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Old 04-13-2009, 06:13 PM   #1
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Honda VX differences?

OK, so here's the deal (sorry for the long intro); A friend of mine has the spare head off his '92 Honda civic CX. I think it was a D16Z6 if I'm not mistaken. He's offered me the head for free, and it should be a bolt on for my '88 CRX with a D15.

I wouldn't mind upgrading the EFI to MPFI anyway, and it would be an opportune time to add the newer v-tech head.

My question is about what mechanical differences the VX engines had and if I could get away with running a VX ecu or VX program in a burned ECU on a CX head? Would I need to change cams? Or are there other differences as well?

Also, I'll probably be keeping the stock DX transmission I have, but running a little taller tires. My goal is to be able to cruise on the highway at 75 at around 3100 rpm (which should be doable with just tires). Does anybody know what rpm the VX would run at that speed and wether I'm in the ballpark for the right rpm to optomize the engine performance on a hybrid VX engine?
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Old 04-13-2009, 06:51 PM   #2
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The D15B8 engine in the CX is 8 valve and is fairly close in fuel economy to the VX except that the CX is underpowered at 70 HP.

You probably wouldn't be able to or really want to use a VX ecu in a CX engine.

The VX D15Z1 has the advantage of VTEC oomph (90 HP) when needed and sometimes it is. My VX can achieve 50+ mpg but this usually means driving at 60 mph or so. I only get about 43 mpg at 70 mph in my VX currently.

I would probably get more mpg if all four tires had the same tread pattern and if they had lower rolling resistance. I have Kumho Power Star 758 and Toyo Spectrum tires on it now. The Kumho tires are quiet but have been wearing down quickly. The Toyo tires are noisy but have twice as many miles on them and about the same remaining tread depth.
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Old 04-13-2009, 08:29 PM   #3
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cx vx or ex... which is it?? i dont know that adding v-tec (note absence of 'h') will solve any of your problems if it is froma vx or ex. the cx head from the d15b8 is extremely similar to the heads from its older brother's 8 valve heads, so again not much of a gain there either.

the only solid gain in mpg will be from the trans

i would just swap the whole motor. forget pulling the head. especially if you plan on keeping your trans. and its the trans and tires that dictate your cruising rpms... so it looks like you have already made up your mind there.

the only reason i would go through with putting the head on is to learn on your own. the other benefits negligible compared to the experience you will gain.
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Old 04-15-2009, 08:04 AM   #4
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OK, so maybe it's not a CX.... He got rid of the car two years ago and I don't remember much about it... It is a 16V head, and it is off a V-tech engine, and supposedly it was a D16 not a D15. I'm a little tentative on this, but have been told that a D16 head will work on a D15 with no problems (though compression is a question I haven't looked at yet).

I'm not interested in doing a full engine swap ($800 car, I just can't justify the cost of doing the swap. I can get the head for free though), but the DPFI on the stock engine has been giving me problems long enough that I wouldn't mind replacing it. If I'm already doing an efi/computer swap, then upgrading to a V-tech head at the same time wouldn't be too much extra work. I just need to figure out if 1) I could make a bastard VX (or VX-lite) and get decent results, or if I'd be better off sticking with another ecu/program. All in all, it may still never happen. I'm just exploring my options at this point.
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Old 04-15-2009, 09:11 AM   #5
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trust me, an engine swap can take just as much time as a head swap... sometimes less...

anyway i am pretty sure that the head came from a ex or si and is a d16z6 and the vx should have something like vtec-e or whatever. what you are talking about doing is what they call a "mini-me" conversion as the idea of putting a vtec head on a non-vtec block was originally done with the honda b series engines and while the b series head swaps would net well over a 30 hp increase. the d-series head swaps do not give nearly as much and since the heads are single cam, they are "mini".

here are some good reads on the proceedure specifically for the older civics:

http://crx.honda-perf.org/articles/z6swap/z6swap.html

http://crx.honda-perf.org/articles/m...oreminime.html
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Old 04-17-2009, 08:01 AM   #6
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Yeah, I'm familiar with the mini-me swaps. Just wasn't using it since this isn't a tuner site. I figured some people might not be familiar with it.

Anyway, I understand that an engine swap might be less trouble. But I don't have a free engine sitting around. I DO have a free head available though. And I'm really not worried about the performance increase. If I can do the swap and squeeze a little more fuel efficiency out of my car by adding a few free parts, then it's a worthwhile endeavor. Since the VX is a newer, v-tec engine, I'd have to swap all the efi gear and wiring anyway, and since I'm considering an efi swap to get rid of the ratty old DPFI, and I have a V-tec head available, I thought I might just be a few components from a substantial upgrade in terms of efficiency, but I need a few "experts" to chime in.

So, back to my original question: Does the VX engine have any major mechanical differences that would prevent me from building a bastard version from an EX head and my stock D15 engine? Or are the differences more than just computer programming differences and cams? If the engine has substantial differences internally, then this idea is a non-starter. I just want to know if it's even in the realm of possibility.
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Old 04-17-2009, 06:46 PM   #7
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i know the vx head operates completely different from the ex. the vx vtec does not open one of the valves or if it does it only opens one of them a little, the ex opens both valves more for vtec and is generally more performance oriented.

i know the cams are way different too, but to be honest i dont know that the vtec head will give you any increase in fuel economy. i would think just the switch to mpfi might, but i have heard from a few people that they were unable to get better fuel economy getting the mpfi on a motor that was originally dpfi.
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Old 04-19-2009, 09:37 AM   #8
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To answer your question, no you can't feasibly make a vtec-e out of a vtec head. Like notjared said, the vtec-e essentially keeps one intake valve closed until about 2000rpm. Vtec opens both intake valves until about 5000rpm, then opens them more. It isn't just ecu and cam differences. On the other hand, like a friend of mine used to say, nothing is impossible - some things are expensive.
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Old 04-21-2009, 08:22 AM   #9
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That's exactly the info I was looking for guys. Thanks.

My reason for doing the MPFI swap is more for the sake of getting away from an increasingly unreliable fuel injection system. I just spent like two months getting it running right. The next time it goes south, I'm just going to rip it out and replace it with MPFI. I honestly don't expect any additional fuel economy out of the swap. It could actually lower my mpg as a matter of fact since it'll yield more hp.

Like I said, I was just thinking that as long as I'm tearing the intake off.... Maybe I'll still do the V-tec, maybe not. It'll all depend on my mood when I pull the trigger on the swap.
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Old 04-21-2009, 01:41 PM   #10
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Quote:
Originally Posted by Hal9000 View Post
It could actually lower my mpg as a matter of fact since it'll yield more hp.
mpg is NOT always inversely proportional to hp.

case in point: 99 civic hx makes more power AND gets mpg than a 99 civic dx
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