I've decided to have my differential rebuilt with a 3.9FD VLSD. It's going to be expensive, but I really think this one mod is going to give me what I'm looking for - quicker acceleration. I don't mind having higher rpm's while highway cruising, nor do I mind giving up some fuel economy.

I thought about trying to find a 3.7 diff from a 370Z or G37, but it's difficult to find a nice one, and then quite expensive when you do. You never really know the condition of the used diff, or how many miles are on it.

My car has only 23K miles, and if I rebuild my diff rather than switching out with one from a 370Z, I get to keep MY diff, which is original to the car. Even though the cost will be quite a bit higher, I'll know that in the end I'll have a part that will work as good as my original one. Z1 makes a rebuild/upgrade kit for around $1,300 (for the 3.9).

Z1 350Z / G35 Final Drive Differential Upgrade Kit - 350Z Parts - Z1 Motorsports - Performance OEM and Aftermarket Engineered Parts Global Leader In 300ZX 350Z 370Z G35 G37 Q50 Q60

I thought it would be nice for Z1 to do the install, but they are booking for September right now! I don't want to wait that long, so I need to find another shop that can do the work for me.

Here's another option from Z1 - fully built diff's. Looks like about $2,250 plus a core charge of $1,000 that I should get back. There will be some shipping charges both ways as well. Then charges on my end for installing the new diff. Gotta figure $3K

Z1 350Z / G35 Fully Built & Complete R200 Differential - 350Z Parts - Z1 Motorsports - Performance OEM and Aftermarket Engineered Parts Global Leader In 300ZX 350Z 370Z G35 G37 Q50 Q60

Looks like I need to find a local transmission shop that understand bearing pre-load and clearances to factory spec. Wish me luck.

I bought a Hotchkis front sway bar today. Just the bar for now. $263. It is adjustable to four different settings. I'm just looking for a bit less body lean into corners and bit more precise feeling in the corner. Can always add the rear and/or links later.

Greg

 

Chose Motul fluid and Z1 bushings with that kit.
That's a long wait time, you know what the wait time is on the built units?

 

Facts!

 

You won't be giving much mpgs. Stock 350z gear ratio is loooooong, longer than many econobox manual cars. 5th gear is 1:1 while its usually 3rd or 4th on most mt. Shorter gearing = torque multiplier.

 

I think it would be best for me to find a local shop to do the work. Z1 has made such a business for themselves they cannot keep up with demand.

Greg

 

I can only speak to my personal experience. I was in the dyno room while the AEM engineers were trying several different bends, lengths and diameters on the 2003 Tiburon V6. They didn't stop until they found a design that had some peak gains without any losses. They rejected a few designs that had better peak gain but suffered losses down at the bottom. I assume they did the same testing with the 350Z. This is why I default to AEM intakes on most of the cars I buy. I know they actually do R&D and don't just slap together something that fits. Am I suggesting AEM is the best? Nope. Never tried anything else (other than stock). I just know they know their sh**.

 

Does anyone here know how to calculate the differences in rpm's between the 3.5 and 3.9 diff? I'd like to know what the difference is in third gear at a particular speed and fourth gear, etc...

Greg

 

Like the top speed in each gear? There are several numbers in that video about it but not every gear.

 

https://my350z.com/forum/engine-and-...ml#post4390297

https://my350z.com/forum/engine-and-...s-3-9-a-2.html

Bunch more on here, I'll leave that up to you to see what you need.

 

This was a good read, and made me happy to still see answers and support flowing in like back in the day. You have good taste Greg your car is gonna feel ALIIIIVE after you get those mods and tune. and you weren't kidding, those back roads ARE beautiful

 

The numbers are for a 3.7 and I'm considering a 3.9.

In the video it was shown that the speedometer gets the reading from the axle hub on the differential and therefore the speedometer reading would not change with different FD gears. But that's not true. The speedo sensor is in the transmission and changing the FD does change the speedometer reading.

Greg

 

Thanks mr. sparco, that first link was a big help. I think.

I could be wrong about this . . . . but . . . .

In the chart provided in that thread, I looked at the numbers in the 3.5 section and compared to the numbers in the 3.9 section. If I took the top speed of a particular gear and a particular rpm limit in the 3.9 section, and divided by the corresponding number in the 3.5 section I get a factor of around .91 every time.

So using that .91 number, if I'm going along in any gear at 3500rpm with my current 3.5FD, at the same speed in the same gear with the 3.9FD my rpm's would be about 3850.

I think.

Greg

 

I agree, I think this thread has a lot of good info in it and I'm glad to be a part of it. Nice to have support from so many knowledgeable people, even if they do disagree a bit from time to time.

