Straight sixes and fuel economy

[jonrpick]

I've been reading a bunch of threads on here, looking for info about swapping a straight six in place of a V8. Some folks seem to be interested in the great low-end torque, and others seem to be chasing better fuel economy.

Everytime I see someone mention the sixes, especially the 292, it seems that the first thing mentioned is that it's fuel economy is no better, if not worse than some V8's. I can't help buy wonder why that is.

Is there something specific to that engine that causes it to guzzle gas? I may be over simplifying, but in my mind, it goes like this: It'll take "X" amount of torque to turn my wheels at 2000rpm. If both a 350 V8 (shorter stroke) and a 292 L6 (*very* long stroke) are capable of producing that torque at that RPM, shouldn't the 292 do so using less fuel?

I don't want to get TOO awfully deep into engine theory but there should be some mathematical way to determine on paper what kind of torque an engine should be producing if supplied with "X" amount of fuel.

So, to simplify...WHY would a 292 not get better fuel economy?? Is there something in its design that causes it to be very poor at converting fuel to torque?

I do have a little too much time on my hands today

 

[Jrgunn5150]

To put it very simply, in order to drive in the same style and accerlerate at the same pace, you have to put your foot alot further down with the 292.

My dad swapped his in a 80 C10 with a 3 on the tree for a regular junkyard 350 and picked up noticeable economy.

 

[jonrpick]

To put it very simply, in order to drive in the same style and accerlerate at the same pace, you have to put your foot alot further down with the 292.

My dad swapped his in a 80 C10 with a 3 on the tree for a regular junkyard 350 and picked up noticeable economy.

Maybe I'm misunderstanding what makes the 292 a torque monster. The long stroke is a big part of why that thing pulls so hard. It's more effective at turning fuel into rotational force (torque) because of the added leverage on the crank, and does so with less displacement than another engine with comparable torque/rpm specs.

And everything I've heard or read indicates that a stock 292 will out-pull a stock 350 any day of the week, so that part of the equation doesn't make much sense to me. You shouldn't have to put it to the floor to make it move...

 

[3 on the tree]

Think part of the equation has to do with fuel distribution. On the inline six, the carb is placed at the midpoint of the engine's length. This means 2 cylinders are VERY close to the fuel source, and 2 are 3x as far away. Therefore, the carb has to be jetted much richer in order to assure that the 2 end cylinders receive enough fuel to function properly.

It would be interesting to see if a mpfi on a 292 would deliver more power and economy on the same amount of fuel as a carbed model.

 

[6.2Blazer]

Not sure who thinks that a stock 292 will outpull a stock 350...though keep in mind the a "stock" 350 can vary quite a bit from an old anemic 150 hp carbed one versus a 255 hp fuel injected version. In any case I think a 350 was always rated quite a bit higher in power than any 292

I've driven a several straight 6 (292 Chevy and 300 Ford) trucks and they definitely felt doggy and had absolutely no extra power. You basically used the accelerator pedal as an on-off switch. When you start from a stop, put the pedal to the floor, when you go to stop again take your foot off the pedal.

I think the reputation of being a "torque monster" was not from them having more overall torque than other engines, just that they made most of their available torque at lower rpms than other engines, thus they felt torquier while lugging around at 1,500 rpm.

 

[jonrpick]

Think part of the equation has to do with fuel distribution. On the inline six, the carb is placed at the midpoint of the engine's length. This means 2 cylinders are VERY close to the fuel source, and 2 are 3x as far away. Therefore, the carb has to be jetted much richer in order to assure that the 2 end cylinders receive enough fuel to function properly.

It would be interesting to see if a mpfi on a 292 would deliver more power and economy on the same amount of fuel as a carbed model.

This is exactly what I'm looking for... NOW we're getting somewhere.

Deficiencies in the intake setup could surely rob some power that only overfueling could overcome.

There are some seriously custom FI setups for these motors, but I found a company that makes custom throttle bodies. I'm wondering if somethat like that on a triple-1bbl intake might provide better fueling. I would be concerned that an intake like that would cause problems with low-end torque though, seeing as how they're generally made for racing and/or higher RPM applications.

The only problem with simply adding MPFI to a 292 (or any GM "six") is that the intake ports are siamesed, so you basically have one huge port for every two cylinders. But I have seen it done. It just wasn't a sequential FI setup.

