Strange Acceleration

[Guest Roger NE]

I'm the guy who put a 2JZ-GE in my Mk3 Supra (manual with a W58 gearbox)

 

The way I installed it is pretty much as in a Mk4, using the correct ECU, igniter, MAP Sensor and Fuel Pump ECU, all from a Mk4 Supra. But both cats are removed, and using a stock Mk3 2.5" exhaust.

 

Generally I've been really pleased, it runs really well, everything is as it should be, starts instantly, perfect tickover, sounds smooth at all revs, no misfiring or anything, stays at normal temperature, plenty of oil pressure (we have a gauge on a Mk3 !), clean exhaust, etc etc, and pretty good performance.

 

But I have noticed something rather strange . . . this may have always been happening since I fitted this engine, but perhaps I'm only noticing it now if I really boot it, given that the roads are now dry!

 

If I am accelerating hard, full throttle, in any gear, suddenly when I reach exactly 3750 RPM it's as if someone flicks a switch and is injecting N2O or something ! In other words the car suddenly has LOADS more acceleration . . . almost like a turbo kicking in, but instantly and with no delay.

 

I'm wondering what on earth this could be?

 

It obviously implies that something is WRONG at lower revs . . . but what?

 

To be honest I haven't checked anything since fitting the engine. I was assured it was running fine in the car it came out of, so thought I'd wait and see what happened when I'd done the transplant. I was delighted that all my wiring interfaces worked properly, so as everything seemed perfect I just left it, just did a normal service.

 

Could it be something as silly as the static ignition timing being retarded . . . but at higher revs it kicks in more advance and overcomes the problem? (must admit I never checked it, as I didn't touch the distributor and it seems to be running fine)

 

I figure without any Cats I may be producing around 240 - 250 bhp, which is quite a lot more than the 200 from my original 7M-GE engine . . . which is probably why I thought the power was OK. But now I'm thinking there must be something wrong at lower revs.

 

That 3750 RPM must be a clue . . . . anyone who knows these engines have any idea?

Edited July 17, 2014 by Roger NE (see edit history)

[David P]

Probably the ACIS moment.

[thomo8568]

Is it not the second butterfly opening/closing in the inlet. Think that's around that point. Would probably notice more in the mk3 due to less weight.

As above.

[Guest Roger NE]

I did wonder about the ACIS changeover point (is that when it is?)

 

I know it operates in one mode to give best performance at low revs, and then changes the intake chamber to give best performance at high revs. But I can't believe it would make that much difference ! (The Mk3 engines have an almost identical ACIS setup, but I've never noticed a pronounced sudden change like this).

 

Probably the best analogy I can give is that it's as if you've been driving with the HT feed only to 4 cylinders, then suddenly at 3750 RPM someone switches on all 6 !! It really is that dramatic.

 

(By the way, a Mk3 is much heavier than a Mk4 Supra . . . they're really solidly built !)

 

What do you notice in your N/A Mk4s . . . do you get a sudden change at 3750 RPM?

Edited July 17, 2014 by Roger NE (see edit history)

[2soops]

 

(By the way, a Mk3 is much heavier than a Mk4 Supra . . . they're really solidly built !)

 

Very true, mine weighed 1690kg

[LOGIE]

I had 2 mkIIIs both 7MGE with w58 and a 7MGTE with R154. So I know about the weight issue. I also noticed a surge in my mkiv NA at around 3750ish maybe a little higher but I dont think it was as dramatic as your sounds.

[Guest Roger NE]

So has anyone any ideas as to what could be causing this?

 

(not the extra power higher up . . . but the fact that it's not there lower down !)

[David P]

Maybe the torque doesn't like your header pipes?

[Guest Roger NE]

Maybe the torque doesn't like your header pipes?

 

Well I don't have headers pipes . . . just no Cats

 

But anything to do with gas flows wouldn't be so on/off like this is

 

I'm not exaggerating . . . when you reach 3750 rpm it's exactly as if someone has flicked a switch and given you 30% extra power . . . you get pushed back in the seat and hear a sudden change in the engine sound.

 

It does it every time you open the throttle fully, when you reach that exact rev point the power suddenly increases, regardless of what gear you're in. (I must check, but I don't think it happens if you just accelerate gently at half throttle)

 

Really hope someone can help, as I'm really baffled !

 

Does anyone know if something happens at that rev point?

 

Is the ignition timing suddenly advanced?

 

Does the Fuel Pump ECU suddenly switch to full power?

 

Is the HT voltage suddenly increased?

 

I'm just wondering if there is something slightly wrong with the way my engine's running, but that whatever the ECU makes happen at 3750 rpm over-rides the problem.

 

(eg if my ignition timing was a bit retarded, but at 3750 the ECU advances it a lot, so corrects the problem.)

Edited July 17, 2014 by Roger NE (see edit history)

[David P]

I seem to remember that it has extremely long secondaries.

[Guest Roger NE]

I seem to remember that it has extremely long secondaries.

 

The two pipes that normally join in the first cat continue a bit further before they join (as I am using a Mk3 exhaust system), but firstly I don't believe that could cause this huge change in power at an exact rev point . . . but also lots of people have put full blown headers on and don't have this problem.

 

The more I think about it, something must be being changed by the ECU at 3750 rpm . . . and I guess it must be highlighting or perhaps correcting a slight issue I have with something on the engine.

Edited July 17, 2014 by Roger NE (see edit history)

[Graham1984]

I thought that was the rpm point the butterfly valve fully opens, giving maximum airflow which happens throughout every gear.

 

There was a thread on here that mentioned possibly having it left fully open all the time but iirc it would make the car slower up until that precise rpm.

