Small block gas mileage DD magnum/LA ?

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rumblefish360 said:
Between my 360 vs the 5.9, both stock long blocks, the LA had better mileage. Why? I don’ t know for sure. I think on paper, the Magnum has all the marbles. But it didn’t pan out that way. Your results may vary.

My LA was a ‘79 mill. Added on to it was;

A ‘72 trap door air cleaner w/a K&N filter
OE ‘79 cast iron intake
625 Federal Mougal AFB
JY distributor triggered by a MSD 6 and backed by a MP Chrome box. Because MSD stands for “May Suddenly Fail” in which they did for me, constantly.
2-1/4 H piped dual exhaust with twin catalytic converter’s (that was the law) and thrush mufflers.

A 904 w/the MP shift improver kit, 2.76 gears riding on 235/60/15’s. The car was a ‘79 Magnum. Best gas mileage was 20 mpg’s Hwy.

Run as much initial as you can, limit timing to approximately 34/36*’s, open up your plug gaps to .050 unless it misfires, then close them down .005 until it runs right, tune tune that carb!

I find the Magnum a little more thirsty. But power is much better! IMO, I think it is the cam specs.

IMO, your use of a 318 cid engine and the overdrive, in a lighter car, I don’t have a reason as to why 22 & better mpg’s can not be seen. I’m sure elevation has a role in this. As I live on an island, it wedge shaped maxes out at approximately 400 or so feet. At best. So I looked it up for ha-ha’s. 400 feet! Highest elevation on long island - Google Search
Click to expand...


^^^^^THIS^^^^

I’ve been driving a 73 Duster for a bit before I swap out the 318 for a 340, and it gets 20-21 MPH and I don’t baby it. That’s running 75-85 down the freeway, and hot rodding around town.

I don’t have much info on the engine because it came with the car, but I do know it is a 318. Cam I couldn’t even guess but it can’t be much over an OE grind if it’s not an OE grind. It has a Holley Street Dominator intake, a Carter 625 purchased form someone here on FABO, an analog Mallory HyFire VII ignition and Mallory Comp 9000 Unilite distributor. And cast iron manifolds. And I almost for got it has dual exhaust.

That’s it. When Muhammad buddy bought the car I was a bit leery over what it would run like, but after a little tune up that thing runs like a top.

Almost a shame to swap out the 318 but the 340 is ready, so I it goes in a couple of weeks...maybe by thanksgiving.
 
MopaR&D said:
For the topic at hand, I noticed you said the LA 318 you can get is a 1986 which means it most likely has a roller cam. In that case you could just swap in a small aftermarket roller cam (or stock replacement 5.2 Magnum cam if you don't need mech fuel pump) and reuse the stock lifters if they're in good shape and bolt on a set of Magnum heads. Probably have to get custom-length pushrods but that's something to check anyway for any build that isn't bone-stock IMO. Heck if the stock heads on that 318 are in good shape they should be 302 castings which are already closed-chamber they just don't flow as well as Magnum heads. Probably not very noticeable in a DD with mild cam. And I believe they don't have as much of a tendency to crack between the valve seats like stock Magnum heads do.

In an A-body with an O/D trans (42RH/A-500) and 3.23 gears I'm willing to bet you could break 25 MPG on the highway, possibly more.
Click to expand...
the 1986 truck 318 has a roller block...but not a roller cam.
 
rustytoolss said:
the 1986 truck 318 has a roller block...but not a roller cam.
Click to expand...

Cool, I wouldn't use the stock 318 roller cam anyway it's just a copy of the old 2-bbl cam (tiny!) in roller-tappet form.

I just remembered I think Hughes sells a cam snout extension so you could just run a stock replacement 5.2 Magnum cam with that extension and all the normal LA stuff (mechanical fuel pump) would work.

Edit: just realized you'd need roller lifters though which aren't very cheap...
 
