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  #1  
Old Jul 3rd, 09, 05:24 PM
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rad454 rad454 is offline
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Louie
 
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Default 454 losing power at high RPM question

I took the "other" car out for it's first ride after the long winter. I merged onto the freeway and nailed it. The engine ran smooth up to 38-4k when I noticed it starting to lightly miss or stutter. Dont know how else to describe it. No backfire or big drop in acceleration, just a feeling that the power curve had hit the wall. The car has a stock LS-5 and the Q-jet was replaced with a 650 Holley spread bore.
I bring this up in the 1st gen forum because I have a similar motor in my 69 Camaro that I'm just putting together. The engine builder says that 650 CFM is enough for the 454 but the majority of guys here say thats not enough.
Is the LS-5 sucking wind with too little CFM or is it another problem?
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  #2  
Old Jul 3rd, 09, 05:33 PM
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KEN
 
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Default Re: 454 losing power at high RPM question

how old is the fuel ?
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  #3  
Old Jul 3rd, 09, 05:34 PM
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Default Re: 454 losing power at high RPM question

You are under carburated!....That big bad boy should have at least a 750-800 cfm carb and preferably a double pumper!
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  #4  
Old Jul 3rd, 09, 05:40 PM
ace's68 ace's68 is offline
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Default Re: 454 losing power at high RPM question

Could be valve springs or lifters giving up... was the LS-5 a hydraulic cam?
But I also agree that 650 is pretty small to feed 454 cubes, stock or not, especially a spread bore design. Wasn't the old quad an 800, even being spread bore it would prob suit the application better than a 650 spread bore.
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  #5  
Old Jul 3rd, 09, 05:52 PM
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Default Re: 454 losing power at high RPM question

At 5500 rpms using 83% Volumetric Efficiency a 454 only needs 600 cfms.Check for bad gas or clogged fuel filter.
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  #6  
Old Jul 3rd, 09, 06:01 PM
fatblock fatblock is offline
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Default Re: 454 losing power at high RPM question

Quote:
Originally Posted by rad454 View Post
I took the "other" car out for it's first ride after the long winter. I merged onto the freeway and nailed it. The engine ran smooth up to 38-4k when I noticed it starting to lightly miss or stutter. Dont know how else to describe it. No backfire or big drop in acceleration, just a feeling that the power curve had hit the wall. The car has a stock LS-5 and the Q-jet was replaced with a 650 Holley spread bore.
I bring this up in the 1st gen forum because I have a similar motor in my 69 Camaro that I'm just putting together. The engine builder says that 650 CFM is enough for the 454 but the majority of guys here say thats not enough.
Is the LS-5 sucking wind with too little CFM or is it another problem?
Its another problem.You can still point the finger at the carb..but it is not a cfm issue if it goes to wot.
Fuel delivery/secondary ignition voltage/weak valve springs/air in,air out/bad gas and the list goes on and on.
I would focus on the miss/stutter under load.Lightly mist the cap/coil/wires and boots with water while a buddy brake torques her in low light levels while viewing underhood.Look for spark energy jumping to ground.
My initial thought was just to go out again and blow the snot out of her after sitting..your call on that one.
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  #7  
Old Jul 3rd, 09, 09:10 PM
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Default Re: 454 losing power at high RPM question

I put new fuel into her today, the good stuff. Supreme non ethanol.
The engine only has about 35k on it and has an aftermarket mild/stock hydraulic cam.
As Chad wrote, the engine should need about 6-650 CFM. But...that may be appropriate for an engine that GM wanted to fit multiple apps and needed to be more reliable than muscular.
The guru's usually say that engine problems tend to be more on the electrical side than on the carb side. My plugs look very good. A mild brownish tinge with no oily deposits. Breakerless ignition is new as is the ignition wires. New lifters and springs were installed at the time of the rebuild. I guess there is alway the possibility of a weak spring that made it past the QA process. Thats why I am narrowing it down to the carb.
Does anyone know if there is a carb, regardless of brand , that is a spreadbore, but over 650CFM?
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Old Jul 3rd, 09, 09:16 PM
ace's68 ace's68 is offline
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Default Re: 454 losing power at high RPM question