 

[Jrgunn5150]

I only know fromm driving experience, mver gave it much thought, but in order to say get on the freeway, you had to floor it, and even then it wouldn't accelerate quite as quickly as a 350 with moderate throttle.

 

[jonrpick]

Not sure who thinks that a stock 292 will outpull a stock 350...though keep in mind the a "stock" 350 can vary quite a bit from an old anemic 150 hp carbed one versus a 255 hp fuel injected version. In any case I think a 350 was always rated quite a bit higher in power than any 292

I've driven a several straight 6 (292 Chevy and 300 Ford) trucks and they definitely felt doggy and had absolutely no extra power. You basically used the accelerator pedal as an on-off switch. When you start from a stop, put the pedal to the floor, when you go to stop again take your foot off the pedal.

I think the reputation of being a "torque monster" was not from them having more overall torque than other engines, just that they made most of their available torque at lower rpms than other engines, thus they felt torquier while lugging around at 1,500 rpm.

Yeah, I should've clarified. I meant at lower RPMs. I'm also avoiding using the term "horsepower". I generally refer to torque and RPM only, so... Lots of folks I've talked with say the 292's have more torque at the lower RPMs than a stock 350. You're right about "stock", as they do vary quite a bit. It sucks that I don't have any decent dyno figures for a few 350's and 292's as they came in trucks...especially at lower RPMs.

At higher RPMs, yeah sure, the 350 will outpull a 292, because the 292 will be well past its torque peak.

 

[vortec]

the best comparison i've ever been able to do was on a friend's k5 during college. he had a 292 in there and it would not move. when he crammed his foot down, you could feel it in your pants, for a few seconds, but after that, it was just straining. after he put a 350 (stocker out of an 83), the off-the-line feel was similar, but it kept pulling for much longer. and, there was a huge improvement in passing ability on the highway.

 

[thatK30guy]

Check into the 250 for fuel economy.

I had one that got me 18 mpg on the highways.

 

[tarussell]

The whole cylinder/port arrangement is not very efficient on that family of GM in-line motors. 292's were made only for work trucks not much anything else - different cam grinds and carb jettings most likely are the main reasons for less economy on a bone stock engine.
As far as a 292 outpulling a V-8 - that is only applicable in a very narrow RPM range , like BELOW a 1000 RPM's. The 4.120 stroke does pull extremely well but it is all at really low engine speeds. A good running V-8 will out do a stock 292 on the street each and every time.
292's are great engines but only if you need power where they make it.

Tom

 

[johnnychimpo]

My first truck was a '78 K10 w/ a 250 465/205 on 33's. Mileage was good & pulled well except on the interstate. If I had left it in there instead of building a 350 like everyone else had I'd still be driving it. But that was twenty yrs ago.

 

[ryan22re]

Why not a 4.3 then? The TBI 4.3 in my DD has pretty good pep and gets around 20 mpg on long highway stretches. Since it's 3/4 of a 350 it wouldn't be hard to get some decent power since you can use speed parts that are cheap compared to a 250/292.

I'm thinking a 4.3 with a hydraulic roller, vortec heads, 9.5:1 and a 500-600 cfm carb (or TBI with custom chip) could make ~180-200 hp and 230-250 lb-ft of torque. Which would be the ouput of a TBI 305 or older 350.

Plus you could use all of your accesories if you allready have a V8.

 

[Element]

Why not a 4.3 then? The TBI 4.3 in my DD has pretty good pep and gets around 20 mpg on long highway stretches. Since it's 3/4 of a 350 it wouldn't be hard to get some decent power since you can use speed parts that are cheap compared to a 250/292.

I'm thinking a 4.3 with a hydraulic roller, vortec heads, 9.5:1 and a 500-600 cfm carb (or TBI with custom chip) could make ~180-200 hp and 230-250 lb-ft of torque. Which would be the ouput of a TBI 305 or older 350.

Plus you could use all of your accesories if you allready have a V8.

Could even swap in a Syclone/Typhoon 4.3L. I've been told they make pretty good power

An SFI, or even a CPI, 4.3L swap wouldn't be bad, either. My old CPI ZR2 had a lot more pep than my 4.3L TBI S-10. A lot of people rag the CPI because the spider injectors and nut kit can go bad, but from what I've seen, that's not really that common.

