We've had a few discussions about plenum volume and power, but I'm wondering about something slightly different… what is optimum for low/midrange torque and gas mileage?

I'm thinking that in general, you'd want to keep it a little smaller then normal would be optimal, but I'm wondering if with a relatively undersized throttle body things wouldn't be happier with a fairly large plenum.

FWIW, if someone is wondering where I'm going with this, I'm thinking of experimenting with my K1500 truck, (33" tires and 3.42 gears aren't helping it), inspired by the fact that I can typically put between $50 and $80 per fill up in it and it rarely does better then 13mpg. The fact is that I need to drive this thing fairly often, tow stuff… and I could care less if it falls on it's face somewhere north of 4000rpm. In normal driving I rarely see higher then 2500rpm, so I'm thinking that I'd see best gas mileage with the torque peak in the 1500-2500rpm range.

 

I get 2100 rpm @ 60 with a 1:1 top gear and about 1500 with a 0.7 OD with your combination. OD should help uless you are towing heavy.

I don't think plenum size change is going to do much for moving your torque range. 1500-2000 is very low for a 350 SBC for PEAK torque.

Assuming your 350 is pre-Vortec, rated less than 250 hp and with fairly low CR, you are fighting an uphill battle. Processing more air and raising CR as high as possibe with the octane you want to use (87?) wold be the logical steps, with valve timing set for low-mid range torque. Vortec heads wold be a good step with an appropriate manifold. Good cat-back wouldn't hurt, but headers are probably not required.

FWIW, if your truck has a gross weight, as you drive it,approaching 5000 lbs, 13 isn't far off. An open pickup box is a big speed brake and gas eater at any highway speed. A tonneau cover, and no cap is worth a few mpg @ 60+. A quicker fix is to remove the tailgate or run with it down.

My $.02

 

The rpm range is right, but the results are not. I generally get slightly better mileage in D then OD lightly loaded. Towing heavy I might get slightly better mileage in OD, but don't do it much if the conditions are hilly. I know, this makes no sense. To be honest, the results are soo close that it might just be my imagination. The worst it's ever gotten was 11.9mpg pulling a loaded, 16' enclosed trailer around NYC in traffic, and the best has been a low 13, all highway, nice and smooth… there really isn't enough variation to see what is really happening. I haven't even been able to document a real difference in city and highway mpg under normal conditions.

Well, yes and no. You don't see it often, but there are plenty of cars from the mid '80's with torque peaks in the 1800 range with 302's and similar engines…

'91, LO5 (TBI), 350, 9.1:1, 210hp. On a hot summer day, heavily loaded it just barely wants to detonate on the cheap stuff. I suspect that it's relatively tolerant of crappy gas and heavy loads partially because it's got the supercool package, a different radiator support and a radiator that has a 34" wide core rather then the normal 28" that the full size trucks get (all I know is that the truck was originally bought in Arizona…, but couldn't have been there long since it has no signs of being out in the sun at all).

Well, this is what I'm asking about. WRT to the cat back, I did go with a fairly nice setup that I made, decent y, no cat (well, this was all inspired by the cat blowing chunks out the tailpipe and not having a spare around) and all 3" from the cat to the flowtech muffler. Funny, if anything it did something weird here and lost mpg.

2 door blazer. It's actually a relatively light one for a fully enclosed setup, but weighs just sort of 6K normally loaded (yea, I carry some crap around with me… at one point I found out that the 33" tires weigh over 100# each… starts to add up over something with the standard 245's or the optional 265's…). Brakes and other goodies were upgraded with 2500/3500 parts making me happy when it's heavily loaded.

 

One other note, I have thought about the whole "ragged edge" compression thing in an effort to maximize efficiency, and the thought I keep returning to is that this thing has the tank in the back and the normal gas tank location is empty. I could really get away with mounting a 10 gallon or so tank under the driver's side floorboards and run water injection if that does the trick… Yea, it's 80# that I'm not carrying around now, but I don't know how much a little more then a 1% change in weight will really make…

 

I am running into a simular issue with my 81 GMC Caballero. I got a killer deal on a used GM Crate "Good Wrench" motor. I stuffed it in with a 700R4, 2,200 stall (with lock-up at 35MPH) and 3.23s.

After wasting a great deal of money I have decided short of yanking the whole damn thing apart and installing new pistons and heads I am just freaking stuck with a best of 14MPG on the Hiway

I have one more step before that comes about. A cam change. It seems the cam for this motor is set up to run at a higher RPM then what I run it at(2,000@60mpg). The motor is desighed as a drop in replacement for 70s work trucks with 4.10+ rear end gears and pilling heavy loads. It does not come on till 2,800 or so even with its tiny cam shaft.

My delema is why change the cam if I need to bump the CR for a more efficent engine and will have to pick a new cam for the new CR?

