Recently had my 68 l-71 on the dynometer, just for a couple runs, it ran 280 hp @4750.I know rwhp ratings are much lower than gross but could this gal be getting tired?
Nick
Nick
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Re: rear wheel Hp vs gross Hp (L-71)
Being that you were about 1500 RPM below where the power is peaking, I would think you could be showing a lot of horses lower than it could have been.
Horsepower is a function of engine RPM, and the faster the engine goes, the more horsepower basically, given a lot of variables. -
Re: rear wheel Hp vs gross Hp (L-71)
On an inertia dyno you need to pull from 1500 to 6500 (or whatever the tach redline is on other engines) in fourth gear to get the whole power and torque curve. This only takes a few seconds and will not harm an engine in good condition. The L-71 should show peak power about a thousand revs higher than you revved. You barely made it past the torque peak. Based on 280 HP @ 4750 the torque is 310 lb-ft at that speed.
HP = T x N/5252, where T is torque in lb-ft and N is revs.
What was the peak torque @ revs?
A rough rule of thumb is that SAE net is equal to about 80 percent of your SAE gross 435 HP, and RWHP is 85 percent of SAE net, so a decent SHP big block should peak at around 300 RWHP, which is about what a modern Corvette LS1 will do.
This is just a ballpark empirical estimate - not a Theory of Relativity type analysis synthesized from the primodial muck of physics. Your results may vary.
DukeComment
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Re: rear wheel Hp vs gross Hp (L-71)
Thanks for the response Duke,
Max torque was 326.3 @ 4000, and I believe the tech went through all four gears.The reason I had the engine dynoed was i was getting a rattling sound from the engine at around 5000 rpm. The tech suspected weak valve springs and was right because upon observation we could actually see exhaust coming out of the rear carb at 5000 rpm.After replacing the springs the engine no longer made the noise ,and picked up horse from 267 to 280.
Yes these results are 1000 rpm lower than the factory rating and after finding the weak valve springs I'm wondering if anything else is tired also, perhaps the camshaft. The engine was overhauled in 1990 and doesn't smoke or go throughmuch oil. Any thoughts?
thanks, NickComment
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Re: rear wheel Hp vs gross Hp (L-71)
Camshafts don't get "tired". They can wear at the nose, but OE cams typically don't show much wear, even after 100K miles - at least on SBs. Using the 80 percent gross equals net and 15 percent drivetrain loss, the 460 lb-ft @4000 OEM SAE gross rating would be 313 at the rear wheels, so it sounds like your engine is nominal. Let it rip!
Assuming this is an interia dyno, do three top gear pulls from 1500 to 6500 (with about 30-60 seconds between) so you can characterize the entire torque/power curve. Make sure you have at least 35 psi in the tires or the maximum cold pressure placarded on the sidewall. Depending on gearing the tires will spin up to 130+ and the more air pressure they have, the lower the tire hysterisis loss. If it's an old fashioned water brake dyno that requires the engine to be stabilized at each point to get a power reading - pass. They are tougher on the engine. Inertia dynos are about the same as running to redline in second gear.
If it will cleanly pull from 1500 to 6500 in second gear, it should pull from 1500 to 6500 on an inertia dyno in fourth gear in about the same time. In fourth gear the transmission is 98-99 percent efficient versus 95-96 in the lower gears.
Have plenty of fans pointed at the radiator. You want to keep the fan clutch from tightening up, which will rob power from the rear wheels - easily 5-10.
I prefer to do dyno pulls when temps are below about 65 degrees since, with plenty of external fans, this will usually keep the fan clutch from tightening and eating power. The engine should be fully warmed up from normal driving with no more than about 30 minutes off time before you start the pulls - assuming a couple of minutes of run time before the first pull. The temp gage should be at its normal warmed up reading.
Many local clubs/groups set up "dyno days" with local dyno operators where they set up a car about every 20 minutes and do three quick pulls - usually for about 40 bucks. They are definitely interesting to attend, and I wish I saw more vintage Corvettes at these events.
DukeComment
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make sure the oil temp is up to 200 +
because i have seen 18 HP difference between hot oil and colder oil. using this trick a lot of dyno operators sell tune up and mods.Comment
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Re: make sure the oil temp is up to 200 +
I think that's why you typically see a slight improvement in power with each successive pull. The oil heats up pretty quickly without road speed air flow under the car.
If you have two pulls within one percent, then you probably have good data, but if the temp gets over 180 and the fan clutch tightens, that's going to cost!
On my Cosworth Vega the OE plastic flex fan aborbs about 5-7 HP at 7000, which is a lot for a 120 RWHP engine.
DukeComment
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i am used to dynoing late model corvettes
with electric radiator fans that do not come on till 230+ degrees. the clutch fan does suck up HP.Comment
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GM went to electric rad fans on some of their
light duty pickups in 05 to get better milageComment
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Re: GM went to electric rad fans on some of their
I don't think electric fans need to operate under the SAE net power rating method, so that reduces parasitic losses, and during the EPA emission certification procedure, which is also the origin of the EPA fuel economy estimates, I don't think the electric fan is needed due to external fans, so EPA fuel economy numbers may go up if a clutch or flex fan is replaced with electrics.
The "80 percent of gross equals net" was a rule of thumb in the sixties, but it varied quite a bit. For example, some cars had engines that were available with both single and dual exhaust, and the dual exhaust version would generate more "installed" power than the single exhaust version due to reduced backpressure pumping loss. (Note: Vintage Corvettes have pretty efficient exhaust systems.)
With nearly all modern cars having very efficient exhaust systems and electric fans the difference between gross and net on modern engines is probably less - on the order of 10-15 rather than 20 percent, and air density correction alone is about 4.5 percent, so the actual difference could be as little as five percent for a car setup with very efficient inlet and exhaust systems.
BTW based on a pretty good body of dyno pulls for LS6 engines it appears that the driveline loss is about 13 percent, so LS7s should average nearly 440 RWHP once a couple of dozen pulls are reported.
DukeComment
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