Is it possible for an aggressive streetport to have a brapping idle?
 

I've got a new motor I bought off someone, a full setup w/ AEM EMS, 5k mile rebuilt rew w/ 2mm RA seals by Formula Drift in Florida. 1680/850 injc and a tune at 400whp at 14.5psi on a T04S.

The motor runs great throughout the rev's, but at idle after it warms up/sits in traffic for a while at 180-200F it begins to have a pulsing idle. Noticeable brap brap brap. AFR reads around 13.8 at idle.

Motor pulls about 12-14 inches of vaccuum at 900 rpm. Needle bounces from from 12-13 during this brapping idle period. If I rev the motor up to 2.5k and hold it, it sounds smooth, no pulsing and smooth vaccuum.

Also, I hear clearly audible even pulses when cranking the car w/ fuel cut.

Just want to make sure this is a normal possibility of having a rougher idle w/ a larger port, and larger injectors.

The previous owner told me its always done this and its the ECU "chasing" timing when the car is warm.

I have heard a few that were lumpy but we are talking a dangerously large, early opening streetport with late exhaust closing, not brappy. Perfect storm. That would also explain the vacuum as I am seeing around 12-13 at 1600 with a half bridge.

But if compression checks out then enjoy.

I'd check the tune as well as compression test. It sounds like the temp correction or something else is dumping a bunch of fuel in at idle.

If the porting caused any additional over lap, it's gonna brap a little bit. Overlap causes an imbalance in consistant air flow to the engine at low speed. This flow imbalance also causes inconsistant readings with the map sensor. That's why your ecu is chasing timming because the low rpm flow embalance is causing a pressure differential with your map sensor and your probably hitting two different points on your timming map as a result. Also when you don't get consistant air flow to the engine, the engine sometimes misfires as the intake charge is never consistant. Reving speeds the air velocity and helps over come the turbulance.

What does it behave like if it's just warmed up to normal temp/idling, i.e.: not sitting in traffic/hot? Is it temp dependent, or all the time.

In my experience a really big street port usually wants more fuel than 13:1 at idle, really low ignition advance and you may have to raise the idle to 1,000rpm.

You should be able to get it to sound more like a 2 stroke outboard (BrrrrBrpBrrrrBrrrBrrrrBrpBrrrBrrrBrrrBrp) misfiring every once in a while from intake dilution instead of the full braap... braap...braap firing every third rotor face.

Additional factors that can add braap are intake leaks and poor idle air control implementation.

Thanks for the great info!

Some notes,

Car starts right up when warm, is a tad hesitation when cold. Must have a goood battery to fire up. Stock motor would fire up instantly even w dying battery.

Idle is super bouncy at low rpn and wont idle until its ran for a few mins. Runs great and idle increases up to 900 at 180f.

Car pulls hard and super smooth in all gears when moving. Just this idling.

Brap is inconsistent, sometimes when warm is stable other times bouncing just like a half bridge, consistent pulsing bounce exactly like a small bp.

I noted the brapping at 190f the other day idling in traffic, strong brap made me think I blew the map line or the motor. Then a few mins later after cruising and sitting at 200, smooooth normal idle, 14 inch vac.

As t-von suggested with the inconsistent idle vacuum you may want your idle map to have several cells the same or very close in timing and fueling right at the idle load range or implement a zero throttle map.

Yes, that sounds like it is in the tune.

You need massive amounts more fuel with a big street port on cold start because it has no intake velocity so the fuel falls out of suspension in the intake charge.

You need to tune the coolant correction map aggressively enriching up to the warmup point and also the idle fuel and timing as I outlined above or a zero throttle map.

Ill take a look at the map in the aem once I figure it out lol. The brap actually sounds amazing, and performs just fine. I do believe its a combo of fuel/timing and iat settings w the aem.

Car does smell really rich during warm up, it just seems overall the idle needs to be raised. I will have to check afr while completely cold.

This falling out of suspension is one of the reasons I'm gonna start experimenting with bottom dead center fuel injection with an injector on top of the housing. Would be cool to get a pp to purr like a kitten.

tuning issues

post the map and a datalog

Ray wilson from pfs will hopefully retune it for me in may, in the meantime ill try to log the map