Most stock or all stock Q-jets ARE 750-800cfm, even on the small 305's they were 750's...
35k, I wouldn't doubt the springs are bad, remember these are performance vehicles and depending how you drive it and the car is set up, many of us lose clutches as little as 8,000miles
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  #9  
Old Jul 4th, 09, 04:43 AM
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Default Re: 454 losing power at high RPM question

I am voting on the carb. is too small, how can 600 -650 cfm be ok for a 350 and a 454, doesn't make sense to me. I did the Holley carb. calculator for my application and it said I needed 815 cfm for optimal performance, so I went with 825 cfm demon, for what it is worth it picked up a full 1/2 second in the quarter mile over a 780 cfm Holley. I'm sure you can run the 454 on a 650 if you are looking for fuel economy, but come on its a 454 not a Honda Civic. The question was high rpm not cruising at 2000 rpm.

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  #10  
Old Jul 4th, 09, 06:04 AM
bikedude3 bikedude3 is offline
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Default Re: 454 losing power at high RPM question

they stuck 780s on the 302 small block i would have at least a 750 on a big block
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  #11  
Old Jul 4th, 09, 07:31 AM
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Default Re: 454 losing power at high RPM question

A 6212 Holley is a spreadbore double pumper 800 cfm.
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  #12  
Old Jul 4th, 09, 07:36 AM
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Default Re: 454 losing power at high RPM question

Quote:
Originally Posted by rad454 View Post
I took the "other" car out for it's first ride after the long winter. I merged onto the freeway and nailed it. The engine ran smooth up to 38-4k when I noticed it starting to lightly miss or stutter.
3800 to 4000 is not high RPMs. He is not running out of CFMs at that RPM.I have a 6210 Holley on my 408 and it will turn 6K all day long.It's not the carb. CFMs.Too small of a carb. will cause it to fall on it's face not sputter and miss.Not enough fuel (lean sputter and miss) caused by old gas or varnished jets?Is then a new install or have you ran it before?You may just need larger jets the vac.sec. spreadbore Hollleys are set up lean.

Last edited by 67 Plum; Jul 4th, 09 at 08:08 AM.
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  #13  
Old Jul 4th, 09, 04:50 PM
Larger Dave Larger Dave is offline
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Default Re: 454 losing power at high RPM question

My first inclination is ignition as it has been gathering cob webs for a while, second thing that came to mind would be valve springs and lifters sitting for a while (lifters bleed down and springs take a set until they have been heat cycled a few times). But a 650 is way to small a carb for a 454. Factory used a 800 cfm Rochester QuadraJet or a780 cfm Holley carb on the 454 with hydraulic cams and a Holley 850 double pumper with a solid cam. I don't think you have run out of carb yet (your vacuum gauge would tell you if you have one hooked up), but most likely ignition wires creating a ground path.

Larger Dave

Last edited by Larger Dave; Jul 4th, 09 at 05:01 PM.
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  #14  
Old Jul 4th, 09, 04:56 PM
fatblock fatblock is offline
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Default Re: 454 losing power at high RPM question

[QUOTE=Dan E.;1245461] The question was high rpm not cruising at 2000 rpm.



[QUOTE=67 Plum;1245523]3800 to 4000 is not high RPMs. He is not running out of CFMs at that RPM.

Right on 67 Plum..
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  #15  
Old Jul 4th, 09, 05:05 PM
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Default Re: 454 losing power at high RPM question

I replaced the Q-Jet with the Holley last fall. It sat all winter, so I'll check the carb for problems. If I dont find any problems, I'm going to check ebay for a holey 8212. I may be able to find a good used one, put in a rebuild kit, and swap it. I'll check the fuel pump pressure at that time also.
Thx for the input everyone. I'll update this when I resolve it.
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