I have a 70 Duster this engine is sitting in. 727 with 9.5:1 pistons. The Rear Gears are 3:55's. The tires are at 27" Dia. right now, but may go down an inch or two. These are 60 Series, will be putting on 50 Series in place of them, 15" Rims. The car, with me and tools weighs about 3,200lbs.
I just got an AirGap Eddie, will be using my Holley 600 CFM 4150 (Mech) and standard headers, Electronic Ignition/Better than Stock. I am not a cam expert, but I would like to have a hydraulic roller cam as big as I can get for the street. Don't plan on using the Duster as a daily driver, just for fun. But I don't want to only get 8mpg either. It gets 12 or so at 65 on the highway now with the above and a Performer intake and very mild cam. I do understand I will lose mpg, but don't want it to take a serious dive.
What is a reasonable expectation for mpg with a decent cam?
With 3.55s and 27s, your hiway rpm @65mph,will be 2800 to 3000 depending on TC slip, grade and wind. This is the first and worst problem to achieving fuel mileage. Rpm just kills it. You only have 2 options; 1) an overdrive, and 2) less rear gear.
I read in a book once that for every 2% that you can reduce your cruising rpm, you will gain 1% in fuel economy. IDK if that is true, but one time I installed a combo that reduced my final drive from 3.55 to 1.97, reducing my cruise rpm @65, from ~2900 to ~1600 which is a loss of 45%....... so should get me in increase of 22.5 % in mpgs. Well it did waaaaaaay better than that.
So bottom line is gears make the second biggest difference to economy; the first being driver-control. Third used to be the loose TC but I hear things have gotten better, but it's probably still better to have a stall lower than your cruise rpm.
But the hitch in this program is that with only three gears in the trans, if you lower the TM in the rear , then you quickly run outta gear for blasting off with, unless your combo is built with lots or low-rpm power in the engine.
So whenever fuel economy is a consideration, the WHOLE COMBO has to be considered. When power with economy is considered,normally aspirated, it gets doubly hard,and sacrifices have to be made.
So with that in mind, and assuming your pockets are deep, here is something to think about.
Scenario #1
3.23s are 10% less rpm at cruise, upping your cruise-economy ~5% or from 12 to 12.6mpg. Lets build a combo around those 3.23s. The first thing you are gonna need to do is ;
swap out the 727 for an A998wide-ratio-non-loc-up, with gear ratios of 2.74-2.54-1.00 . This will restore your first gear TM from 8.7(with the 3.55s) to 8.85(with the 3.23s), so you actually gained a wee bit there. Your cruise rpm falls from 2800/3000 to 2600/2740; about 200rpm.
But we have a bit of a problem; the splits on the 727 were .59-.69, and on the A998 trans, the splits are; 56-.65.... so your engine is gonna have to work a tad bit harder at the extremes of the power band. For a 360 this is only a problem if you select the wrong cam for your compression ratio. And you are not gonna post a low ET with this combo no-how. You can still post a good mph, but would have to be content with being just a tad less quick.
Now we get to the engine/stall/cam
A true 9.5 Scr (as opposed to 9.5 rated pistons), is gonna be a bit of a challenge . To reach it's upper potential for pumpgas and iron heads, @160psi cylinder pressure, would require an ICA of 56*..... which is a pretty small cam. The upside to this is the bottom end will be very strong. So strong that you could run a TC with as low a stall as perhaps 2200.
However, our cruising rpm with 3.23s will be say 2600 lightly loaded, so we can lose a bit of cylinder pressure and not miss it with a 2600TC. This will allow us to run a tad bit more cam, say an ICA of 60* for 155psi.
You lost some cylinder pressure and ultimately a wee bit of peak power, but with the 2600, you will have plenty enough to have fun with at lo rpm.
So if you are with me so far, then all we have to do is find a fast rate of lift cam with a 60* ICA.
Well, as it happens, this ICA can be found on all the most popular 268/110 cams,lol
But here is the rub; there are fast 268s and there are turd 268s so buyer beware.