[Guest Roger NE]

I know how the ACIS system works - what the butterfly valve does is change the intake from a DUAL chamber input system to a SINGLE system . . .

 

As you say Graham, if it wasn't better to have a twin system at lower revs it would be much simpler and cheaper NOT to have the Y pipes and the dual chamber Intake Plenum . . . let alone the whole ACIS system that does the switching.

 

I just find it really surprising that IF it's the ACIS valve opening that is causing this, that it suddenly makes such a HUGE difference. When I tried experimenting with the ACIS system on my 7M engine (ie always closed or always open) it made a SLIGHT difference, but nothing dramatic.

 

What on earth could be causing this big power difference? It's obviously not normal . . . if you plotted the engine power output on a graph there would suddenly be a huge jump !

 

When I first noticed it I thought a couple of HT leads must be breaking down, then suddenly working - that's how instant the change in power is. But when I started realising that it wasn't just at random high revs, it was at exactly 3750 every time, I figured it couldn't be that. (plus there are no other symptoms of faulty HT leads)

 

To be honest the reason I'm so concerned about this is because most of the time I never rev that high, and so I have always thought this engine was a bit lacking (compared to my previous engine, given that it should have 20% more power) . . . so need to find the reason for this lack of power at lower revs.

 

Surely there must be some brains out there that have an idea?!

Edited July 18, 2014 by Roger NE (see edit history)

[Guest Roger NE]

Just been scouring the internet for info on the ACIS system . . . and it seems that it's not that simple.

 

The butterfly valve in the plenum is OPEN as standard to start with (ie one big intake chamber). It's only at low to medium revs and if the throttle is open more than than 30% that it CLOSES, creating a split intake, and hopefully gaining more torque by utilising the acoustic pulses from the valves effect.

 

BUT according to the info I have found it OPENS the valve again once you reach 4500 RPM (ie NOT the 3750 where I am noticing this strange effect)

 

Does anyone have more info about the Supra's ACIS system? Because I'm now thinking that this CAN'T be anything to do with this issue.

Edited July 18, 2014 by Roger NE (see edit history)

[David P]

Unplug it and find out.

[Guest Roger NE]

How spooky David . . . I had decided to do just that, and have just got back from doing tests (even though I should be getting on with work!)

 

Well with the ACIS disconnected (ie the butterfly valve open all the time) it still does it ! So as I figured, that rules that out.

 

Out of interest, it's the first time I've done that on the 2JZ (which has a bigger ACIS system than on my previous 7M-GE) . . . performance is noticeably worse at low to medium revs without it.

 

And the Strange Effect I'm describing of lots more power at 3750 rpm was slightly less pronounced with it disconnected (presumably due to less power generally)

 

Something I also did try was whether this happens if you go through 3750 rpm at half throttle . . . and it DOESN'T

 

So it only happens at full throttle . . . so that must be a clue?

 

Could it still be due a timing issue? (ie is the timing likely to suddenly be advanced a lot at that rev point?)

 

If I didn't have to remove the cover on this engine to get at the spark plugs I would have checked the static timing by now.

Edited July 18, 2014 by Roger NE (see edit history)

[David P]

Try it again with the butterfly closed.

[Guest Roger NE]

OK, with the butterfly forced closed all the time (connected to a permanent VAC line) it's just the same . . . big surge at 3750 rpm

 

(It's actually more like with the ACIS working normally, ie more noticeable than with the butterfly always open)

 

As I say, I think we can rule out the ACIS system . . .

 

Does anyone know if anything happens suddenly at 3750 rpm to the Ignition timing or the Fuel Pump control ?

 

I suppose one thing I could try is bridging the terminals in the Check socket so that the Fuel Pump gets continuous full voltage.

Edited July 18, 2014 by Roger NE (see edit history)

[David P]

There is no permanent vacuum, use a cable tie.

[Guest Roger NE]

There is no permanent vacuum

 

Of course there is ! I used the Vacuum Tank output. This made the valve stay closed all the time, even on tickover.

 

David I've clearly ruled out the ACIS . . . although I never really believed that could cause such a dramatic change like this anyway.

Edited July 18, 2014 by Roger NE (see edit history)

[Guest Roger NE]

So has anyone got any other ideas about this?

 

Has anyone else with an N/A noticed a sudden boost at 3750 rpm, even if slight?

 

(something must be changing at that point, although clearly something is accentuating it with my setup)

[stevie_b]

I've read more than my fair share of threads on the forum, but I've never read anything about a dramatic increase in performance from an NA at any specific rpm. I have an NA, and I don't notice any performance pickup at all. This isn't much help I know, sorry.

[Guest Roger NE]

No it IS a help Steve . . . I'm trying to suss if this is totally unique to my setup . . .

 

Perhaps you could just check next time you're driving though . . . put your foot hard down from 3000 rpm (probably in 2nd or 3rd gear is best) and just check whether you notice any sudden change at 3750 rpm

 

Same goes for any other N/A driver ! I'd appreciate any observations.

 

I'm pretty sure the ECU must be changing SOMETHING at that point . . . even though in most cars it doesn't produce any dramatic difference.

 

If I can find out what is happening it will help me diagnose the problem. (the problem is obviously not the increase in power at 3750 rpm . . . it's the lack of it up to that point!)

Edited July 18, 2014 by Roger NE (see edit history)

[Graham1984]

What ecu are you running? My mines gave a more dramatic feel than my stock ecu.

[Guest Roger NE]

As in my original post, the ECU and everything about the engine is stock Mk4

Edited July 18, 2014 by Roger NE (see edit history)