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how much performance without hurting mpgs or emissions?

COCHEV

COCHEV

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I want to pose this question and confine it to carburated sb350s...cause that's what I drive :D. But that's also what most of us drive.

And so with our high gas prices and the greenies tightening up on emissions, what and how much can be done to the 350 before you start to see a decrease in economy and or emissions? For example, how does a 350 hp small block compare to a stock 200ish hp motor- economy wise.

Thoughts? Stupid question? Go buy a diesel?
 
That is totally up in the air, depends completely on the motor/trans/tune etc.

I averaged ~12 mpg with my stock 180K 350 and Q-jet, 3.73's and 33's with and without a 4" lift.

I actually jumped up to 12-15 (of course depending on how I drove it) when I built my full roller 383 with Vortec heads. The 350 was in REALLY good shape when i tore it down, makes me wish I had slapped TBI or propane on it and put the motor build money towards things that made the truck more capable but it is fun. My 383 needed premium fuel (it's on propane now) so that cuts into the cost of any fuel economy gains. That was all with a full roller 383/465/205 with FF 14 bolt and D60, 4.56's and 37" MT/R's.

A strong Chevy V8 doesn't have to be bad as far as fuel economy, in my experience it got better after building the motor but I believe that is because I could make it over the steep climbs from Denver to Moab easier because I didn't have to downshift, the roller parts and the better designed Vortec heads help as well.

My daily driver '96 Z28 six speed (w/4.10's) just made it's best trip from Carbondale to Denver yet, 34 mpg (not a typo, I promise, overfilled it which I never do and was still at 31 just to make sure it wasn't a mistake, 5.7L LT1 V8). I've been consistently getting 26-30 through that trip, more like 19-21 driving ~7 miles to/from work that just barely lets the car warm up.

V8's don't have to be terribly inefficient. Less weight, lower RPM's (provided you don't have to downshift all the time), better fluids (WAY worth it unless you have to change them often due to water contamination) and driving style have huge impacts.


That's a bit of a tangent but my experience with the carbureted truck was that my tuning really payed off. Stock, it was ok at best (10-13 mpg) and I had rebuilt the stock Q-jet. After I built the motor I spent time tuning the carb so I may have picked up some by spending the time to tune it right with an oxygen sensor, the roller cam and Vortec heads definitely help too.
 
I don't expect to get much above the 10-12 range. So how much torque and hp do you think you could get and still get the same mileage?

The roller cam makes sense. As would roller rockers. Less friction. Lots of mixed reviews on synthetics...
 
I wouldn't count on a roller cam or roller rockers gaining you enough mileage to be worth it unless you drive the truck a LOT.

In a truck, if you want mileage, a diesel is your answer.

Gas motors can do ok (as per my experience) with a good carb and proper tuning (which takes some time, not necessarily expensive equipment). Nothing will beat a diesel for fuel economy.
 
I have no illusions of getting great mileage with a truck. Just wondering at what point do you start losing economy as you add more power, ie heads, cam etc

Can you have say 400 ftlbs in a carburated 350 and still get 12 mpg?
 
COCHEV said:
I have no illusions of getting great mileage with a truck. Just wondering at what point do you start losing economy as you add more power, ie heads, cam etc

Can you have say 400 ftlbs in a carburated 350 and still get 12 mpg?
Click to expand...


Probably, what trans?
 
COCHEV said:
I'm running a th400 right now. Contemplating another sm465 swap
Click to expand...


A TH400, while a strong trans, is probably the least efficient one out there.

An SM465 is the most efficient trans put in these trucks (maybe short of the rare 4 speed manual overdrive put behind 6.2L diesel trucks with a rare OD option).

In almost every situation a manual transmission is more efficient than an auto. Automatic transmissions draw more power by their inherent operation, their pump/gear-train draws X amount of power regardless of how much power the motor makes.

Automatic transmissions need coolers (big ones if they're use in heavy duty situations) and manuals don't. There is some obvious proof in efficiency there.

I'm not saying a TH400 is bad, but if you're looking for efficiency a manual will almost always be your best bet.
 
This question can't really be answered. There are a bunch of other variables that will play into the equation and probably the biggest one is the weight of the vehicle. Obviously a 200hp engine will move a 4000 lb truck easier than a 6000 lb truck and get better mileage doing so.
 
