How do you learn an engine's "sweet spot for efficiency?

Ok math experts,

Let's say that I'm building a car and I have several motor/trans choices - and I'd like to learn more about how to best choose which motor while factoring in hoghway cruising RPM in relation to that motors best known sweet spot for cruising - and then reverse the engineering to help be determine what range of gear ratios would be the best choice.

My motor options would be:

500 caddy with 400TH
292 Inline 6 with 2004r, 400th, or S10 5 speed.

Rear tire height will be about 40" tall.

Is this enough info to create the math problem to help decide what range of rear ends would be best suited for either motor?

Just substitute Sigma for Delta and plug into the equation.

Short answer Yes.

Given the engines it should be a known quantity where the sweet spot is. On a 350 SBC is around 22-2500 IMHO, but I don't know about you motors of choice. Call the variable S for sweet spot

T can be the tranny's final drive ratio (over or underdrive but you get the picture.

So at the driveshaft you get s*t as the transmission's final drive is a coefficient.

I'm not an engineer by education but it seems to me both ends of the drive shaft turn at the same RPM (although not sure how flywheel RPM translates through a torque convertor so you have to take that into consideration).

Then using the 40 inch tire math you can plug some numbers together to figure out the rear ratio you neen.

The kicker is though that tranny doesn't bog down if the gears aren't right. I bought a 1969 Blazer that had been converted from a 3 on the tree to a TH350. 0-60 in 20 seconds (I kid you not) but it turned about 1800 at 75 MPH. you also need to figure out if the auto trans can work with the tire size and rear you pick......

40 inch tire is gonna take some torque,.

500 caddy is 1800-2200 iirc,. a cam upgrade would really help it,.old technology and cam profile,.
292 6, again 1800-2000,. needs better intake and carb setup,.stock 292's were just as thirsty as a 500 caddy,. BUT, i,ve never seen one hooked to low rpm gearing either,.
this is old schoolstuff that comes with its own unique set of problems,. both the caddy and the straight 6,.
The older motors didn't have the low rpm capabilities of later stuff,.
4.11 would be minimum,. 2000-=2150 rpm approx in direct,
1400-1600 depending on your O/D ratio,.
4.56 would give you 2300ish in direct,. 16-1800 in O/D
depending on the weight of your vehicle,.if you keep it light, the rpm can be lower,
but again the early motors liked to spin, made the carbs a little more efficient.

6BT for the win,.

Yea, that's kind of the pickle I'm trying to finger out - if the 6 will be worser than ya'd think because it's being bogged down, or if the caddy would be better than ya think because it's not working as hard as the 6.

Bewber wrote:

Yea, that's kind of the pickle I'm trying to finger out - if the 6 will be worser than ya'd think because it's being bogged down, or if the caddy would be better than ya think because it's not working as hard as the 6.

zackly,.

the 292 can really be woken up,.with a better intake and a header,.
but that shoots your budget,.

Are you stuck in the 70's for the retro feel etc?
you might explore a 5.3 and 4l60e, they are cheap these days,. complete stuff for 500,
fuel efficient, and powerful,.be interesting to put the 500 carbed caddy against a 5.3 for power,
5.3 definitely more fuel efficient,.big bore caddy's are thirsty in stock form,.
Nick

Well Pat swears that when that 500 motor was being used in his caddy, he'd get 23-25mpg driving 70mph with the air on.

I know that the th400 is not an OD trans, but I figure the following...... that if OD reduces your gearing by about a third - and you don't have it, but your tires are taller by about a third - then your tires effectively become yuor Overdrive at cruising speeds. Low end performance will suffer, but that's not one of my goals. I want it to look cool (to me) and let me afford to be able to leave town without the motor or fuel tank screaming for mercy on the highway.

With the 6, I'd split the stock manifold between 4 and 5, add an outlet, and pretty much turn the stock manifold into a set of 4/2 split duals. I also have a small 4bbl carb that I can use with a 4-to-2 adapter plate, which would help alot at both ends of the pedal range.

I like the 6 because I like 6's - and yes, I'm pretty much stuck in the 60's.

Case study worth mentioning - when Pete build his last T bucket, he had a KILLER flathead in it with a T5 and got like 25+mpg - till it grenaded on him. He replaced it with a 258 jeep 6 with auto trans and MPG dropped to like 9.

So I dunno.

This website [You must be registered and logged in to see this link.] use to have these calculators for free, but with today's economy they have opted to charge for the once free calculators. I think I may have a copy of the equations some where, I just have to find them in the depths of my hard drive. Only thing though, it will require the use of pen, paper and calculator.

I guess what I'm trying to finger out is what gear ratio in a rear end with that tall of a tire it'll take to get me to where that rear yolk is spinning at about 1800-->2grand-ish at about 75mph, and from there I can use that info to help figure out what tranny output ratio is best compatible with THAT ballpark number, then from there finger out what motor would be best used, based on torque curves that I've not yet found.

I think I might be able to give it a shot now that I've been noodling on it for a bit, and I also has a calculizer. I also - just a few months ago - tested OUT of retaking a college algebra class that I'd already taken...... in 1994!