A fast solid flat-tappet 268 might have 38* ramps making your 268 a
[email protected] which is a heck of a powerhouse street cam.
A turd 268 might have (I heard of one) 76* ramps, making your 268 into a
[email protected] kindof like a 318cam with attitude. But more typically
a hydraulic roller might have 58* ramps making your 268 into a
[email protected] cam, kindof like a 340cam.
A flat-tappet hydraulic might turn your 268 into a
[email protected] middle of the road cam.
All of these could have the magic 60*ICA.
Now armed with this knowledge, you could back up the bus and go for the previous 56* ICA and just grab the fastest solid lifter cam you can find.
But there is another problem. solid lifter cams are rated differently than hydros and you need to order the next bigger cam usually, to get the NET 56* ICA that you want, because some of the duration is lost in the lashing. And this brings up another little tidbit; the lash plays a huge part in the actual ICA, and the greater the lash spec the more duration is lost. So it sorta sounds like a crapshoot and it sorta is.
So then your search is for a fast-rate cam, with a tight-lash spec and go one size bigger than you planned on.
And that brings us to LSA the lobe separation angle.
Generally, a wide ratio trans requires a wide LSA cam and vice versa. This is to satisfy the powerband requirement of the trans.
Wide LSAs start at about 115. And tight ones at 106 with 110s being the default will-work-in-either. In the above examples I used 110s and 4*advanced, only because this is off-the-shelf stuff.
The thing is; two otherwise identical cams, but one with a 108LSA and the other a 114; the 108 will make more specific power than the same cam ground on a 114LSA. However the 114 will have a slightly wider powerband.
This only matters to you on the 2-3 shift...... which with 3.23s might occur at 86mph. As a streeter, how often do you expect to be racing somebody up there that you would need to be concerned with this? With the proper cam and 15" street tires, the proper combo will spin the tires to 40, or 50 mph, so the LSA could be tightened up, to net you a few more rpm on the cam and thus a tad more power...... but at this power level I wouldn't worry much about it, so long as you stay away from anything over 112...... If we're still on the same page.
solids require periodic lashing to maintain the proper ICA and thus the power. But solids allow the engine to make a bit more power.
Ok so bottom line now is still same as it was earlier. Get the fastest rate of lift solid lifter 268 you can lay your grubby hands on.......... with the previous combo worked out.
That takes care of the power.
Unfortunately, the fast rate 268/276/110 cam comes with a penalty, the overlap at advertised, is 52*. Overlap is the time in the workings of the engine during which BOTH valves are open to some degree. With both valves open, the intake manifold is connected to the header pipes. And headers are designed to yank on the atmosphere during this time, initiating Air/Fuel flow before the falling piston actually gets to work on that. This effect is tuned by the header manufacturer to occur over a specific usable rpm range, usually higher in the rpm range. But it is still working at idle and still working at the lower rpms to some degree.Soooooooo, guess what that does to fuel economy. Hyup gas is going right out the tail-pipe.
But something else is going on. The later your ICA is, the further up the cylinder the piston is able to travel, before the intake valve actually closes. Sooooo it is possible for the piston to push some of the just inhaled AF charge, back into the intake. This effect is most noticeable at idle, and is part of why the idle-vacuum drops as the cam gets bigger. As the rpm increases, there is less and less time for this phenomenon to occur, and eventually the vacuum rises to a peak value, with a street cam usually around 2000 or a little higher rpm. So guess what this does to fuel economy.Nothing good that's for sure.
So that was scenario 1 for you.
Scenario 2 is to keep your 3.55s , and keep your 727, and re-examine the cam choices, the cylinder pressure, and the TC. But one thing remains the same; with a true, measured,9.5Scr, on the start-line, you will have to play the balancing game between gear,Stall and cylinder pressure; and for performance, economy usually is the loser, no matter how fast the rate of lift on the cam is.
