My head choice dilema...

[jimberg]

I have thoughts on timing , and yes, tuning the timing on a dyno is the best thing to do. Too much timing will do more harm than good.

Advancing the timing will net you more power throughout the powerband since you are able to utilize more of the energy being released by the explosion of the air/fuel mixture.

Bear with me since this is probably something that most people know, but it's a good idea to keep in mind and not all people know it.

When we get our basic lesson on how four stroke engines work, we know that there is an intake stroke where air is sucked into the cylinder and fuel is injected. This is then compressed and ignited. Before I ever understood what timing was, I always assumed that the fuel was ignited at the top of the compression stroke and then the piston was just pushed down buy exploding gases.

What they really do is start the explosion before the piston gets to TDC to get the mixture expanding during the compression stroke. It's kind of like compressing a super duty spring that then releases its full amount of potential energy after passing TDC. This is where it becomes relevant to the thread. The danger, though, is that if you start the expansion too early along with high combustion chamber temps, the fuel will spontaneously ignite (Detonation). My guess is that if you could take a movie of the explosion in super slomo, you'd see the explosion start up by the sparkplug and grow downward toward the piston. During detonation, all the fuel explodes at once before it reaches TDC.

There's a lot of important information that can be derived from knowing this.

When you advance timing, you increase the amount of potential energy that can be released after the piston reaches TDC.

Cooler combustion chamber temps as a result of aluminum heads will allow you to advance your timing farther.

Cooler combustion chamber temps as a result of running richer will also allow you to advance your timing farther.

Octane is the lab measured amount of compression that fuel can take before it detonates. This is why you use a higher octane with high compression engines, or with engines that have advanced timing. High octane fuel also burns slower so using it with stock timing will give you less power since the amount of potential energy released at TDC will be less.

This is why timing advance is important as engine RPMs go up. Since the piston is moving faster, it is important to start the expansion of gases earler so that you maintain your maximum amount of potential energy throughout the rpm range.

There's probably a lot more, but I think you guys should have a good idea of what's going on.

Before anyone harps on me about going into such detail on what we consider basic, there are a lot of people on this site and other sites who have no clue about this stuff. It's obvious in a lot of questions that are asked.

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351W 89 Mustang GT Convertible


[This message has been edited by jimberg (edited 06-14-2001).]

[jimberg]

Just so you know what kind of time spans we're talking about here. At 1000 rpms, it takes the crank .06 secs to make a revolution. If advance is at 10 degrees it's about 0.0017 seconds of explosion time we're talking about. At 6000 rpms, it would take 62 degrees of advance to allow that much time, but with the speed and heat being generate, total advance can only be little more than half of that.

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351W 89 Mustang GT Convertible

[This message has been edited by jimberg (edited 06-14-2001).]

[TJR]

jimberg....That's a lot of info, I had to read it twice to catch everything I missed the first time .

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1989 lx notch
89,000 miles
bolt-ons,stock heads/cam
60'- 2.07, 8.90@78, 13.93@99

[jimberg]

I just hope it's useful info.

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351W 89 Mustang GT Convertible

[MiracleMax]

Originally posted by jimberg:
Just so you know what kind of time spans we're talking about here. At 1000 rpms, it takes the crank .06 secs to make a revolution. If advance is at 10 degrees it's about 0.0017 seconds of explosion time we're talking about. At 6000 rpms, it would take 62 degrees of advance to allow that much time, but with the speed and heat being generate, total advance can only be little more than half of that.

don't forget though, combustion chamber shape and volume, rod/stroke ratio, spark plug location, piston shape, ect. effects the total timing needed

I had a shop teacher adamantly maintain that no engine could be revved past 6,000 rpm or so (yeah rite!) since his calculations were based on a steady state burn (gasoline),
unfortunately he didn't any other variables in account when he did the math!

For a high RPM engine, a small chamber with a good R/S ratio is desirable, along with a port/chamber design which promotes good fuel atomization/homogezination.

a. a small chamber reduces the time it takes to burn the A/F mixture across the chamber
b. a high r/s ratio creates more piston dwell time at TDC
c. highly atomized and homoginized fuel tends to burn more rapidly.

[jimberg]

Right, the timing isn't linear for another very obvious reason. By compressing the gases faster at higher RPM you generate more heat which will cause detonation earlier and earlier.

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351W 89 Mustang GT Convertible

[Mach 1]

jimberg - how is the timing being non-linear (advances with rpm) going to discourage detonation?

[jimberg]

Sorry. That was unclear. Advance in degrees can be linear in relation to rpms, but advance in an actual time interval would not be. e.g. The 0.0017 seconds in above example.

The faster you compress a gas, the more heat will be generated since there is less time to dissipate the heat. This extra heat will add to the potential for detonation.


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351W 89 Mustang GT Convertible

[Unit 5302]

I think jimberg's been readin a lotta physics lately.

Mach 1 TFS added me to the payroll 3 months ago, they keep telling me the check's in the mail, but for some reason, it always get's lost?

[Mach 1]

Yeah, the old "check is in the mail story"...heard it a hundred times myself...lol.

I think Jimberg is confusing himself......

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1993 GT/AOD
'93 Mustang GT

[RobertD]

Trick Flows it is....

Thanks!

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Robert
91GT; 88 ASC McLaren #709

[Zack87GT]

Have you thought about AFR heads? Edelbrock is an excellent choice as well!!

[andy669]

This thread is 3 1/2 years old. Whatever he decided, I'm sure its a done deal my now. :lol:

Andy

[Rev]

I use TFTW heads and couldn't be happier. I have enough low end torque to smoke the tires with a C-4/3.25. I don't know what I would do with anymore low end torque. One other good thing about the TFTW, is that they have a raised valve cover rim that allows using roller rockers with poly-locks and still be able to use stock height valve covers.

On the other hand, I've heard that the AFR 165 heads are "kick ass" in the torque and power department. I think I would have been just as happy with those too.

Rev