six barrel

-
budnmic said:
How to adjust for fuel economy? '70 AAR 6 barrel motor in a 1969 fastback with overdrive 4speed. any thoughts???
Click to expand...


Don't open the outboard carburetors...

Just run off the center 2 bbl....
 
It's gonna get lean, the end carbs still draw, unless you turn the idle/air srews all in. I know someone that tried that, mod man single plane intake, she leaned just sitting there and burned the ceramic off some exh tubes. You can try it though.
 
I would imagine if you got the car rolling and stayed out of the throttle except when you need to the mileage would improve . I never worry about it with my V8s I figure I am paying for the power. I don't drive them all the time though I have a commuter car :)
 
I'll be happy with 12 mpg highway. :)

IMG_25071 (1).jpg
 
Got to pay to play. If you want mileage go with a Thermoquad and keep your foot out of it.
 
rumblefish360 said:
Expensive key factor highlighted.
Click to expand...

budnmic said:
How to adjust for fuel economy? '70 AAR 6 barrel motor in a 1969 fastback with overdrive 4speed.
any thoughts???
Click to expand...

3.55 x0.71 x0.78 =1.97; That was my double O/D. It comes with a 3.55 x3.09 =10.97 starter gear.
My thoughts:
Now, with an A500LU, the final drives become;
1.97/.69od=2.85; so anyone can do it on the relatively-cheap, cuz;
2.76s x.69= 1.90, with a starter of 7.56; or
2.94s x.69= 2 .03, with a starter of 8.06; or
3.23s x.69= 2.23, with a starter of 8.85;
So just choose an appropriate stall.
>Who doesn't have at least one of these gears laying around, and if not, people practically give them away.

>But to be fair, OP already has the 3.09-1.67-1.00-.73o/d, so this, to me,when it comes to fuel-economy only; is a no-brainer;
3.55 x.73=2.59; and 65= 2200; which is a GOOD number for a 220* cam, with a long power-extraction cycle. You can still make 300/330 hp on that cam, enough to satisfy all but the the most hungry streeter.And it has a starter of 10.97. The 340 cam being 228/235/114 will want to cruise at 2400 or higher.
So;
>just install a fat-.200, solid-lifter, cam in there of about 220@.050, and call it done. The rest is in the tune. And some is in the chassis. and a little is in the aero.
>Besides, that o/d trans needs a small cam to work right; the splits are wicked far apart, especially the one that counts most for a streeter, namely, the 1-2, which is .54 . I suffered that trans for 3 or 4 years behind my hi-torque 360, before I couldn't take it any more. I bought the GVod strictly to be able to split those wide gears. That it had an overdrive was just icing. I ran it with a Hughes HE2330AL, which was s hydro at 223@.050. I was very happy. I wouldda been a lil happier with a solid, one size bigger at .050. even if it meant sacrificing a lil extraction.
>A 2nd gen manual-trans Barracuda is pretty good on aero. Get one up to 65 mph, and then put it in neutral and see how far it coasts; it can be pretty amazing.

My tip for economy is this;
People put way too much faith in the intake duration, and not nearly enough thought into the extraction and overlap.Here are some cams in all their durations(advertised), arranged as;
intake/compression/extraction/ exhaust/LSA/ overlap/ Ica
268/116/104/276/114/44/64 the 340 cam
268/119/113/276/108/56/61 same cam with LSA tightened up.
>Notice that the extraction has jumped NINE degrees! That is huge.
> notice that the overlap has jumped TWELVE degrees!
> Notice that the intake is closing 3 degrees earlier. That is, in terms of compression building, the equivalent of running one size smaller cam.
> but there is no free lunch; the powerband of this 108LSA cam is NOT suited to those wide od ratios, and it will want to cruise at a higher rpm to slam the door on scavenging with 56* of overlap AND, most importantly, I doubt it will get any better fuel-economy than the 44*overlap cam.......... even geared optimally. It may not be any worse either. But I would never recommend either of these cams for fuel economy.

For fuel economy, in terms of camshafting you need;
to limit the overlap,
to maximize extraction,
optimize the cylinder pressure,
be aware of the powerband requirement of your transmission,
and optimize the cruise rpm according to what comes of your figuring.
Lets look at the 318LA cam;
240/130/122/248/112/20/50
>We know this cam can make great torque in a low-compression 318, because the Ica is just 50* so it traps a lot of just-inducted mixture and idles at a hi-vacuum, usually 18 to 20 inches.
> this cam makes great fuel economy because extraction is 122*, which is a lot; probably a lot more than it really needs, and because the overlap is just 20 degrees, not enough to scavenge with log manifolds.
> and the 112* of LSA, makes for the right powerband for the TorqueFlite splits.
So this cam is great for it's intended useage.

