Stock -> 272 Cams = Massive Richness?

[saliya]

Hey folks,

just added some HKS 272 step 2 cams to my previously-stock-cammed RB26.

Boost is now earlier and smoother (thank God) on the T04Z.

Question is, why did I need to remove a stack of fuel at idle and pretty

much everywhere below 3000RPM? I mean, it's still running of the same

AFMs, so why is the AFR so badly out?

@idle it was below 10.5 before I took out injector duration.

Is there some explanation that has a sound engineering standpoint behind it?

Thanks,

Saliya

[sh@un]

There is more airflow through the engine due to the increased valve timing overlap- hence the ECU is doing what the AFM tells it to do and adds more fuel. Simple.

[saliya]

There is more airflow through the engine due to the increased valve timing overlap- hence the ECU is doing what the AFM tells it to do and adds more fuel. Simple.

Hey,

what I don't understand is why that would make it richer. The AFMs are measuring all the

air that goes through, same as before... more air + corresponding more fuel == same AFR

(but it wasn't, it was richer!)

I'm thinking it has something to do with corrections for RPM... maybe... but I'd really

like a good explanation for why the same air flow has resulted in more fuel being added...

Regards,

Saliya

[sh@un]

It'd have something to do with the way the ECU cross referrences the rev/load tables in your pre-cam tune with the TPS values it expects.

So the airflow has gone up (which the computer sees as extra load through the AFM), but the cylinder pressure hasn't gone up much, due to most of the air flowing straight out the 'zorst valves at low revs, so you get lots of unburned fuel.

[Adriano]

Interesting, but unburnt fuel shows up as lean on an A/F meter. All i can think of is the high load/low rpm cells must be tuned rich before hand(not the rich really, as at 1000rpm the difference in ms between 14:1 and 10:1 is bugger all)These cells are probably rich to assist the accel pump "circuit" im helping the car during hill starts and low rpm takeoffs where you would normally access these cells. Now with the cams you are accessing these cells at idle. Were they different cells than it used to idle at? These can be tricky to tune, as you cant hold these cells on the dyno(you should here a car at 1200 rpm on the dyno, sounds like the car is going to fall apart.

[saliya]

Interesting, but unburnt fuel shows up as lean on an A/F meter. All i can think of is the high load/low rpm cells must be tuned rich before hand(not the rich really, as at 1000rpm the difference in ms between 14:1 and 10:1 is bugger all)These cells are probably rich to assist the accel pump "circuit" im helping the car during hill starts and low rpm takeoffs where you would normally access these cells. Now with the cams you are accessing these cells at idle. Were they different cells than it used to idle at? These can be tricky to tune, as you cant hold these cells on the dyno(you should here a car at 1200 rpm on the dyno, sounds like the car is going to fall apart.

Well, this might be it - I didn't observe idle cells before the cam change. To be honest,

I didn't expect cams to affect AFRs the way they have, but you're right - time-wise

my changes aren't that radical - percentage-wise, they are...

It's now idling basically at N02/P04 although it accesses cells in N01/P03 occasionally.

Anyone with stock cams/AFMs and a 3.5" or bigger exhaust care to post where

their car's idling? Is this a relevant question?

At this stage it's only low RPM and load that I've been investigating; so even

hill starts etc. are in different map areas.

Regards,

Saliya

[Adriano]

I thought that it would have moved more than one cell down, they usually idle at cell 2-3, was it at operating temp when you checked the mixtures?Hill starts should still access the same points, except it will probably take a few more revs

[zebra]

Hey there,

bigger cams = less manifold vacum so the engine thinks its underload all the time and dumps fuel into it.

[T04GTR]

bigger cams have poor cylinder filling before they start to work.

i put in some 260 poncams to mine and had to remove around 20% at low load areas and 20% more once on boost.

nice idle but when first started hehe

[T04GTR]

they need less fuel then heaps more. veiwing the fuel table its just an exagerated mountain side. same curves but the highs are higher and the lows ar lower