Starting to understand why building engines may be best left to the professional

[machoneman]

Why do the GMs like more advance than the Fords? This is just me wondering.

The amount of advance any engine runs best at involves lots of variables, the key one being the design, hence the efficiency, of the combustion chamber (head and piston).  Generally, the better the design, the less advance is needed. 'Course the type of fuel, the amount of quench, engine rpms and more also come into play.

Can't say the BBC's in particular run more than FE's or some other big blocks. We used to race a well-built BBC (std. stroke 427 +.030, open chamber steel L-88 heads, 13.6 c/r) and never went over 40-42 total timing, even on 108 octane race gas. 

   

[blykins]

Last TP engine I did liked 41° total.  Last 427 MR liked 40°. 

[Tobbemek]

Depends of a lot of things going on combustion wise mostly the chape of the combustion chamber. what it needs and then der is to consider what type of fuel octan you use if it can tolerate de advance you are looking for.  low octane fuel burns faster then high octane 
There is a disadvantage to use higher octane fuel then needed. One should always try to get as fast as possible burn in the cylinder, a slower burn is a waist of power vs a fast burn.   
Go in to BBM heads site an look at the head comparison Blair did on a 390  Cj heads vs BBM heads and see the difference in total advance and watt he say about it.
google 101 NASCAR engine on you tube if you have time its very educating at least i think.

[Barry_R]

Personally I do not think running at 38 total will hurt you.  I've gone that far and more tuning on stuff here without issue - just keep an eye on the plugs.  If you hear it rattle you let go of the gas pedal - you'd need to be pretty brutal to get it to fail a piston on part throttle rides around the block working out driveability stuff.  Drive it, tune it, and chase the "perfect" part once you're doing OK at all the other places.

[Yellow Truck]

Barry - what am I looking for on the plugs?

[machoneman]

Not Barry, but I do have a B in my name!

Here you go: https://www.enginebasics.com/Engine%20Basics%20Root%20Folder/Reading%20Spark%20Plugs.html

[Tobbemek]

Just to complicate it a little 


 http://gallery.myff.org/gallery/1486369/Sparkplug+reading.pdf

[Yellow Truck]

I should have been more specific. Since I'm too lazy right now to go and read all the stuff I have on how to read plugs - i.e. all the symptom cross referenced to the causes - I was hoping just for the thing that tells me I'm too advanced and about to blow up my engine!

[Tobbemek]

Just do like Barry suggested. Get the initial timing right first together with idle carb calibration light grandma driving and cruising figured out first
and KEEP YOUR FOOT OUT OF IT before you decide how much mechanical advance and total advance you need and it will be just fine.
 You can not heart your engine with 40 -45 total advance with out loading it remember the vac advance will take most engines to 50 and more in very light cruise
A modern GT 500 Shelby Mustang can have over 50¤ advance just over idle and super lean AF/R but as son as you demand something of it the timing retards significant and AF/R goes richer     

[Drew Pojedinec]

32 is safe.....  fine tuning can come later and you can push the envelope.


Barry, you ran 38 with BBM heads and it was what that engine wanted?

[Barry_R]

Not with BBM heads.  But I have been to 40 with mine, and it has a pretty darn good chamber.  It did not detonate to death or anything, but I have done plenty of them and always tried 36 to 38 trying to find that sweet spot. 

You already know this - but for the benefit of those who are just reading along - - just because the best place might be at 32 or 34 does not mean that it's so fragile that a short run to a higher range will result in destruction.  The best place is what the engine wants, and is dependent upon much more than just the heads/chamber.  You really will not know until you try it, and two  degrees beyond the ideal should not be "blow up parts instantly, deadly critical" in a street combination.  You try it, you listen, and you look at the plugs (or the time slip).

The compression ratio, cam, intake, short block all have an impact on fuel tolerance and timing demand.  A longer runner small cross section intake path will build more cylinder pressure around torque peak than a lazier package would, and would want less timing.  Its one of the gripes I have with relying too much on dynamic compression ratio calculations - because they ignore cylinder filling, chamber shapes, intake designs, piston dome configurations and such.  DCR is a useful directional tool, but not something to optimize around unless you have historical experience with a given combination to know where things want to be.

Reading plugs for detonation is in some ways easier than reading for fuel mixture.  You are looking for little pepper specks - tiny purplish balls - that will form on the white porcelain of the plug nose.  No pepper - no likely damage.  If you hear rattling or breaking up - let off the throttle right now.  No rattle and no pepper - try another degree or two if the performance seems to have improved from the previous effort.  If performance drops off or stays the same there is obviously no reason to look further or push harder in that direction.  Some folks believe that the color change on the plug's ground strap should occur right at the bend when timing is best.  I always look at it because I read the same internet stuff as everybody else, but my dyno and the color of the strap do not seem to agree all the time - so I believe the dyno and look at everything else for verification.

[blykins]

I've had pretty good luck with reading the strap to get close on the total timing.  I have had some situations though where the engine made the best horsepower on the dyno with a certain timing spec, but was a little lazy at the track in comparison to another setting. 