I Paid $110 for the motor(5,000 miles three oil changes with a El Camino wrapped around it) and I have put $500 into it trying to get it to run right. It actually runs pretty good but sucks up $2 a gallon gas faster then I can make money to pay for it!!

 

My $.02

 

FWIW, I have a 93 TBI truck, 305, 4WD, 3.73 gears (all the disadvantages of a big TBI motor, even less power). I was getting about 15, 15.5 highway with a tailwind. It was pinging mildly at full throttle on 87 octane. My truck had an intake leak that I decided to take care of. When I pulled the intake, I looked down the ports in the head and was surprised at how caked they were with carbon. I ended up pulling them and getting them cleaned up. New truck. Now it doesn't ping at all, and it picked up power and torque. Best of all, I pulled 16.5 mpg average on a cross-country trip right after I finished the head rebuild. I think those swirl-port heads are junk. They're OK for low-end torque, but no good otherwise. I'm not sure if your truck has those heads, and I know this doesn't really help answer your original question, but since I was in a similar situation and found improvement, I thought I'd put in a comment. Good luck.

 

Actually, it makes it worse.

I read a detailed article on it.
Didn't understand a goddamn thing.

But I do remember that it was very clear on the tailgate down creating more drag.

 

You may consider just biting the bullet and going with steeper gears.

Like I said, even with the stocker TBI 350 I don't see any worst mileage in d the od, which would be the same as running 4.56 gears in OD. I suspect that 4.10 or 4.56's would help acceleration/moving a load and not hurt any cruising (basically the same as giving me one steeper gear then I have now), and would result in a better overall average.

I can tell you for sure that my friend's dually (same engine as mine but with the 4l80e + 4.10 gears, bigger, heavier truck…) actually gets better MPG then my blazer.

Grrr… statements like that just make me want to work on this harder…

Well, quick example… the CFI 302 fords had a published torque peak of between 1650 and 1800rpm depending on the year/application.

Well, that chamber design is just the traditional SBC wedge/bathtub design… It's not awful, but it's not as good as the more modern heart chamber. It can be made to work, but the first thing is to get something approaching a reasonable quench distance.

Some day I'll understand why GM put their pistons .025" in the hole and then use a thick headgasket… Mopar is even worse.

heh, the best MPG I've ever had was my crossfire 305 TA. That thing would regularly pull down 35-37mpg on the highway, and it always ran a 160 and the temp guage rarely moved more then a fraction of an inch off the bottom stop… I haven't seen any evidence one way or the other but I can't believe that that running cool was hurting gas mileage in that car.

Um, do I really strike you as that dumb??? Anyway, right in the middle of what I wrote I said that it got a "decent y, no cat." Really, this thing almost doesn't have an exuast. It's pretty much just a crossover/y, 3" straight pipe to the muffler and a couple of feet of tailpipe. Amost no bends and very few restrictions… I suppose that I could make it a little better with long tube headers and a turn down behind the muffler, but past that…

You know, the thing is getting long in the tooth but I don't really have a good reason to do an engine swap. To be honest, I doubt that I would completely convert to vortec. Instead I would run vortec heads and some manifold fangle (possibly a stock vortec base and a top plate to work with the TBI). Really, what I really want to do is build a 305 with a similar top end that then add a smallish turbo. The idea is optimize the 305 for "light" cruising and then let the turbo make up for the lack of low end the smaller displacement will give.

I'm not sure what I expect. I have heard of full sized Chevy guys claiming as much as 22mpg with a 2wd truck and high teens with 4wd. I’m not sure what to believe.

And you know, I've said this before on this board… there's something that we're not getting here. I've never seen a good explanation for some of the mpg weirdness that I've seen in some vehicles. A perfect example is my '83 TA vs my '87 Formula. Same body style/gfx, same ride height (actually swapped the suspension from one to the other) same tires/wheels and same exhaust. Same tranny, 3.23 (and later 3.42 gears, got the same mileage but was happier in general with 3.23's) vs 3.27's. A crossfire 305 vs a TPI 350. At it's prime the crossfire ran a 13.8, the TPI is good for 13.59. I can't find anything wrong/exceptional with either. The crossfire could be relied to get about 20/36mpg, the tpi about 13/20, at best.

Yea, it's pretty common with some fullsize trucks. You get a semi stagnant pocket of air created by the turbulence of the airflow coming off the top of the cab, which acts like a smoothly sloped cover to the tailgate top, and once airflow starts that path it continues that past the tailgate… It actually works quite well in some cases. Open the gate or put a net in it's place and suddenly you have no way of retaining your pocket of air and you loose it's drag decreasing advantages. Put a big, square cap on it and you create a sudden drop off and no way of controlling the turbulence/drag behind it.

 

Sorry. Missed those two words.

You probably can improve your mileage a little, but it may cost you more than you will save. For a reasonable goal look at current 6000 lb SUV's "city" mileage. Getting within 80% of that probably ain't easy.


