Increasing displacement = Increasing MPG? (Theory)

[nbleak21]

Ok, I know this sounds kind of counter-intuitive, but I wanted to get some of your expert thoughts and opinions on this.

Preface:
Currently my Acura has the F22b1 engine in it: 2156cc
I received a newer F23b1 engine for literally pennies (traded an old, worthless to me cell phone for it.)

The engine was running when pulled, but replaced with an H22 (DOHC JDM motor) in someone's 98 accord. I've pulled the head, had it cleaned, pressure tested, and milled true, and am planning on boring the cyl walls out from the factory 2256cc to 2306cc (+1mm bore) using aftermarket K24 pistons from a honda CRV to keep the CR the same.

Initially I was building this to be used as a turbo build, and may still take that route, as these blocks/factory internals have been proven to handle a bit of boost...

As for the Theory in question:

I've noticed that with my IM, I have less vacuum losses at 3100 rpms (TPS avg. 14-15%) where as 2800rpms TPS sits at 17-18%

So my theory is that by increasing the engine CC, the VE will be approx the same at 2800rpm in the rebuilt block, as it would be at 3100 now.

The difference in avg cruise mpg is 29 (2800rpms or 76mph) vs 26mpg (3100rpms or 82mph)

In theory I'm looking at a 7% increase in displacement resulting in a 12% (relative) decrease in TPS.

I understand that MAP and o2 readings will be different because of this, and I also don't know the TPS fuel curve adjustment at given percentages either, but I figure the majority of this can be modified via my V-AFC that I run on the ecu anyways.

Just some food for thought, would love to hear your guys' input.

(as an aside: rotating mass of the engine will also be reduced over present engine, as I plan on deleting the balance shaft assemblies, shaving off 9-10lbs, and the K24 pistons are approx the same weight as the OE F23 pistons)

[ps2fixer]

Interesting build, from my understanding, if you up compression over factory, the engine *should* be more efficient. I might do similar with my corolla with over 240k on it some time before 300k.

Since your increasing engine size/power, I think it would be a good thing to look into upping your gearing a bit to put more load on the engine which should make it run more efficient too.

[nbleak21]

Originally Posted by ps2fixer

Interesting build, from my understanding, if you up compression over factory, the engine *should* be more efficient.

my original intention was to increase the comp to 11.5:1 and leave it N/A, but after having done a recent HG replacement on an MR2 turbo, I really want a turbo for those time when I do want to "get on it" so I will be leaving the CR approx the same as stock.

Since your increasing engine size/power, I think it would be a good thing to look into upping your gearing a bit to put more load on the engine which should make it run more efficient too.

I would love to up the gearing a bit, but in the honda/acura world, my tranny is one of the longer gear ratios, so I'm stuck with with over-sized tires as my only affordable option, and I've yet to find anything LRR that will increase my Diameter while keeping the tread width reasonable (205/55/16 is the recommended stock size, though the PO installed some performance 215/50/16's on there which is what I am currently running)

[serialk11r]

Decrease in throttle position = reduced vacuum loss? Hmmm? That doesn't sound right to me... Higher throttle position at the same or lower rpm should be less vacuum loss usually. Percentage throttle position doesn't tell you much because throttle position and load/pressure are definitely not linearly correlated at all.

At lower rpms, VE being low is an indication of the cam bleeding off some charge, which in theory should be good for mpg. So yes your VE at 2800 rpm with the increased bore should look something like your VE at 3000rpm or something currently but that's not going to be good for mpg.

If you're keeping the head the same, it seems like your top end will not see much improvement due to flow restriction. I also don't think that 11.5 is that much of a problem for turbo, you just need to keep the boost down a little, maybe like 0.7 bar? (still a respectable 70% increase in torque) Deleted balance shafts is pretty hardcore, I'm guessing you're going to have the rotating assembly balanced and the pistons and rods balanced as well?

[ps2fixer]

I thought the balance shafts delete is pretty extreme as well, lucky my corolla don't have one, but the Camry does... HUGE difference in how smooth it runs, not sure about power/mpg relating, I guess it should rev quicker with less effort if everything is done right.

[ksa8907]

i second the advise of changing your gearing, a 2.3 I4 is a decent size engine and unless your car is quite heavy i would guess that 1800-2200rpm @55 would be pretty good.

[serialk11r]

Originally Posted by ps2fixer

I thought the balance shafts delete is pretty extreme as well, lucky my corolla don't have one, but the Camry does... HUGE difference in how smooth it runs, not sure about power/mpg relating, I guess it should rev quicker with less effort if everything is done right.

I think I read somewhere online that someone had deleted the balance shafts, and found no detectable change in power output, but it was a hell of a lot rougher.

Do you have a 1ZZ-FE? All I know is that my 1ZZ powered car vibrates like crazy... no balance shafts would sorta explain that.

Originally Posted by ksa8907

i second the advise of changing your gearing, a 2.3 I4 is a decent size engine and unless your car is quite heavy i would guess that 1800-2200rpm @55 would be pretty good.

3000rpm at 80mph is 1500rpm at 40, 2250rpm at 60mph. That's pretty good gearing...I can't think of another 4 cylinder car with a longer cruising gear than that. The CL is not even a very small car either.

[ps2fixer]

My corolla with factory gearing/tires runs around 1600rpm @ 45mph, at 40mph it down shifts out of lockup . Camrys are about the same with a 2.2L but shift out of lockup at around 45mph. My dad's "low geared" 1987 4x4 Mazda pickup (2.6L) runs around 3200rpm @ 70mph (~1830 @ 40mph).

Never comapired rpms before, but it seems like ~2500rpm @ 70mph is pretty ideal gearing for a 4 cyl.

[serialk11r]

Originally Posted by ps2fixer

My corolla with factory gearing/tires runs around 1600rpm @ 45mph, at 40mph it down shifts out of lockup . Camrys are about the same with a 2.2L but shift out of lockup at around 45mph. My dad's "low geared" 1987 4x4 Mazda pickup (2.6L) runs around 3200rpm @ 70mph (~1830 @ 40mph).

Don't worry, my highest gear is 2500rpm & 45mph, 1ZZ-FED. I don't know if you have a 1ZZ or the earlier 4E? (is that correct) but my 1ZZ bogs pretty badly below 1500rpm, so that downshift might actually be a good thing.

[ps2fixer]

Mine is a 7A-FE 1.8L, the old corolla cast iron blocked engine.

It seems to be rather happy working @ ~43-45mph at the current load, my 100% best mpg numbers and engine load (according to scan gauge) is something like 60%. Hills kill it, which is why I really dislike the down shift since I have to go 45-47mph to get back into highest gear + the weird mode I get better mpg in (retarded spark, and less tps). Probably a sensor going bad but w\e tank averages says it is good so far .

Another thing I should note about my engine... it has a very low RPM for the max tourqe, I beleive it was 2600rpm while just about every other 4 cyl has the max at around 4400-5800+. Pretty sure my engine has more of an eco cam in it than the average 4 cyl. Oddly, it does get up and go pretty well, but runs out of steam around 4500rpm, shifting early is faster than R-ing it out to 5500+. Of course I rarely drive like that, only to clear out the system, or trouble shooting, and once in a while the late run to work.


EDIT:

Ok 2800rpm is my max tourqe .

http://www.motortrend.com/cars/1997/...pecifications/

Compaired to the 1.6L same car... 4400rpm

http://www.motortrend.com/cars/1997/...pecifications/

And a newer 1.8L (1ZZ series I think?) - 4400rpm

http://www.motortrend.com/cars/2000/...pecifications/