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

[30psi 4agte]

I have herd of this prob but i cant say i have ever had it happen on my setup.

none of my numerous dyno sheets show anything that would resemble a dip.

[kingmick]

takai, what cams are you using?

[takai]

Im using a grind close to the Grp A grinds which they used to run in the AE92s. About 275@50.

Changing timing around does help somewhat, but not overly. I was having a chat to another guy at the club meeting tonight and he runs a Ford 1600 in his Escort and has a similar dip, just lower in the rev range. So maybe its something completely unrelated. But it does seem fairly intrinsic to the 4A range at least.

[kingmick]

and make? is it much of a dip?

[Ben Wilson]

I've noticed a similar thing playing around with 4AGs with my engine analyser software, inlet length and cam sizes don't seem to get rid of it. I'm not sure about exhuast lengths. it's been a while since I've played with it...

[takai]

Its not a huge dip, but i only brought it up because i remembered back in the day (3ish years ago) that there was some talk on Twincam.org trying to figure out what the dip was. The consensus was that it was a vibration inducing some ignition retardation in the knock sensor.
But my engine doesnt actually use the data from the knock sensor (just logs it), and so no ignition retarding there.

Hence why i think its something intrinsic with the 4AG.

Ben: Was that with Engine Analyser Pro? If so i might break out the software again.

[Ben Wilson]

Ben: Was that with Engine Analyser Pro? If so i might break out the software again.

Yeah, it was when I was doing Mark's Trueno motor...

[The Witzl]

I am tending to think that perhaps the addition of a TURBOCHARGER virtually eliminates or smooths out this dip - whether it be from harmonics or otherwise.

The dip you see on my turbo graph is due to knock control. The knock control used to go NUTS on the old turbo AE71, cos the engine was old, it was a stock smallport 4age with a turbo, running smallport AE92 gze ecu + emanage, and using the stock smallport 4age knock sensor.
I think without the knock sensor problems i encountered i wouldnt have had this power dip.

[myne]

My theory, as told to takai is that the exhaust gasses might be going supersonic at around this RPM.

Reason I say this, is the exhaust ports are a mere 32mm. At 4500rpm, each cylinder should be doing in the region of (4500/2 * 350cc = 787500cc per minute) And that's assuming it's getting ~80% cylinder filling (I guessed 350cc for the sake of arguement) and that's also not counting the expansion of being 900*C.

Is nearly 1000L/min through a 32mm hole enough to go supersonic?
What does air do as it reaches those speeds?

[oldcorollas]

My theory, as told to takai is that the exhaust gasses might be going supersonic at around this RPM.

Is nearly 1000L/min through a 32mm hole enough to go supersonic?
What does air do as it reaches those speeds?

but that would mean all rpm above this would be even more so!!

remember that the speed of sound increases with temperature also....

i'd be curious to see how changing bore size and stroke affects the dip in EAP....

[Ben Wilson]

but that would mean all rpm above this would be even more so!!

remember that the speed of sound increases with temperature also....

i'd be curious to see how changing bore size and stroke affects the dip in EAP....

There might be more of a difference around the speed of sound though - shockwaves and all that. I remember reading Chuck Yeager describing the first time he broke the sound barrier, the plane felt like it was about to shake itself to bits, then it all smoothed out above that speed.

The speed of sound (and the corresponding dip), would be a lot lower in turbo cars due to the increased pressure and temperature in the exhaust manifold...

[Ben Wilson]

i'd be curious to see how changing bore size and stroke affects the dip in EAP....

'twas just my memory playing tricks, I finally remembered to have another play with it last night and it was an inlet harmonic.

Playing some more, it looks like if you close up the inlet port, 4AGs really respond to big valves and small cams....

[nick.parker]

Valve sizes hmm..ONE major difference between say a Honduh B16 (118kW) and 4AGE (100kW). Is that the Honduh has inlet and exhaust valves (33.0/28.0mm respectively) compare this to Formula Atlantic 4AGE (32 / 27.5 I think). And a stock 4AGE is 29.5 / 25.5 inlet/exh respecitvely. Its still food for thought..

Cheers, Nick

[oldcorollas]

'twas just my memory playing tricks, I finally remembered to have another play with it last night and it was an inlet harmonic.

Playing some more, it looks like if you close up the inlet port, 4AGs really respond to big valves and small cams....

how much smaller?
inlet to head is (from memory) 40mm vs 43? 45?mm diameter equiv... how small is too small?

can you see what size ports are good for say.... 18psi?

[myne]

Valve sizes hmm..ONE major difference between say a Honduh B16 (118kW) and 4AGE (100kW). Is that the Honduh has inlet and exhaust valves (33.0/28.0mm respectively) compare this to Formula Atlantic 4AGE (32 / 27.5 I think). And a stock 4AGE is 29.5 / 25.5 inlet/exh respecitvely. Its still food for thought..

Cheers, Nick

There are only a few differences between those 2 motors.
The valve angles are only 2* different
The stroke is only .6mm longer on the honda.
The B16 uses rockers, but I'm not sure how much difference that'd make.
I believe the B16's ports are larger than the bigport
The B16, because it has vtec also has the advantage of running 290* cams and a lot of lift once it hits 5500rpm. I think the low end cams are closer to 220* and 6mm.

Such a pity we cant simply stick the B16 head on the stronger 4a bottom end