MPG Motor Magnum vs. Hemi
AJ, help is much appreciated.
I've only driven with this 360 engine (and cam) for about 3 weeks. And had 3 tank fill ups.
But till now the mileage has the same with the bone stockish 318 ('88 block w/ stock rollercam) and early '80s "302"-heads.
I have no issues with swapping cams, but if I do I want to step up to a (solid) roller.
Propane make less power in the same built engine, but if the build is optimized for propane it can come very close or sometimes surpass gasoline power.
When I got my Dart it had the 318 in it with a 904 transmission and a 7-1/4" 3.23 rear axle.
It could (once) manage 17mpg on a long highway trip, keeping it below 65mph.
(Fact was the engine started running hot if went faster so had to drive just over 60 all the time).
Found out later the engine was running very lean too.
18 years ago, I was thinking of going LPG/CNG with my 11.3- 360 as well, but as you probably know, 11.3 is on the low side for propane....... they say. I threw out all my propane/CNG literature, but memory says those gasses liked 13/14 to one with iron heads. I can't say if that is true or not, that's a long time ago. I also seem to remember the companies pedaling the equipment were also recommending a very wide LSA; 115 IIRC.
In your case; if in fact the 11.3 is on the low side, then a solid lifter cam would be part of the answer. This is because you can run a smaller advertised cam that will trap more charge and increase your cylinder pressure,yet maintain the same .050 specs.
Stated another way; with same advertised specs, a solid will yield more .050 duration , and so ... more absolute power.
However, when going from a flat tappet to a roller you have to bare in mind that the rollers usually have quite a bit slower opening/closing ramps, and this doesn't usually cause a problem........ until you start trying to increase cylinder pressure on an already built engine, while trying to maintain the same absolute power.
Going back to the former examples; see #1 and #2 below; Firstly I couldn't find a roller XE256, so Ima guessing your 360 is an LA block with a flat tappet hydro in it. So when swapping to a Solid roller,you will want to maintain that 212/218 .050 spec if at all possible. I have seen some solid roller with ramps of 38*, so 212/218 plus 38, yields 250/256, and adding a couple for lashing, I get 252/258, and we want to spread the Lsa to 114. Lets see what that looks like; see #3 below. Ok, so, we see that the pressure dropped 9psi/14VP , not the best . Lets advance the cam to, in at 110 and that would be example 3b). And that is sneaking back up to your current specs.Ok great, but I see that the power duration is down to 116, and I see an opportunity to extract a bit more energy here for economy, and that gets us to 4), which looks like a fair trade.
So as you can see, the rollers cannot maintain the same pressure/VP, as your current combo, with the same .050s
What about a solid Flat Tappet? Glad you asked, but you won't gain anything over a solid roller, and the solid roller will make more power than a same-sized solid FT, size for size...... so that's kindof a bust.
On the off chance that you currently have a preponderance of unusable low-rpm power, you could up-size intake duration one size, with the solid roller,and gain some high-rpm power. And that takes us to #5.
But I'll bet your 11.3Scr was achieved with a zero deck and the Fell-Pro .039 gasket for a Q of .039. And....... I ran a Q of .028 for over a year on 87E10 in mine; so that's 2.48cc right there. Recalculating your Scr for the difference, I get a new Scr of 11.67....... and you see where I'm going right... #6
Of course with the new higher Scr, that leads to #7,lol. OOps, we gave up some fuel economy on that one, but we got a ton of absolute power over the baseline.
Ok so #7 is a bust. How about #8?
So #8... is a full size .050 bigger Solid roller than your current FTH. That should be good for around plus 18hp. The powerband is also widened for the automatic, and so will generate more average hp from the torque peak to the power peak and a bit beyond. The 8* less overlap will help prevent LPG from escaping across the piston. The pressure/VP is down some, but Ima thinking that will be a non-issue. Ima thinking; winner!
Ok so,
this is just an exercise in what ifs. I did not go looking for any of these cams, and so far as I know, yours is the only one that actually exists.
You can slam the door on escaping LPG just by running it at a higher rpm.You don't necessarily need a different cam. If you slowly rev it up in Park, while watching the vacuum build on a gauge, you will see the vacuum climb to plateau somewhere around 2000. When it does, this is the lowest rpm you want to cruise at for best fuel efficiency. At/above this plateau is when the overlap period is no longer allowing monkey-business in the intake, and the theory is that time is no longer great enough for significant LPG loss across the top of the piston. So you want to spend as little time as possible, below that rpm.Especially at hiway cruising.Sometimes increasing your hiway cruise speed nets an INCREASE in mpgs, just because of that steenking overlap period.
>Also IIRC LPG wants early ignition timing.... and lots of it.
Try this: get the rpm up to about 2200, on the fast-idle step. Then advance the timing until the rpm no longer rises. Make sure the Vcan is on the sparkport and is being activated. At 2200 rpm it should be; but just pull it off and see if the rpm changes.If it drops, that's good. Ok check the timing now at 2200. That is the advance, that the engine wants, at that rpm with no load on it. It may not want quite that much when cruising. Ok kick her off the fast idle and return the timing to your baseline.
Now think about it.
I bet you didn't have nearly enough,lol. My engine accepts up to 63 degrees at 2150 (on gas) for cruising at 65. I don't run it quite that high. The more I advanced it, the faster the car drove... with the gas pedal locked. So I reduced the foot-feeder, to drop the mph back to 65; so now I was cruising at the same speed with less throttle opening..... and the fuel economy increased. But even better, at the new timing and throttle opening, I was able to lean her out some more. I won't tell you that I got her up to 32MpgUS, point to point, over a 10 hour cruise ; nobody believes that. But I can say over 20USg, and somehow that sounds more believable. Ok I cheated; I had a double overdrive manual trans with a final-drive of a tic under 2.0,lol, and a
[email protected] cam. Your results may vary.......
>So, no matter what cam is in it;
Perhaps run a little more rpm at hiway speed. This will slam the door on the LPG escaping across the piston, as will keeping the Rs above say 2200/2400 rpm around town.
Put a low-pressure gauge on the crankcase (leave PCV installed), and run it up to where you can see it from the cab. Leave it there for a few days, and make sure you never see pressure on it below about 3500 rpm.
Comparative Chart
.....int.....comp.....pwr....exh...o/lap...in at/ICA....Lsa......050s ............................psi/VP
1) 256...126.....118...268...42.....106/54... 110... 212/218 current FTH.. 202/190
2) 250....122......117....256...25..... 113/58.....114... 208/214 theoretical FTH 195/178
______________________solid rollers below_________________
3) 252....121.....116....258...27......113/59.....114... 212/218 solid roller........ 193/176
3b)252...124.....113....258...27......110/56.....114....212/218......................... 198/184
4) 252....123.....119....256...30......111/57.....112....212/216....................... 197/181
5) 258....118.....112....266...34......113/62.....114....218/226....................... 188/167
_________________________ below at 11.67Scr _______________
6) ............... same as #5 .............................................................................. 196/174
7) 262....118.....108....270....38......111/62.....114....222/230....................... 196/174
8) 260...118.....112..264....34......112/62...114....220/224..................... 196/174
1 to 5 are at 11.3Scr and 6 to 8 are at 11.67Scr and; All are at 500ft, for comparison.
10) 250..129...115...262....36......106/51....114....206/212 your current
cam, converted to "solids" with lash allowance of 3* at each end
11) 252...127.....116....260....32.....110/53......112....206/214 the factory 360 2bbl cam