Good luck!

 

Man, no disrespect but I'm sure that we (and most engineers, since I know the engineering mantra about manifold vacuum, engine size…) are really missing something WRT to gas mileage. There are enough examples out there that in no way follow the published information on gas mileage that I'm sure of it.

I also know that there are people out there that do know it (I was hoping to weed a couple out here and on a few other lists), since some race classes are getting BSFC's that appear to be much in excess of what most conventional practices predict is possible out of IC engines in that size range. I had a longer discussion about this with the guy that just flew the RC airplane across the Atlantic (only allowed 11oz of fuel, and had something like 1.5oz left after flying across the atlantic in 30some hours), but he was pretty defensive about some of it since apparently he's gotten in some big arguments about it with some of the contractors designing drones for the military.

FWIW, I've checked out the gas mileage for new, similarly sized SUV's and, well, they're rated to get the MPG that I'm complaining about…

As a side note, I took it on a road trip last weekend and it averaged 14.2mpg… that's easily the highest that it's ever gotten

 

Maybe it's OPEC surpressing the mileage secrets.

A wise engineer friend of mine said it well: "You can't rape Mother Nature." Folks continue to try, however. Good luck in your quest.

Keep on truckin'...

 

This deviates from the topic a bit, but....if you're considering an engine swap, i'd just swap in a diesel and be done with it. While it might be tough, i'm sure you could find a newer model in a junkyard or something for a good deal. The diesel aftermarket has grown considerably in the last few years. And now it's to the point that you can buy one off the lot and with a simple module you can add some 100hp and 200lb ft of torque in minutes. Imagine having some 600lb ft of torque ready to go . And decent milege on top of that. IMO, with a truck that big a diesel is the way to go.

As a side note...some guys on the ford boards with newer powerstoke deisels and aftermarket mods (new exhaust from turbine-back, computer mods, propane injection, etc...) are pulling over 800rwtq on the dyno... with close to 600 of it right off idle . And still nabbing close to 20mpg on the highway. The duramax guys are most likely pulling the same numbers too.

Just a suggestion

 

More like nascar…

yea, but this is not like I'm trying to get blood from a stone or break any laws of physics… I'm trying to find maybe a percent more efficiency out of what is being lost to the cooling system, exhaust, pumping losses…

I've thought about it (Cummins chevy… ), but it just doesn't make sense. The swap would cost more then just buying a diesel outright, and the diesel 2 doors are damned rare and $$$. A wasted 2 door diesel will end up costing more then my good shape gas truck. I'd end up shoveling out a load of cash, end up with the weakest of the diesels (and yea, I am hooked on the 2 door thing…) and around here diesel costs more then gas.

Barring all that and say I find all the parts for some insanely low price, and do the swap, I'm still screwed. It will never in a million years pass emissions…

 

Ok...

- Ceramic coatings throughout
- aluminum reverse-flow cooled heads (12:1 compression )
- forged, lighter pistons
- big rear gears
- OD tranny w/lock-up
- torquer cam
- longer intake runners.. perferably coated sheetmetal
- alumium wheels w/ road tires (my 17x11" 315's push 52lbs)
- aluminum drive shaft
- aluminum fly wheel
- beehive springs
etc....

There's plenty of room for weight reduction, heat retention, and tunign the engine to maximize VE and reduce rpms at cruising speed. (use torque as a guide).

Quote:

I've thought about it (Cummins chevy… ), but it just doesn't make sense. The swap would cost more then just buying a diesel outright, and the diesel 2 doors are damned rare and $$$. A wasted 2 door diesel will end up costing more then my good shape gas truck. I'd end up shoveling out a load of cash, end up with the weakest of the diesels (and yea, I am hooked on the 2 door thing…)

hmmmm... you should be able to special order a 2-door cummings from GM. I know a guy who special-ordered a manual tranny for his race car's hauler. (yes, I know, I know... we all told he the same thing GM probably did: goodbye clutch, hello syncro repairs ). If you buy new (and on warranty you should be able to pull some strings if your at the right dealership.

While your at it, order a dually truck and swap in a 14-bolt Suburban rearend... that should fit some fat Pro Street tires with a factory flared fender (hell, I should be working at GM with these ideas floating around)

Quote:

and around here diesel costs more then gas.

holy crap. For as long as I've lived here diesel's been as cheap as regular 87 octane... and off road diesel is even half that! (no DOT tax, but your fined if they find the dyed stuff in your tank... who's they I have no idea ... but "they" have the power)

Quote:

Barring all that and say I find all the parts for some insanely low price, and do the swap, I'm still screwed. It will never in a million years pass emissions…

Well it will if you order it from GM, and I imagine a diesel swap would make you subject to diesel testing... not gasoline. Probably need a state inspection after the swap, but that would do it.

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