An Easy good mileage la 318 would be a roller block regrinded towing camshaft Advanced 4 degrees with edelbrock sp2p intake 500CFM edelbrock recurved dizzy whole lot of V-can with MSD ignition. Some 2.93 in the rear with a 904. or 3.23s with a 4 speed. or 3.91 with the O/D gear box.

Magnum motors also can give great mileage a well!

My Mild roller 318 wit 3.55 gears gets 14-15 Mpg city driving on the highway around 17-19 if i swapped to 3.23 highway mileage would increase but since i stay of the highways and drive red light to red light 3.55 it is! it currently has 3.91s in it now and i get about 10 smiles per gallon!
 
I have a 97 ram with 5.2 and 87 Ramcharger with a fresh 88 roller cam 318. All stock rebuild except for a cast spreadbore and 650 Holley and HEI conversion. The ramcharger gets better fuel mileage than the 97 which has EFI and OD trans. Same gear ratios, tire size and 4wd. I dunno, but even before installing the rebuilt roller motor, the LA still got better mileage.
 
I get 21 highway with a stone stock 5.2 magnum, m1 dual plane, thermoquad, and a set of dougs headers in a 71 dart, 904, and a 3.23 rear gear.
 
Honestly, IMO, I think it has to do with the fuel injection. The carb can run leaner if so set that way. The FI may be a tad richer, the cam slightly more aggressive making the whole package slightly thirstier? Perhaps the shocks itself has some small disadvantages as well.

Just like looking for HP, it can be a lot of little things adding up? Looking at the longevity aside of things, those Magnum engines are really something else! Cylinder bore wear is near nil on the ones I opened up with 80K, or even 150K.

From a performance stand point, as delivered, my 5.9 has been really good. An excellent driver on the street (even though it is converted to a carb) with very good power. I think it is a bit better than any 360 LA I have had in the past. No drag slips to support it. Just the butt dyno.
 
Here are, what I found in my quick search, for factory originally installed Mopar cams .
5.2 cam; 251/264/113Lsa
5.9 cam; 249/269/109Lsa
Can anyone verify if these are correct??
And does anyone know,for sure, what the original installed centerline was?
I keep bumping into some crazy-looking numbers.

The reason I ask is,because, if you run log manifolds, that 5.9 cam has got some economy potential.
With logs, and for this application; you can time it however you want. I mean 269 exhaust degrees is a ridiculous number, for the size of the intake.

See if you can follow along;
this is the 5.9 cam;
the overlap on this cam is ; (249+269)/2 less (109 x2) =41*
That looks not gas-mileage friendly.... on paper.
but lets install this cam at 108*. This would make the compression cycle to be 128*, and the Ica to be 52*
The Power cycle would be 115* ( a lot) and the
Effective overlap shrinks to 34*

Now, lets look at the 5.2 cam. In at near split overlap this would be an install of 110*
The Compression would be 125* and the Ica comes to 55*
The power cycle comes to 111.5, and effective overlap is 31*.

Here they are stacked up so you can see what's going on. The 5.9 first
249I/269E/128C/115 P/ 34eO and 52* Ica 5.9cam (109LSA)
251I/264E/125C/111.5P / 31eO and 55* Ica 5.2cam (113LSA)

So lets analyze this;
Intake duration is a wash,
Exhaust is a wash
effective Overlap is a wash
Compression degrees is plus 3* for the 5.9 which will translates to about plus 5 or 6 psi cranking cylinder pressure
Power extraction is plus 3.5* on the 5.9. which ordinarily nobody cares about, but 115* at a low cruising rpm, IS a big deal, cuz, this provides additional time to get rid of the exhaust thru the logs.

