4AG[Z/E] Deadspot at 4500rpmish - Theories and Postulations

[takai]

Ok, so there is that little flatspot or dip at 4500ish rpm which seems to irk everyone with 4AGs.
Originally I, and many others, thoguht that it was a resonance in the block causing the knock sensor to go haywire and retard the timing slightly. However, as of yesterday when my car was on the dyno, i dont think this is the case.

My cams start coming on at abotu 4000rpm, and then extend to about 8900rpm. But i still have that dip in the power curve. No matter how much tuning, or playing with cam timings i have done i just cant seem to remove that dip.
I remember having the same dip on my stock 100kw and also my GZE, so im starting to think that it has something to do with either the inlet or exhaust on the 4AG heads. Now the problem seems to also manifest itself on the bigports, so by powers of deduction i wonder if it is in the exhaust. Possibly a harmonic resonance or similar.

Anyone have theories or ideas on what causes this? Also, what about 4A-GTEs? Do they display the same dip in the powercurve?

[YLD-16L]

None of my 4AGTE power curves have any dips in them mid RPM (from factory ECU through to current state over the past 6 years), I will check the curves when it had the factory ECU again though to be sure.

I will try and find a graph from when it was a ZE with factory ECU too for comparison.

[takai]

Interesting, i remember Witzl finding that his had a dip in the powercurve too at 4000ish when it was tarbroed.
Can you guys post your engine types.
i.e. mine is an NA smallport, aftermarket management.

[YLD-16L]

Can you guys post your engine types.

Big port (ZE now TE), after market management, custom exhaust manifold, RWD inlet manifold with a RWD throttle body bolted to the front of it

I will double check my power curves tonight, but I am dead sure the most recent trip to the dyno a couple of months ago netted me the same boring curve I have had at every dyno day for the past few years. Increases cleanly to about 170rwkw then tapers off.

[nick.parker]

Huh?
On the GZE peak torque is around 4400 rpm, so I can't imagine a dip in torque around this point. I can understand a dip or rather a step in torque with TVIS on the bigport NA.

I would expect on almost ANY motor with a 'plenum + runners' style manifold to have interested effficiency vs RPM curve due to the interactions between the main mechanisms that are employmed to give good performance, namely wavelength tuning of the runner length and the helmhotlz resonance due to inlet manifold volume. Harmonics of the runner wavelength are also useful.....I guess thats why its silly to make a custom manifold unless you do some calculations first.. Chances are this dip could be related to the inlet manifold if everyone has it.....

The exhaust will have a bigger impact if exh / inlet overlap exists, the inlet geometry will be more significant with short duration (normal 240deg) cams.

Cheers, Nick

[Vios-GT_07]

just a quick question since we are on pleniums and intake causing a flatspot.. doesn't the TVI-S and "small port" design eliminate flaws that would cause a problem like this "dip" ??

i've noticed on a dyno of a stock bigport and a smallport, it's more significant on the smallport for some reason.. although i didn't think u could feel it and it would be of any concern... hmm..

[takai]

Huh?
On the GZE peak torque is around 4400 rpm, so I can't imagine a dip in torque around this point. I can understand a dip or rather a step in torque with TVIS on the bigport NA.

I would expect on almost ANY motor with a 'plenum + runners' style manifold to have interested effficiency vs RPM curve due to the interactions between the main mechanisms that are employmed to give good performance, namely wavelength tuning of the runner length and the helmhotlz resonance due to inlet manifold volume. Harmonics of the runner wavelength are also useful.....I guess thats why its silly to make a custom manifold unless you do some calculations first.. Chances are this dip could be related to the inlet manifold if everyone has it.....

The exhaust will have a bigger impact if exh / inlet overlap exists, the inlet geometry will be more significant with short duration (normal 240deg) cams.

Cheers, Nick

Ah yes, but my point is that it seems to exist across multiple different setups. Even my quad throttled massive cammed NA engine has it.
Anyway i cant stay up now, racing tomorrow.

[Talasas]

Intersting ideas here and I'm also one crusader against the 4A-GE power dip @ 4000. Although mine does it from around 3000 until it hits 4000 at which point it feels like it's coming on boost. After 3 years of poking and adjusting (just non-dyno settings) I've narrowed it down to a couple of things.

