Hi guys.

I had a quick dyno-tuning question on forced induction cars. .

When you have a car ('01 ls1 6spd, p1-sc@ 12 psi, 218ish 114LSA cam, meth/water inj) on the dyno and you increase spark advance....

Will it keep making more and more power right until it detonates (and breaks stuff on your motor)?

-or-

Will it eventually level off (stop making power). Eventually I assume it too will detonate if you continue to add more timing.

-or-

Is there an 'optimum' spark advance point where once you find it, you will start losing power if you continue to increase SA (and then eventually engine will detonate).

any help is appreciated!

 

Think of it this way, you advance the timing to time the explosion to peak at TDC. Too much timing and it peaks too soon and you are essentially trying to push the piston right back down instead of after TDC.

 

ok. after reading your post i thought this out.

The answer is c.

There is an optimum timing (esp. for low rpm torque...ie too much timing and it will lose power.). Once beyond that it will lose power and eventually detonate if you keep increasing the timing.

On an overly advanced engine (but not enough that it detonates): Some of that power gets 'wasted' if piston is still coming up a little bit when the flame front pressure hits it.

Definitely a peak/optimum exists.


Now... the next question..on a forced induction motor....

a. should timing be steady across all rpms (ex. 15 degrees from 2800 and up)

-or-

b. should timing increase slightly with rpm's (like on naturally aspirated engines)...ex. 14 degrees at 2800 rpm, 15 degrees at 4000 rpm, 16 degrees at 5000 rpm, 17 degrees at 6000 rpm.....Keep in mind, stock bottom end and the supercharger (linear w/ boost) is making more psi as rpm goes up...

-or-

c. should timing hit a peak around 2800 rpm (for torque) and then decrease (to play it safe as boost increases)?

Ex. 17 degrees at 2800 rpm, then 16 at 4000 rpm, then 15 at 5000 rpm, then 14 at 6000 rpm?


Remember this is an 01, stock bottom end ls1, with 10.5:1 compression, p1-sc making 12 psi at 5500 rpm, 50/50 meth water inj. etc.

Any help is appreciated!

 

c... keep the EGT's down..

 

Thanks. Good info

 

Measure the gains from each degree added. Stop when gains start to diminish. Pull a couple degrees b/c **** happens, and the dyno is not the real world. Mod the timing table vs iat and ECT to remove timing under sub optimal conditions.

Less timing at peak torque. More timing leading up to it while boost is lower on a centri car.

Take things a step farther than almost all tuners do and and log some pulls when the weather heats up or cools down. Modify the IAT and ECT vs spark again. Also, short, 3rd gear pulls don't count. Stay in the throttle long enough to find out if it wants to knock or not.

This is what I do. Hasn't failed me so far.

 

Good advice here. You should have less timing around your torque peak. Make small changes checking for knock after each change. Real-world testing under varying temperature conditions is the only way to get it optimized and safe.

 

Ive heard this too.

On an aggressive (good-fuel-only) spark table you want to take out 2 degrees around the torque peak.

In my case 5000 RPM.

Then add those 2 deg back in say at 5400 RPM.

 

Most LS engines, especially without meth will be knock limited. ie, you'll never see optimum timing because knock will occur first.

But when tuning, basically you will add timing until either MBT is reached or knock is detected. If MBT is reached then there is no point adding more timing whether knock is detected or not.
If knock is detected you must take measures to prevent it. That can be retarding timing, better fuel, water/meth, lower compression, less boost etc etc.

If you are forced into running very low timing figures because you are trying to avoid knock, you must monitor EGT's, as too low timing can cause these to skyrocket which can lead to engine destruction, when you think you're creating safety.

 

This isn't a correct way of thinking. If I misunderstood your meaning with this, p1bz, apologies. You never want an explosion event, this essentially is knock/detonation. Also you don't want peak cylinder pressure at TDC, as this does not help rotate the crank.

Op. A peak or nominal timing for a particular setup would have to be discovered in testing. There are far too many factors involved. What you want is to time the burn rate to expand effectively across the crank rotation driving the piston down. If you plotted cylinder pressure vs. crank angle, think of it like the area under the curve in a way. When you advance the timing, you start the burn sooner and increase the peak cylinder pressure and brake thermal efficiency. Advance too much and the you can elevate the cylinder pressure to cause the air/fuel mix to burn too quickly or "detonate". The knock you then hear is cause by the percussion of the explosion. Having done in-cylinder pressure monitoring before, knock looks like a sinusoidal wave of pressure oscillations.

Increase the dynamic cylinder pressure with higher static CR, or by adding forced induction, and that is one more piece to push your fuel capability. So, consequently you either decrease timing advance to reduce peak cylinder pressure so the fuel doesn't burn too quickly and detonate, or you use a slower burning fuel.. i.e. methanol injection, E85, race fuel mix, etc..

No. Retarding timing will increase EGT's. Just as stevieturbo notes above.

Sounds like a plan to me. Bumping things at the track is one thing, but for safety definitely take into account the everyday environment.

 

See Josh's notes... To much will raise it and too little will raise it.. that's what he's saying...

it goes too lean or too rich... which is why your headers glow red under WOT as the RPMs builds

 

Sorry, don't follow. Is stevieturbo, Josh?

It still is not true though, for a constant fuel ratio advancing the timing decreases EGT's and retarding timing increases EGT's. Retarding timing puts the combustion later in the engine cycle and more heat in to the exhaust. Too far retarded, and air/fuel mix can still be burning in the manifold.

Headers or exhaust manifolds get hot, and can/will glow red in a properly tuned engine when it is under WOT. It is a result of increased thermal load, not fueling condition even though this can further exaggerate the effect. Peak burn temperature for any fuel is at stoich, or a lambda of 1.0. So, EGT's can go up by approaching stoich or what is considered a "lean" condition at WOT.

 

I am not Josh, I am Stevie...

And fully endorse everything JDS is saying.

In it's most simple form. Too much timing will destroy your motor in a split second. To little timing will destroy your motor, although it will take a few seconds at WOT.

High compression/boost/pump fuel will give you a much smaller tuning window where things are deemed safe.
Lower compression, less boost, better fuel, or any combination of the safer options will give you a much wider tuning window. It's just finding the best balance that suits your budget and needs.

 

Your fuel curve will follow the torque curve. You will need less timing at peak torque than after that.

With a centri blower however you are making more boost as the RPMs increase which is why it's not common to see the timing keep being increased.

If there is one trend I have noticed on this forum its that way too many people are running not enough timing and way too much fuel. Less timing means less power and also higher EGTs...to rich of fuel doesn't always mean safer.

Tuning is a process....it's never really done IMO