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Dirty Biker 08-17-2010 12:08 PM

3 inch stroke motor project, rod ratio, and mpg
my name is Austin, and this is my first real post on here. I hope it doesn't make you guys think I am nuts or have a screw loose. Any ways I have a couple of questions that you guys can help with. I have acquired for a song 2 283 motors, a 327 small journal motor and a 307 from craigslist. These things go super cheap here in Louisville and I know why. It's because everybody wants big cubes to get the power way down low in the rpm range. Anyways, I have been reading about how all the people on here are very much against using these motors in a full size car, and I dont mean in drag racing, but even just a daily driver. I don't mind that I wont win any land speed records thats not what Im looking to do. I just want to get better fuel economy, I cant afford the gas. So here are my questions: #1 If I get 6 inch rods and mate them to stock 350 chevy pistons in that 327 block and hook them to the 283 crank, would that work? #2 Compression height calculator shows about .040 higher in the bore with them, but dropping from a 3.48 stroke of a 350 to a 3 inch stroke is gonna lose alot of compression so is +040 closer to the heads too much? #3 In my mind, rod ratio increases dwell time at tdc so I can run less ignition advance and still burn all the fuel and the same with compression ratio up to a point correct? I would like 11:1 static compression, I think if I watch how much ignition timing I am running it will get better gas mileage especially since I have a huge radiator to keep the heads cool. #4 If nobody thinks a 3 inch stroke is good enough to be in a heavy car then why did gm put them in the 1957 chevy and lots of other heavy vehicles like trucks studebakers and checker taxi cabs until 1968? #5 According to wiki the 327 was the most powerful small block combo ever made until the LS6 in 2001. Even the paltry 307 managed 200 hp (149 kW) SAE gross at 4600 rpm and 300 lb�ft (407 N�m) of torque at 2400 rpm. The question is, with the short deck height of the small block, is any more stroke than 3.25" really helping peak power at all, and is more stroke actually hurting fuel economy do to rod ratio? The main question is will 350 pistons work in my 327 with 6 inch rods and 283 crank. Thank you for reading, Austin

turbolover 08-17-2010 12:47 PM


Originally Posted by Dirty Biker
The main question is will 350 pistons work in my 327 with 6 inch rods and 283 crank. Thank you for reading, Austin

Building an engine for fuel economcy is a bad idea. If you want good fuel economy run a stock engine, or better yet take $500 and go buy yourself a 15 year old Ford Escort with a 5 speed stick, Presto! 30MPG.

To answer your question The compression height for a 350 piston with 5.7" rods is 1.550-1.560"

The compression height you would need for a 302 with 6" rods is 1.525" Having .030" or so above deck isn't a problem AS LONG AS you use a head gasket with about .065" compressed thickness.

ericnova72 08-17-2010 01:15 PM

If the piston is a full 1.560" compression height piston, the parts stack equals 9.060", in a block that is 9.025" tall, leaving the piston sticking .035" out of the top of the block, it will crash into the head with typical head gaskets. If you do it with typical rebuilder grade pistons with 1.540" comp height you are still .015" taller than the block, which could work if you hunt down a gasket thicker than .050". Not going to work otherwise without modifying some parts.

All this amounts to is you are making a low compression 302.

You could cut a little off the top of a flat top piston to get this to work, I have done it in other engines, but with typical heads available you will have a hard time getting much compression ratio due to the small cubes from the short stroke. To get 11-1 you will need a dome piston, or a really small chamber head like the Dart or RHS 49cc chamber race heads with flat pistons. The dome will make it a lot of work to shorten the piston so it will fit in the given block deck height and keep the dome, so that idea is kind of blown, leaving you only the flat top/small chamber route as feasible.

The 283 got put in the '57 Chevy because it was all there was, the SBC was only 3 years old at this point and there wasn't anything bigger even being made yet. The 283 got continued work horse duty into the 60's, but it was the lowest option on the V8 list, only a inline six was smaller. People also drove a lot more relaxed in those days, 60 mph was pretty fast for road conditions back then, roads were narrower and Interstate highways didn't even exist yet, they just started to build them in the mid 1950's.

