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K2e2vin 01-18-2010 12:58 AM

D15Z1 build suggestions
Ok, I'm new to the economy thing but have been wanting to build a car that does +75mpg hwy for a while. My current 88 Civic did 57mpg with a cammed D15B VTEC and a VERY short transmission(70mph at 4000rpm). Anyways, I had owned two 88 CRX DXs prior and now I regret scrapping them.

Anyways, I got a hold of a D15Z1 that needs to be rebuild. Previous owner says valve seals were leaking so he had it replaced, changed it, and now it leaks more. He thinks it may be incorrect seals since the parts lady gave him ones that all look the same(little does he know the exhaust and intake use the same size). I think it's either the rings or PCV valve.

Anyways, I plan on swapping in TL1000RR or GSXR750 injectors and have them pointed towards the "primary valve" or the valve that stays open during VTEC-E mode. The injectors are 4-hole type and flow about 270cc at 3bar IIRC. I also have a 7lb flywheel currently on my D15B VTEC that I'll swap over(being replaced with even lighter multi-disc clutch setup) One thing I'm thinking about doing is swapping the pistons from my D15B VTEC and throwing them into the D15Z1. They're slightly dished and will bump compression up to about 11.5:1. The problem with the P08(D15B) pistons is they're not designed to promote swirl like the P07 pistons. Another thing I'm looking to do is extend the runners on my aftermarket header(Z1R header) or would it be better to build a custom one(maybe runners are to large in diameter?)

Would it be more beneficial to have a higher compression ratio that possibly results in higher cylinder pressures and a more complete burn vs. a swirl-flow setup that promotes the ability to run leaner? Any other suggestions to build this "beast" D15Z1?

PS: The stock ECU is ditched and replaced with a chipped P28 or P06 running Chrome along with a Innovate LC-1 wideband O2 sensor instead of the stock one.

btw, I don't have a chassis to put it in but looking for a CRX HF.

Thanks, Kevin

rkcarguy 01-20-2010 12:35 PM

Depending on your budget, maybe look at going with a low mile japanese Vtec-E or 3-stage vtec motor and for-go messing with the rebuild?
Too bad your east coast, local guy has a 3-stage for sale here in town.
Good call on the HF. Get a 88-89 model to get the lightest ones. The DX is also very close, and you can remove and swap some parts to get to the HF weight.

K2e2vin 01-20-2010 05:44 PM

Not interested in a 3-stage. I don't have any need any high-rpm usage. Plus, I bought the Z1 specifically to rebuild and attempt to make it better. For $100, it's a hell of a deal.

greasemonkee 01-24-2010 12:50 PM

I've toyed with the very ideas you have for some time. To really get an answer I suppose one would need to test themselves, which we would expect to be a long term experiment - stock vs, different internals ect.

I considered the 3 stage vtec-e myself, but for the extra cost it was hard to justify. I now have a jdm low mileage (regular) vtec-e on the floor which I purchased for $565 shipped. The problem with rebuilding those engines is that by the time you get new valveguides, which almost always are well past the service limit on high milage hondas, buy the new internals and go through the complete rebuild you could have 2 fairly fresh jdm engines for the same price.

The solution I've been considering is to go turbo with the setup, a very small turbo and non-intercooled, in hopes of reducing pumping losses and operating at leaner AFR's while picking up a bit of power (within reason). I am anxious to see if these theories are true about the turbo/economy benefits.

I believe I would investigate milling the head before changing pistons, the valve reliefs seem to be in an unorthodox location with the z1's. And for your success, I'd recommend getting a real engine management program that you can actually do something with, I'm sure the fuel savings would pay for it.

K2e2vin 01-27-2010 07:36 PM

I'm against the idea of milling the Z1 head; it already looks pretty shallow along with a unique quench pad design that compliments the piston and looks to promote swirl, yet the piston has a fairly large dish. Aim here was just to improve the combustion/power cycle.

I haven't had time to inspect the head myself, but if the valve guides are the problem I can get it done pretty cheap;(I paid $100 for the motor and transmission btw). I hope it's the rings that are bad though(maybe even just be the PCV!). The internals are sourced from my D15B VTEC, so only items that would need to be bought are bearings(if out of spec), seals, rings, etc. My goal was to have something better than a OE engine.

Crome should be fine for my needs. I'm not doing anything special so no need to get something more powerful than what the stock ECU is capable of. My D15B VTEC, on the other hand, is waiting on Megasquirt-III/MS3X. Surprisingly, the MS system looks to fit my needs better than the +$1000 ECU systems out there(yes, it'll make use of many advanced features). ;) Only reason I'd switch ECU is for flex fuel though; in which case I'd probably go with Megasquirt also.

MetroMPG 01-27-2010 08:48 PM


Originally Posted by K2e2vin (Post 154937)
Ok, I'm new to the economy thing but have been wanting to build a car that does +75mpg hwy for a while. My current 88 Civic did 57mpg

"hwy"... that covers a wide range of possibilities. Are you talking cruising at 55 mph? 70?

How easily you can meet that goal will depend a lot on your "hwy" definition!

K2e2vin 01-27-2010 09:02 PM

70mph on the interstate!

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