My 90 Q is back from the shop, having had, among other things, cleaning the plenum, cleaning throttle body, new hoses, new a/c compressor, and, (ugh) a trannie rebuild. On the whole this has returned the car to the performance level I loved and remembered, although there are a couple of things that are different. First, is that there seems to be a "flat-spot" in 2nd at low speed - that is, at about 20 mph/3000 rpm or so, when I put my foot down the car just doesn't seem to respond - it just sort of slowly accelerates, then when the rpm get up a little higher, it takes off. Second, and perhaps this is related, the throttle just doesn't seem as responsive - there is a lag between when I put my foot down, although it responds well (modulo comment abobe) powerfully when it does go.

Any ideas what may be at work here. Is this something about the computers, or it is something mechanical that can be adjusted/repiared?

Second gear in a 90Q is 740 rpm per 10 mph so 3,000 rpm would be 40 mph [20 mph?] And 20-22 mph WOT should allow a full downshift to 1st. Does it?

Maybe you are speaking of the typical problem with 2nd gear start where if you are trying to down shift at say 23-25 or higher mph [nothing happens unless you manually pull the gear to 1st.The Q is pretty slow from 23-45/50 mph [till you exceed 3500 rpm] in 2nd.

Tranny rebuilds always scare me as few people in the US can do it well -- most work for Nissan/JATCO in Tenn.Measure the 50 mph to 80 mph acceleration [locked in 2nd] with a stop watch [3 times to get an average] and report!

I am assumming you have had the engine scanned with a Consult to look for codes [knock sensors, etc].You mentioned lots of things but not chain guide replacement when were they done?

Q45tech wrote:Second gear in a 90Q is 740 rpm per 10 mph so 3,000 rpm would be 40 mph [20 mph?] And 20-22 mph WOT should allow a full downshift to 1st. Does it?

My mistake; it is down around 2000 rpm. (Sorry - but what does WOT stand for?)

Quote »Maybe you are speaking of the typical problem with 2nd gear start where if you are trying to down shift at say 23-25 or higher mph [nothing happens unless you manually pull the gear to 1st.The Q is pretty slow from 23-45/50 mph [till you exceed 3500 rpm] in 2nd.[/quote]

Could be. The flatness is particularly noticable when I slow but do not fully stop at a stop sign, so there is no downshift to first. The car is then really slow pulling away, until the RPMs get up and the power drops in. I have the JWT 1st gear start chip, so from a stop the car picks up pretty well, except for a short lag before the power drops in.

Quote »Tranny rebuilds always scare me as few people in the US can do it well -- most work for Nissan/JATCO in Tenn.Measure the 50 mph to 80 mph acceleration [locked in 2nd] with a stop watch [3 times to get an average] and report! [/quote]I'll do that next time I'm out on the freeway and report.

Quote »I am assumming you have had the engine scanned with a Consult to look for codes [knock sensors, etc].You mentioned lots of things but not chain guide replacement when were they done? [/quote]Yes, I have. On chain guides, my car, while a 90, had a new long block put in mid 1994. After much work with Infiniti corporate, we were able to cross the part number of the long block (. . . U02), which is supposed to have the upgraded quides and my car's VIN to determine what was put in. I know, and indeed Infiniti corporate stressed this, while this clearly indicates the proper part, there is no telling without actually looking. This I haven't done.

WOT=wide open throttleAll 32 valve V8 trade some low rpm torque for the 20% increased efficiency they get from 3500 to 6900 rpm. You have to either use low gear [1st] and or wait till 3500 rpm.

Due to the economy gearing the Q are slugs to 25 mph even in 1st gear [9.85 compared to modern 5 speed 11.5-13.0]. Same with the torque convertor its stall is set at 2200 rpm [with sufficient torque] to enhance around town and highway fuel economy.

Don't Q's have a slight flat spot in power at arouns 2500? It starts to roar (technical term for the bisection point in RPMs of the torque and power curve divided by the cube root of the exhaust decibel level at the exact time the bisection occurs...top that Dennis!) above that.

Something simpler:

:rolleyes Q45 tech is the answer " Do you want fries with that sir?":D Damn I thought I got away from equations like that when I left physics and calc. Looks like a classic cranial - rectal inversion to me. Not to be confused with cranial rectalalasis.

Y'all have a great day now, ya hear.

firedane

Are the throttle cables set correctly? and is the throttle potentiometer within range? I agree that Q's are sluggish at thebottom end. especially from a slow roll to about 35. However, the lag you describe seems excessive. As everyone pointed out to me check hoses, connectors etc. it is easy even for a skilled mechanic to miss an item down in the plenum area.

firedane

Actually the equation is the proof that the velocity of an air particle changes thru a nozzle with a constant volume of flow.

