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.