I want to build a 305 for max MPG on 87 octane for a daily driver.
I would like to use 6 in rods for better geometry and 58 cc heads and max compression for the combo. I have a custom cam available for mpg that uses a late intake closing of 37 at .050 to reduce pressure so I can use more squeeze on cheap gas. Looking at 416 heads and KB pistons but need some input. Any help would be great - Thanks in advance

 

You'll want a low RPM intake and exhaust (like TPI) and a MAF. Match all the intake, runner, head, and exhaust ports closely at the flanges, but try to keep the passages smaller to maintain gas velocity. You'll want lower ratio gearing to compliment that low RPM torque engine. Consider fully rollerized rocker arms, a roller cam, and true roller timing set. A needle bearing distributor and bronze gear can lower distributor and oil pump drive frictional losses. Keep all the bearing clearances tight, oil pressure as low as possible, and use the lowest viscosity possible to maintain safe pressure. Use an oil cooler if necessary. Use thin rim piston rings and low expansive pressure oil rings. Full balance of the rotating assembly will help.

You'll want a wideband O² sensor. You should have programming that enables Highway Fuel and Spark modes at about 16.7:1 and 48° (a 305 will take a LOT of advance). You'll want a hotter cooling system. Your cam should have fast ramps, short duration, and some retard. You'll want a free flowing exhaust. You should consider synthetic lubricants in the crankcase, transmission, and axle.

Hard tires and disc brakes on all the corners will help, and no ground effects (the trim package). If you use an automatic, consider dropping the stall RPM to about 1,500 RPM or slightly lower.

Of course, that's just a start.

 

Thanks for the reply, I'm on track with what you're thinking, How much compression can I go with 87 octane using the later intake closing ? I'm trying to improve the expansion ratio without pinging it to death. Cam I'm looking at is Isky 202 int and ext ground at 112 .420 lift no advance ground in. I would install it 4 deg retarded for intake centerline at 116. Or I might run a tadd less compression and install it straight up. I like the KB145 D-cup piston vs oem dish because of better quench area and fairly smooth top for less mixture to hide and remain unburnt, but I would also like to maybe run the KB145 for more compression. Ideally I would like to run 6 inch rods for better geometry, KB153 has a flat top, but has 2 large reliefs, not connected that might
hide some fuel. Main concern is to use piston that puts some turbulence in the mix and burns fast and completely. What would you use ?? Thanks

 

Two additional things I'd consider when building for economy would be:

1. Combustion chamber design. The efficiency of modern cylinder heads is well above older designs and most aftermarket "performance" heads. Vortec heads would be the most obvious example. They can typically take about 1/2 point more compression than older "bathtub" designs without increasing the chances of detonation (which is a mileage killer itself). They're also cast iron which means they don't "waste" heat (energy) into the cooling system like aluminum heads do. I would think you could get away with 9.5:1 compression easily with Vortecs on 87 Octane in that combo. Maybe even a smidge higher if you keep a good tight quench area and watch the details. It's better to have compression a little low but be able to use optimum spark advance than it is to have too much compression and have to pull the spark back. That applies for both power AND economy.

2. Piston rings. LOW TENSION! The factory has used low tension rings for YEARS in their motors. Why? Becuase over HALF of all internal engine friction comes from the rings! If you can use thin rings that are low tension that's great, but if you have to choose between thickness and tension- go with the lower tension rings.

I agree with your piston choices. If you can avoid "domed" pistons of any kind in favor of flattops, I would. But Vortecs actually do a pretty good job of getting the tip of the plug "on top of the dome" vs. some other designs. I built a Vortec-headed motor a while back with some BIG domed pisatons and you could look right at the top of the piston dome through the plug hole. Other heads I tried you were looking at the SIDE of the dome. My untrained eye told me the Vortec plug location would probably be better for combustion with the domes.

BTW- smoothing everyting out in the combustion chamber and on the pistons faces will also increase octane tolerance and allow you to squeak a little more compression into the motor. No peaks or sharp edges that can quickly heat up and go "glow plug" on you. Barrier coatings on the piston faces and combustion chambers can also help efficiency but now you're talking pretty serious money.

Running the cam straight up vs. 4* retarded I can't advise you on. I haven't tried playing with the cam like that on an economy-oriented engine.

Of course, it goes without saying that once you have this all sorted out it's time to tune the snot out of it. Best motor in the world won't run to it's potential without a sharp tune.

 

Yep heads are big part of the deal, I would love to run vortecs, but wouldn't the chamber overhang the bores on a 305 ? or is that not a big deal ? Seems if they do, fuel could be left unburned in those areas ? Do the world torker 305 wilh the 58 cc overhang the 305 bore ? They say to use 350 style gasket so I assume they really don't fit the 305 bore very well, any thoughts on that ? How about some 305 swirl port 87 up style heads, Whats the cc on those ? Everyone hates those for performance, so they should be cheap too, but might they be good for MPG as they claim ? Thanks.

 

Here's an easy one.

