Redline Question? Where does Engine Power Level Off
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,084
Likes: 8
From: Illinois
Redline Question? Where does Engine Power Level Off
No matter what engine you have when do most engines stop accelerating to a great extend as you approach redline. If the car redlines at say 6000rpms where you you expect the motor to seem like it is no longer accelerating and it stops pulling---at 5000, 5500, or 6000 rpms. Is it the same in all 4 gears of a 4 speed. Or does it just start to float and level off with little increase as you near redline. This for a stock corvette not a highly modified one. 350cc/ or a big block--is there a difference. Thanks Always wondered about this!!!!!!!!!!!!
Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
Redline and powerband have little or nothing to do with one another. I can make a motor capable of spinning to 8000 rpm with very good components and put in a small cam and heads and make its peak power at 4500 rpm. Other way you can take an L48 and put on fantastic heads and a huge cam that makes peak power at 7500 rpm but I would not spin a L48 much past 5500 rpm or it will come apart. Redlines were set at the factory to hope that owners would not spin the motors past the capabilitys. Redline just usually means it will start to come apart above the stated rpm or severly reduce the life. Peak power is determined by heads and cam and induction systems.
On stock motors, which stock motors are you talking about. Late 70s smog motors don't make much power beyond 5000 rpm. An early 70s LT1 might make peak power above 6000 rpm. It is not a simple answer.
On stock motors, which stock motors are you talking about. Late 70s smog motors don't make much power beyond 5000 rpm. An early 70s LT1 might make peak power above 6000 rpm. It is not a simple answer.
Melting Slicks
Joined: Jun 2009
Posts: 2,709
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From: Savannah GA
Wow. How to answer this one.... Redline is a speed beyond which the manufacturer (or builder/tuner) worries about breaking the "weakest link" parts in an engine. Engines can be tuned to achieve maximum power well before max RPM, at max, or beyond max if some component which is safe to say 5000 RPM is used in an engine which peaks at 6800. If your cam reaches maximum airflow at 4500 rpm, but the engine has all components safe to 7000 in it, redline is 7000 but it will gain you nothing to spin it that fast.
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,084
Likes: 8
From: Illinois
The exact engine I am interested in is a 71 LT-1; 350cc/330 hp with a newly rebuilt carb by Jerry MacNeish, newly rebuilt TI system, and it does have a newly rebuilt AIR pump. The car goes like mad but seems to flatten out at about 5000-5500 rpms. The distributor has stock springs so that centrifugal advance is: 0 at 1060rpm; 2 degrees at 1340rpms;17 degrees st 2400 rpms; and 24 degrees at 4800 rpm. Set at 8 degrees at 700 rpms which is what manual calls for.
I know redline is where things can go wrong but wondered if most engines continue to pull to redline or flattenout at some number below. Mine redlines at 6500 rpms. I have new ignition wires and stock plugs.
Just trying to understand--I guess it would be called the power curve?? No use pushing the engine past Xrpms and will shift into the next gear if it starts to flatten out at some point. My LT-1 sure doesn't make peak power at 6000 rpms----it least doesn't seem like it and I don't want to such any higher than 6000rpms--original motor--don't want to blow it. I HAVE HAD ONLY ONE OTHER CORVETTE A 64 327/300HP and therefore not much to compare to. I did test drive a 2003 ZO6--all I can say is WOW!!
Thanks again. I really appreciate you experts explaining this
I know redline is where things can go wrong but wondered if most engines continue to pull to redline or flattenout at some number below. Mine redlines at 6500 rpms. I have new ignition wires and stock plugs.
Just trying to understand--I guess it would be called the power curve?? No use pushing the engine past Xrpms and will shift into the next gear if it starts to flatten out at some point. My LT-1 sure doesn't make peak power at 6000 rpms----it least doesn't seem like it and I don't want to such any higher than 6000rpms--original motor--don't want to blow it. I HAVE HAD ONLY ONE OTHER CORVETTE A 64 327/300HP and therefore not much to compare to. I did test drive a 2003 ZO6--all I can say is WOW!!