The flow in the runners is designed to not impede power at 5500-6500 rpm. The higher the redline the worse performance you get at low rpm.The VE is just low at low engine speeds unlike 2 valve engines with a 1,000 to 1,500 rpm lower power peaks.You overcome this with gearing so that in 1st gear you get thru the sluggish point faster

Uncle...My mathematical abilities peaked with econometrics in college. I'm not even sure how you were able to type the symbols in the equation!

I'd also look into the cables (they should have some slack) but not a tremendous amount. Hope your tech didn't mess with the fast idle cams etc. on the side of the throttle body. I'd say it's time for a good checkup with Consult to tweak everything out. Might need an injector flush... the power balance test will tell you. More than +/-12 RPM variation on a cylinder means something needs cleaning...

Heath

If your Q45 is pre-93, I predict you'll be really pleased with an upgraded tranny controller (TCU).

The '93 TCU I put on my '92 Q was the best $250 I've spent on it so far . . . fist-second shift quality varies from "no different than any other shift" to "firm" to "extra firm" depending on throttle openning. That may have something to do with a 1st that had essentially 0 miles on it and a 2nd that had 100,000. Wouldn't put old TCU back on if you paid me!

AZQ45 wrote:Uncle...My mathematical abilities peaked with econometrics in college. I'm not even sure how you were able to type the symbols in the equation!

ahh...he didn't type that, it was an attached image. probably from Maple or something.is derivatives all you got dennis..throw some triple integrals out there.

Ah yes the age old problem of air flow. Get it at the top end and the bottom turns to a dog. Tune for torque and low RPM and redline starts to fall on it's face. Q45 tech - hasn't there been experimentation on dual runner manifolds? Seems to me I remember something about a speed sensitive manifold. Or did I cook too many brain cells smelling nitro at the track?

Quarter pounder with cheese- no onion please.

firedane

Sure dual runners [were / are ] used on many engines, as is a dual or variable displacement plenum.But you can only get a 5-7% boost from the tuning .Lots of it has to do with specmanship as for every peak there is a dip!The question is what is the average area under the curve over the rpm range you are measuring!

Obviously the Q was optimized from 3700-6000 rpm as best they could with a single runner.Actually the 90-93 Q has a single 14" runner which divides into 2 -4" runners feeding each intake valve. The gasket is mismatched to reduce low rpm reversion and the slope and shape of each is slightly different to broaden the power peak over a 100 rpm range

At a cetain temperature of inlet air and cylinder head temperature a 90Q engine has a peak at 5500 rpm, a dip at 5750 rpm and another equal peak at 6,000 rpm......then a long slow decline to around 6700, then a steaper decline to 6900-7200.There are other peaks at 3700, 4000, and 4600-4700 rpm with dips between them each varies with temperature.

I recall my old '89 SHO had dual intake runners. Short and long, not sure about diameter. That car definitely had a big power rush at at 4400 RPMs. I think it was 3.0L w/220 HP.

Q=61.7hp/lSHO=73.3hp/l

Since SAE uses 60-70F as the norm you can be assured that the tuning is done in that range. In the dyno labs they would be forcing [supplying] large quanties of cooling air in front of the car to simulate the air velocity as speed.

Why it is so critical to maintain the lowest possible air inlet temperature to achieve factory performance.As the power [due to lower air density] drops off at 1% per 11F.When you are traveling less than 60mph the engine compartment air has time to heat the low velocity air in the intake system.

It may take a second or two for the 10x higher flow of WOT to cool the tract.

Interesting to install thermocouples to measure the REAL air temp in the plenum vs say infront of /after MAF vs outside as you think it is.You would think 100 ambient vs 32F is 68F or 6% difference but internally it is more like a 100F difference or 10% in the plenum......alot has to do with the AC system heating the air 30-40F in the engine compartment and the radiator efficiency going up in cold weather........200-220F coolant heats the 140F condenser air to 170-180-190F flowing over the motor, compared to 176F coolant having minor effect on 32F air bring it up to 60F or so.Thus the plenum is relatively cool 100F vs 200F in summer.

Ever notice what seems like a significant power boost when it is cold outside and the converse in the summer ....this is one of the main factors.

1/5-1/3 of NOX power addition comes from dropping the air temperature thru the system [going from 150-180F to 40-50F] as it passes by the injector going into the cylinder.People say they notice a different sound when NOX hits sure they do the whole tune of the motor has changed due to the lower air temps.

Fix this topic because I want to see the equation Dennis posted again.

I posted this in the moderator forum to see if anyone has any ideas, but I suspect whatever formula Dennis posted there has moved on to the intergalactic bit bucket long ago...

Heath

We were able to fix the thread's appearance, but we have no idea what forumula Dennis posted or how to get it back. Probably due to this:

Mayhem_J30 wrote:

AZQ45 wrote:Uncle...My mathematical abilities peaked with econometrics in college. I'm not even sure how you were able to type the symbols in the equation!

ahh...he didn't type that, it was an attached image. probably from Maple or something.is derivatives all you got dennis..throw some triple integrals out there.

Heath