NOT!!!

Seriously, figure out what RPM the engine spends most of it's cruising time within a 500 RPM range, and design the engine to have it's highest VE in that range.

 

I know this isn't an easy one, but given the gas prices, performance takes a back seat for me now, I've build several performance motors, and the combiations are well known, aiming for max mpg is a different set of rules, any help is greatly welcome.

 

If you want to keep the stock 305 heads you might want to port/polish them youself. When I did that I gained 2-3mpg on the highway, not to mention a significant HP gain.

 

Why dont you buy a honda or something along those lines as a daily driver, and keep the camaro as your sunday car?

-- Joe

 

Lots of people have successfully used Vortecs on a 305. As far as "overhang" of the chambers beyond the bore I don't know but I doubt it. Remember that the hole in the head gasket is typically much larger than the bore of the engine itself.

 

If you're responding to me, you need to better educate yourself on VE.

If not, carry on.

 

here is something for you to think about though
sometimes the gains from using higher octane gas can overcome the price difference

and also the higher gas prices get the less of a difference in mpg it would need to make
like right now in idaho gas is around 2.97 for regular 3.17 for premium
so there is around a 6 percent difference in price if your gas mileage goes up by more then 6% at those prices then it's more worthwhile to run the premium
and on my ol'honda going from regular to premium made something like a 13% difference in fuel economy. of cource tht was with timing adjustments to make up for the gas price differences.

 

I've always heard the opposite.
something about having a faster burn design where you don't need as much spark advance would be better being that when that spark ignites the fuel your not constantly applying a pressure against the piston. and since the piston is still moving up towords the head the pressure is ever so slightly pushing down against the piston causing a loss.
a fast burn design head allows the a/f to burn quickly allowing for less spark advance, less chance of detonation, and should result in better fuel economy

 

Basicly what this boils down to is what piston top is best for MPG, and geting one that produce the best compression ratio with any given head. These are some options, cast dish with 4 reliefs, has little quench,but cheap, cast flat top, may have to much compression, better quench, KB D-cup good quench, but compression too low? That type of thing, there are tradeoffs with all but what is best compromise ?

 

one of the gods (aj, vader, 57kid someone) way back in the day and I don't remember which one talked about using a small chamber head and a dish piston I think

but this was long ago I don't remember the application or the details so sorry

 

Gods? ME??

Maybe a "*** da-- mo' fo' ".

But yes. You're correct. A "D" shaped dished piston with a smaller chamber to achieve the same compression ratio will result in better swirl and less detonation due to the better squish of the larger quench area.

 

what can I say you guys are always here giving a hand and always some of the more knowledgable people around here.



just wish you could bestow some of your collective brain power onto me

 

I figure d-cups and 58 cc will get me about 9.2 compression. Is that the max for 87 octane on a 305 ? If I run flat tops will that require premium ?

 

Why are you building a motor? It will cost more to build the motor than you will ever save in gas.

But use TPI use a short duration cam (202 @.05) with lots of lift and very steep cams, a roller would be awesome. Also run some 1.6 or bigger roller rockers to get more lift without added duration. Then use some dished pistons with proper quench distance and a very small fastburn chambers around 48cc.

LT1 heads would work great but would transfer more heat to the cooling system. But would allow more compression for a higher VE.

Then do all the little tricks vader said.

If you wanted to get crazy and do some really cool stuff. You could always put a throttle stop on your induction system. So that throttle would be limited to 1/2 throttle. Then you could run 87 and run 12:1 compression. Then add a water injection system to fight knock, cool the induction, and take up combustion volume. This would provide pretty decent gains in mpg.

 

almost think heating the induction would help out though
yes colder air is dense air and dense air means more power.
but with warmer air to get the same MASS of air in you need to open the throttle more, which in turn should reduce pumping losses because now the motor has to work less to suck air past the throttle blades.

also with the warmer air it should help out with fuel atomization though the EFI should be doing a decent job of that already


also wouldn't the water injection kinda hurt fuel economy being it is going to take quite a bit of the heat out of the combustion process leading to some dull burn?

 

Well thanks.

I can say that ~70% of what I know now I learned AFTER joining this site. I would suspect that's because I learned 70% of it here.

I usually know what I know, but sometimes I don't know what I think I know. You know?

Regardless, it's nowhere near what some of the other guys here know (Vader, RB --vanished??--, five7kid, FastBird, ME Leigh --who I suspect learned a great deal here also--, etc.) We all seem to excell individually in certain areas, even those who play with their wankels.


You seem to know a great deal more than you realize. Which may be a good thing. Keeps the mind open for learning MORE!!!

 

Burn that heratic at the stake!!

J/K


But seriously, on the topic of cams: Comp has a Magnum roller cam with 206 duration at .050 and .500" lift. Stated powerband is like 600 to 4600 (?). Me personally, I'd like to try that cam in a 305 with vortec heads. You get the agressive ramps of a Magnum cam but short duration for lower RPM power (and MPG!)