I am so lucky to have roads like that just a few miles away from my home. Gotta watch out for deer though. We spotted at least 15 on our trip last night - none in the road, but out in the fields. Also one rabbit, lots of birds, and a pack of five dogs right on the side of the road.

Greg

 

So for those of you who have been following this thread, you know by now that I have been changing my list of things that I want to do to my car almost daily.

I was all excited about having my differential rebuilt with the 3.9FD gears yesterday, and now not so much. Two things - the cost, and the fact that my speedometer will not read correctly. Also, I have a bit of hesitation with the higher rpm's in each gear. I was thinking about this during my drive last night - about how the car is so easy to drive in fourth gear. I don't have to shift at all through sharp corners that slow me down a bit. I've still got plenty of torque to come out of the corner nicely. Yes, it's even more fun to go down into third and really rip it out of a corner. But that's my point I guess - that right now with the 3.5 FD, it works really well to do either of those things. Keep it in fourth and cruise around the corner and out with some nice acceleration, or downshift and rip it through the corner and slingshot out the other side.

The other thing I was thinking about during my 30 mile loop run was why I was thinking about doing anything at all to my car! It's a really well balanced sports car in so many ways. It's got plenty of power - and would I really go any faster if I had another 30hp? No I wouldn't.

Just a peak into my mindset right now.

5:30 - time for a top-down cruise.

Greg

 

I did every bolt-on to my '06 RevUp just to see what I could do. Made 281 whp. If money had been a concern, it wouldn't have been worth it. The only suspension mods I had were a front strut tower brace and Hotchkis sway bars. That's it. I had no brake mods (except RBF600 fluid). Friend of mine who used to race open wheel cars said it was one of the best balanced cars he'd ever driven on the track. His daily was a modded S2K.

 

Late to the party…

A few other details that are key with the RevUp -
It does have a different plenum / intake … the plenum airbox is bigger and the ‘trumpets’ are slightly modified to feed more air in at higher RPMs and gain horsepower compared to the DE. This comes at the cost of torque … the RevUp has a lower torque output with the modification to the OEM plenum.

The Motordyne MREV is essentially a DE intake with some very minor modifications.

There are some ECU differences on how the ECU controls the engine. A DE engine will not run on a DE RevUp ECU. The RevUp has wideband o2 sensors (which some of the later model DEs have widebands as well).

There are obviously a few more sensor circuits and output circuits to account for the exhaust variable cam timing but the pinout between the DE and RevUp are very very similar.

nissan did revise the method of exhaust cam timing between the RevUp and HR engines. I have heard 1st and 2nd hand of a few people losing their exhaust cam timing spring (within the cog gear) and it grenades the engine. I often think that Nissan knew of this but couldnt roll out the technology of the HR engine in time for the 2005.5/2006 model years.


Also, the RevUp has more aggressive cams compared to the DE - this also is due to the higher redline and higher hp output.

I think the revup has slightly higher compression ratio as well (but I could be wrong).

I’d also say stick with the OEM filter & intake … oiled intakes are bad, oil comes off the filter and will eventually gum-up the MAF sensor and cause mis-fires…further, you want your engine to ingest the coldest air possible (yes, colder air does contain more o2 per unit but) the value in having the colder intake charge is that the ECU can advance timing more, compared to a hot intake charge and more timing means more torque and more hp.

Im pretty sure your speedometer is based off your rear wheel speed sensors so your speedo shouldnt be negatively effected by a differential change

Nissan doesnt have gear sensors to determine which gear youre in (aside from reverse & neutral). In boosted applications (mostly?? Boost by gear) where an ECU determines the gear of the car, it’s deduced using the engine speed compared to the wheel speed, and an assumed final drive ratio.

 

Hi Beal,

Thanks for the added detail on the Revup VQ vs. standard DE. I never heard about the exhaust cam vvt gear spring causing a problem on these before. Under what conditions could this potentially happen? (Ex: Lack of oil change services, excessive redlining or over-revving the engine?)

Are you sure about the speedo sensor pick-up? I've tried to research this myself and what I was able to find all pointed to a sensor at the transmission, not the rear ABS sensors. I could be wrong, as I know many late model vehicles on the market rely upon the ABS (wheel speed) sensors to calculate the vehicle's speed. But I'm not positive the 350Z falls into that category. I might need to dig out the FSM for this one.
Cheers!
-Icer

 

Is the K&N oem replacement air filter an 'oiled' filter?

Greg

 

If I'm not mistaken, the Z gets its speedometer reading using the 2 sensors at the rear axles outside of the differential. Had to remove and reinstall those when doing my LSD a while back.

 

I was just thinking the same thing.