Ok so what does that 5.9 cam look like installed ?
Did I read that you are at 5300ft elevation? If yes, here we go...
EDIT; ok no WIKI says 950 ft so , here are the proper figures;
According to the Wallace Calculator;

Here is the 5.2 Magnum/ 5.9 cam
Static compression ratio of 9.2:1.
Effective stroke is 2.82 inches.
Your dynamic compression ratio is 7.98:1 .
Your dynamic cranking pressure is 157.95 PSI.
V/P (Volume to Pressure Index) is 128
This is bang on!, about as good as it gets

Here is the 8/1 318LA with the 5.9 cam
Static compression ratio of 8.0:1.
Effective stroke is 2.82 inches.
Your dynamic compression ratio is 6.95:1 .
Your dynamic cranking pressure is 130.93 PSI.
V/P (Volume to Pressure Index) is 106
158psi versus 131psi, and 128VP versus 106VP. With compression being the ONLY change..... See what I mean? That 27psi is a huge sacrifice



For
"on the cheap",
at 950ft
If the cam specs I found are accurate.
really, IHMO, you only have this one option;
5.9 cam into 5.2 engine, at 9.2 or better Scr.




Now, IMO, for fuel-mileage, you need three things
1) as low a cruising rpm as possible
2) as high an Effective cylinder pressure as is possible
3) as drastic a friction reducing program as you can muster
4) a heckuva tune

I could tell you that my 367, on one point-to-point, long-distance trip, got me 32 mpg without much trying. But I've been called a liar for saying that.
But if I got that, here's how I mighta done that;
1) geared 65=1533 rpm , using an A833od-box and a GVod behind that.
... (3.55s x.71od x.78od= final drive of 1.97)
2) about 11/1 Scr, with cranking cylinder pressure over 180psi@ 900 ft elevation
3) My Barracuda, takes forever to coast down from hiway speed. I haven't measured it, but I would estimate it takes between 5/8 to 3/4 mile, to drop from 60 to 20mph
4) a heckuva tune.

FYI
Here is what the 318LA with the 5.9cam actually wants.
Static compression ratio of 9.3:1.
Effective stroke is 2.82 inches.
Your dynamic compression ratio is 8.06:1 .
Your dynamic cranking pressure is 160.08 PSI.
V/P (Volume to Pressure Index) is 130
But getting to 9.3Scr into a 318LA, is not gonna be cheap




 
Last edited:
AJ/FormS said:
This is what I found in my quick search, for factory originally installed Mopar cams .
Can anyone verify these to be correct??
And does anyone know,for sure, what the original installed centerline was?
5.2 cam; 251/264/113Lsa
5.9 cam; 249/269/109Lsa

I keep bumping into some crazy-looking numbers.
Click to expand...
Everything I have seen is that the factory 5.2 cams are slightly more aggressive than the 5.9 cams.
 
gliderider06 said:
Everything I have seen is that the factory 5.2 cams are slightly more aggressive than the 5.9 cams.
Click to expand...
I read that too, but could not find the numbers, and OP is not looking for a more aggressive cam, anyway .......
but looking at the numbers that I did find, yes, the 5.2, if installed right, could have an edge, but there is no way on earth, going on the numbers I found, that I would pull a 5.9 cam out of a 5.9, to swap for a 5.2 cam, with NO OTHER Changes.
 
AJ/FormS said:
I read that too, but could not find the numbers, and OP is not looking for a more aggressive cam, anyway .......
but looking at the numbers that I did find, yes, the 5.2, if installed right, could have an edge, but there is no way on earth, going on the numbers I found, that I would pull a 5.9 cam out of a 5.9, to swap for a 5.2 cam, with NO OTHER Changes.
Click to expand...
Yes! Also, I forgot to mention the LA 318/360 roller cams are the same from what I was able to research when i rebuilt the engine for my ramcharger.
 
With all this information, it looks like the 1986 truck 318LA might be worth keeping. I also found out it may have 302 heads +++.
 
I looked up the elevation for Clinton, Ohio, and WIKI says it's about 950ft. In my previous post I got 5300ft in my head, so I will go back and rework the numbers, no big deal for me. Ok all fixed.

