L-series using such a wide range of total timing figures, even if you didn’t know it, and you were pretty close on the rest of it. Keep reading up). 46 degrees is totally realistic and not uncommon for high output open chamber dished piston N/A L-28’s. That doesn’t mean that much timing is a good thing, actually it indicates the chamber is quite inefficient, but the open chamber dished pistons combos require that much timing for maximum output and they do produce some impressive power, even with their inefficient chambers. With the squish chamber of the P-79/P-90 and flat top pistons, (like Bubbles application), the chamber is much smaller so the flame front doesn’t have as far to travel, even with its slight squish design, that also helps chamber efficiency, hence less ignition timing is required. In my opinion, 35 degrees seems a little conservative for an N/A P-90 flat top combo on 110 octane race fuel. I’m not saying yours is not ideal but you might be leaving a little more power on the table. It has been my experience for most P-90/P-79 flat top piston combos, around 37-38 degrees is where the engine prefers to run on pump gas, but this is dependant on many factors which could influence it to a little more or less timing. With the even smaller, more efficient, better squish chamber of the MN-47 or P.Reiths welded N-42 chamber pictured here, ideal optimum total ignition timing could be as low a 34-36 degrees.

 

Chamber flow efficiency, plug location, squish, fuel octane, cylinder pressure, all dictate how much ignition timing an engine will require/can take for maximum output and also how much timing it can take without destroying itself, (detonation resistance). Some are more efficient than others. Even within the L-series family there are drastic variations as to what ideal timing is depending on the cylinder head and pistons used.

 

 

Edited April 1, 2011 by MikeBZ

[curbsquirrel]

5-6 inital seems a little low to me. Use a dial back timing light to figure out your total ignition advance. 34-38 degrees total is a good ball park. But really, detonation determines how far you can go.

 

I searched "total ignition advance" and found some great topics.

 

Here is some great info from BRAAP

 

thanks mikebz, thats good info, im still hoping someone with my same setup ( or close to it) can tell me what worked best for them...i dont really like to let my engine suffer detonation if i can help it...but if i must, i must, i'll give the post a little longer before i start messing with it more..

[curbsquirrel]

thanks mikebz, thats good info, im still hoping someone with my same setup ( or close to it) can tell me what worked best for them...i dont really like to let my engine suffer detonation if i can help it...but if i must, i must, i'll give the post a little longer before i start messing with it more..

well after playing with it alot........im sticking with 17 btdc, that setting has given me the best power and drivability, while maintaining expected gas milage. total advance is 38-39 degrees. still gonna hook up a pertronix soon!

Edited April 11, 2011 by curbsquirrel

[DAW]

How did you get 10.5:1 c.r. in an L26 with dished pistons? Maxima N47 head?

[ozconnection]

How did you get 10.5:1 c.r. in an L26 with dished pistons? Maxima N47 head?

 

Yes, I'm curious to! And running 38-39 degrees total advance...wow. Stock dizzy?

[steve260z]

If the head and pistons are stock, CR is about 8.3

[rayaapp2]

All the L26 engines I have seen (4 total) have flat top pistons like the L24 not dished. That said 10.5:1 would still be hard to achieve with the stock cylinder heads that fit an L26 bore. Some of the thinnest head gaskets available are .3mm crush and even then 9.5:1 is pretty typical of most heads that will clear the bore in an L26/L24.

 

Im not sure any 'dished' piston would yield that compression ratio with any available cylinder head. Domed, typo?