Highlander

I am very curious with the timing advance and what it depends on...

Why a car with same boost and no intercooler, but with higher CR can run more timing before detonation than a car with less CR? Same... everything?

I dont think that is very likely but its what I saw...

Is there any rule of the thumb... CR/PSI for each degree of timing??? Like in nitrous applications that is 3 degrees / 50HP?

Thanks

Damon

Just about everything affects timing. Cam, intake, boost, heads, compression, combustion chambers, exhaust, turbo or blower type/size/style, amount of boost, starting intake air temp, ambient air temp, etc, etc.

Real complicated. Too complicated for a semi-serious hobbyist like me. I try different things until it blows up and then I don't do it like that again.

Trial and error. Those with real sophisitcated software can probably get closer to perfect on the first shot than I can just using the old "bio computer." Not always, but usually.

Before this thread spins outta control with theory is there some particular combo you had in mind? Maybe we could help you if you had something specific in mind.

Highlander

I dont get why I can only run 25 total advance with 9 psi and 186F IAT since I have 9:1 CR.... I thought I would be able to run a bit more advance than that... I've tried to raise more degrees of spark, seeing a lot of gains in HP about 20HP from down low... just to see later on a knock retard...

I once heard that due to the nature of the kidney shaped cylinder heads it would allow you to run a lot of timing since it was a very bad designt to spread the explosion...

So.. what is the most important thing with spark advance?? IAT??? CR?

The bigger deal is that I want to run more timing due to the fact that there is where the real power is...

Will more timing result in needing more fuel or vice-versa? I heard that the less advance will make the car consume more gas or use more gas and it will make less power or so...

SStrokerAce

The only way to know is on a dyno. There is a point where the timing will lose power adding or subtracting it, right there is where you want to be.

Now if you get KR then you have too much boost, too much compresion (Static and Dynamic) and too little octane.

Now the chamber design and piston design along with bore size, quench area all have to do with the timing requirement. A Winston Cup engine with SB2.2's or Yates heads can get down into the 26-28 total area at certain tracks, while old school Fuelie heads angle milled, with .040 quench and flat tops running race gas and 12:1 can need 39 total to work perfectly.

Engines can also be very flexible in their tolarance for max timing and can be very finniky. In the engine masters contest Joe Sherman said he could run 32-36 in his AFR 215 headed engine, while my Brodix T1 headed engine wanted 35 no more no less. There was a big drop off on the dyno on both sides of that. Having a slight small dome on the piston with a 9:1 DCR probably contributed to that.

On a turbo engine, IAT play a role in this whole equation too. The colder denser charge will most likely not need as much timing, but all the parts in the system play into that.

Bret

Highlander

I get your point... actually... the less timing you need the better is your combustion and/or your engine...

The point that I would love to stress is... if you get more power out of more timing and then knock retard, how can you have too much advance and not have knock retard? I always thought that the limit of timing would always be knock...

If I remove the btm just a tiny bit I can gain up to 18rwhp from down low till 5200 that I get the knock retard... I would love to have all those 18rwhp more and still be able to push it up top...(thats what we all want though)

Anyways... 9psi too much boost??? I have a 9:1 CR and I think my dynamic is 8.27:1 with the 224/236 XE cam that i got.

I have been thinking its the fuel here... they are mixing reg with premium.. cuz.. at first I was only able to run 2/psi and I changed fuel and I was able to run more timing.

Anyways... Can you have too much timing and not get knock retard?

SStrokerAce

Yes, but in a boosted engine it's a different game. When you are not limited on Octane and limited on SCR then yeah it's easy.

What kind of boost is it? Centrifigal Supercharger? I'm guessing that since the engine has more KR up top, which is really odd. But that blower would be why.

With a 9:1 SCR and a 224/236 cam you don't have a 8.27:1 DCR, I have a 383 LT1 in here with a 11:1 SCR and it's only a 8:1 DCR engine. SCR has alot to do with DCR.

KR will always kill power, I've run knock sensors on test engines to set a timing curve, and when you can run the engine in Knock you will still make more power but it will not last as long.

Bret

Mindgame

Could you please rephrase that... it's tough to tell what you're asking there.

Quote:

If I remove the btm just a tiny bit I can gain up to 18rwhp from down low till 5200 that I get the knock retard... I would love to have all those 18rwhp more and still be able to push it up top...(thats what we all want though)

Anyways... 9psi too much boost??? I have a 9:1 CR and I think my dynamic is 8.27:1 with the 224/236 XE cam that i got.

