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Valve float due to exhaust back pressure?

General engine tech -- Drag Racing to Circle Track

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PRH
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Re: Valve float due to exhaust back pressure?

Post by PRH »

If lighter pushrods were indeed the cure...... that sure would seem to point towards the upper rpm problem being some sort of valvetrain control issue.

That’s a good looking curve.

I would have pulled it a little higher to see that it didn’t run into the wall right where it’s in the meat of the big power(and if it did, that would help determine where the rev limit needs to be set).
Somewhat handy with a die grinder.
Merc Man
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Re: Valve float due to exhaust back pressure?

Post by Merc Man »

PRH wrote: Tue Oct 13, 2020 3:23 pm If lighter pushrods were indeed the cure...... that sure would seem to point towards the upper rpm problem being some sort of valvetrain control issue.

That’s a good looking curve.

I would have pulled it a little higher to see that it didn’t run into the wall right where it’s in the meat of the big power(and if it did, that would help determine where the rev limit needs to be set).
I think it must have been more a geometry issue but still I need to go through the procedure again to figure out where it went wrong.
The engine was pulled for another 500 rpm to see that power dropped calmly without hitting the wall, remember this is supposed to be a rather tame street machine with the cam only in the 240's O:)
Also I had no data on the converter other than it has been sitting behind a tired 454 so stall speed was guessed to be around 2200-2400 rpm
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Re: Valve float due to exhaust back pressure?

Post by F-BIRD'88 »

The 12" stock GM converter from a Th350 will show a max stall of 2500 rpm when behind a 454 BBC
Merc Man
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Re: Valve float due to exhaust back pressure?

Post by Merc Man »

F-BIRD'88 wrote: Tue Oct 13, 2020 4:09 pm The 12" stock GM converter from a Th350 will show a max stall of 2500 rpm when behind a 454 BBC
Only thing I know is that transmission is a slightly beefed up 4L80 :)
PRH
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Re: Valve float due to exhaust back pressure?

Post by PRH »

The engine was pulled for another 500 rpm to see that power dropped calmly without hitting the wall
So they pulled it to like 6600?
Somewhat handy with a die grinder.
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Re: Valve float due to exhaust back pressure?

Post by Merc Man »

PRH wrote: Tue Oct 13, 2020 4:37 pm
The engine was pulled for another 500 rpm to see that power dropped calmly without hitting the wall
So they pulled it to like 6600?
Hp is dropping off at 6200 and it was pulled to 6500
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Re: Valve float due to exhaust back pressure?

Post by F-BIRD'88 »

Merc Man wrote: Tue Oct 13, 2020 4:23 pm
F-BIRD'88 wrote: Tue Oct 13, 2020 4:09 pm The 12" stock GM converter from a Th350 will show a max stall of 2500 rpm when behind a 454 BBC
Only thing I know is that transmission is a slightly beefed up 4L80 :)
If you can get the 4 digit letter code off the converter and measure the conveter diameter I can tell you the factory " K factor" rating which tells you what you can expect for stall performance.

Eg: 245 mm (9.5") 298mm (11.75") etc......
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Re: Valve float due to exhaust back pressure?

Post by My427stang »

Merc Man wrote: Tue Oct 13, 2020 3:57 pm
PRH wrote: Tue Oct 13, 2020 3:23 pm If lighter pushrods were indeed the cure...... that sure would seem to point towards the upper rpm problem being some sort of valvetrain control issue.

That’s a good looking curve.

I would have pulled it a little higher to see that it didn’t run into the wall right where it’s in the meat of the big power(and if it did, that would help determine where the rev limit needs to be set).
I think it must have been more a geometry issue but still I need to go through the procedure again to figure out where it went wrong.
The engine was pulled for another 500 rpm to see that power dropped calmly without hitting the wall, remember this is supposed to be a rather tame street machine with the cam only in the 240's O:)
Also I had no data on the converter other than it has been sitting behind a tired 454 so stall speed was guessed to be around 2200-2400 rpm
I think you are correct, a lighter pushrod shouldn't have fixed it. A short pushrod could be taming the cam a little from the valves perspective or correcting something that was causing something goofy, or maybe your preload adjustment was corrected with the pushrod swap? Just spit balling. Maybe look at the tip pattern when you get it back and see how wide it is and/or measure preload

Glad to see it runs well though
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 461 FE, 4 speed, port injected EFI, 3.50s
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