That 318LA is an 8/1 nominal engine. As compared to 9.2Scr for the 5.2Magnum.
For fuel mileage this is a big deal, because the higher compression makes more power everywhere in the rpm band.
That translates to you being able to cruise at a smaller throttle-opening, which usually translates to less fuel being consumed. At 950 ft, your 8/1 LA will already be down on power, so IMO, the LA is gonna need to bring the compression up; Whereas the 5.2 was born at 9.2Scr already. Plus the Magnum has better heads.
On the cheap, meaning not installing hi-compression pistons, to me, the 9.2 Magnum is the clear choice.

Furthermore;
With that A500 comes a new set of gears, loc-up, and an overdrive. The ratios are:
2.74-1.54-1.00-.69od. compared to
2.45-1.45-1.00 in the A904.
The deeper low is 11.8% lower, meaning your first gear will be quite a bit peppier. With this new low gear, you could drop one rear gear size, and not lose any first gear performance,as compared to the 3.55/904 combo, yet this will drop your hi-way rpm about 9% Even before the overdrive goes in. For example from 3.55s down to 3.23s is about 9%, and from 3.23s to 2.94s is also about 9%

Looking at it in another way, in first gear;
3.55 x2.45=8.70 starter gear for the A904, and
3.23 x2.74=8.85 starter gear for the A500. So both about the same.
But on the hiway, in third, with 26.4" tires, before loc-up;
The 3.55/A904 combo might cruise at 65=2940 (*), whereas
the 3.23/A500 combo might cruise at 65=2670 (*), there's that 9% again
(*) is plus 50 to 150 rpm slip, depending on terrain.
After overdrive,in loc-up mode, the A500 looks like;
65=2030 with 3.55s and
65=1840 with 3.23s ..... there is that 9% again

Because the 318LA is rapidly losing power below 2000rpm, the 8/1 318LA is gonna take more throttle opening at 1840, than at 2030, so in all likelihood would NOT Get much better mpgs.

The 9.2Scr 5.2 on the other hand is still very torquey at 1840, and WILL get better mpgs at the lower rpm, with the right tune.
 
Last edited:
AJ/FormS said:
I looked up the elevation for Clinton, Ohio, and WIKI says it's about 950ft. In my previous post I got 5300ft in my head, so I will go back and rework the numbers, no big deal for me. Ok all fixed.

That 318LA is an 8/1 nominal engine. As compared to 9.2Scr for the 5.2Magnum.
For fuel mileage this is a big deal, because the higher compression makes more power everywhere in the rpm band.
That translates to you being able to cruise at a smaller throttle-opening, which usually translates to less fuel being consumed. At 950 ft, your 8/1 LA will already be down on power, so IMO, the LA is gonna need to bring the compression up; Whereas the 5.2 was born at 9.2Scr already. Plus the Magnum has better heads.
On the cheap, meaning not installing hi-compression pistons, to me, the 9.2 Magnum is the clear choice.

Furthermore;
With that A500 comes a new set of gears, loc-up, and an overdrive. The ratios are:
2.74-1.54-1.00-.69od. compared to
2.45-1.45-1.00 in the A904.
The deeper low is 11.8% lower, meaning your first gear will be quite a bit peppier. With this new low gear, you could drop one rear gear size, and not lose any first gear performance,as compared to the 3.55/904 combo, yet this will drop your hi-way rpm about 9% Even before the overdrive goes in. For example from 3.55s down to 3.23s is about 9%, and from 3.23s to 2.94s is also about 9%

Looking at it in another way, in first gear;
3.55 x2.45=8.70 starter gear for the A904, and
3.23 x2.74=8.85 starter gear for the A500. So both about the same.
But on the hiway, in third, with 26.4" tires, before loc-up;
The 3.55/A904 combo might cruise at 65=2940 (*), whereas
the 3.23/A500 combo might cruise at 65=2670 (*), there's that 9% again
(*) is plus 50 to 150 rpm slip, depending on terrain.
After overdrive,in loc-up mode, the A500 looks like;
65=2030 with 3.55s and
65=1840 with 3.23s ..... there is that 9% again

Because the 318LA is rapidly losing power below 2000rpm, the 8/1 318LA is gonna take more throttle opening at 1840, than at 2030, so in all likelihood would NOT Get much better mpgs.

