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What was the primary difference between the '74,'75,76, and '77 L48 engines? Is the dip in power (on the '75) related to cat converter and exhaust or more a result of differences in rating points to make it appear lower than it really is? I'm anticipating a rebuild on the '75 that I just picked up, and really want to stay faithful, but don't want a snail either.
Cat Converter, acts like a potato in the exhaust, also single pipe vs dual, plus '75 is heavier somewhat due to the metal floor which is used due to the heat of the converter.
As far as I know there are no major differences in the L-48 configuration from the carb inlet to the exhaust manifolds from 1975 to 1980, and even back to 1971 - same basic block, heads, pistons, camshaft, inlet and exhaust manifolds, and carburetor. It was also essentially the same as the '69 base 300(gross) horsepower engine other than having a reduced compression ratio to operate on unleaded regular.
The biggest problem with post '74 configurations is the single catalytic converter. It's a real power killer. Dave McClellan, in his book, stated that the single catalyst added "half an atmosphere" backpressure to the exhaust system. That's about 7.5 psi, which would place total exhaust system backpressure in the 10-15 psi range.
At LOT of power is consumed pumping against this backpressure and exhaust gas dilution of the fresh charge when exhaust pressure is this much higher than inlet pressure kills volumetric efficiency.
The gradual improvement in L-48 power as the years progressed was primarily "tuning" issues - fuel and spark advance maps" and improved air induction systems including, I believe, a cold air induction system in later years that bypassed the inlet air heat stove at WOT.
It's easy to make a pre-'75 L-48 make 300 net HP without affecting idle characteristics or killing the low end torque - same way as I do on 327/300s, but without a low restriction full dual exhaust system it's DOA.
Vehicle exhaust systems are an afterthought to most guys, but I am here to tell you that they are a BIG DEAL, and a restictive vehicle exhaust system will absolutely kill top end power, no matter what you do to the engine. Replacing the OE catalyst with modern monolithic catalist will help, but the engine really needs TWO low restriction catalysts, like modern V-8 engines.
Fortunately most pre-catalyst Corvettes have very efficient exhaust systems - especially the SBs with 2.5" manifolds and pipes. But if you're stuck with the single catalyst system it's a whole other game with big odds stacked against you.
The 1975 has a fiberglass floor except for an area under the passenger seat. There is a aluminum insert there which is located directly above the Cat converter. This factory modification(design change)was implemented because of the high heat emitting from the Cat. converter; it tended to soften the fiberglass. The amount of metal and it's weight is negligible, however it is more than the fiberglass of earlier years.
Am I correct in assuming that the power and torque figures you quoted were measured on a lab dynamometer with no fan and no mufflers and the observed data was corrected to standard sea level conditions?
When quoting torgue/power figures, the context should always be provided. Otherwise, the data is pretty meaningless.
All of that being said, I'll probably opt for a "straight up" rebuild (providing the bores and pistons are o.k.), select a cam close to stock lcs, and a true dual exhaust system (the original converter is long gone anyway...I can always replace it with stock system if it ever becomes an issue). I'm saving all of the "hot rod" stuff for the '74 (the ZZ4 383 is patiently waiting)
A rebuild will likely require cylinder boring, which allows the opportunity to increase the CR, and I recommend about 9.75 (actual measured, not based on some piston catalog specification) with the OE cam, but this will required premium fuel.
As with any Chevrolet V-8 the best single "modification" is pocket porting/port matching/chamber relieving the OE cylinder heads along with multiangle valve seat geometry.
With the heads massaged as above, the 3.48" stroke configuration responds nicely to retarding the OE 3896929 cam four degrees, and I do not recommend any other off-the-shelf camshaft. Exact reproductions of the 929 cam are available from several manufacturers.
With a 2.5" full dual exhaust system the above will yield about 270 net horsepower and useable revs to about 5500 with only a slight loss of low end torque and should work very well with the typical Turbohydramatic transmission and 3.08 axle - even better with a four-speed/3.36.
Keep in mind that removal of a catalytic converter and conversion to dual exhaust is illegal under federal law. So, if you have to undergo emissions testing, either periodic or at the time of transfer, you'll have problems.
It's also illegal under federal law to even add true dual exhaust WITH catalytic converters (i.e. dual converters). If the application originally used a single converter, you cannot legally add dual converters.
Also, the 1975 application originally used a pellet-type converter. This is the most restrictive and performance-robbing type of converter. I don't know if these can legally be replaced with a monolithic type converter, or not.
It would be an interesting exercise to test your car on a chassis dyno with SAE air density correction. Your peak RWHP would probably be near 300.
Lab dyno data is usually corrected to standard sea level air density (29.92", 60F dry air), which is sometimes called "standard correction". SAE air density (990mb, 25C, dry air) correction (or "SAE corrected") reduces torque and power values by about 4.5 percent relative to "standard correction", and the front end accessories and exhaust system further reduce output to yield SAE net at the flywheel. And, on average, rear wheel data is reduced a further 15 percent to account for drivetrain and tire loss.
I have both lab dyno data and chassis dyno data for two "327 LT-1" configurations that are essentially identical including the massaged 461X heads. The first produced about 360 SAE gross horsepower (standard correction) @ 6500 on a lab dyno. (The massaged 461X heads from this engine were tested, and the 236 CFM @ 28" H20 depression @ 0.5" valve lift is very good!) The second produced 278 SAE corrected RWHP @ 6500 on a Dynojet 248C, so estimated flywheel net power is 278/.85 = 327 net horsepower at the flywheel.
And 327/360 = .91. So the ratio of SAE net to SAE gross is only .91 rather than the 0.80 that is usually quoted as an average number. Since nearly half the difference is due to air density correction alone, the other half is exhaust backpressure and front end accessories, which indicates that the Corvette exhaust system is very efficient for a high output 327.
A strong 427 pumps more air, so exhaust backpressure will be higher and the ratio of SAE net/SAE gross with be lower.
If you run your car on a chassis dyno, you can do the same analysis and determine how much power your engine is loosing in the chassis due to exhaust system restriction.
BTW, that 327 LT-1 chassis dyno test was done in 90 degree weather with NO external cooling fans. The owner reported that the fan was "screaming" (but it didn't overheat after several pulls). With the fan clutch "loose" the fan will slip at about 1500 RPM and probably consume no more than one horsepower. When the clutch is fully tightened the fan will not slip until about 3500 PRM, and since fan power dissipation increases with the CUBE of speed, it was probably consuming about 10-15 HP and this power loss would be near constant from 3500 to peak revs.
So the 327 LT-1 tested on the chassis dyno is probably making closer to 290 SAE corrected RWHP with the fan clutch loose, which means the exhaust system loss is virtually nil. This particular car has the OE 2.5" manifolds and pipes, but the OE shaped mufflers are straight through types with 2.5" cores and tailpipes.
As another point of reference I have lab dyno data for a "stock rebuilt" 327/340. The only change from OE was a 0.040" overboard. It made 295 SAE gross HP @ 5500, which gives you an indication of how overrated the gross power ratings were of that era.
The above also shows the importance of providing full data context when dicussing engine performance data.
Duke..Thanks for all that info.I never really got into all the ins and outs. I am sure that my RWHP is lower than the numbers pulled on the shop dyno.The one thing that i do know is that it gets up and goes.And it leaves all those mustangs far behind me..That is except for those guys that run that bottle in the trunk.
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