Theres a really fine line where the mods will help performance and economy, and where they will really help performance, but economy goes out the window.

I think the biggest issues with the poor economy of these trucks, with regards to the engine itself is the cam and cylinder heads. Those parts make or break any engine build if improperly selected.

I would like to see a healthy, bone stock, smog 350 with only a change to a modern camshaft, then to modern cylinder heads, on a dyno. then compare the improvement with the basline of the stock long block. I know magazines have done this type of thing, however it seems the aftermarket parts selection is more drag strip oriented than what would be ideal for a 5500+ lb 4x4 truck.

I would tend to believe that more torque production, lower in the PRM range should help economy , depending on tuning and how the vehicle is setup of course.
 
At least they are very aerodynamic......:whistle:
 
as Scott and the others mentioned, the other variables make all the difference...

but I will throw out my thoughts on the mill itself... imo, this is by no means every case, in fact, the majority of mills don't follow this parameter, but when the right parts combo's are used, V-8's can be made highly efficient up to about a 1 to 1 ratio, cubes to hp...

after 1 to 1, it will start to fall off, no matter how well matched, etc. the aggressiveness of the components makes it inherent.. the nature of the beast and all...

so you can actually be more efficient mileage and emission-wise with a 350 hp 350, than a 200 hp.. now, throw a firebreathing 2 to 1 700hp version into the mix, and you have a party...
 
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I agree that all the other variables make a difference- gear ratios, trans, tire size etc. you could ruin the ecomony of any engine with those facors.

But speaking just of the motor, I just think there has to be a better sweet spot- like the 1 to 1 hp to cubes for example.
 
I've seen 60's Corvette's with 375 HP 327's get close to 20 mpg on a trip,provided the driver refrained from using the secondaries ...the engines then had up to 11:1 compression,some had a bit more!,that is the key to greatest efficiency,the higher the compression the more power you get out of every drop of fuel....of course we had gas back then that could run an engine with that compression--todays engines are lucky to have 9:1 compression,and the computer and fuel inhection helps make up for the loss of efficiency...imagine what we could get from a higher conmpressioin engine with todays technology...

Cams also determine how good an engine will be on fuel,there are cams specially ground for better fuel economy,and often engines with one not only get better gas mileage,but also see a profound increase in low end torque and driveability,and outperform an engine with a "hot" cam in street driving .....I've had big blocks get better mpg than small blocks I've owned too,a smaller engine dont always mean it'll be a fuel sipper...a big engine loafs along at a fast idle to propel a 3 ton truck,while a 305 will struggle to get one moving and use more gas doing it....you can get away with gearing a tad too "high" with a big block,that would smother a smaller engine,and that will gibe better fuel economy..

I would say 15 mpg in a full sized truck with a properly built engine should be attainable...the size of the carb is critical too--too many people plop on a 750 CFM double pumper and wonder why it wont run good or get over 8 mpg--when a much smaller carb would be much more efficient ,possibly even increase power too...
I had a friend with a 455 Olds with a 2 bbl that got 17 mpg ,in a ;68 Delmont 88,a big tugboat,that had 10,75 :1 compression..the 2 barrel was far superior to the 4 barrel Holley he swapped on it with a 4 bbl intake...he lost 5 mpg and it never ran as well,so he ditched it and put the stock 2 bbl back on it...I had a 450 CFM Holley Economaster on a '74 454 and I got 16 mpg a few times on my 75 mile commute with it...you can get some impressive MPG if you learn not to give it more than 1/4 throttle--floor it once though,and you'll watch the gas gauge drop!..
 
and so to get even more specific, If I wanted to squeeze some more performance out of my great big white brick of a truck and not dip below my chevy standerd 10 mpg and still be able to pass emissions...

what mods do I do?

currently its a a crate 350, performer rpm (po installed, don't know why), edelpuke 1406. stock everything else...

I have aq jet for it. it does "ok", but I've driven stronger 350s before. I'd want mods that gave me more torque, i haul tools and sometimes a 10' trailer full of tools too. city driving with short jauts up to 60
 
diesel4me said:
I've seen 60's Corvette's with 375 HP 327's get close to 20 mpg on a trip,provided the driver refrained from using the secondaries ...the engines then had up to 11:1 compression,some had a bit more!,that is the key to greatest efficiency,the higher the compression the more power you get out of every drop of fuel....of course we had gas back then that could run an engine with that compression--todays engines are lucky to have 9:1 compression,and the computer and fuel inhection helps make up for the loss of efficiency...imagine what we could get from a higher conmpressioin engine with todays technology...

