mr08353

Does the addition or deletion of catalytic converters affect the tune?
If the car is tuned without cats, will it need to be re-tuned (Dyno-tune) if cats are added back on?

joecar

The difference is small and can be made up with LTFT's.

If you observe the LTFT's you can edit the VE according, and carry the trend to upper VE.

mr08353

Ok. Thanks. I'm not a tuner and not familiar with the abbreviations. LTFT & VE. Example, my last tune was without Cats. So if I change and add a cat and roll without a new tune, will that be ok? Or would it be better if the tune were with the Cats and changing later to remove cats and roll without having re-tuned. Make sense or have I messed everything up? Lol. The point is that I like variety in my exhaust system and don't want to pay $85 for a tune every time I change.

kingtal0n

iTrader: (1)

Woudn't it be carry the reverse trend to upper VE, since if you have more VE at lower rpm with a cat (or smaller exhaust) You expect less VE as rpm increases? On the other hand, I can see how a cat might actually degrade VE across the board (and raise exhaust gas pressure). It could go either way in theory (never ran a cat, don't know for sure)


Its kind of sketchy to just guess based off near-trip "long term fuel trims" anyways. To the thread op, if your car has a maf sensor, I Wouldn't worry about the tune when adding a cat or not. On the other hand if you are pure speed density, you will want a wideband to be sure you arn't just drowning out your power with extra fuel (if you are naturally aspirated it matters much more, whereas a little more fuel in a turbo application probably wouldn't worry me). And if your vehicle was dyno tuned there is a chance you have a bit too much timing advance on the street anyways, which when adding backpressure/temperature increase to your exhaust side (exhaust gas temp or EGT) you will want a bit less timing.

2xLS1

iTrader: (25)

Unless it is tuned in SD it isn't using the VE table at all over 4K rpms with the stock settings.

jimmyblue

iTrader: (11)

True for steady-MAP cases, but somebody with a busy
foot could be popping back and forth between MAF and
SD based modes a lot, picture a road course where they
may be wanting to stay between 4000 and 5500 RPM
but vary torque around the corners etc.

Cats punk the high-RPM, high-MAP corner of the VE map
and removing back pressure can be seen to bring on ping
(like, I tuned mine up real tight with cutout closed, then
went to the track and made passes with it open and saw
KR that didn't appear before). You get not only more air
mass, but better quality air mass because the preceding
cycle evacuated better.

kingtal0n

iTrader: (1)

In my opinion sir, if you remove an exhaust restriction and gain VE, and suddenly your speed density tuned engine starts to ping/knock, you have too much timing in that region to begin with, for more than one reason. One of those reasons is that your fuel is too volatile to be on the bleeding edge, imagine if the air temp dropped 20*F or 30*F you could wind up in the same predicament.

Darth_V8r

iTrader: (4)

I can speak to removing plugged up cats. Pinging went away completely.

jimmyblue

iTrader: (11)

No doubt it's too much spark or too little fuel or both.
All of this happens at a point on the VE map that can't
be accessed without the cutout open, meaning it's a
special tuning exercise (or, you extrapolate and see).

kingtal0n

iTrader: (1)

exactly the opposite of what the mod posted above. He claimed to have removed cats and gained pinging- indicative of too much timing advance.

Which is why I brought it up.

So this is a huge waving red flag. You need wayyy more headroom. I would back off the timing around wide open throttle at least 5* maybe more, then compare back to back EGT readings to confirm diagnosis. (run the car the same as you did before and the EGT will not increase significantly with 5* less timing, then you are safe to remove even more timing) Only use enough timing to keep the EGT from creeping up. There are some situations where this is not possible (too much VE i.e. forced induction) the graphs are NOT linear (combustion pressure vs octane vs temp vs compression ratio) so in those situations you are better off spraying water to hold down EGT.