Howdy guys. Years ago, I used to alternate carbs.
One was a 2bbl Motorcraft...great off idle power but quickly ran out of breath on my large engine.
The other was a Quadrajet...made a very large difference above say 3000 rpm.
When I converted from the Quadrajet to the single 425 last weekend, it effectively felt like a put a 2bbl carb back on the truck--the low end is excellent, quite possibly snappier than with the Quadrajet off idle, but the 460 cfm of the Impco 425 simply isn't the same as a 750 cfm Quadrajet. Due to that, I'm strongly considering running dual 425's in the next few months. My thread with more driving impressions is here:
http://www.ohio4x4.com/forum/showthread.php?1690-Propane-Conversion&p=70815#post70815
Brief engine specs:
460 bored .040 = 469 cubic inches
9.5-1 compression (1969 heads on stock later model pistons)
Early 460 timing chain set (more low end torque...later models were retarded...literally)
Comp Cams Extreme 256 camshaft
Matching Comp Cams valve springs
L&L Headers
Stock cast iron intake.
When building the engine, every time I came to a potential "Y in the road" regarding maximum horsepower or maximum torque, I went route to get maximum low end torque.
I'd estimate 340-ish horsepower (that's pretty low for such a large engine).
I'd estimate 525-ish foot pounds of torque.
I'd consider it to have a 5000 rpm redline.
I rarely need to wind it above 4000 rpm.
With my lack of low gearing, when all four 42's are turning with the engine at 4K or 5K, they are turning pretty quickly.
Regarding my engine, it's nothing high tech...I built it for maximum grunt in 2005. The engine is able to compensate for my lack of gearing (crawl ratio of less than 20-1, not counting the torque converter). I understand that a 10% loss in power is common when switching to propane. I'm quite happy with the propane so far, but IMO the single Impco 425 just isn't enough to properly support a 7.7 liter engine even at moderate rpm.
I'll be interested to follow this thread.
Roy