So the question is;
how can we incorporate this information into your 340/od trans combo?
Ok first, the 268/276/114 numbers are not bad numbers by themselves. What is bad is that they are big compared to their .050 ratings. Yeah I realize that those ramps are there for a long street life. But com'on, who puts 120,000 miles on their hotrod engine without ever freshening it; the rings will be gone LONG before that 268/276 cam wears out. So IMO, those long ramps are costing me money at the pumps every daymn time, that I would rather put into new rings from time to time.
Ok so that 340 cam is advertized at .050, as 228/235/114. These are the important numbers. Those are good numbers. What is not good is 268-228=40 degrees, which is actually pretty good, until you realize that the intake valve on that hydro cam is still open .008 x 1.45effective rocker ratio = .012, at that 268. it may not close until 20 or more degrees on down the road, (IDK the actual number so guessing), and that is why that 340 hydro-cam is known to be a gas-hog.
If you could find a 268 cam that actually closed the valve tight at that duration, and still be 228@.050 you'd have the best of all situations. But that is not possible. So when it comes to fuel economy with a small engine, AND performance, you gotta do three things;
1) yur gonna have to bias your combo one way or the other
2) swap that 340 cam out. You didn't mention what cam is in your engine, but only offered 71-340, so I'm guessing the cam and compression are still 1971 status. and
3) the best way to cover those TWO bases of performance AND economy, AND those wide split of the overdrive 4-speed, is with a short-period, wide LSA, solid-lifter cam.

To that end, IMO, I recommend to KEEP the 114 LSA. And that is gonna be the first hurdle. But, it will allow the later closing of the exhaust, and the earlier opening of the intake, without the overlap penalty.
Next, assuming a true Scr of 10.5, you can afford to decrease your compression degrees ,a lil.
But you want your extraction degrees to be as large as possible, certainly more than 104*, but does not need to be as high as 122*, lol. 112 is a good number.
Ok so I get something like this;
262/116/112/266/114/36/64
Notice the extraction is up to 112, and
the overlap is down to 36, but
I left the compression at 116, for an Ica of 64, to work with your current Scr. At 10.5 your pressure is already borderline too high for iron heads and pump gas. So I cannot make the compression duration any bigger.
Now, the 340 cam is 228/235 At .050, so we don't want to sacrifice too much power, so on this 262/266 cam, it will need similar specs, and that means the acceleration ramps from advertized to .050 need to be 262-228=34* And that means your only choice is a flat-tappet cam. And @Wormryder has the scoop on which one to get, with the fatest .200 numbers. But I warn you, a cam like this will idle almost dead smooth, with a lil clatter from the valve train. Just like millions of slantys do. But be not deceived, the only thing you give up with this is the slight 340 burble.

Is there a cheaper way to do this?
Well as it happens, yes.
Just install a 273 rocker gear, and run your hydros loose on the exhaust side, and retime your cam. You can gain up to about 8 degrees of extraction, going from 104 to 112 extraction, and give up nothing on the intake, and the 340 cam is already ground on a 114LSA.
But there is no free lunch here; you will have to keep your eye on the valve lash, and if it suddenly increases, the cam is done. No one can predict the day and the hour when this might occur; I said might occur.
Would I do it?
I already did, twice even. I'm coming 68, so binaround for a while. And I started out a broke-azz, high-school-dropout, kid,with no internet, and living on the cusp of technology, making 85cents an hour part-time. So with no learning and no schooling, and no one to tell me "that won't work", I tried stuff out for myself. Time is the one thing I had lots of.
 
Last edited:
MPG can be a fun pursuit. Pursuing MPG with a musclecar is a cringy pursuit.
 
The heck with with fuel economy. No one stops at a car show to look at the car getting 32 mpg. But they stop when they see a 6 Pack. (Hey Honey look it's a 6 Pack.) :poke: I rest my case. :)

IMG_25071.jpg
 
Let’s see, you’re running a 6bbl setup, and you’re worried about gas mileage? Something drastically wrong with that on every level!
 
The real question is how many 6 Pack cars does it take to get 21 mpg combined? LOL
 
I’d like to apologize for AJ’s novel being cut short. His internet went out.....
 