As far as strokes longer than 3.25" and your perception of the SBC having a short deck height not helping peak power, you are looking at it all wrong. For example, look at World of Outlaws Sprint car engines - 410 cubes, 8000+ rpm redlines, and 800hp,... something a 283, 307, 327, or 350 can't even get close enough to touch.

If you want to build a small motor for gas mileage your idea will work(with piston mods), but you will just have to live with the fact that it won't have any low end torque. It will work, but it won't be great on acceleration in a big car. I put a 2 barrel 283 in a heavy 1970 Chevelle in high school, it got good mileage(20-22) but wouldn't even turn the tires, even when power braked or even after adding a 4 barrel carb and intake. Engine was a 200 hp version IIRC.

As far as the 327 being top dog for SBC hp, that is simply a paper shuffle of the way GM put ratings on the engines, the later '69-70 LT-1 350 at 370hp will run the pants off a 375 hp 327 due to the 350 being a bigger cube engine.

The reason people are building bigger and bigger stroker SBC's is because that is where the power is, cubic inches are king.

cobalt327 08-17-2010 01:16 PM


Originally Posted by Dirty Biker
Compression height calculator shows about .040 higher in the bore with them, but dropping from a 3.48 stroke of a 350 to a 3 inch stroke is gonna lose alot of compression so is +040 closer to the heads too much?

I hope you take this in the spirit in which it's offered- constructive criticism- rather than just criticism. It is meant to give you some other angles to look at.

I haven't checked your math. But any time you run the piston above the deck, attention needs to be paid to the quench clearance, as well as where this places the top ring. Some piston designs will have the ring too far up to properly cool, so you'll need to do your homework.

AFA all this 3" stroke x 4" bore for mileage goes, you still have to spin the engine over fast enough to make the power required to get down the road. I don't know of anyone who builds a 302 cid SBC for the mileage potential, but hey- go for it! Might knock down 20-22mpg w/mixed driving- depending on what you put it in.

Keep the intake velocity high and the port X-section small. Use a 2-bbl carb or small 4-bbl. A Q-jet, if you can stay out of the secondaries. Use a set of 1-1/2" diameter tri Y headers hooked to 2-1/2" exhaust pipes w/a connector between the two sides after the collector and before the low-restriction muffs. An OD trans (w/the right rear gear ratio) may help as much as anything you do- even if running a 350 SBC.

11:1 CR will require E85 or race gas, and/or aluminum heads, tight quench, etc. plus the ignition timing may need to be limited- and if you have to do THAT, there goes the HP. This will also limit the cam timing to something that won't put the running compression through the roof. Cam selection becomes even more important- and the type cam required will not be a "MPG" cam.

You mention the 307 and the 327. BOTH those engines use a 3.25" stroke- not a 3" stroke.

They used the 3" stroke in '57 because that's all there was! It was used beyond those years by manufacturers who bought the engines because they fit the needs- not because they were high performance.

The HP ratings of that bygone era were determined by a much "relaxed" formula than post-early 70's, and of what's used today. The HP was gotten by using 100-plus octane fuel, solid lifter cams and when run to the RPM required to make real power, MPG was in the single digit range.

If you need to ask if a stroke (and thus the CID) larger than 3.25" really helps peak power, I would suggest you do some more research on engines in general and the SBC in particular. CID rules, period.

The rod ratio- including any "dwell time"- has a lot less to do w/mileage than what you seem to be giving it credit for.

Bottom line- to get the best of what you have, just build a 283 or a 327 if you like. To get back the money you'll spend on custom parts, etc. over a straight 'budget' rebuild will require you to drive this engine around the world a couple times times to see a positive return on your investment from better MPG.

Dirty Biker 08-17-2010 04:23 PM

Thanks for the replies! Theres a guy on ebay that has some rods for sale, they are crazy long 6.2 inch rods, I sent him a question asking about them and he came back with a reply and said he has some other sets as well, including some 6.08 inch small journal rods. Now, could those be used with 383 pistons for 5.7 inch rods? 383 pistons are easy to find and cheap enough thanks to all you guys who think the 400 crank is a gift from the heavens, and have a 1.425 compression height according to Thats alot closer to coming out even with 6.08 inch rods and 3 inch stroke crank isn't it? How about that? Cubic inches might be king but I swear sometimes I think some of you guys must be working for the oil companies... hehe ;)

ericnova72 08-17-2010 05:43 PM

The 6.08" rod will work with 1.425" height piston and 3" stroke if you are bound determined to do this. Comes to a 9.005 stack height, which is no problem.

