VX Swap..........3000rpm @ 60mph

[dj1000]

Well, I finished my JDM D15B VX swap into my 95 DX. I was pretty excited to finally start driving it, until my first couple fill ups. I pretty much spend 90% of my daily commute on the freeway, so I expected to see some pretty good results seeing as my 99 Saturn SL2 consistently got 44-45mpg. Well, I am only getting about 39-40mpg with my VX. Started checking over everything and I know there are a few things I used from my DX engine, but didn't think it should have made that big of a deal. I am using my stock air box and rubber intake tube which I thought I read is a little larger than the VX's. I am also using the stock DX exhaust manifold, due to not wanting to change the new exhaust and cat that were installed previously. SO, after doing some reading on here my issue has to be the fact that the S20 trans that I have can't be geared correct. I have been reading and most of the VX's on here are around 2000rpm @ 60-65mph. I can't maintain lean burn at 65 or 70 because I am well over 3000rpm.

For the VXperts, does this sound like a correct guess as to where my lack of fuel efficiency is at???

Thanks for all the help so far, I couldn't have finished my swap without a lot of reading on this site.

Dave

[dj1000]

Should have added that I have 14" HX wheels with 185/65/14 LRR tires 42 psi, so that can't have a 1200rpm adjustment.

[suspendedhatch]

Those things you mentioned will easily account for that 4 or 5 mpg deficit. The smaller VX intake tube increases airflow at the very low RPM the VX was designed to cruise at. Same goes for the VX exhaust manifold to a lesser extent. And the VX transmission's gearing is absolutely critical to proper operation of lean burn mode and the low RPM that gives the VX it's incredible mileage.

I think your mileage is very good for what you've done. Besides the parts you mentioned, did you get the VX rear diffuser and front lip spoiler? Is the EGR functional?

Keep in mind that the techniques that you used on your Saturn will not be as effective on the VX. Most people are getting in the mid 40's with the VX. With your low mileage engine; once you've acquired those remaining items I'd shoot for 50+.

[PaleMelanesian]

I think the gearing is 90% of your problem.

[R.I.D.E.]

My VX tachs 1000 at 30 MPH with a speedo adjustment up 1% due to the tires.

3000 would be 90 MPH.

I think mine looses lean burn at about 75 MPH, right at 2500.

gary

[dj1000]

I think the gearing is 90% of your problem.


That is my thought. I see too many running after market exhaust manifolds that I figured it can't make that big of a difference. The fact that I can't keep the rpm's low enough for lean burn mode is were my problem has to be.

[dj1000]

My VX tachs 1000 at 30 MPH with a speedo adjustment up 1% due to the tires.

3000 would be 90 MPH.

I think mine looses lean burn at about 75 MPH, right at 2500.

gary

Yeah, something is really off with my gearing. Has anyone else with a JDM swap run into this problem??

I am searching right now for a VX trans to swap out and test the results.

[dkjones96]

Is it an automatic or a manual transmission? If automatic, are you sure it's going into overdrive?

Over 3000 at 65 is high for anything but small displacement motorcycles. Even my mom's 3 speed auto neon used to turn 2750 at 65. It didn't have overdrive but the final drive on it was shorter so it was able to turn low without it. That car was such a dog.

[nowhhs]

I think the gearing is 90% of your problem.

I would say closer to 98%. I did the same swap into a 90 hatch, using the stock DX air cleaner housing and tube, as well as the DX exhaust manifold and am averaging 48mpg without trying too hard. On the other hand, I'm turning 2200 at 70mph with the HF transmission. Air velocity before the throttle is irrelevant, whats important is the velocity between the throttle and the intake valves, so the intake manifold can have a huge effect, but not the air cleaner housing. As for the exhaust, I think the worst part is that the O2 sensor is farther away from the valves, so reaction time might be slightly longer, but otherwise it shouldn't matter much if at all. Find the right transmission and you'll be fine.:thumbup:

[dj1000]

Thanks, I am glad my thinking of the wrong gears (5 speed trans) were off. I know my casing says S20 on the id tag towards the front. I read in another post to look for a J** type number for a correctly geared VX trans. Anyone with a VX transmission that can confirm this?

Thanks again for all the advice!!

[theholycow]

I've been watching this thread because I'm interested in gears ratios and how they affect FE and I don't like it when a transmission doesn't have tall enough gears, as yours doesn't. I haven't posted because I don't have anything productive to add, but I do have a couple questions on my mind...
- Did the DX come geared like that from the factory, or do you think that was a modification by a previous owner?
- ......drat, I forgot it while writing the above question... :(

Ok, so I forgot my second question, but in the process I did think up something that might help you: Keep in mind also that the transmission might have the right gears but the differential's ratio could have been changed. I suspect that the differential gears are more common to change than the transmission, as in the non-Honda world I usually inhabit.

[dkjones96]

I suspect that the differential gears are more common to change than the transmission, as in the non-Honda world I usually inhabit.

I don't know about Honda specifically but I'm a lover of RWD and in that case the differential gears are always easier to change out than the transmission. You drop the drive shaft and that's all you need to separate the diff from the rest of the driveline. And I've never seen a RWD differential that even needs the rear end taken out to change gears. It can be done in the car.

