Just curious what a decent setting is for the rev limiter.

The trans is programmed to auto shift at 6800RPM but I'm warned that it may take a bit longer to actually make the shift so I'm planning on an actual 7000 shift point (4R70 with Quick4 controller).

I have the Rev Limiter set to 7250
Soft Limiter set to 200

Under full boost the total timing for the engine is 11* so I thought to really shut the engine off I set the Fixed Angle to -5* ATDC but not sure if this is too much.

Are these numbers acceptable if not what are some "safe" values that I should use? Basically: set it and leave it values..

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Use fuel cut limiting. Every OEM engine uses fuel cut. If you cut spark it will send fuel to the exhaust which will burn there or buildup and then burn (backfire / explosion). Negative timing on the way out means rapidly rising EGT it could melt the engine, rings lost tension.

Rev limit depends on many factors. Piston max velocity is first one. Then, valvetrain. Transmission internals. TH400/4l80er won't like high rpm without lightweight parts inside for example.

For street duty I always keep near 6000rpm for every engine. Even factory skyline 8000rpm redline I will stop 6200 to 6500rpm. This for safety and reliability. You can jam into a low-limiter anytime and be fine. After 6500+rpm there is more chance of random issues causing catastrophic failure.
No sense is taking risks when you are just messing around on the street.

For the track at high RPM You never want to touch the limiter. Set it higher than the engine should ever be ideally. Tapping a limiter at 6k is fine but at 7k like I said its a risk, not a good idea, too many momentum forces involved inside the mechanical engine parts to be reliable no matter what engine. Just do not exceed limit for valvetrain, trans, or piston speed. And the oil system is another consideration, oil pressure can be high and still have little or no flow at high RPM depends on clearances and engine design.

 

I really don't understand the fuel cut I can see it in a new direction injection engine. But some thing like my old small block Ford I assume there is some reversion fuel in the manifold. Therefore cutting fuel and keeping spark would cause a very lean mixture with left over fuel from previous cycle.

This conversation has makes me think I need to revisit my limiter settings and safeties. Sounds like they can do more damage than good?

I saw a post on IG where a guy was on a PB pass hit the 8500 limiter and it end in a blown engine. With out the limiter he may have set a personal best. In this case it didn't save him. Not sure what the value train etc. Would have done at higher rpms.

 

It is very difficult to achieve a combustible ratio of air and fuel inside a cylinder without precise metering.
Leftover fuel cling to manifolds and active reservoirs, all engines have these things. They will always induct some fuel even during fuel cut. But it cannot combust because the a/f ratio is like 100:1 or 50:1. It needs to be in the range of 17:1 or less, which if calculated you will see is quite a lot of fuel, much more than can cling or sit around freely in a manifold, even 10x of that fuel isn't enough.

Next, a single lean event doesn't ruin an engine. The cylinder temperature will increase slightly for each lean event. Combustion occur at say 5000rpm to 10,000rpm , many cylinder events happen within a second. At just 5k rpm isnt that around 40 times per second? So within say 3 to 4 seconds, something like 160 lean events, now the cylinder is getting hot, and ready to start detonating. You won't reach a critical temperature threshold with a couple lean events, the cylinder temp won't rise enough to cause any harm. Lean combustion is harmless, we run lean for cruise and idle, the energy is less and when rich and fewer pressure can be made. The danger is the temperature rise caused by running lean with high input oxygen mass, as a sequential building up of heat throughput inside the cylinder, causing high temp to initiate combustion prematurely or by increasing the reaction rate of combustion due to temp input, like any chemical reaction the more temp input the faster the molecules collide with higher velocity and the more products they will form quickly, causing a pressure spike in the cylinder eventually, blowing the engine apart. If an engine is proper compression and tuned properly, the lean and high temperature will not cause this behavior in my experience, anyways, the high temp will cause piston ring tension lost but no resulting damage to the cylinder or detonation. I've seen an engine 22psi of boost to 7200rpm at 16:1 air fuel on 93 gasoline for 7 dynometer passes and have no detonation or engine damage other than lost piston ring tension due to high temperature. I witness this and experimentally determined that If the tuning is safely done to minimum best timing and the compression adequately low enough (9:1 in this case) the high temp isn't an issue causing detonation or high pressure, only the heat can become a problem to engine parts not designed to take that kind of heat.

At high RPM the limiter is a guess work at best. You don't want to tag the limiter at high RPM, unless it has been configured perfectly to be gentle for the engine under specific circumstance, which is difficult to do. There are too many momentum forces at high RPM no matter what type of engine, but heavier internal parts contain more energy so the limiter is even more difficult to pull off at high internal energy. It is difficult to describe the myriad influence of internal energy distribution at high RPM. Remember that each combustion event speeds up the piston and rotating assy, and between events the rotating assy slows down. It is a harmonic system, a frequency changing constantly, speeding up and slowing down as it rotates, a single missed event can induce an unwanted resonance or rapidly changing frequency, when internal forces are very high the force can focus on a particular engine part, such as rod cap or wrist pin or cylinder wall, causing excess unexpected force local to that particular region, leading to excess wear and failure. The higher the rpm the more smooth and controlled the coming on and off the throttle and combustion needs to be, no sudden differences and no rapid slowing down of particular leverage arms due to some issue with misfire or limiters is tolerated easily.

My recommend, again, is set the limiter very low for daily duty and fooling around. This way you can tag it by accident all the time without issue at say 6200rpm or 5800rpm. You can ignore the limiter and basically engine can never die anytime you smack it. That will keep the fear of damaging the engine out of your mind for these times.
Then for the race track, remove the limiter or set it very high so you never touch it unless a driveshaft breaks or something. Its better than nothing In the case of broken engine load transfer but you don't want to be on the verge everytime you shift or at the end of the track trying to get the last MPH On a PB because.... it may damage the engine just as easily to touch the limiter as to bypass it.

 

For me the rev limiter is used in case of mishap so the rev's are more controlled when shit happens.
I have the limit set to 7700 but my shift points should automatically happen around 6800-7000 max for now.

ks

 

So I would make 2 changes based on your screenshot:

1) change spark mode to progressive. This should gently retard the timing in the soft zone versus dropping straight to -5°
2) enable fuel cut (keeping spark cut also) since you have a bunch of extra mufflers and a full exhaust system

Have you reviewed a log to see what RPM it actually shifts at and how much it overshoots? I would guess having the soft limit 1-200 above your shift point would be enough.

 

Not yet. I have always manually shifted it about 5500 when out testing. Last weekend I was prepared to let the Quick4 autoshift/lock converter at a programmed RPM and also test the MSPro boost control settings but the meth inj system took a dump before testing started so no more testing this year or until I get it retuned w/o the meth inj.

ks