 

[dyeager535]

From what I recall, the Vortec heads actually first appeared on the 4.3, so fairly very early on (well before 1996) they may already have had those heads. May account for why they feel so strong compared to similar older vintages of 350.

 

[buffblazer]

Why not a 4.3 then? The TBI 4.3 in my DD has pretty good pep and gets around 20 mpg on long highway stretches. Since it's 3/4 of a 350 it wouldn't be hard to get some decent power since you can use speed parts that are cheap compared to a 250/292.

I'm thinking a 4.3 with a hydraulic roller, vortec heads, 9.5:1 and a 500-600 cfm carb (or TBI with custom chip) could make ~180-200 hp and 230-250 lb-ft of torque. Which would be the ouput of a TBI 305 or older 350.

Plus you could use all of your accesories if you allready have a V8.

why not a 4.8 then? im gettin 21 on they highway with mine, and can smoke the tires from a dead stop,all stock. but also am i forgeting something or missed it already, but nobody has mentioned gearing. i have heard a lot of talk about 292's in work trucks hauling more than 350's in burbs and blazers, now i know work trucks are LWB and not a good comparison to k5's but people also forget to check what gearing you have. for example i have a 68 2wd LWB chevy with a 292/sm465/eaton rear end (dont know the name of the diff) with 4:56 gears and it will pull the house off the foundation if i wanted it to, but my 350/sm465/208/3:73's in my 10 bolt wont pull half of what that truck will. so i guess when comparing trucks, make shure the gearing is involved because lots of times to make up for the small motor, GM will stick steeper gears in them.

 

[ryan22re]

I think they became available in ~93 with the CPI 4.3 in the s-series trucks. They were rated around 190-195 hp when the tbi 4.3 was in the 150-155 range.

Nothing against the 4.8, except the work to put it in. That and you could probably build 2 sixes for what it cost to buy a decent used motor and swap it in.

 

[jonrpick]

The whole cylinder/port arrangement is not very efficient on that family of GM in-line motors. 292's were made only for work trucks not much anything else - different cam grinds and carb jettings most likely are the main reasons for less economy on a bone stock engine.
As far as a 292 outpulling a V-8 - that is only applicable in a very narrow RPM range , like BELOW a 1000 RPM's. The 4.120 stroke does pull extremely well but it is all at really low engine speeds. A good running V-8 will out do a stock 292 on the street each and every time.
292's are great engines but only if you need power where they make it.

Tom

This is also what I thought--in terms of the way the head is arranged. When I finally get a 292, it won't be going in the truck immediately. I'd like to play around with it on a stand for a while, maybe take it to a local dyno a few times and see what I can do with it.

I'd probably start with a stock build as a baseline and going from there, possibly with some mild head work and a different cam to start with. I know they can push well in excess of 400hp when built accordingly. I don't want it to be that extreme, as drivability would probably suck.

There is an *excellent* aftermarket head available for that motor, but it's damn expensive. Over $3000 as I recall...but it's all aluminum and has individual intake runners and ports for each cylinder. It's also got a real nice heart-shaped closed combustion chamber.

I guess my main point is that "where it makes power" can be shifted around in the RPM band. I only really want one because they are different. It's more or less an experiment--something to play with, because I do know that dollar for dollar, it's much more economical to stick with a 350.

 

[jonrpick]

My first truck was a '78 K10 w/ a 250 465/205 on 33's. Mileage was good & pulled well except on the interstate. If I had left it in there instead of building a 350 like everyone else had I'd still be driving it. But that was twenty yrs ago.

Has anyone had a "six" backed by an OD tranny?

 

[tarussell]

Has anyone had a "six" backed by an OD tranny?

That head is way cool but as you said it ain't cheap ! Also you are on your own for an iintake and injection system.

I have driven a few Ford FI 300 c.i. with O/D tranny's . Most of them really were fairly lame do to the axle gearing ( 3.00:1 ) and 31" tall tires. Gear any motor to be happy at cruise RPM's and it will respond well - it's all about using the power band of the motor and the effectiveness of the gear used.

Since this is a play/project motor for you I would not waist much time/effort on a stock build - you might be dissapointed in stock form. Usually they have to be tweaked to make them fun - and that always cost more money .......

Tom