Bewbs,
I had a buick apollo with a 250 inline 6 in it. with 2:93:1 posi rear gears and a THM200c trans, the car averaged 13-14 mpg combined (and this was driving semi conservatively in a car with a 3000 lbs. curb weight)
so whats the point of this?- GM 70's inline 6's were not great on fuel economy (which would be the only reason to pick an inline 6 over the 500). I will say, that 250 had awesome low end torque for what it was (and the car was pretty quick for an economy car)

Bewber wrote:

I guess what I'm trying to finger out is what gear ratio in a rear end with that tall of a tire it'll take to get me to where that rear yolk is spinning at about 1800-->2grand-ish at about 75mph, and from there I can use that info to help figure out what tranny output ratio is best compatible with THAT ballpark number, then from there finger out what motor would be best used, based on torque curves that I've not yet found.

I think I might be able to give it a shot now that I've been noodling on it for a bit, and I also has a calculizer. I also - just a few months ago - tested OUT of retaking a college algebra class that I'd already taken...... in 1994!

just google speed rpm gear ratio calculator,. there,s all kinds of them,.
2000 rpm at 75, depends on your aerodynamics,
no good asking for lots of hp to drive a weird brick thru the air,.rpm will be a moot point,.
i based the previous post at 60mph,.

Nick

Quote :

Well Pat swears that when that 500 motor was being used in his caddy, he'd get 23-25mpg driving 70mph with the air on.

Did he transplant it in a Nova with a Gear Vendors overdrive? The guys on the Cadillac Forum with 500s report mileage in the mid teens, at best. Even if he is calculating with the big Imperial gallon, it still won't see those numbers (if it was a factory 500 car).

jayoldschool wrote:

Quote :

Well Pat swears that when that 500 motor was being used in his caddy, he'd get 23-25mpg driving 70mph with the air on.

Did he transplant it in a Nova with a Gear Vendors overdrive? The guys on the Cadillac Forum with 500s report mileage in the mid teens, at best. Even if he is calculating with the big Imperial gallon, it still won't see those numbers (if it was a factory 500 car).

I had a 75 coupe, it was horrendous,.
course i had the aircleaner lid on upside down so i liked to listen to the quadrabog moaning away,.

I had a 250 inline 6 in a 68 Belair wagon with a 1 barrel carb and a powerglide trans, and I loved that motor. Once I added an HEI dizzy to it, it felt like I'd added a second 250 under the hood and I loved it even more. I never did the math on the mileage, but I drove it like a normal person and it seemed like I could go FOREVER on a tank.

I guess the point Im trying to learn here is - while I know I'm not building a Prius - I'd at least like to put forth an amatures level of effort into the engineering to learn what zipcode my gearing needs to be in if I'm offered more than one rear end.

Like am I better off with a 3.xx:1, a 4.xx:1, or a 5.xx:1 based on what little else we DO know, ya know?

It's not what you want to hear, but the engine you have on the stand in the garage would be your best bet to try first, because you have almost everything.. Just get a harness. No carbed engine is going to come near the power & economy of a computer controlled one. 350 cubes can only eat so many gas molecules, and it has plenty of fp's of torque. With a little work, you can make it look retro.

Yea, just get a harness (that I don't have), and a computer (that I don't want), and a bunch of other sensors and misc. crap that I don't want on my build.

Thanks, but I'm not interested in going LT1, or LS1, or using anything other than what I either already have or can get for free and requires nothing other than a little fabrication to put it in place.

I pretty much have the 2 engines avalable to me that I can plop in and go, and am trying to finger out if one would have significant mechanical or economical advantages over another, because I think both offer equal COOL factor.

the 250 6 cyl is way better on fuel than the 292,.

i dunno john,. i ve seen people do this before,.put up wild and wonderful goals, with wild and unreasonable goals,.

Do what you want,...you're gonna anyway,.and this is all just interesting speculations and conversation,.

economy is an interesting science,. different motors have different":sweet spots" its true,.
but everything changes when you add in large heavy tires, different transmissions

t400's are harder on fuel than t350's or powerglides,.

A large truck sized rear end will take torque and fuel to spin it,.

a 250 6 cyl turning a powerglide (1 set of clutches and 1 band) and turning f78 14" tires probably did get good fuel mileage,60mph was probably as fast as you went in those days,.. now hook it to a t400, then a huge cast iron rear end,. and hang some 40" tires on the end,.put 75mph as your cruising speed,.
well you probably get my point now,.

Nick

Nick

All true. I still have the powerglide too, but I don't really like it as an option on this build since starting off in 2nd every time would kill it aruond town.

As I said, I know I'm not building a prius and I'm not asking what the most efficient setup on earth would be in a build so otherwise economically irrisponsible - I was just hoping to learn what ballpark of a rearend would be the most user friendly using tires that size - and from there looking to see if I had a combo available to me that would have a sweet spot closer to the other - where it needed to be.

Truth be told, I have a couple 4 speeds I could use - one with overdrive too, but I'm hoping to do some cruising at the bricks and woodward with it at some point - and don't really like the idea of dealing with fading clutches and clutch legs.