Really a 6 pack is just like a spreadbore 4bbl, except harder to tune. With the outboards closed, as in cruising, it is just a 2bbl so I don't see a problem, or even a challenge; mpgs as far as a given engine is concerned, is in the efficiency of the particular engine-combo, which is;
Effective cylinder pressure, rpm, and tuning.
Effective pressure depends on; Scr, Ica, sealing, and throttle opening.
Throttle opening depends on load, rpm, AFR, and ignition timing.
Tuning at this point, is pretty much just AFR
__________________
At 800ft a stock 318LA is predicted to generate 134 psi cranking cylinder pressure at 8.0 Scr, which is enough for most of them with 2.76 gears to break into the 20s mpgUSg
Also at 800ft, a stock 340 is predicted to make 169psi at 10.5 Scr.
So pressure is not the issue.
Imagine (jus saying) what would happen if you increased the pressure in the otherwise stock 318LA, also to 169psi. That would take an Scr of 9.55.
Really, about the only thing handicapping the 340 in terms of mpgs, is the 340 cam, with it's 44 degrees of overlap, compared to the teeners 20;
and the meager 104degrees of extraction, versus 122 for the 318.
With any cam, you can trade compression degrees for extraction degrees, by changing the ICL/Installed Centerline; which sometimes also pukes the effective overlap, and moves or kills some power.
For instance, this 340 cam in at 110, has 116* compression +104* of extraction= 220 degrees to play with. If you advance the cam another 4* to in at 106 you will swap 4 degrees FROM extraction, giving it to compression; 116/104 to 120/100. If you retard the cam 4 degrees, to in at 114*, that will trade 4 degrees away from compression, giving it to extraction; 116/104 to 112/108. While you could certainly do the latter, because 169psi is borderline too high anyway; watch out for what happens to the EFFECTIVE overlap.
In this case, it goes from 40* to 40*, but trades away early opening intake giving it to a later closing exhaust; which IMO, for this combo is probably a good thing. So to recap,
in at 114*, the events are;
268/112/108/276/44overlap/114LSA/Ica of 68* which drops the predicted pressure, with a 10.5Scr, to a much safer 161psi. BadaBOOM! That's about as cheap a change for gas-mileage ,as can be made. But I'm surprised by the change in cranking compression that, in this case, comes to 8psi for a 4* change. I checked it twice, with same results so, it is what it is. 161psi is a good number for iron heads at WOT, and you could run less for cruising.
___________________
Did I mention, that I once put an entire 1969 318LA topend and cam into a 340? Yeah that made it a hi-compression big-bore 318, and it went like snot. That was about 1975/76, and back then, I didn't keep track of anything much less the fuel-mileage, but Ima guessing it was waaaay better than the stock 1970 Swinger 340/4-speed, that I had just sold.
Oh and BTW, it had the sweetest thoop-thoop slow-idle you ever heard,lol. I still have that engine, now torn down, and in storage.
 
CRUZE 418 said:
I'm sorry, but I just can not believe your figures, and I can not come out to verify any of it. Sorry.
Click to expand...
I understand that perspective. The only one I can verify personally is the 440+6. That was my own daily driver and I'd wager the mileage was lower because I beat the **** out of it pretty much every time I started it for two years. My 383 4bbl newport did get past 20mpg on the highway when my foot wasn't in it and the nitrous was empty. The others are reported from customers I built the engines for so I took their word for it.
 
It's all in the tune. Jets is usually NOT the place you will find the right set up on a holley carb.

It's usually not an issue to have a car cruise at 15.5-16:1 A/F ratios.
 
Noworries Moper.
Cracked, your right there! Not for nothin.... IF you can get a nice and lean AF ratio going down the road without issue, this is where you'll get the most mileage.
 
AJ/FormS said:
You didn't mention your rear gears.
But if you are running the factory 114 LSA cam, and getting 18mpgUSg out of her, that is phenomenal.
The factory 268/276/114 cam on the advertised numbers and in at 110LSA, has only 104 degrees of exhaust duration, which is really lousy for fuel economy. But the compression degrees while only 116,with a true 10.5 Scr , the Dcr is nearly 8.4@800ft elevation, which is really good, if you manage to keep it out of detonation at WOT.
The thing is; that cam likes to be spinning,
at the very least, 2200rpm at your chosen cruising speed, 2400 would be better.
The slower you spin it, below ~2000 with headers, the worse the economy will be. And the faster than about 70 that you drive, also the worse it will be. Taking all those things into consideration, and with the 3.09 first gear, the optimum rear gear will be about 3.55s; for 65= 2120rpm with 27" tires.
3.73s would get you 2230, 3.91s would be 2330, but now the starter gear, at 11.53 is getting a lil excessive.
At these low rpms, it is doubtful that your MJs will be flowing more than a trickle, but since it's a 6bbl, you can lean them out until the you can't any more,lol.

