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Discussion Starter #1
I have a few questions about the design benefits/drawbacks of the D15b6 head (8 valve roller rocker) and the D15z1 head (12-16 valve roller rocker vtec e). If you know about the "D15z7"/3 stage Vtec e head, please throw your 2 cents in. Backstory: I have a cali CRX HF. The rings are shot because I had a valve job done and didn't replace the rings. I know it's a weak argument, but I didn't really have the money for a rebuild. The HF engines are not too plentiful, either. I ended up swapping a D16a6 in it, but I miss the mileage. Now to the meat of the discussion:

1) Since the HF has half of the same size valves in the same location, do you get the same swirl effect as the VX on the intake stroke?

2) The VX has a very small combustion chamber making it difficult to swap on pretty much any other stock block (as far as practicality goes). The dismal chamber requires dished pistons to run 9.3 CR. Are there any advantages to this particular combination as opposed to a conventional piston with valve reliefs and a pentagonal/ somewhat hemispherical combustion chamber?

3) If number 1 is yes and 2 is no, would I come out ahead running an HF engine with a z1 ecu, distributor, oxygen sensor, and egr? I would only be running 8 valves instead of 12. From what I understand I could use my HF injectors with the resistor box, or use vx ones without the box. Please correct me if I'm wrong on this.

4) Would I get any more MPG with the HF head on my D16 or the HF's 1.5? My theory (however flawed) is that the extra torque from the increased stroke and CR would overcome the extra friction that comes with it without costing any more fuel.

I look forward to reading the answers.
 

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Discussion Starter #3
Ultimately, I'm looking for better gas mileage. I was getting 55mpg with my d15b6. I know I can do better.
 

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I've thought about this. To utilize the VX head most effectively, it is meant to be used with VX pistons, as they compliment the funky chamber shape by providing correct valve clearances and maximum quench.

Stick with the HF head. Whack it until it looks like a Y8 head (over 2mm). Clean up the ports. I am pretty sure the Hf heads just didn't drill the valve guide for the second valve per cylinder and side, so, I might actually go so far as to fill the "dead" space with devcon on the intake side.

Re-ring the bottom end, paying attention to maximum efficiency by re-working the oil pump, modding the water pump, and making your own crank scraper. Do some Aero work on the crank. Have the bottom end rebalanced.

Chop, port and the weld the intake manifold, paying particular attention to shaping the mouths of the runners. Open up the throttle body port to 4mm larger than the throttle body.

Actually, I would probably get a D15B7 manifold, portmatch the intake ports to that, and do the same chop/port/weld/throttle body mismatch as I mentioned above.

2.5" cold air intake.

Small primary 4-2-1 header with at least 30" primaries and a good collector leading to a larger than normal exhaust.

And, that should be about it. You'd just have to watch for piston to valve contact, and this build would benefit from a custom cam.
 

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maybe it's just me but simple answer to the me would look for a low mileage VX engine, and swap it in the HF. if your ultimate goal is gas mileage. granted you will have to swap everything over to OBD1 for greatest effect

to answer the question on the whole swirl effect of a HF head vs VX, really don't think the hf head has the capability to swirl the mixture.

whole point of running 2 valves on the intake of the VX head was to allow one valve to barely open below a certain rpm (to prevent gas pooling) and induce a well mixed ratio of air to fuel. above a certain RPM both valves open the same.

3 stage vtec is what it sounds like, below a certain rpm one valve barely opens, between that rpm and a higher rpm they both open the same, above a certain rpm the valves are obviously lifted higher (different lobe)
 

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maybe u should go with a full swap a A6 bottom and a Z6 head with a dx tranny youll get 55-60 at like 2 (x1000) rpms and it will have some jump when u want to have some fun with it
 

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Discussion Starter #10
Excellent! I have gotten other people's attention.

I've thought about this. To utilize the VX head most effectively, it is meant to be used with VX pistons, as they compliment the funky chamber shape by providing correct valve clearances and maximum quench.
I was wondering if this design is better as far as efficiency goes. If you put a VX head on any other bottom end your CR goes to 17+:1. Great for diesel, very bad for 87 octane unless your into the whole grenading thing.

Stick with the HF head. Whack it until it looks like a Y8 head (over 2mm). Clean up the ports. I am pretty sure the Hf heads just didn't drill the valve guide for the second valve per cylinder and side, so, I might actually go so far as to fill the "dead" space with devcon on the intake side.
Define "whack". This may sound stupid, but I'm not sure what you're going for there. As far as filling the dead space, I hadn't thought of that. That's a great idea. Waht is devcon, though?

Re-ring the bottom end, paying attention to maximum efficiency by re-working the oil pump, modding the water pump, and making your own crank scraper. Do some Aero work on the crank. Have the bottom end rebalanced.
The next time I do an engine rebuild, I would like to knife edge and chamfer the crank. I would probably replace the oil pump. What do you mean by modding the water pump?

Chop, port and the weld the intake manifold, paying particular attention to shaping the mouths of the runners. Open up the throttle body port to 4mm larger than the throttle body.

Actually, I would probably get a D15B7 manifold, portmatch the intake ports to that, and do the same chop/port/weld/throttle body mismatch as I mentioned above.

