496 flat-lines at 5200 rpm - Chevelle Tech
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post #1 of 42 (permalink) Old Jul 1st, 10, 5:51 PM Thread Starter
Nash
 
Join Date: Feb 2010
Location: Michigan
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496 flat-lines at 5200 rpm

Having some trouble with my 496 bbc not wanting to spin over about 5200 rpm. In first gear itll go to about 5400, second only 5200, and third bearly hits 5k. I built this motor to spin to nearly 6500 and i need to know where to look for a cause.
Specs:
xr280r cam, solid roller
10.2 cr
Air gap
Oval port heads, opened up
850 mighty demon
Stock hei ignition (for now)
170 gph holley mech fuel pump
regulator sitting at 7.5 psi always.
Turbo 400 rmvb
3.08 gear (for now)
Its in a 4900 lbs truck.

Seems like its running out of fuel? maybe ign? maybe gearing? Cam??
This motor zips up to 5000 so quick and really pulls till it just stops. Not sputtering, no back-firing, just wont go anymore. Where should i look first?
Thanks for reading
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post #2 of 42 (permalink) Old Jul 1st, 10, 6:32 PM
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Brad
 
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Re: 496 flat-lines at 5200 rpm

Are you sure that the fuel pressure is good at that RPM? Check that first. Floats are set to the bottom of the sight plug when idling? That should start you on the path to troubleshooting. I dont think it sounds like ignition, but would not hurt to try someone else distributor if the carb/fuel checks out. Is the battery in good shape? That can affect performance also.

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post #3 of 42 (permalink) Old Jul 1st, 10, 6:36 PM
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Re: 496 flat-lines at 5200 rpm

If the pressure's constant and the spring's are the right ones for the cam ,I'd look at the stock HEI.
You do have full 12v to the distributor while running - right ?
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post #4 of 42 (permalink) Old Jul 1st, 10, 6:45 PM
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Re: 496 flat-lines at 5200 rpm

The distributor is what came to mind to me, many crap out at 4500 as far as good spark.

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post #5 of 42 (permalink) Old Jul 1st, 10, 6:54 PM
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Dave
 
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Re: 496 flat-lines at 5200 rpm

Check the valve spring height. I had the same problem. You'll be happy when you get the extra rpm... I sure was.
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post #6 of 42 (permalink) Old Jul 1st, 10, 7:34 PM
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Todd
 
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Re: 496 flat-lines at 5200 rpm

My HEI crapped the bed at 4500 until I rewired the power wire using a relay and direct batt voltage, now I'm comsidering a rev limiter.........

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post #7 of 42 (permalink) Old Jul 1st, 10, 8:10 PM
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Re: 496 flat-lines at 5200 rpm

Are the secondaries opening all the way?
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post #8 of 42 (permalink) Old Jul 1st, 10, 8:24 PM
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Re: 496 flat-lines at 5200 rpm

Double check the valve spring pressures (installed height and pressure) is set correctly for the cam used. If it doesn't have enough spring it'll float and kill RPM way to soon. I assume whomever you had do the head work set them up for you???

1968 Chevelle Sb
1969 Chevelle 496
1973 SS 454 Chevelle
1974 S-3 454 Chevelle
1955 Delray LS1
1967 Nova 496
1969 nova 427
1969 Camaro 350
1968 Camaro Convertable 350
1968 Camaro Coupe 427
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post #9 of 42 (permalink) Old Jul 1st, 10, 8:33 PM
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Re: 496 flat-lines at 5200 rpm

Here's the recommended spring with lb ratings. Although I would like a little more open pressure.


....... Seat Load Open Load Coil Bind Rate
#954 210 @ 1.900 524 @ 1.250 1.170 483

1968 Chevelle Sb
1969 Chevelle 496
1973 SS 454 Chevelle
1974 S-3 454 Chevelle
1955 Delray LS1
1967 Nova 496
1969 nova 427
1969 Camaro 350
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post #10 of 42 (permalink) Old Jul 1st, 10, 10:05 PM Thread Starter
Nash
 
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Re: 496 flat-lines at 5200 rpm

Thanks for the replies,
Yes the guy that did the head work set the spring heights and checked the pressures. I am for sure getting 7.5 psi all through out the rpm range, it does drop to like 6.5 after 5000 if i can get it that high but seems pretty steady. From tank to pump im using stock 3/8 fuel line..
I was thinking it must be the HEI giving me all the trouble. I have a 6al box to hook up but i was waiting to get the msd billet dist.
Also hooked up my buddies Air/fuel ratio gauge, seemed to be just over perfect in the high R's. (a little rich) But reasonable
Timing: 16 initial and 36 overall. seem good? No vac advance is used cause it bumps my overall timing to nearly 50 degrees.
I want to hear and feel this thing scream to 6500!
Thanks
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post #11 of 42 (permalink) Old Jul 1st, 10, 11:47 PM
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Dave
 
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Re: 496 flat-lines at 5200 rpm

I would double check the spring height specs and confirm it the best you can by pulling the valve cover. It's a pain but it won't cost a thing. It doesn't hurt to double check someones work.
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post #12 of 42 (permalink) Old Jul 2nd, 10, 9:49 AM
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Re: 496 flat-lines at 5200 rpm

X2, I've came across other peoples work that was supposed to be done correctly just to find they took a short cut and just installed blindly and the heights were way off.

I've even seen supposenly correct roller springs on a set of heads that were actually flat tapped springs and no where near the correct pressures. When a solid roller is needing 200 plus at the seat and 600 open (give or take a bit) why would someone charge a person good money for those correct springs and then I find the heads to have 125 at the seat and 350 open? Yes, it's happened and will likely happen to people in the future.

1968 Chevelle Sb
1969 Chevelle 496
1973 SS 454 Chevelle
1974 S-3 454 Chevelle
1955 Delray LS1
1967 Nova 496
1969 nova 427
1969 Camaro 350
1968 Camaro Convertable 350
1968 Camaro Coupe 427
.

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post #13 of 42 (permalink) Old Jul 2nd, 10, 10:28 AM
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Re: 496 flat-lines at 5200 rpm

I would have to say, if it flat lines in 1st, 2nd, and 3rd, I'd be looking at the springs also. Fuel issues show up in the higher gears when the motor is really loaded down. Timing could/distributor could also be an issue.

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post #14 of 42 (permalink) Old Jul 3rd, 10, 3:11 AM Thread Starter
Nash
 
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Re: 496 flat-lines at 5200 rpm

Yes if it's not the ignition i'm going to go ahead and assume its valve float. Which sucks cause i payed my machinist good money to set/check all that.
Whats the best way to double check his work, and what damage could i have done already? I have not heard any slamming or strange noises, it just Stops making power and stops revving. hmm
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post #15 of 42 (permalink) Old Jul 3rd, 10, 11:28 AM
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Re: 496 flat-lines at 5200 rpm

what exact cam do you have? does the part number end in -8? those are cast iron rollers and Comp recommends a softer spring for the same grind if it's on the cast core. Verify what cam you have and let us know.

Do you have the part number of the springs?

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Last edited by Tom Mobley; Jul 3rd, 10 at 11:49 AM.
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