Thanks again. I really appreciate you experts explaining this
Le Mans Master
Joined: Oct 2006
Posts: 7,123
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
What carb? A 650 on that engine will give you great low end, but it'll run out of air in the upper RPM range. I've got a 350 hanging on a stand that the 650 was great for low end, ( 5500 was it's end of pulling), an 850 was terrible on the low end but fine on top, (I quit before it quit pulling) and a 750 was a decent balance of both. (Made 7000 easy)
Burning Brakes
Joined: Jan 2007
Posts: 1,057
Likes: 4
From: Concord Calif
No matter what engine you have when do most engines stop accelerating to a great extend as you approach redline. If the car redlines at say 6000rpms where you you expect the motor to seem like it is no longer accelerating and it stops pulling---at 5000, 5500, or 6000 rpms. Is it the same in all 4 gears of a 4 speed. Or does it just start to float and level off with little increase as you near redline. This for a stock corvette not a highly modified one. 350cc/ or a big block--is there a difference. Thanks Always wondered about this!!!!!!!!!!!!
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Race Director
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In addition to the distributor change mentioned above, did you check the float levels after installing the carb? I have a 70 LT1 shortblock and it has no problem going to 6,500 and still feels like it wants to go higher.
Last edited by MelWff; Sep 16, 2009 at 03:33 PM.
Race Director
Joined: Oct 2000
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From: central FL, near the beach.
my tach was accurate EXCEPT at max acceleration. it was slow to respond, so in 1st it said 5500, was really 7000,
i noticed my mph was too high for 1st, like 65 MPH
so i NOW use a shift light
i noticed my mph was too high for 1st, like 65 MPH
so i NOW use a shift light
Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
If you take that crappy exhaust system off and put headers on it and let it breathe it will easily pull to 6500 rpm. I had a stock system on mine. I measured the exhaust pipe as it reduces down right at the exit of the manifold into the pipe. It was down to 1 7/8 diameter. yes it went out to 2 1/2 from there but you are trying to squeeze all the airflow through 1 7/8. My headers now have 1 3/4 pipe per cylinder and you are trying to push 4 cylinders through 1 7/8. 
It will reduce your upper rpms power. After I put headers on mine it was a different car. Power was good to 6500 rpm dropped my 1/4 mile time 1/2 second and picked up 4 to 5 mph.
It will reduce your upper rpms power. After I put headers on mine it was a different car. Power was good to 6500 rpm dropped my 1/4 mile time 1/2 second and picked up 4 to 5 mph.
Le Mans Master
Joined: Dec 2003
Posts: 5,727
Likes: 38
From: Fremont CA
I would start looking at the secondary diaphram of your factory Holley 4bbl carb for failure or a binding of the secondary throttle shaft. There may also be a slight chance of valve spring or timing chain problems ANY LT-1 should run to 6500RPM easily with the factory Solid lifter cam, even a 71/72 low compression motor.
Last edited by Solid LT1; Sep 17, 2009 at 02:47 AM.
Burning Brakes
Joined: Jan 2003
Posts: 755
Likes: 32
From: Roswell Georgia
If I did this correctly, there should be a clickable thumbnail below of HP and torque curves of three Vette engines: a 91' L98; a '70 LT-1; and a '92 LT1. Gives you an idea of the as installed HP of these three engines. 
Last edited by mr.beachcomber; Sep 17, 2009 at 09:01 AM. Reason: Thumbnail didn't work so I just added the image.
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,084
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From: Illinois
The carb is a stock #4801 holley and Jerry M. assures me that everything is checked on the carb prior to shipping. How much difference does the springs of the distributor have?
This might sound dumb--but you are saying that even at 5500-6500 rpms the motor will continue to accelerate--but how much compared to 2-3000 rpms or 3-4000 rpms.
The pipes are 2.5 inch on the LT-1 except a small section--if I remember right when I replaced the entire system with a aluminum one and magnaflows.
Thanks
This might sound dumb--but you are saying that even at 5500-6500 rpms the motor will continue to accelerate--but how much compared to 2-3000 rpms or 3-4000 rpms.
The pipes are 2.5 inch on the LT-1 except a small section--if I remember right when I replaced the entire system with a aluminum one and magnaflows.
Thanks
Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
Yes the motor will continue to rev higher even if it is out of the power band. But why do it the car will accelerate slower this way vs shifting to the next gear and getting back into the power band. If you look at the chart above there is no reason to take the rpm even to 6000 rpm. The power levels aredropping off quickly right before 6000 rpm. They took the motors higher but it is making less power. If you shifted these into the next gear and put these motors back in the power band it will move the car quicker.
Last edited by Gordonm; Sep 17, 2009 at 07:38 PM.