In the case of FWD I'd figure that you are looking at the same amount of work to do either one. I've seen Toyota transmissions and, for the ones i've seen anyways, there is no way you are going to change out that diff without at least unbolting the transmission from the engine and pulling out it to get to it.

[itjstagame]

Hey, I was thinking about changing to a taller gearing in my pickup and figured changing the differential would be easiest. Do you know where to look for good, used diff gears and what size will link up and fit? I'd really, really prefer to find someway to cob up a 2 speed rear so my 4x4 will still be operable, but I bet that's out of the question, huh?

Acutally I guess it should be just as easy to change my front diff gearing too. I'm not even sure what my stock gearing is, it's certainly not too bad, but it can definately be lower, I don't need so much acceleration and if I need lower range for 4x4 I still have low 4x4 setting on my transfer case.

[theholycow]

A GM full size truck forum can tell you exactly what to get for your truck (I like gmfullsize.com). You might find someone who would be willing to trade straight up, since lots of truck owners still would prefer lower gears.

[SCoupe]

Well, I finished my JDM D15B VX swap into my 95 DX. I am only getting about 39-40mpg with my VX. SO, after doing some reading on here my issue has to be the fact that the S20 trans that I have can't be geared correct. I have been reading and most of the VX's on here are around 2000rpm @ 60-65mph. I can't maintain lean burn at 65 or 70 because I am well over 3000rpm.

For the VXperts, does this sound like a correct guess as to where my lack of fuel efficiency is at??Dave

Dave,
You are correct about suspecting the RPM levels are too high. You're seeing nearly 50% higher revs than the VX transmission would deliver to you. I was unclear in reading your post if the S20 trans is from your existing DX. If so, it has much lower gearing than the VX, so there's the problem. If not and you retained the tranny came with your JDM engine then who knows...all bets are on that it came with something less than optimal.

To a much lesser degree, those wheels/rim combo are doing you no favors either as they are too large for the VX.

My VX gets ~42 miles per gallons with 85% hwy, a number that I am not too please about given way its driven and the all stock drivetrain. I've got a gremlin to be deterimed, its a stubbling, gutless, torqueless wonder under 2k.

[dj1000]

Thanks for all the great comments. I could only wish fixing my problem was as easy as changing out rear end gears in a RWD vehicle. I have found a VX transmission local and am thinking about rebuilding it with new gears (has over 200k on it) and then swapping mine out.

The S20 transmission that I installed came with my JDM engine, so I am not 100% sure what gearing is in there. My DX was orginally an auto, so I sold that trans a while back. Kind of a fun swap all in all.

As far as the wheel and tire combo, I have seen plenty with the 14" HX rim setup getting good mileage compared to the original 13" VX rims. If I am currently getting 40mpg with the transmission issue I am having, switching to the correct gearing "should" put me in the magic 50ish area. I can only hope.

Overall the swap runs excellent. Lean burn is really a cool feature, it feels like the engine just shuts off. I am very happy the way everything turned out and will be even happier when I fix my gearing issue. I am also glad to hear that my intake tube and header won't make that much of a difference.

Thanks again!!

Dave

[itjstagame]

Yeah I don't see how larger tires could ever hurt you. I mean as long as you have sufficient power to accelerate and keep you happy then how can a lower RPM be worse?

Cow: I can't seem to register on those forums but they look good. I just looked it up and apparently I have the 3.73:1 gears, that suprises me but probably explains why it has so much get up and go. Not sure it'd be worth the hassle for the ~9% gain, but the reason it crossed my mind was because I've been noticing drippings from the rear diff and I'm sure it could use a flush, fill, seal anyway.

Any ideas how such a change would affect my torque converter lockup? Right now it locks at about 43-45, but I'm not sure if that's rpm based or some kind of pressure (rpm related). I mean if it's VSS based I don't want to be locking up at speedo 45 which will actually be 49 now.

[theholycow]

3.73 is probably the most common ratio that GM puts in their full size trucks. 4.10 is optional for those who would sacrifice FE for power, and I don't know if they usually offer a taller (lower-numbered) ratio for those less interested in going fast.

I suspect it would increase the speed at which you lockup. I think lockup is controlled by engine load and RPM, similar to how gears are selected. My truck uses a PWM signal from the PCM to lock the TC, but if you're lucky then yours uses a plain 12 volts, which would make it very easy to hack in a TC switch that allows you to take control and manually lock/unlock it.

[StorminMatt]

Here's something else to keep in mind. From what I have heard, 3000RPM is sufficiently high to engage VTEC on a D15Z1.

[theholycow]

3.73 is probably the most common ratio that GM puts in [...] and I don't know if they usually offer a taller (lower-numbered) ratio

I was bored last night and looked at the GM truck forum and saw that indeed, there are 3.42 and 3.23 ratios available factory equipment, at least in some years and models.

[dj1000]

Here's something else to keep in mind. From what I have heard, 3000RPM is sufficiently high to engage VTEC on a D15Z1.

That is my fear, that I am bouncing in and out of VTEC......:(