bottom end

[windy1]

need to know how much , a b3 block can be bored out, and still get good milege? dont want the engine light coming on. theres gotta be a compramise.thought i could get more power,by boring it over size. anyone ever done this,on a b3? [i know someone has]

[Christ]

Boring the engine won't change much in terms of runnability or CEL. It increases the air mass that can be drawn based on engine displacement, but the ECM can compensate for the increase by adding more fuel. Boring the engine basically means using a (albeit slightly) lower throttle position for the same power output, so by that means, your fuel economy could be reduced by increased pumping loss, but you likely won't even see the change, if any.

[link5186]

For mileage you'll want the minimum overbore possible within the amount of wear on the cylinder walls. Measure all your bores top, middle and bottom both front to back and side to side. Write them all down and look for the highest number. Get it bored out from there. The differences are very minimal from a mileage standpoint. Any fresh B3 is going to do much better on mileage when compared to a worn out one. Oversize bores are +.25mm, +.50mm, +.75mm and +1.0mm. If you're really trying to eek out the extra decimal points of fuel mileage then a +.25mm bored engine will do better than a +1.0mm simply because the +.25mm one moves less air (thus needs less fuel). Take your measurements, then make a decision from there. Parts availability may be an issue on the B3, so you may have to take that into consideration as well. I've found that you may be able to get pistons one size and no rings to match.

[windy1]

i was afraid of that. id be wasting money boring it oversize. can anything be improved/added to engine? i was going to do a mild port job ,3 angle valve job,and stock cam. i could mill the head a hair more ,but then id have to use hi test. any ideas?

[Christ]

Anything you do, for the most part, to increase power output, is going to potentially reduce fuel economy. The exception is anything which makes the current setup more efficient, such as valve jobs, etc. The idea is to reduce pumping loss and frictional loss as much as possible, and keep your torque range as low as possible.

Some of the easy things you can do to improve fuel economy without sacrificing power are 195* tstat, aerodynamic improvements such as a belly pan or air dam at least as low as the lowest hanging parts of your under chassis, partial grille block, blocking the areas in the engine bay next to the radiator so that air that enters the area has no choice but to go through the radiator (which prevents excess flow through rough areas).

You can also reduce electrical load by replacing lights with LED's, not running headlights during the day if you feel comfortable doing so, using the wipers only on the low setting if comfortable, etc.

May I suggest joining ecomodder.com and reading up on the stickies there, including the "Master List of 100+ Ecomodding tips and tricks". Some of them remove power, some move the power range lower in the RPMs, and most are basically driving lessons that teach you to drive more efficiently. The single largest change you can make is in driving habits. You could see up to a 20% improvement in FE with just a few small changes in how you go about your driving day.

[windy1]

if i bore over size, ill have to locate pistons,with rings first. i never considered parts problems with pistons. i used to bore datsun 510 l16 s to 60 over and use hi compression pistons. because they were available. festy s another story. thanks

[Christ]

There are a good many piston options available from "other" sources, but it does pose a bit of a problem if you're not familiar with Frankenstein engines and trying to use non-market bits to build something.

I'd suggest that unless your engine already needs machine work, you just leave it be and concentrate on "low-hanging fruit" for FE gains.

[blkfordsedan]

Everything said is correct in theory, but we need to put things into proportion. Boring the block even +1mm will not make a substantial difference in displacement. It will, however, increase the compression and help to unshroud the valves. Both can actually improve fuel efficiency. If the block is worn, reboring it will allow the best possible chance for optimum ring seal as well, and thus efficiency,well beyond just honing it. Just order a new set of +1mm pistons with rings from Rock Auto. You'll be fine on 87 Octane as well. On the other hand, if the block and old pistons are in good shape, don't waste your money. The difference in MPG would be negligable.

[FestYboy]

windy, what's wrong with the engine you have now? what's your MPG with it now? have you done any compression or leakdown tests on the engine? given it a proper tune-up and time it lately?

if you're that concerned about fuel economy, build a B1 with the GTU head and shave everything to the limit for maximum compression (probably 10.2-10.4:1) and tune it for 87oct.

but you wont do that...