Don't think you are going to find some magic silver bullet as far as gas mileage or HP with this, it is still just a 302. If you aren't going to put heads and valvetrain on it to spin to 9500 rpm don't expect it to shine, and if shooting for gas mileage alone expect it to be right about the same as a stock crank and rod 305. The long rod is not going to provide some miracle mileage.

When looking at Ebay rods, make sure you note the width, a lot of these odd length Nascar program leftovers are narrower than a normal rod, they use piston guided rods rather than crank cheek guided rods, and require a special piston or special machining to use. You can't just throw them on a stock crank with off the shelf pistons, the side clearance will be huge. many also have smaller pin bores for small wrist pins also, make sure you know all the specs before you bid.

Gas mileage is more in the tune than it is in some magic parts combo or engine size. My low 11 second 1/4 mile 406 gets 17 mpg highway, with 4.56 gears. no overdrive, just a race spec TH-400

gearjammer38 08-17-2010 06:09 PM


Originally Posted by Dirty Biker
I have acquired for a song 2 283 motors, a 327 small journal motor and a 307 from craigslist.

if your shooting for just gas milage, why not build the 283 and save the others till time gets better? im only taking a shot in the dark here but wouldnt the 283 be the best on gas? just my opinion

Dirty Biker 08-17-2010 10:17 PM

Ok I think i got it... pistons for a 350 chevy with 6" rod=1.26" CH and my 327 block with 283 crank and 6.25" rods! Bingo!! Would this work???

Oh yeah I already have a 283 that runs good, I just want to do something with this extra 283 and 327 I have that need rebuilt, I been wanting to do this for several years now. My girl lives in Bristol TN in nascar country, 4 miles from bristol motor speedway. She is down there now selling hotdogs in her little hot dog cart. I wanna show off a little bit one day, get a s10 and equip the 301 with solid lifters in it and such. I do not want her family to think Imma sissy cause I have long hair and I speak all correctly and all. As far as cubes being king go, I have a 454 in my van now that runs great, the gas stations around here all love me. Ide love to sell this 454 one day, people love them, I dont really know why. Maybe a tall deck 427 might be ok if I were rich...

ericnova72 08-17-2010 11:44 PM

Yes, 6.250" rod, 1.25-1.26" piston height, and 3" stroke works out to 9.000-9.010" stack and will fit.

Make it a 13.5-1 compression 302, solid cam or solid roller cam, put it on E-85, in an S-10 with a Tremec 5-speed trans and 4.56 gears and it could be a lot of fun. Sure wil like to rev. Could probably manage 16-18 mpg and maybe better if you are good with tuning.

Ever been to Thunder Valley Dragway??

turbolover 08-18-2010 06:19 AM

If you're after mileage build a 383 and gear it accordingly. It can return better mileage than a 302.

If you want to spin it to the moon and make a lot of smoke and noise build a 301.

topwrench 08-18-2010 07:01 AM

I think Cobalt is right, you r goin to blow more money trying to get good fuel economy than ur goin to save on fuel,so if fuel economy alone is what ur looking for, then i agree with what turbolover said.
Buy a small 4 cyl car with standard trans.
For your info on rods.
350 with 3.48 stroke max rod lenght=6.00 inches=1.7:1 rod ratio
3.25 inch strke max rod lenght=6.25 inches=1.9=1 rod ratio
if u use the 3 inch crank from 302 engine max rod lenght=6.50 in=2.1:1 rod ratio
If u increase rod lenght beyond above mentioned lenghts, the ring package will be too tightly bunched up to allow proper functioning and reasonable reliability.
So my advise buy a small s#1t box 4 cyl. n save ur money,keep the motors u got n build a hot rod.
I personally never did like sticking a piston above block not even .001 n I love quench with flatops.