However, the BIG secret is timing. At 2000 rpm it will be impossible to give the engine optimum Cruise Timing.
Even at 2400 it will be tough.
I estimate your engine will like 54* at 2400 rpm.

If you have set your PowerTiming to 34* at say 3400rpm,
And she idles on 18*
And doesn't start to advance until 1000rpm,
And has a single-stage curve;
THEN
the rate of advance will be;
(34 less 18)/(3400 less 1000) x 1/100 = .6* per 100 rpm, therefore
at 2400, the mechanical in the D is 8.4. and so, to that you add the 18 =26.4. to get to 54, your Vcan needs to bring in ~28 degrees, which, AFAIK is impossible.
You can mod almost any Vacuum canister to get 22 degrees, maybe 24 tops, and so if you did that, you would only be short 4* from my estimated 54*.

How much Cruise-Timing your engine actually wants is easy to figure out.
1) determine your cruise-rpm.
2) rev it up to that, and put it up on the fast idle cam.
3) grab the D and start advancing it. you will notice that the rpm goes up. Just keep advancing it until the rpm no longer rises, then
4) put the rpm back to your cruise rpm with the fast-idle screw.
5) now reset the timing to get the highest rpm.
6) adjust the mixture screws for the highest rpm
7) put the Rs back to Cruise-rpm, and one more time; adjust the timing for highest rpm.
8) finally, set your mixture screws 1/4T rich; and read your timing on the balancer. Subtract 3 degrees, and write it down!
9) return the engine to idle, restore the timing to what it was at the first, and let it cool off.

What you have just done, is determine the ideal cruise timing at cruise-rpm at speed. The 3 degrees I took out is for load compensation at speed.
If the mixture screws are too far from in the center of their adjustment range, or the engine will not idle on the new setting, then you will have to fix something.
If your engine truly is at 10.5Scr and has a factory 340 cam, in at +4*, then the compression readings should be close to 170 psi. And that is a great number to make fuel economy with; BUT; the engine has to be reving high enough to be out of reversion. How can you know when this is? But a vacuum gauge on the intake , then slowly rev the engine up until the vacuum peaks, and no longer rises. The lowest rpm that it peaks at should be considered to be your lowest cruise rpm. Just match that to your chosen cruise roadspeed , and yur done. A lil higher is Ok, but slower, will lose efficiency, until you change the Ica, by either changing the Cam, or the installed Centerline.

If your engine was a 360 with a fast-ramp 268/276/110 cam, I'm pretty sure that it would not yet be on the mains at 65 mph. But your 340 might be dribbling some. Since it hasn't maxed out the PMJ; for cruising on, it don't matter what size MJ is in there...... don't even bother with them, for this mode of operation.
Well maybe I shouldn't be so cocksure of myself; cuz one time I got a tank of something that was not 87E10, and I had to stop on the side of the road TWICE, to upjet the primaries, just so it would cruise. Man that was awful.

But I just gotta say, 18 mpgUSg in a 340 is like Mike said


BTW;
For cruising;
If you get the engine revving fast-enough, and the AFR set right, and the timing set right; then it should not matter what kind of carb you are using, nor how many barrels it has, nor how many total cfm it has...... as long as the primaries are sized decently for cruising. How you get to your cruise speed is an entirely different matter,lol.
Click to expand...
 
Just played with some things and went 89.2 miles on 4.9 gal with some city driving. Thanks for all the info . Will try more later
 
budnmic said:
How to adjust for fuel economy? '70 AAR 6 barrel motor in a 1969 fastback with overdrive 4speed. any thoughts???
Click to expand...

The best tool for optimizing your air/fuel mixture is a wideband analyzer. Install one and use it to dial in your set-up.

Also, getting your timing set correctly including the vacuum advance is the first step in getting peak performance and economy, and insure you don't have any vacuum leaks.
 
You must log in or register to reply here.
-

Back
Top