[BigElCat]

if you're that concerned about fuel economy, build a B1 with the GTU head and shave everything to the limit for maximum compression (probably 10.2-10.4:1)

What's a B1? What's a GTU?

My plan for a max econo B3 includes an overbore, and shaved head. Start with a carby motor w/9.4 to 1 CR, get it up to 10 plus. Advance the cam timing 4 degrees, and keep the rpm down. Extra torque and driveability up to 4000RPM.

I didn't know a B1 existed. Did Mazda make an 020, below the 121?

[Christ]

What's a B1? What's a GTU?

My plan for a max econo B3 includes an overbore, and shaved head. Start with a carby motor w/9.4 to 1 CR, get it up to 10 plus. Advance the cam timing 4 degrees, and keep the rpm down. Extra torque and driveability up to 4000RPM.

I didn't know a B1 existed. Did Mazda make an 020, below the 121?

There's no need to keep RPM's down during acceleration or under load. In fact, the most efficient way to turn fuel into motion is achieved at or near the peak BSFC (where each lb of fuel is converted into the most energy). The low RPM thing is for steady operation, such as when cruising.

Changing an engine so radically, I would suggest a few dyno runs to establish an average of BSFC for the new setup, as well as hard tuning to reduce pumping losses where you can, including port matching, paying attention to air velocity.

Specific tuning for air velocity can create a "stacking" effect wherein less vacuum is required to move a volume of air because it's already in motion, thereby reducing pumping losses. For this reason, a /smaller/ intake diameter can be beneficial in some cases to both FE and BHP at lower RPM ranges.

High compression isn't always beneficial, either. I'm not suggesting that it won't be for your particular setup or desires, but there are circumstances where the extra force to compress the mixture is excessive and wasteful, although I can't cite specific instances at the moment.

For both FE and HP, pay very specific attention (as long as you're actually building an engine for either) not only to quench spots, but areas where mixture can accumulate, such as the area besides the piston crown and above the first compression ring.

This area is seemingly insignificant, but can represent more than 15% of total volume at TDC. The same can be said of the area directly above the spark plug electrode in the combustion chamber.

Larry Widmer has a whole series of tech articles regarding this "dead" space at www.theoldone.com that you can review, as well as many other topics. Check the "archives" section there.

Cam overlap and duration also play a good part in fuel economy, although I don't have enough experience with cams to really form a valid assertion on what's good and bad. Unfortunately, it's almost a cookie-cutter proposition to build high-HP engines anymore, which is the limit to my experience with it, as I just don't (and likely never will) have the resources to dedicate to pioneering or researching new or improved engine dynamics. I pretty much do what I know or suspect will work and hope for the best with input from many places.

[blkfordsedan]

If you're really concerned with max MPG and have the money, convert a B3 to a hot vapor engine per Smokey Yunick's design. Based on the results of his other cars, you should develop around 140hp and probably get 80-100 MPG. Link to the info below:

http://www.legendarycollectorcars.com/featured-vehicles/other-feature-cars/smokey-yunicks-hot-vapor-fiero-51-mpg-and-0-60-in-less-than-6-seconds-see-and-hear-it-run-in-our-exclusive-video/

[FestYboy]

What's a B1? What's a GTU?

My plan for a max econo B3 includes an overbore, and shaved head. Start with a carby motor w/9.4 to 1 CR, get it up to 10 plus. Advance the cam timing 4 degrees, and keep the rpm down. Extra torque and driveability up to 4000RPM.

I didn't know a B1 existed. Did Mazda make an 020, below the 121?

B1 is a 1.1L B series engine (essentially a B3 with a B5 crank) available in the 121 and Pride in Japan and far east areas. the GTU was a special edition festy with a DOHC B3 in it. and Christ is right, you want to keep the engine in it's power band durring accel up to the point of desired cruising speed then immediatly get to your cruising RPM.

[Movin]

Isn't the 1.6 sohc essentially a b3 bored over? So any over bore in between
would yield results in between those two, barring the many improvements
that should be made in such a project.

[windy1]

wow, efficient advice. getting more air to radiator is smart. what u,think about synthetic oil,for engine efficiency? a k&n air filter? when i do head,ill do a very mild port job,and a
competition valve job. and stock bottom end. thanks for tips.