Timing, Octane and Overheating

[J Witt]

I've been having some thoughts about overheating in our flatheads and the relationship between compression, fuel octane and ignition timing.

As I recall, generally speaking, as octane increases, advance needs to increase. As compression increases, advance needs to decrease.

Given the stock advance of 4 deg. and original fuel octane of about 60 (don't know what method used to get this number), and current octane of about 87 (R+M/2), I think that we'd need some more advance.

Added advance would ensure the fuel burn had finished by the time the mixture had left the cylinder and entered the passages in the head. My thought is this would reduce the heat load on the engine by having a somewhat cooler exhaust stream. The above is keeping stock compression of 6:1 and assuming the fuel burn is finished by the end of the power stroke.

The question for the expert pool is, how much extra static advance, if any, is needed for a stock engine run on modern fuel and keep the engine temperature manageable. If the fuel is still burning on the exhaust stroke it will reduce power and mileage and also be heating the exhaust system on the way out.

A further question is the effect of alcohol laced fuel. I make another assumption that the added alcohol increases the octane number and the fuel will still be maintained at 87 octane with less of whatever other additive (benzene, toluene, xylene, etc.) is used to establish the right octane. I think I remember reading that the alcohol burns cooler and slower and that is the source of the increase in octane number.

So, fellas, what is the right static advance and max advance number? I realize this is like asking "how long is piece of string", but tell us what you think.

[Ol' Ron]

I like this question. I've been working on the economy of our engines. Richard had a modified engine (276,L-100, 8.5 CR) the tuning process of this engine went on over several years. Richard tries several carburetors, and ignition systems, spark plugs, etc over a period of time and expense that should give him the gold metal for perseverance . I won't go into any detail of everything he did, but the results for HIS engine was 4 degs initial, 16 degs max mechanical @ 2K, and 8 degs vacuum. These numbers gave the best results for power and economy. However, The fuel system (carburetors) played a large part in this tuning process. The engine never overheated in the hot Florida summer and the Octane requirements was marginal with 87 Oct. with ethanol. I'm sure he can add to this if he feels it's necessary. I on the other hand am working on the stock engine. My present engine is a 290 ci engine with stock cam and milled EAB heads. GM dist and 2GC carb. I don't have any mileage Ys yet but the truck is now on the road. I have found the torque to be quite impressive and it may need a rear gear change. I do have a 36% OD that may save the day here.. I have another engine in the works, a 59 with 60 over pistons. Now most people are probably wondering why the increase in Size? well the 280 was a 276 street engine. The cylinders rusted and I got a set of Egge 3 5/16 plus 20. Thes light weight pistons come with Metric rings from a Volvo truck. The 248ci 59 engine will also add to the CR after I modify the heads. The only way to increase torque is to increas compression and/or cubes. Trying to get as close to 8:1 as I can. This will have an affect on the fuel burn. Now I'm not saying we've solvet the problem, but I keep trying to find the answers. One thing i will do is try the relocated spark plug heads. Just to see if this is helpfull. I'm just a putzer at heart.

[JWL]

What must be remembered is that all fuel reaction before TDC is, at best, wasting fuel and can likely be doing negative work against the rising pistons. Best power timing is a balance between getting a major portion of the produced cylinder pressure at the most favorable crank angle and burning the most fuel. In other words, burning the most fuel can not be the determining factor for best timing. The turbulence inside the flathead chamber accounts for the lower required total advance where a typical OHV might need 36 or 38 degrees compared to our Flatheads at around 18 to 22. Then there is the factor of A/F ratios. In a light load, cruise, condition a very lean mixture can be utilized requiring a good deal more advance to get such a mixture to react at the most favorable crank position. Hence the practical use of a vacuum advance system. Adding a radical camshaft can require more initial advance but the total advance changes little.

[J Witt]

I probably didn't phrase the question well. What I was thinking about, perhaps to exclusion of other factors, was expanding the gases on the power stroke to maximize the power and minimize the exhaust gas temperature.

I've seen an engine running with the ignition well retarded and the exhaust manifold was glowing red. In that case the distributor had lost its retaining bolt and had spun around abut 30 degrees or so.

[JSeery]

Think the objective is to maximize EGT.

[Old Henry]

Sponsored Links (Register now to hide all advertisements)

Don't forget that the correct static initial timing has a lot to do with elevation. 4° BTDC is way too late for my engine where I run it at 5,000 feet and above. I have to advance ahead of that to get optimum performance then retard it gradually to 4° as I go down to sea level.