Basically, you just try different stuff and see what works.  (Plus, the ground strap on an Autolite X plug is a little hard to read....)

[Tobbemek]

I agree with Drew 32 is safe
When i do tuneups for customer and often there is a cost limit and time is money. First thing to do fore me  is "Mapping" the carb so i know what direction it supposedly  has to go in calibration then then sett the proper transferslott opening on pri side. After that i  figure out the initial advance by looking on  Vacuum readings at idle, high as possible with no misfiring  and no starting problem when hot.
then i take a qualified guess depending on type of engine, heads, comp, cam,and so on and restrict the mec advance in dizzy and if the soft spring
has a little problem to pull weights back properly in idle ( erratic ) initial timing readings i often weld up the slot a little on low side first to get just a little more spring tension.( its not all dist you have acses to bend a spring tab after its back in the engine )That will also delay the advance just a tad. this is something i decide playing with idle and of idle how quick or lazy the advance response is. Very often the dizzy looks terrible inside weights and reluctor  rusty and weights not moving freely and that can explain  the erratic behavior of the timing. Any way i have a god idea of what it needs for a initial setting and in your case i would have sett the total advance to 32¤  there is allot of experienced people on here that showed us best performance with BBM heads is just about right there 32-33¤  total advance. the last couple of degrees of timing you can decide up on later testing
After the timing is settled its time to start calibrating the carb because fuel delivery in idle and light throttle is depending up on intake vacuum your fine tune calibration will go out the dor if you fiddles with timing after carb calibration, and a right fine tuned idle surcuite needs allot of vac-advance to burn properly.when that is done    Next step is to de term what power valve opening point you need and like Barry sated in his post about timing goose for carb calibration as well you have to take a swing in both directions to find a sweet spot but more with main jetting than idle jets,Tslr and IA. There you go to to lean and find where the engine starts to surge and go back up a little richer to find the sweet spot.
Same thing with pri main jets and power valve opening point   find to lean surge and go back up to perfect running.
For a daily driver or a cruiser this is for me very vital to get sorted out real close to optimal before ultimate WOT performance testing inkl sec main jets and pri powervalve restrictions.
This is just how i do it, others has theirs  way to perform a proper tune up and remember  a universal carburetor is nothing but just  universal calibrated and most of the time takes allot of idle sercuite calibration to suit your specific combo. The old style Holleys useley is  allot closer in that term then the new HP and ultra stuff.

[Yellow Truck]

Drew, I quite agree to your post on FTE. I put things up there for a few folks who are in a similar position - in fact you offered some help to one of them today. He is a good guy.

So I got motivated and went and started the truck, checked the timing and it had regressed - the timing demon that caused me to go to the MSD distributor was back. The advance when I locked it down last was around 18, now it was back down around 16 or so, and the vacuum was back at around 6 inches.

I loosened the clamp, turned it up to 20, then 22, and at 22 I was loosing RMP and vacuum, so went back to 20 and locked it down. I was by myself so I don't know the RPM, but with the light on the timing marks I gave it throttle until the timing stopped moving and it seems to be around 35 degrees, so I'm not too worried about driving it. Took it for a short spin and the drivability is reasonable.

I closed the throttle screw a little more to get the idle down to 850-900 as it had crept up to 1,000 again, but then I killed it and started it a few times.  Twice I saw it turn the engine backwards. Each time it had turned backwards it took additional cranks to start - I assume this is because it had expelled exhaust gasses into the intake and it had to flush. In the video you can see it turn backwards. Is this something I should worry about? Otherwise, fairly warm it started easily enough.

https://youtu.be/jDbAYbCMQOo

I'm curious about this - I assumed the pushrods got longer when hot and tended to get tighter, i.e. increase the tendency to hang the valves. Called T&D for their advice about oiling, in addition to confirming that I should have either pushrod or oil passage oiling, but not both, I asked about the tick. He said it would get worse when warm.

I see there is another thread asking about the size of the oil passage on BBM heads, what I notice is there is a difference between mine. On one the restrictor dropped all the way down the passage, on the other side it got caught at the top and stopped. I'll have to try and get a dimension when the rocker stands are off.

If I can get it to run quietly I'll get someone to help with the throttle and watch the tach - keeping it around 4,000 rpm, and I'll confirm what the max timing gets too. Hope I can find the T&D installation instructions - they only have the race version on their website.

[Drew Pojedinec]

It's bizarre your timing changes between runs.  I haven't had that issue.
(I will mention again that some guns aren't friendly with msd boxes, I have chased my tail with that).

Sounds like 18-20 is your happy place for initial.

Your continued run on/wants to turn backwards on shut down can be caused by a few things.
-Idle too high
-lean causing hot spots in cylinder (possible minor vacuum leak?)

those are the most common.  Start simple, drop the idle rpm 100 or so and see how it feels.  If your primaries are set perfect on the slot just drop the secondaries down a lil.

I've heard people say this "pushrod gets longer" thing before.
Which do you think grows more, the steel pushrod or the aluminum heads and rockers?