What MPG are you 340 guys getting?
AJ, kinda surprised about the 340 fuel mileage comment. My experience has been the low compression 360's were the gas suckers. I guess it is the overall combo. Do you think that a 360 scienced out combo would make significantly better mpgs over a 340 optimized combo?
How did I miss this?
Long overdo, but
as to fuel economy, on the hiway, the factory low-compression 360 should theoretically annihilate the factory hi-compression 340; never mind after being scienced out.
There are several reasons for this;
Firstly; the Scr of the 340 even tho advertised at 10.5, it never was.
But it doesn't matter to this conversation because to go 65 mpg with a certain car, requires an exact amount of power. which means it should require an exact amount of fuel.
If you ran those two engines in identical cars so that the load would be the same, now you are down to camshafts and timing.
The throttles will control the power output, by the EffectiveCompression ratios while being throttled, so the Static Compression ratios are meaningless, same for Dynamic Compression Ratios ...... because both are calculated at WOT.
That leaves just;
the duration of the PowerStroke and The amount of Overlap, and ignition timing.
To that end,
As to the Power extraction,
installed at 4* advanced, the 340 has just 104* of Power extraction, to the 360 at 116*, which is a huge difference of 11.5%.
As to overlap;
the 340 has 44* to the 360 at 32*, which is just 73%= 27% less for the 360.
With log manifolds most of that is gonna go dead, but deader for the 360 .
As to ignition timing.
I don't have the timing curves but, this I know, NEITHER of them ever had an optimum curve.
Lets say the 340 ran 3.23s as was typical, and the 360 ran 2.76s which was also typical. and lets assume both on same 27" tires. and both with autos and the 340 at 3% convertor slip to the 360 at 2.55, BOTH at 65 mph.
Ok so the 340 is cooking along at 65=2690, and the 360 car at 2180.
At 2690, the 340 car might want cruise timing of 56 degrees, but only gets say 35.
At 2180 the 360 car might want 48, but only gets say 29*
So where is the Fuel economy going?
Well, the 340 is opening the exhaust way too early, dumping pressure, which could have gone to the crank. But to it's credit it is cruising at an rpm at which the reversion should not be an issue, is running at 500rpm higher than the 360, and, it's way short of cruise timing.
Whereas the 360, is pulling more useable pressure out of the expanding gasses, is cruising 500 rpm slower, is also clear of reversion, but is operating closer to ideal ignition timing.
Now, if you build both engines the same, and tune them the same, then about the only difference is the stroke ...... right? Which means the 360 gets more effective stroke, runs a smaller throttle opening, and gets better steady-state fuel economy because of it.
Where most guys fail, is in not giving their engines enough cruise timing. Mine likes over 56* at 2800=63 mph, in direct. In overdrive at 85= 2160, it has liked over 60 degrees, up to 63 actually
How do you get there with a factory-type distributor?
Answer; you cannot. .
If you max your power-timing out at 36*, say at 2760,to run 3.23s, and modify your VA to the max of say 24*, that totals 60* allright .................. but now just try and drive it from idle to 65mph without detonation, with all that PowerTiming, in so early. You'll need best gas, which around here is at least 20% more money. Well then, you'd have to make at least 20% better fuel economy to make it worth while..... right?
How then do I do it?
My secret is a stand-alone, dash-mounted, timing module that has an electronic range of up to 15 degrees.