[Lawrie]

Ron ,are you saying that static plus the 16 equals 20 degs,then the vacuum 8 will be 12 or 28degs
Lawrie

[Ol' Ron]

Lawrie
Yes, you have it. By varing the inital you can tune the WOT timing. Depending on the AF the 8 degrees of advance works well for economy. but this is adjustable. Now having a tight piston to head clearance also helps

[BUBBAS IGNITION]

Problem with vacuum and mechanical at the same time . RPM is up causing full mech advance but very little vac advance . Vac when high is at low rpm . Pretty difficult to tune the over lap........

[Ol' Ron]

Jim, we've been doing this for years. The trick is to keep the mechanical advance short, 18-22 Total at WOT. Too much Mechanical can cause allot of detination with all that Vav adv. Depending on the CR of the engine.

[BUBBAS IGNITION]

Originally Posted by Ol' Ron

Lawrie
Yes, you have it. By varing the inital you can tune the WOT timing. Depending on the AF the 8 degrees of advance works well for economy. but this is adjustable. Now having a tight piston to head clearance also helps

Thats 8 degrees engine , or 8 degrees distributor??? Most vacuum units are 7-8 distributor or 14-16 engine.........

[BUBBAS IGNITION]

Not real sure we are understanding the vacuum advance and how it works. Looking at a vehicle spec sheet (sun tester cards) We show the total factory engine spark advance and then show each setting etc.

1-Intial spark advance usually BTDC as a static advance number
2-Mechanical spark advance set with a limit of how many degrees at x amount of engine rpm.
3-Vacuum advance , usually direct manifold advance but in some cases ported (controlled) vacuum.

Then they give a total advance which doesnt add up to the three numbers above. Reason being that you usually dont have vac advance at wot and when vac is highest (idle) theres no centrifigual or rpm .
These need to over lap each other , a spec card on a (gasp) small block chevy shows 8 degrees of intial , 24 degrees of mechanical and 16 degrees of vacuum. Thats 48 degrees, the engine wouldnt like this at all!!! The engine however might like 36 degrees and thats what most have which means the centrifugal and vacuum must overlap each other . When rpm is up vac is down and when vac is up rpm is down.

[40cpe]

But it is more simple in a flathead where all the advance is in at 2000 rpm. At cruise, where vacuum helps, the advance is already in.

[Ol' Ron]

I never discuss distributor advance. It doesn't help understand what the engine wants. The overlapping is of little concern as the engines only in this area for only a few seconds of operation. One of Richards contributions for our understanding of what goes on during an engine cycle (Start, accelerate up to speed, and cruise) was recorded in the road rig. This instrument recorded all the engine parameters during this cycle . It included a Tac, AF meter Vac gauge and a video of the actual advance from a timing light and camera mounted on the timing cover. Watching the replay. At start up the advance went directly to 10 or so adv, acceleration went up to 20 degsas the vac fell yo zero on the gauge. The AF meter would show the acc pump and PV in action. Cruising along, the AF was around 15/15.5 and the advance would be around 28. Theas numbers varried allot when learning the affects of all the information. and it took a few months, several carb setups and afew dist mods including the adjustable mechinacal advance design. Like I said. Perseverance . Now yousr guys have all this info and I'll try to post a video of one of the runs. One of the confusing numbers is the 16 degrees in the dist. Everybody thinks you need more, Well you do, but you get it from other things, Like the inital, and the Vacuum. On hard acceloration 20 is all you need for a street engine. Noe if your racing, then you tune for power not econmy

[flatjack9]

Originally Posted by BUBBAS IGNITION

Not real sure we are understanding the vacuum advance and how it works. Looking at a vehicle spec sheet (sun tester cards) We show the total factory engine spark advance and then show each setting etc.

1-Intial spark advance usually BTDC as a static advance number
2-Mechanical spark advance set with a limit of how many degrees at x amount of engine rpm.
3-Vacuum advance , usually direct manifold advance but in some cases ported (controlled) vacuum.

Then they give a total advance which doesnt add up to the three numbers above. Reason being that you usually dont have vac advance at wot and when vac is highest (idle) theres no centrifigual or rpm .
These need to over lap each other , a spec card on a (gasp) small block chevy shows 8 degrees of intial , 24 degrees of mechanical and 16 degrees of vacuum. Thats 48 degrees, the engine wouldnt like this at all!!! The engine however might like 36 degrees and thats what most have which means the centrifugal and vacuum must overlap each other . When rpm is up vac is down and when vac is up rpm is down.

You do not want or get vacuum advance at WOT. And vacuum is not necessarily highest at idle. At cruise, you have a lot of vacuum and that is where you need the extra advance. And the vacuum advance Ron talks about is crankshaft degrees. It really isn't that hard to set up.