First, a big thanks to Paul Vanderpot and Mike McCagh for responding to my request for info late last week. Paul took a bunch of tape measurments, and Mike took both tape measurements and measured runner volume, which is a good input for the Engine Analyser simluation program to compute equivalent round runner diameter. WHAT A GUY!!!
And the results?!
Plenum volume works out to around 350 cubic inches. From a practical standpoint consider it equal to engine displacement, which is very generous. The FI manifold is essentially a "tunnel ram", but doesn't suffer from the fuel distribution problems that tunnel rams with carbs suffer at less than WOT and high revs. It's measureably more efficient than any reasonably streetable manifold and carburetor combination.
Equivalent round section runner diameter works out to about 1.8" for the FI manifold versus 1.6" for a good four barrel manifold. The head is also about 1.6" - average equivalent round port. The FI manifold's generous plenum and large and straight runners represent very little of the inlet system's overall restriction. The FI manifold runner length (including the adapter) is close to 6.0" - about the same as a typical four-barrel carb manifold, which yields a total inlet passage length to the valve of about 11" with either system.
I've never seen first hand flow data for the air meter, but anecdotal evidence indicates about 600 CFM @ 1.5" Hg. Jim Gessner told me that a modified air meter he had on a vintage racer flowed 750. I used 600 for FI and 585 for the carb.
So what, you say?! Well, there's that age old controversy: Does FI really make more power than its SHP cousins with a carb, and if so, how much?
Plugging this data into the Engine Analyser simulation program yields some interesting results. The long block is a 30 over 327 with 10.5:1 CR (real, not GM's marketing number) forged pistons at .003" clearance, windage tray, and LT-1 cam, topped with reworked heads with flow numbers as published in Vizard's book and 2.5" cast iron exhaust manfolds. I looked at net power so the engine has a clutch fan and water pump and a good exhaust system that doesn't create more than about three psi backpressure at peak revs and SAE net atmospheric conditions.
I hate to quote numbers because the relative differences are more important than the actual computed numbers, but let me put it this way: With a well massaged set of OE heads, a SHP engine is not that far behind the modern LS1 and FI will get you partway to a LS6, so GM's higher ratings for FI are probably reasonable, except '64 - '65. More on this later.
To be fair to the modern engines, their 80 or 90 percent torque bandwidth extends much lower, so the vintage SHP/FI engines are well behind in stump pulling low end torque, particularly FI because it is essentially a 360 degree or "single plane" manifold (as is a tunnel ram) that does not have the favorable low rev wave dynamics that help 180 degree "dual plane" manifolds make good low end torque.
One "fact" that is very important to keep in mind is that these engines are limited by the HEADS - not the carb or inlet manifold, not the FI system, not the cast iron exhaust manifolds, and definitely not the LT-1 cam. Also, it is very important to do everything possible to minimize exhaust back pressure, and the OE 2.5" exhaust with a set of low restriction mufflers should meet this requirement.
Another important conclusion: As head flow improves, the relative advantage of FI becomes GREATER, at least as far as upper rev range power is concerned. With OE unmodified heads the difference is not that great, but the FI will pull a greater percentage increase in the upper rev range as the heads are improved.
I suspect that the modest 10 HP advertised rating difference for '64-'65 SHP/FI engines was part of the "conspiracy" to kill FI. The big block was in the pipeline, and I'm sure GM was losing money on every FI system it built by that time, and I even suspect they may have lost money on every FI system they built since '57. The approximate $400 premium that FI cost in its last two years versus its carbureted SHP cousin probably steered a lot of potential L-84 buyers to L-76, and the production quantities support this conclusion.
Regardless of whether you're a carb or FI guy, these engine, with massaged OE heads and the LT-1 cam are damned impressive for 35-40 year old technology. Chevrolet Engineering understood the strengths and weaknesses of the heads very well by the late sixties, and the rather unusual LT-1 cam timing mitigates the heads' biggest weakness which is exhaust flow.
Duke
And the results?!
Plenum volume works out to around 350 cubic inches. From a practical standpoint consider it equal to engine displacement, which is very generous. The FI manifold is essentially a "tunnel ram", but doesn't suffer from the fuel distribution problems that tunnel rams with carbs suffer at less than WOT and high revs. It's measureably more efficient than any reasonably streetable manifold and carburetor combination.
Equivalent round section runner diameter works out to about 1.8" for the FI manifold versus 1.6" for a good four barrel manifold. The head is also about 1.6" - average equivalent round port. The FI manifold's generous plenum and large and straight runners represent very little of the inlet system's overall restriction. The FI manifold runner length (including the adapter) is close to 6.0" - about the same as a typical four-barrel carb manifold, which yields a total inlet passage length to the valve of about 11" with either system.
I've never seen first hand flow data for the air meter, but anecdotal evidence indicates about 600 CFM @ 1.5" Hg. Jim Gessner told me that a modified air meter he had on a vintage racer flowed 750. I used 600 for FI and 585 for the carb.
So what, you say?! Well, there's that age old controversy: Does FI really make more power than its SHP cousins with a carb, and if so, how much?
Plugging this data into the Engine Analyser simulation program yields some interesting results. The long block is a 30 over 327 with 10.5:1 CR (real, not GM's marketing number) forged pistons at .003" clearance, windage tray, and LT-1 cam, topped with reworked heads with flow numbers as published in Vizard's book and 2.5" cast iron exhaust manfolds. I looked at net power so the engine has a clutch fan and water pump and a good exhaust system that doesn't create more than about three psi backpressure at peak revs and SAE net atmospheric conditions.
I hate to quote numbers because the relative differences are more important than the actual computed numbers, but let me put it this way: With a well massaged set of OE heads, a SHP engine is not that far behind the modern LS1 and FI will get you partway to a LS6, so GM's higher ratings for FI are probably reasonable, except '64 - '65. More on this later.
To be fair to the modern engines, their 80 or 90 percent torque bandwidth extends much lower, so the vintage SHP/FI engines are well behind in stump pulling low end torque, particularly FI because it is essentially a 360 degree or "single plane" manifold (as is a tunnel ram) that does not have the favorable low rev wave dynamics that help 180 degree "dual plane" manifolds make good low end torque.
One "fact" that is very important to keep in mind is that these engines are limited by the HEADS - not the carb or inlet manifold, not the FI system, not the cast iron exhaust manifolds, and definitely not the LT-1 cam. Also, it is very important to do everything possible to minimize exhaust back pressure, and the OE 2.5" exhaust with a set of low restriction mufflers should meet this requirement.
Another important conclusion: As head flow improves, the relative advantage of FI becomes GREATER, at least as far as upper rev range power is concerned. With OE unmodified heads the difference is not that great, but the FI will pull a greater percentage increase in the upper rev range as the heads are improved.
I suspect that the modest 10 HP advertised rating difference for '64-'65 SHP/FI engines was part of the "conspiracy" to kill FI. The big block was in the pipeline, and I'm sure GM was losing money on every FI system it built by that time, and I even suspect they may have lost money on every FI system they built since '57. The approximate $400 premium that FI cost in its last two years versus its carbureted SHP cousin probably steered a lot of potential L-84 buyers to L-76, and the production quantities support this conclusion.
Regardless of whether you're a carb or FI guy, these engine, with massaged OE heads and the LT-1 cam are damned impressive for 35-40 year old technology. Chevrolet Engineering understood the strengths and weaknesses of the heads very well by the late sixties, and the rather unusual LT-1 cam timing mitigates the heads' biggest weakness which is exhaust flow.
Duke
Comment