2.5" cold air intake.

Small primary 4-2-1 header with at least 30" primaries and a good collector leading to a larger than normal exhaust.

And, that should be about it. You'd just have to watch for piston to valve contact, and this build would benefit from a custom cam.
Port matching will not help because I only have one intake valve per cylinder. It can only flow so much. If I was going to go through the trouble of chopping and welding the intake manifold, I would probably make a custom one similar to a Corolla (VE I believe). It looks like a header. Equal length runners into a throttle body. No plenum.
Cold air intakes are tricky because the throttle body is so small. I would have to find an adapter. Does somebody really make cams for an HF?
 

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Discussion Starter #11
maybe it's just me but simple answer to the me would look for a low mileage VX engine, and swap it in the HF. if your ultimate goal is gas mileage. granted you will have to swap everything over to OBD1 for greatest effect
I could just swap the ecu, distributor, and O2 sensor and get the same thing. All electronic gadgetry aside, I was wondering which engine is mechanically more efficient and why.

to answer the question on the whole swirl effect of a HF head vs VX, really don't think the hf head has the capability to swirl the mixture.

whole point of running 2 valves on the intake of the VX head was to allow one valve to barely open below a certain rpm and induce a well mixed ratio of air to fuel. above a certain RPM both valves open the same.

3 stage vtec is what it sounds like, below a certain rpm one valve barely opens, between that rpm and a higher rpm they both open the same, above a certain rpm the valves are obviously lifted higher (different lobe)
Actually, one valve barely opens to keep gas from pooling there. The_Acid_Beaver's solution of filling the dead space in my intake ports would probably solve that in mine. I doubt I have that much of a problem, but it would streamline the air flow. I thought that the point of opening one valve was to "swirl" the mixture into the cylinder. One side benefit is opening one valve saves frictional drag on the cam. The HF has one intake valve on the pulley side of each cylinder. The vx opens mainly the intake valve on the transmission side. Does anyone know if that makes a difference. One part of my brain says it just swirls in the opposite direction. The other part hurts trying to figure it out.
 

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actually no you wont. different motors, different maps

still i would have to say the d15z1 even though you have the extra drag on the cam, becuase of the electronics associated with the engine

rationale would say it would just swirl in from that side and not make a difference, but of course you might have to take into account head and piston design

should have worded that different i guess, point being above the rpm range it acts like a normal 16 valve engine to allow the engine to breath better
 

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Discussion Starter #13
maybe u should go with a full swap a A6 bottom and a Z6 head with a dx tranny youll get 55-60 at like 2 (x1000) rpms and it will have some jump when u want to have some fun with it
I have an Si tranny, but I'm not gonna use it. I could care less about power and fun. My wife drives a SE-R Spec V and I'm putting an old big block Ford truck together. There's my fun. My main priority is getting MPG with either parts I have or parts I can somewhat easily attain. I'm halfass looking for a 49 state HF trans, so I'm not even considering a DX.

Ya know you CAN have the best of both worlds. You don't have to sacrifice hp by running the HF head just to get good MPGs. You can, with the correct tune, make great power and over 50mpg with your run of the mill a6/z6/y8/etc head
I'm guessing for the same price I could swap a d15z1. I don't want anyone to assume that I'm opposed to buying a z1 swap. I've had trouble finding a reasonable price(I'm talking $600 just for an engine). People hear that it's a Vtec and think that an arm and a leg sounds about right. I'm wanting to find out if I use the same electronics as a z1 on an HF head would it be better than a z1 engine.
 

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Discussion Starter #14
To qweakers: You said different engine, different maps. I was talking about swapping a vx ecu dizzy and O2 on an HF engine. (It sounds like you misunderstood but I might be crazy). I was hoping (long shot) that someone could explain the difference in piston and head design. As far as I can tell, Honda only did this for the z1. There has to be a reason. Everyone mentions the "swirl" when talking about the z1, I kind of assumed the HF did the same thing.
 

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the engines are designed differently and would use different maps in the ECU.

looks to me like the head and piston combo was designed for the most tightest quench they could get. if that makes sense. from what i remember most engines do swirl the mixture in the chamber to create and ideal burn mix. so the HF included should.
 

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Discussion Starter #16
An electronics swap would work though, right?
 

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nope, soon as it does it's system check and realizes there are some system components missing, it will flag the ecu and more then likely go into limp mode. even if it did run it would be less then idea and more then likely effect MPG

two options for sake of easiness

rebuild the HF motor or find a complete d15z1 swap, there have got to be some people around that realize that the motor is a vtec-e and not just straight vtec and could get it for a good price.
 

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whole point of running 2 valves on the intake of the VX head was to allow one valve to barely open below a certain rpm and induce a well mixed ratio of air to fuel. above a certain RPM both valves open the same.
The whole point of cracking one valve open was to prevent fuel from pooling.
 

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we already covered that.
 

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What would Bone do?????

D15B7 short block, .060" Cometic HG, Z1 head, 12:1 on 87 octane.

D15Z1 long block, .060" HG, 01-03 GX pistons, 13:1 on 93 octane

many ideas to be tested.......or should I say REtested :)
 
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