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]

Originally Posted by PaleMelanesian

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]

Originally Posted by 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

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]

Originally Posted by PaleMelanesian

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.

[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!!