Cams also determine how good an engine will be on fuel,there are cams specially ground for better fuel economy,and often engines with one not only get better gas mileage,but also see a profound increase in low end torque and driveability,and outperform an engine with a "hot" cam in street driving .....I've had big blocks get better mpg than small blocks I've owned too,a smaller engine dont always mean it'll be a fuel sipper...a big engine loafs along at a fast idle to propel a 3 ton truck,while a 305 will struggle to get one moving and use more gas doing it....you can get away with gearing a tad too "high" with a big block,that would smother a smaller engine,and that will gibe better fuel economy..

I would say 15 mpg in a full sized truck with a properly built engine should be attainable...the size of the carb is critical too--too many people plop on a 750 CFM double pumper and wonder why it wont run good or get over 8 mpg--when a much smaller carb would be much more efficient ,possibly even increase power too...
I had a friend with a 455 Olds with a 2 bbl that got 17 mpg ,in a ;68 Delmont 88,a big tugboat,that had 10,75 :1 compression..the 2 barrel was far superior to the 4 barrel Holley he swapped on it with a 4 bbl intake...he lost 5 mpg and it never ran as well,so he ditched it and put the stock 2 bbl back on it...I had a 450 CFM Holley Economaster on a '74 454 and I got 16 mpg a few times on my 75 mile commute with it...you can get some impressive MPG if you learn not to give it more than 1/4 throttle--floor it once though,and you'll watch the gas gauge drop!..
Click to expand...

I thought about a big block. most get the same mpgs as a small block. peanut port head, long tube headers and a mild cam. wonder how that would effect my DEQ testing...

I've heard a lot of stories about 2b carbs like that too. I'm hoping the q jet will give me the best of both worlds there. most time when i'm under a load and put it to the floor, there is no change. just more gas dumping in

also, isnt it true that high compression and towing don't mix will?
 
it's not mods.. it's a package design.... if the bottom end is sound, and the rings/walls are up to the increased pressures, you could do a highly efficient top end on it...

so at a minimum, your at the match the cam profile to whatever head you wanna run... EVERYTHING else is secondary and can be easily changed, induction, ignition, etc... performance and efficiency start with that combo... and in a nutshell, the better the heads, the better the cam you can run... bolt ons are not the cure...

I can't think of a more ideal carb for what your talking about than a q-jet... if it was just "ok" and being outperformed by a 1406, than it wasn't set up right or had a prob..
 
I bought it with all the edelbrock stuff on it. I'm a q jet guy. just haven't got around to swaping it out. the last edelbrock i had was on my burb, a 750. I hated it. put a qjet on and it ran better across the board.

I'll start with the carb swap, but i was thinking a cam swap would be nice too. but if its wasted on the stock heads, then I won't waste my money untill i can do both. right now i get 11 mpg on the highway with with 35s and 4.10s and the th400. If another 465 comes my way, i will be scooping that up too...

I know everyone here are big vortec head fans, but has anyone run the SR Torquers?
 
yeah, I really don't like the Carter design.. believe me, I see a fair amount on boats, Merc ran them on quite a few gasserrs over the yrs... I just think the quad is a much better design...

I was just trying to get across the point that the heart of the mill is what needs to be in the back of your brain... and then ya have, how much coin am I gonna throw on top of that bottom end, that certinly wont be ideal compression-wise, etc, etc..

that's what makes the Vortec's such an appealing proposition to many.. if you base your "bolt on" decision on what cam/heads you want in the end, you can keep babysteppin it, till you decide on the heart of the matter...

I seem to remember hearing good things about the s/r's, but it's been quite awhile since I delved into head flow characteristics..... I'm a huge fan of AFR's, really great heads, but obviously your paying for some great numbers.. I've also heard surprisingly good things about Edelbrocks offerings too..
 
I hear what your sayin. :thumb: I will ride this crate motor out till she dies and get the most out of it I can. i'd like a little more torque with the same mpgs and emissions, but if thats a whole nother build, then i'll stick with q jet swap and live with what I got :D

who knows, in a couple more years, I might have to swap in an electric motor to keep driving my truck in portland :haha:
 
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