DoubleVision 08-18-2010 07:33 AM

You want mileage huh? Here`s the layout I did on my 86 Olds cutlass.
350 SBC 1996 880 casting Vortec block, OEM Roller cam.
Decked .010
906 Vortec heads PM 5.7 rods. Factory windage tray.
KB Hyper pistons, 14cc D dish. .015 down in the hole at TDC.
GM head gasket .028 thickness. .043 Quench distance.
Hooker super comp jet hot coated headers.
Comp Cams Hydraulic roller. .206 duration @ .050 .500 lift.
Cloyes Vortec street single roller timing chain set straight up.
Edelbrock performer Vortec intake.
SMI Induction Quadrajet carb.
HEI Ignition. Pertronix flame thrower coil and module.
Moroso spiral core 875 ohm plug wires.
TH350 Tranny, 3.23 posi rear.
This combo knocked out excellant mileage and the car was a pleasure
to drive with it`s really smooth acceleration, throttle response was
sensitive as fuel injection.
Now, if you built this same combo and applied it to a OEM roller 305
and used L30 Vortec heads, you`d come out even better.
There`s no need to worry with high rod to stroke ratio`s to have good
mileage, the trick is the engine combo and the cruise RPM.
The lower the cruise RPM the better. If you build this on the 305 platform
with a 2004R OD transmission with the right rear gears for a cruise RPM of about 1800 at 55 mph, you`d knock off some really impressive mileage numbers. With fuel injection you can get down to really low cruise RPM`s, but with a carb it needs to be a little higher. with a carb and a 1800 RPM cruise, the carb would be riding almost on the idle circuits, and that`s the whole idea.

oldBodyman 08-18-2010 08:53 AM

your assumptions are incorrect on both counts:

Chevy had the 302/ 3in and the 307/ 3.25in so they built the 305/ 3.48in, guess which one gets much better fuel economy, that would be the 305 / 3.48in, torque moves the car.

6.0 in rods do increase the dwell time at TDC, SO, they raise the apparent CR, you will need aluminum heads OR drop CR to run pump gas. they also raise the power band up the RPM scale.

For gas mileage just build a 305.
Keith Black does make pistons for 305 w/6.0 rods KB 305 X 6.0

turbolover 08-18-2010 09:29 AM


Originally Posted by oldBodyman
Chevy had the 302/ 3in and the 307/ 3.25in so they built the 305/ 3.48in, guess which one gets much better fuel economy, that would be the 305 / 3.48in, torque moves the car.

:pain: :pain: :pain:

Can someone explain what this is trying to say?

Dirty Biker 08-18-2010 12:03 PM

I think that it takes a little bit more power to turn a transmission in overdrive than it does to turn one without overdrive in high gear because the tranny without over drive is more directly passing the the torque to the driveshaft instead of thru another set of gears. Wasted energy = less gas mileage. I cant afford a 4L80-E or the controller anyways. Here is something, a 1958 Rambler had a 198 straight six, now it wouldn't break any land speed records but, quoted from wiki "Tom McCahill wrote in Mechanix Illustrated, "There isn't a better buy in the world today." He continued, "The Rambler American ... is an ideal-size small family car... It will give up to 30 miles on a gallon of gas (and more, with overdrive) and will outperform any imported sedan selling for under $2,000 except in the cornering department... ". Ok that led me to research the 198 six, some more. Sure heck of alot heavier than a ford escort and cooler looking too right? Anyway I found this later down on the page that it got what? 58 mpg? insane... In 1959 they set a record with that motor? Whats it have in it? What cam? What rods? right? Ok well to research that obscure motor that they quit making soon after that (Im sure they wanted more power, and maybe the oil companies had a hand in helping them hurry to develop a motor that would burn more ga$? eh?) I deciphered well close enough, 199 six has a 1.96:1 rod ratio. Now I dont know what sized intake runners, how long the header tubes or what cam they used but I set out to build a chevy v8 equivalent. This was like 7 years ago I started building this dream motor, and I dont care if it works or not at this point. If I had a dollar for every person to tell me to build a 383 or that a 305 is the same as a 302 chevy, then I would propably not care anymore and just buy the dang 427 tall deck or better yet just buy the cummins turbo diesel. Oh and btw, did you guys see the website about the farmer who puts tractor engines in his one ton pick ups and claims 50+ mpg? Watch the vid!! Its hilarious!!!

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