I have been thinking its the fuel here... they are mixing reg with premium.. cuz.. at first I was only able to run 2/psi and I changed fuel and I was able to run more timing.

Anyways... Can you have too much timing and not get knock retard?

If you have "too much" timing, the engine will "detonate"... the knock sensor will then pull timing. Everyone's aware of that...

If you are running for instance.... 34* of timing and no detonation, then bump that to 36* and detonate, you would assume that the ECU might only pull 2*, but that's not the case. Because of the temperatures at that "detonation" level.. the ECU may need to remove 3 to 4 times as much timing to quell it. Big loss of power there. So it's best to establish a baseline and move 1* at a time.
Tune it at the track... you're looking for best MPH not ET.

-Mindgame

Highlander

Now that you explained, there is no need to rephrase.. I just thought that you could have too much advance and hurt power without having detonation at all(without the PCM pulling timing)... I always thought that the "too much advance" will lead in detonation and because of the detonation and what the pcm has to pull is where you loose power...

Damon

Just a quick clarification question.......

You say 25* total advance. Are you talking your initial lead at the distributor (say, 5*) and then 20 more from the computer's advance map (assuming you have a distributor)?

Have you verified a true 0* with no computer advance? I have lately gotten in the habit of not only degreeing in the cam (as usual) but also degreeing in the balancer and timing pointer so that I know that TDC for the ignition is really TDC.

Admittedly, that would be tough to do on an engine that doesn't use a distributor (or the LT-1 opti) but.....

My gut tells me you are either running lean at WOT (making detonation more prevalent) or you've jsut got too much squeeze due to perhaps an incorrectly calculated compression ratio, too small a cam or a number of other possible factors. Or maybe you've got some sharp edges in the combustion area or the dome of the piston that start to "glow plug" when the heat is laid to them.

Highlander

I get what you mean... but.. no... no miscalculated CR and no difference in cam or all that sort.. I CC'd the heads and pistons... so that is about right I would say.. acutally the car off boost feels like a 9:1

Anyways... I just wanted to learn a bit more about timing and how timing is everything in an engine and power... in fact.. timing is everything in life...

CAJUN-Z

I'll try to throw my $.02 in the discussion from what I know about octane and detonation. Basics: TEL ( TETRA ETHYL LEAD) and MTBE (METHYL TERTIARY BUTYL ETHER) was produced as a cheap alternative (vs. super refining of standard fuels to increase octane rating) for the purpose of increasing the octane rating of fuel and to cushion valves to help prevent valve recession of the head mating landings (softer materials). Many variables come into play...it is almost impossible to predict how an engine will run until the fuel and compression ratio is set. Test engines with sliding cylinders (to vary CR's. The engines are actually run at a specified rpm and the cylinder lenght decreased until the engine "knocks") are used to establish the "motor octane" at refineries. A "research octane" is the fuel producers best guess considering BTU and burn rate figures, and the two are combined and averaged to get today's octane rating. Most theroies on "knock" consider the "flame front" travel. Higher octane fuels are generally slower burning and "controlled, whereas lower octane fuels tend to "flare" when ignited (under a compression-type situation). Where the intake valve closes (timing event) and DCR seem to be two variables that have to be closely watched, along with piston speed and cylinder and exhaust gas temperature. If an engine's effective powerban is 4500 to 5500 rpms, the design of that engine will require a different octane fuel compared to an engine that can effectively use the 6500 to 7000 rpm band. If all the variables are isolated except for piston speed, and the timing is the same for the two engines, the spark event will ignite the A/F charge in exactly the same point, but the piston speed in the 6500 to 7000 rpm engine will cause the complete burn to event to happen closer to TDC (maybe even exceeding it) with the same fuel octane. I think this is a simple way to explain how complicated the question of "any rule of thumb" is. In the "old days", we just waited to hear knock or for better tuning, installed a new set of sparkplugs after getting the ideal timing close and made one full throttle run for a few blocks and shut down the engine, coasted to a stop and pulled the plugs. If the plugs had spots of aluminum on the insulator, too much timing. Back down a degree or two and rinse and repeat until you got a clean plug. That's about the best that could be done way back when....
Hope it helped a little...

Highlander

Actually it did... actually i knew almost all that but I needed a reminder...

But I didn't know and the piston speed and its obvious...

That is why I think I should be able to run more timing with my 5.7" rods and the 383 9:1. I am running about 25 of total advance at 9psi.. which I should be able to run more timing and it likes it because I gain about 18-20rwhp with just .5º of less retard per boost.