Honda, Higher compression = better mpg?
 

Hey guys I drive a Honda Civic with a D16z6 sohc vtec engine with ~9.2:1 compression ratio. I have read a few times in a few differant places that higher compression ratio can lead to better mpg, as long as you still drive "correctly".

I am feeling as thought my stock engine needs a rebuild to remain efficient and was wondering about the compression=mpg saying that Ive heard. Would a rebuilt 10.5:1 D16 allow for better mpg than a 9.2:1 D16 if correct driving methods were retained? Also, would a slightly larger cam help as well? Being vtec, the engine runs on "small" cam lobes for cruising, but has "larger" cam lobes for wot/passing situations.

I am looking to rebuild my engine anyways and looking for slightly better mpg, but also slightly more passing power so,
Will slightly higher compression help with mpg?
Will a slightly larger cam help aswell?
Anything else I can do while rebuilding to help with mpg?

i dont think the cam will help with mileage. ive heard the A6 cam is the best (smallest?) for mileage.

as for the compression ive heard the same as you have, that higher compression will help. only real way to tell with your individual vehicle is to do it and see what happens. do as much recording of your current mileage as you can and either do the head gasket or rebuild the entire thing. you can get all different thickness head gaskets and you can plug the numbers into a d series compression calculator. i did a d15b7 with a z6 head gasket and shaved the head and came up with about 10:1 compression. i couldnt do a comparison because i got the car with a bad gasket. but i averaged 42-43 normal driving and highest tank i ever got was 50 mpg while EOC. i was using regular fuel too. i was happy with it. but now im doing a d15z1 swap

As long as you don't use the cheapest, lower-octane fuel which would lead it to knocking, raising the compression helps the MPG.

Depending on how you drive, disabling or avoid getting the vtec to kick in would be a start.

The newer Insight uses like a 11 to 1 compression with its fancy 2 plug per cylinder spark plug setup. It doesnt have the torque Id expect with such a high cr, all the power is around 5 grand unless you got the IMA system enabled and a full charge.:turtle:

Increasing compression might increase fuel economy. I say might because its not a simple yes/no answer. In an ideal engine with no other changes, increasing compression will increase power output and efficiency. However, there is no such thing as an ideal engine in the real world. Increasing compression has effects on preigintion and just ignition timing in general. Usually, with high compression you need to retard timing at higher engine loads so you can avoid knock/preigintion. This reduces efficiency. IMO Honda makes great engines and I'd be a bit surprised if you could eek out a noticable gain.

A shorter duration cam would give you more lower rpm power while sacrificing high rpm power. Your vtec helps negate this though. This also will return better mileage, and this can be a sizable gain. The Metro guys with XFI cams see a fairly decent gain.

IMO leave the engine alone. Do a compression test or leak down to see if you really need a rebuild. "Feeling" you need to rebuild an engine is just not possible. There are tests you can and should do to be sure.

Higher compression ratio is more complicated than it seems. First of all in the ideal thermodynamic cycle one assumes constant heat ratio, but the more you compress the intake air the hotter it gets and the more the heat ratio drops. This is why in theory it's useless to increase compression ratio on spark ignition engines past a certain point; the temperatures get too high and the heat ratio drops.

Now for the practical side: You have to remember that higher pressures increase friction (particularly, at the rings). Additionally, reducing combustion chamber volume can cause a decrease in burn efficiency, and the higher burn temperatures result in greater rejection of heat to the cylinder walls and piston.

On the other hand, higher compression ratio given the same piston/rod geometry means more time is spent at lower chamber volume and thus higher pressures and so more of the charge burns at higher pressure. This effectively makes the exhaust valve act as if it were opening later, since more work is extracted on the expansion stroke. This effect gets more and more precious as the revs climb, because you have less and less time for the charge to burn. The "better solution" is to just increase the expansion ratio a la Atkinson cycle but this obviously decreases specific power, and you lose mechanical efficiency as a result.

Empirical data suggests that most of the time the benefits outweigh the drawbacks, until you get to the 13-14 :1 area, where careful attention has to be paid to combustion chamber shape, interference with in cylinder flow, etc. or else you can actually lose power/efficiency.

Mazda is increasing the compression in their petrol Sky-Active engines.

And reducing it in the Diesel Sky-Active engines ;)

In the Diesels it might be intended to reduce the NOx emissions, altough sacrifices some fuel-efficiency :mad:

Also the <s>Sky-activ</s>Ecoboost engines are cast iron so they will hold heat longer. At least last I read unless they decided to change and go the weight saving route.

What? I've never heard of the Skyactiv engines being cast iron anything... I'm pretty sure they're all aluminum.

The gassers are all-aluminum, but nodular iron is still prevalent for Diesels.

Oh, you were talking about diesel. Over here in the US we don't have many of those :P

Yea the Skyactiv diesels are aluminum to save weight (probably doesn't hurt that cooling is less of an issue too), and the reduced compression ratio reduces the strength requirement.

Since it leads to an easier cooling it also reduces the NOx amount. A win-win deal...

Back in the bad when I worked at a machine shop it was common to stroke an engine. THis is grinding off the back side of the crank journal so the rod move up and down more in the cylinder. Then again these were engines made right after the non leaded gas revolution so their compression ratio was like 8 to 1 and we bumped it up to 9.x to 1 or as much as 11 to one. Makes a hell of a different in torque, acceleration and the heat an engine will produce.

For the smaller "rice burners" they just decked the head a few thousands.

Well the d16z6 in my car has a lot of miles and I was wanting to rebuild the engine regardless, just trying to see if a little increase in compression could help with fuel economy while cruising. I guess Ill just rebuild it at around 10-10.5:1 (compared to the stock 9-9.5:1) and just see

How do you know the engine needs a rebuild? As others have pointed out, specific tests must be performed to determine a rebuild is necessary. How many miles are on the motor? I've had a Subaru 2.2L go nearly a quarter million miles, and it was still running fine when it was rear-ended and totaled.

I applaud your "build it and see for myself" attitude, but unless you have plenty of money to burn I would find out more before jumping into the project. That said, please report your findings here if you do end up doing the rebuild. Your results whether positive or negative are valuable information to share with everyone.

You were right the Skyactiv are aluminum I have corrected my post to reflect so.

The engine I was thinking was the Ford Ecoboost 1L:
"The engine block is cast iron instead of aluminium for up to 50% faster warm-up, at the expense of additional weight." Ford EcoBoost engine - Wikipedia, the free encyclopedia

I've heard raising compression helps efficiency as well. On the '93 DX/VX I had the head on my d15z1 shaved and got a 3-angle valve job at the same time (had the car apart anyways).

Unfortunately I did a lot of other upgrades at the same time so no before/after mpg results, but the seat-of-the-pants felt nice after - was very happy with its ability to stay in lean-burn on small hills and headwinds (didn't have to step on it). Passing power was decent for such a small engine - the d16z6 car I now have doesn't seem much better FWIW.

I dont think the engine NEEDS rebuilding, just wanting to do it for knowledge and to know I have a fresh engine, but wondering if higher compression pistons (can be had from other oem d16's for pretty cheap) can help with both low throttle cruising mpgs and for a little more passing power

In this range, yes, increased compression ratio will help those. The difference will be slight, but it'll be there. If it's not expensive and you're rebuilding the engine anyways I'd just do it. The stuff I was talking about applies when you go over 13:1 or something like that.

By the way do you have adjustable cam lifters or whatever? Maybe a fuel economy regrind while you're at it ("Atkinson cycle" slightly increased intake duration)? Deltacam quoted me ~175 dollars for a weld + grind job, if you have lash adjustment or whatever then it would be less.