I've noticed that the only time my engine does it (only apparent) is when it is low on oil. Since I had an oil leak in the distributor as well as the oil cooler lines I found myself topping up the oil often and when it was low this "hesitation" became more apparent. However this is ALL I could deduce from it and since I've fixed the oil leaks the problem hasn't been around. I can't back this up with any theory really except that I know 4AGE's love a good batch of quality oil. At a guess I figured it had something to do with the oil pump but again not much to back it up there. Of course if you're using a dry sump setup (and I'm sure your race engine has been built properly and is running an appropriate oil) I imagine this isn't a likely cause for you. Perhaps increased chance of picking up vibration at these revs?

The knock sensor was always something people have mentioned and to confess I haven't actually tried testing it yet so could be that, I agree that I feel the engine does vibrate more but as above for me the engine is rough around 3000 rpm.

This brings up another thought, what kind of engine mounts do you have? If they are too rigid perhaps the knock sensor can flare up?

Also, say for argument's sake I was to fit a bigport intake to my smallport, should I expect the dip in power to change to another rpm area?

[roadsailing]

I remember Bill mentioned this once before, if memory serves me correctly he said "the dip in the torque curve is characteristic of most multivalve engines, but i tuned the engine to put it where i wanted it to go"

maybeye he can shed some more light.

[allencr]

This is on different dyno's, right?
So if this happens with different intake systems, large & small port heads, different piston tops, different cams and different exhausts, plus all the different ideas about how to "help the ports with a grinder", then the only common factors would be the valves, bore & stroke.

I don't have a clue.

[YLD-16L]

Checked my old charts over the weekend, none had the dip and that was on 5 different dynos. The only one I had with an odd curve was where the clutch died and started slipping at about 6500rpm.

[beige_bandit]

My NA bigport 16v with tvis connected has this mystery dip at bout 4500rpm aswell on the chart, well not really a dip but more of a Flat spot then continues to rise.

[LeeRoy]

My n/a smallport 16v has a dip/flat spot/hesitation at around 3500-4000. When i knocked the ign timing forward a little it reduced it heaps. But from what your talking about it seems as though this is not your problem.

- LeeRoy

[The Witzl]

My power trough was most noticable from 5000rpm to 6000rpm..... after 6krpm the power shot up to sky high levels (compared to other stock 4age's on the same dyno day).

That engine used to have "Kellam-brand" 4-1 mandrel bent extractors, no cat, and 2" press-bent exhaust. Intake was a cut-n-shut smallport item, and everything else was stockasarock just how it come out of the bin i bought it from


Then when it was turboz... everything was the same, just add a boglog manifold and a RB25 turbo.

Here's the dyno graphs..

N/AZ - http://www.rollamods.com/witzl/sml-ae71-dyno.jpg
TURBOZ - http://www.witzl.com/pmp80y/dyno01.jpg

[kingmick]

Ok, so there is that little flatspot or dip at 4500ish rpm which seems to irk everyone with 4AGs.
Originally I, and many others, thoguht that it was a resonance in the block causing the knock sensor to go haywire and retard the timing slightly. However, as of yesterday when my car was on the dyno, i dont think this is the case.

My cams start coming on at abotu 4000rpm, and then extend to about 8900rpm. But i still have that dip in the power curve. No matter how much tuning, or playing with cam timings i have done i just cant seem to remove that dip.
I remember having the same dip on my stock 100kw and also my GZE, so im starting to think that it has something to do with either the inlet or exhaust on the 4AG heads. Now the problem seems to also manifest itself on the bigports, so by powers of deduction i wonder if it is in the exhaust. Possibly a harmonic resonance or similar.

Anyone have theories or ideas on what causes this? Also, what about 4A-GTEs? Do they display the same dip in the powercurve?

you might find its the good old sonic bounce coming out of the chamber. the big commodore i have here cracks the hat from the reasonance so you might find its that. it depends on so many variables that unless i see the setup and it on the dyno, i cant tell you for sure. but im sure its what i think it is!