The 9.2Scr 5.2 on the other hand is still very torquey at 1840, and WILL get better mpgs at the lower rpm, with the right tune.
Click to expand...
the roller cam la318s sposed to have 9.2 compression,..pistons only .040 down in the hole!!
 
Per post #1
rustytoolss said:
Or should I use a LA 318 (which I have 1986 truck engine)
Click to expand...
Per post #9
rustytoolss said:
Just was not sure if the 5.2 magnum would be a better choice....or 318 ? I do have a 1986 truck 318 (roller block engine)
Click to expand...
per post #27
rustytoolss said:
the 1986 truck 318 has a roller block...but not a roller cam.
Click to expand...
pr [post #39
rustytoolss said:
With all this information, it looks like the 1986 truck 318LA might be worth keeping. I also found out it may have 302 heads +++.
Click to expand...

I took a stab at it
 
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Dude!!! I'm gonna pick your brain when the time comes to work out an performance/mileage engine for the Dart!! Very impressive!! @AJ/FormS
 
rumblefish360 said:
It’s all in the calculators.
Click to expand...
Is that a dig on me?

Math was always my best subject,
with geometry a close second.
Algebra helped me find numbers I never knew existed. I was a bit baffled when they introduced it in school, but I was a fast learner.

All three of those helped me, in 1998, design an 11.3 compression ratio engine, when the speed shops said it would detonate itself to smithereens.
They said 9.5 was the ceiling.
They were, of course, wrong.
And today, everybody knows how to do it.
Had I listened to them in 1998, I would still be driving the same lazy-dog engine.

But if it's not a dig on me;
absolutely yes it is all in the calculators.
If we didn't have calculators, we'd still be living in the dark ages.
 
Last edited:
RabidScott said:
Dude!!! I'm gonna pick your brain when the time comes to work out an performance/mileage engine for the Dart!! Very impressive!! @AJ/FormS
Click to expand...
I'll be glad to give you my recipe, which speaks for itself. And then you can run it by all of FABO, to get as many opinions as will satisfy you. It's no secret, I have published it here on FABO several times. And you can find a very similar combo in the Hughes Performance archives.
#11---360 Dyno Test (Feb 2000)
It's just too bad they stopped the test at 6000, cuz if you extrapolate the torque line down to 6600 or 6800, and convert what you find to horsepower, it is amazing how high this HE3038 cam (same as what I got, but with OOTB Eddelbrock heads), is willing to rev to.
And check out the torque curve; over 400 ftlbs from ~3800 to ~5700, 400/438=91.5%; simply amazing.


land_dyno.jpg
 
Last edited:
AJ/FormS said:
Is that a dig on me?
Click to expand...
No
[QUOTE]
Math was always my best subject,
with geometry a close second.
Algebra helped me find numbers I never knew existed. I was a bit baffled when they introduced it in school, but I was a fast learner.

All three of those helped me, in 1998, design an 11.3 compression ratio engine, when the speed shops said it would detonate itself to smithereens.
They said 9.5 was the ceiling.
They were, of course, wrong.
And today, everybody knows how to do it.
Had I listened to them in 1998, I would still be driving the same lazy-dog engine.

But if it's not a dig on me;
absolutely yes it is all in the calculators.
If we didn't have calculators, we'd still be living in the dark ages.[/QUOTE]

Engines/car performance doesn’t always do what the calcs say there going to do. The calcs should get you really really close to balls dead on accurate. But that is not always the case. Calcs are extremely useful. Just not law. Many/most people think it is because there numbers, that are an absolute definitive, a no argument more than rock solid answer. What the problem is, is what happens after the calculations are run and what the actual physical item does.

This is way I say that some people are in the calculator to much. An excellent example is when I have (or anyone really) dyno an engine, showy he numbers to a calculator head Genius! and they come up with a time slip that you should run..... but never take into consideration anything other than the engine itself. If you know Scientist! They’re always right! Why? Because they use the calculator!
 
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