Cuda's Cars, v2.0

[CudaMan]

Now that both Zs are running properly at the same time... I've just noticed something.

It's most noticeable while blipping a downshift. Regardless of whether the brake is applied at the time. The '91 requires little pedal movement and it zings up the rev counter to where I want. The '94 requires no different rhythm, but a much deeper prod, down to full throttle at the briefest moment, to accomplish the same rise in revs. Under acceleration in gear the '91 feels eager, willing. The '94 feels labored. Not slow, just like it's having to work hard where the '91 engine isn't breaking a sweat.

The '91 is 100% stock except the holes in the mufflers. The '94 has a cone filter, headers, X-pipe and cat-back, lightweight crank and water pump pulleys, a brand new fan clutch, and has been dyno tuned to optimize A/F ratio under WOT. I'm not actually convinced the '94 is that much faster below 6000rpm, if at all. But primarily I want to know why the difference in engine response.

Is it possible there's a vacuum leak which the engine and IACV is able to compensate for? What am I overlooking? Think a MAF swap might reveal anything? The '94 runs smoothly and pulls well to over 7k RPM, so I don't think I'm looking at something like ignition or fuel.

I want the free, willing spirit of my '91 engine in my '94!

[thesameguy]

With mods involved on the '94, it could be anything!

o The '94 could be differently tuned from the factory - different cam on the throttle body, different spring on the throttle body, different weight of the rotating assembly, different weight on the flywheel
o Could be a goofy air measuring system on the '94 not responding quickly enough to changes in intake volume
o Could be a pre-throttle body, post air flow meter vacuum leak
o Could be a weak fuel system not providing sufficient flow or pressure resulting in sluggish throttle response
o Could be a weak ignition system bogging at low revs or not providing optimal spark
o If ignition timing is adjustable, could be a difference there
o Could be a result of the ECU mods
o Could be an obstructed catalytic converter

Do the pedals feel the same with the car off? Can you have someone watch the throttle body while you work the pedal or vice-versa to verify 50% pedal is 50% throttle on each car? That would work towards eliminating #1!

As a guy who has frequently had multiple instances of the same car at the same time, I sympathize with your plight!

[CudaMan]

I know, I'm kind of all over the place lately... but...

How important is gravity with a catch can routed between the valve cover vent tube and the PCV valve? I just realized my 6ft of hose will go up and over the engine from the vent tube. Is there enough vacuum that it won't be a problem?

[thesameguy]

On a fuel injected car it shouldn't be an issue at all. Obviously less hose and elevation changes are preferable, but with EFI at sea level you should have plenty of vacuum. Just remember the size of the hose has a bearing on vacuum at the end of it, and you don't want a situation where the crap in the hose could cool and solidify before reaching the catch can. You don't need this car to run tens of thousands of miles between inspections, so just keep an eye on it with tire pressure checks or whatever to be sure it is still doing its job. Based on how much oil you plan on processing, it will probably get plenty of attention.

[CudaMan]

Hmm. Well it's not terribly difficult to disconnect the hoses at the catch can. But the ends that go under the intake manifold are another story, and that's where the oil would pool on the side in question. Those hoses are not coming off without getting destroyed. Just no room for any leverage. So um, tens of thousands of miles between inspections would be ideal. Obviously draining the cans will happen more frequently.

The hoses are 9/16" for that side. Passenger side has the short hoses, one is 9/16" and the other is 3/8".

One of the old PCV hoses was already split and so probably leaked a bit of oil. It was wet with the stuff when I took it out. Hopefully this reduces the size of my oil leak(s) now that I have a new hose on there.


And now for a 2-week break before I can get back into it. Continuity!

-

Now on the throttle response deal...

- I'm not aware of any throttle cam changes. In gear, 20% throttle seems to give similar acceleration in both cars. There is just one flywheel for 90-96 NAs.
- It's the same design as the '91 air measuring system. Same parts. Only difference is the '91 has long tubes inside the bumper that snake around before meeting the air filter box.... while the '94 just has a cone filter off the MAF.
- There are vacuum hoses that come off the intake pipes. This is the only place I can think of there'd be a vacuum leak. Maybe in the EGR vacuum modulator system, or something. Two vacuum hoses that looked slightly cracked were replaced a few months ago.
- Fuel pumps aren't known to be a weak spot. Both cars have new Nissan fuel filters.
- It's coil-on-plug with new plugs. Coil packs aren't known for going bad either, but I suppose it's possible. New ones aren't cheap.
- Timing should be the same on both cars. Unless one of them slipped. Doubtful, given the adjustment mechanism with locking washers on the CAS.
- Maybe. I'm not sure how part-throttle was handled. If it was changed from stock maps. Full throttle definitely was.
- Cats look good from the rear on the blue car.

[thesameguy]

Does the starting speed of the engine have anything to do with it? Is 2k-4k different from 4k-6k?

[CudaMan]

I don't think so. It's just... by the numbers, the blue car should feel faster. It doesn't.

I was doing a thing to it today, and accidentally left a small vacuum line disconnected. I took it for a drive and then remembered when I got home. It didn't seem to drive any differently than normal. This line goes from the passenger side intake tube to the EGR vacuum modulator. Wonder if this means something's wrong with my EGR system?

The long green hose on the left:



My '94 doesn't have the AIV system, though, so part of this diagram doesn't apply.


Oh yeah, and despite nut-and-bolting the entire front suspension 2 weeks ago, and loctiting the adjustable upper arms, I still have a ma-hoo-sive clunk in the RF over bumps. Primarily low speed. I'll put the front in the air and try to find it again, but I'm stumped. Unless the shock is very much broken somehow. Which it shouldn't be. I still can't feel anything funny through the steering either.

[thesameguy]

I'd look at strut bearings, shock bushings, and sway bar bushings & links. Every car I've ever had with a clunky front end has been the swaybar... EXCEPT the PT Cruiser, which had an obliterated inner bushing on the LCA.

I wouldn't read too much into the EGR scenario. On a 20 year old car, I think it'd be a miracle if it even worked right anymore. If it does, it wouldn't make any tangible drivability difference. I blocked off the XR's EGR like six or seven years ago. I remove the blockoff for smog because it won't pass visual otherwise, but it's nonfunctional at best, and leaking boost at worst. It hasn't done anything for that car's emissions in a long, long time.

I'm slightly confused as to why you'd have that green hose if you don't have AIV. The EGR vacuum circuit should just be the yellow hose to the solenoid and the red hose to the valve. What would the green hose be doing if it didn't connect to an AIV solenoid?

[CudaMan]

It connects directly to the EGR solenoid directly, with the blue hose. The metal T-pipe isn't a T on my 1994, since there's no AIV for it to go to. Everything else is the same.

I'm not 1000% sure I checked the tightness of the top nut on the shocks last time, but everything else I guarantee was torqued and inspected. I hope that's it... [I did disassemble the coilovers to cut bump stops and measure shock travel].

[thesameguy]

Ah, got it. So the nipple on the TB connects directly to the second port on the EGR solenoid. That's weird. I'm not sure why that would want to be there. Maybe intake manifold resonance at big throttle openings causes low vacuum and tying the EGR valve to pre-throttle body airflow provides the vacuum needed to operate the EGR valve? That's my best guess. If so, the side effect of leaving that green tube disconnected would be a small amount of unmetered air (causing a slightly lean mixture) and an EGR valve that would probably stay closed at big throttle angles. Not great, but probably no terrible side effects. On a modern car, that'd be a more or less instant CEL, though!