K3NnY_G

Hey all, got an EG8 the other week, and while I don't plan on doing much of anything to the stock engine it has (as I'm well aware it's an economy engine, and hardly responsive to bolt-ons) I do however want to do one thing. My brother bought a randomly modded si years ago, the car ended up in a friends scrap yard and I still have access to the remaining parts; on that car is a D15B Vtec, intention is to take the top-end for myself.

Now, I've done my research, I know I need D16 wires, VX timing belt, P08 ECU etc. etc., I'm not here to get told to use the forum search.

I'm here to ask this one simple question I couldn't get a clear answer to in some excessive searching:

Will swapping to the Vtec head have any genuine negative effect on fuel economy? My brother keeps going on this will be the issue, I personally don't see it being a problem.

Thanks in advance and feel free to post your reasoning for your response, I'm relatively new to all this and always looking to learn!

Caoboy

Why not just take the whole engine and throw it in your car? It seems a waste to try to mix and match something to end up with a 'stock' engine.

Also, economy is based more on your driving habits than swapping the head, in this case.

It's not like all of a sudden you're ending up with a turbo engine that is guzzling fuel for breakfast.

TomCat39

Just taking the head is asking for folly.

The JDM D15B vtec has the same 137mm long rods to safely allow the 7200 rpm as the Z6.

Your B7 has 134mm short rods that is only reliable up to 6500 rpm or so.

Also the B7 is using a flat top piston with valve reliefs while the D15B vtec has a dish piston which pulls in more volume.

The fuel economy will be very much the same IF you keep your foot out of the pedal, the fuel economy will go much lower if you hot rod it consistently in comparison to hot rodding the D15B7.

It's the same displacement so if you drive it economically it will be almost identical in fuel economy.

This is also assuming using the respective ECU for the JDM D15B. Using your P06 on the D15B vtec just won't ever see the added power and might drop a touch of economy due to the longer rod and dish piston.

If you however chip and tune your P06 you can get vtec, and get the most of the motor.

Now lets talk about your proposed direction. Just slapping the vtec head on the b7 block with it's shorter rods and flat top pistons. Assuming you chip your ecu and wire vtec etc.... You just might kiss the pistons to the valves when vtec engages due to the higher lift and the valve reliefs are designed to the non vtec valve train and lift.

The other area is you will be in the 7000 rpm range having a ball and suddenly one of the pencil thin B7 rods snaps and goes through the block or worse.

Take the full long block, even the Intake manifold is designed for the extra demand of the vtec motor.

Of all the SOHC vtec's the JDM D15B is the strongest of them all in stock formations.

In short, purely to answer your main question, it depends on if you keep your foot out of the pedal, if yes, then yes same economy, if no, then definitely not the same economy.

K3NnY_G

Thank you very much for this post, extremely informative; I definitely am no longer keen on my original intentions, gonna give the Vtec engine a good once over in the car and see about the shape it's in. Main reason I was just planning to swap the head is because the engine in my EG8 is completely stock and runs like a top.

Thanks again for the posts, both of you.

Cheers!

tony_2018

Economy didn't lose...you did.

K3NnY_G

Being Canadian I feel this to be an adequate response:

Eh?

94EG8

Eh, lots of guys ran this setup 15 years ago. Z6/D15B vtec head on a B7 block, with a stock P28, they're reasonably reliable. There's only a 400RPM difference in the redline, small enough that it's still most likely within Honda's margin for error.

That said I'd still swap the whole engine.

As far as the mileage goes my mileage either never changed or got better, it's been 7 or 8 years now so I really can't remember. My mileage did drop however when I swapped in an EX/Si tranny by a noticeable amount.

TomCat39

Mileage should be totally dependent on driving style. If both motors are driven mellowly there shouldn't be any difference. However redlining it most of the time, there should be being the B7 isn't tuned for wild cam lobe so wouldn't push as much gas even in the top range over a vtec motor. The wild cam lobe breaths more which in turn needs more fuel. That's why I pointed towards the foot habits of the driver.

The tranny would affect it no matter what the foot habits are.

I don't know what the rev limit is on the vtec motors. I know redline is 6500 with rev limit of 6800 on the B7. If rev limit is only 7200 on the vtec then I can see only the 400 rpm difference. I was under the impression redline was 7200 so rev limit is 7500 or 7700 and figured might be a bit more than the B7 rods would safely operate for extended periods.

But also, even at the rev limit of 6800 on the B7 the peak power was had at 5200 rpm with X amount of fuel and burn and pressure etc etc. With the vtec motor there is going to be higher pressures so even more stresses as more fuel and air and stronger burn etc etc from the wild cam and higher power output. So not only are you putting more strain on the setup from the higher rpm but you are also adding more strain from the higher power output. It will work, but for how long? How many times can you stress it to the max before it goes pop?

But then again, I also assume a person will push their motor as I tended to do the first year or two of getting SRI and Exhaust. I spent a lot of time in the top ranges of the motor most of the time. I'm sure if I did that with a standard mini setup, I'd have snapped a rod by now if I didn't at least shot peen the rods. But I also have some serious hills I climb to really test out the motor capabilities in 2nd gear. And I don't fear hitting the rev limiter like some people do.

Maybe I should have indicated that it can potentially shorten the life of the motor but might not as people have run it without issues for years. I just wouldn't likely be one of them.

One thing that is good to know is that the valve reliefs of the PM3 (B7) pistons are deep enough for the vtec wild lobe. I wasn't too sure on that even though people have done mini-me's for ages without issue. I obviously had a brain fart there.