Engine LOSS OF POWER AT 4,000 RPM.

[Jarhead67]

EDITED 1/5/25
See edited text in CAPS.

I have a 331 N/A stroker built about 4 years ago, and am LOOSING POWER at 4,000 rpm. At 4,000, it will not rev any higher, and FEELS LIKE THERE’S A REV LIMITER (there isn’t one). This started about a year after the engine build and everyone thought it was electrical/ignition.

Everything electrical has been replaced since then, including installing a Terminator X system, Holly Dual Sync distributor, MSD 6AL, plugs, wires, etc… My tuner has verified that the ignition system is working as it should, and the tune is good. He also thinks it may be valve float, or possibly fuel starvation at high rpm. That being said, the only thing I haven’t checked is the valve train.

The Engine has a Scat 331 stroker kit with:
Trickflow Track Maxx Stage ll cam,
Overhauled & ported Ford GT40 heads (with unknown beehive valve springs),
10.5:1 Compression Ratio
Scorpion 1.6 rockers.
HOLLY 350lph FUEL PUMP

I’ve spoken to Trickflow tech support, and they said that with those heads and that cam I should be running HEAVIER SPRINGS.

INSTEAD OF JUST CHANGING SPRINGS, I THINK I’LL CHANGE HEADS.

 

[General karthief]

Install a fuel pressure gauge and monitor pressure when acting up.
Is there a way to check spring pressure on the engine?
Anyone?..............
Well?...........
WELL??.....

 

[General karthief]

Install a fuel pressure gauge and monitor pressure when acting up.
Is there a way to check spring pressure on the engine?
Anyone?..............
Well?...........
WELL??.....

 

[Jarhead67]

Install a fuel pressure gauge and monitor pressure when acting up.
Is there a way to check spring pressure on the engine?
Anyone?..............
Well?...........
WELL??.....

Thanks for response. Good idea. I have a gage I can use, I'll have to get some hose.

I heard there is a way to check springs on the engine, I don’t have the tool.

 

[Jarhead67]

Thanks for response. Good idea. I have a gage I can use, I'll have to get some hose.

I heard there is a way to check springs on the engine, I don’t have the tool.

The fuel pressure is holding at 43psi at the rail. I also swapped the fuel pump with a Holly 350lph with no change with high rpm.

It feels like there’s a rev limiter set to 4,000rpm, but there’s not one on the engine. I’ve been told by some people that it’s probably valve float.

 

[Mcmahst]

After trying everything, just a crazy idea, you might try putting the car on a dyno and see what the data says at 4k. ecs-performance in your town has a car dyno. Just a crazy idea.

 

[Jarhead67]

After trying everything, just a crazy idea, you might try putting the car on a dyno and see what the data says at 4k. ecs-performance in your town has a car dyno. Just a crazy idea.

Been there, done that. All they said is that it was misfiring. Not much help, I already knew that.
They work mostly on rice burner’s and know nothing about mustangs.

 

[AeroCoupe]

If it were me I would pull a set of the valve springs off and have them checked for closed and open pressure. You can do this with the heads on the car just make sure you have the cylinder at TDC to prevent the valves from dropping into the cylinder. A Google search will give you some ideas on how to swap valves with the heads on the motor.

If you have spark and fuel and you are sure the engine management system is 100% valve float would be a possibility. Are you sure you don’t have any broken springs? If it’s not just obvious you can sometimes see it when the motor is being rolled over with the starter as the tip of the rocker arm will shutter when the valve is coming up to close.

You say the 331 was built 4 years ago so can we assume this has been a problem the whole time or did it just crop up?

 

[Noobz347]

Close up the gap on your spark plugs to like... 0.050 and see if that makes a difference.

 

[Noobz347]

You say the 331 was built 4 years ago so can we assume this has been a problem the whole time or did it just crop up?

^this^

 

[Jarhead67]

If it were me I would pull a set of the valve springs off and have them checked for closed and open pressure. You can do this with the heads on the car just make sure you have the cylinder at TDC to prevent the valves from dropping into the cylinder. A Google search will give you some ideas on how to swap valves with the heads on the motor.

If you have spark and fuel and you are sure the engine management system is 100% valve float would be a possibility. Are you sure you don’t have any broken springs? If it’s not just obvious you can sometimes see it when the motor is being rolled over with the starter as the tip of the rocker arm will shutter when the valve is coming up to close.

You say the 331 was built 4 years ago so can we assume this has been a problem the whole time or did it just crop up?

Thanks for all the info.
It started about two years ago, before that I was making 300bhp and had power up to 5,500rpm. I don’t think it’s an ignition problem, the only thing I haven’t done is check the valve train.

I’m going to pull the valve covers and check the springs. I’ve intended to pull some springs and have them tested at a local speed shop, just haven’t had a moment.

 

[Jarhead67]

Close up the gap on your spark plugs to like... 0.050 and see if that makes a difference.

Right now I have NGK TR55 gapped at .045
I was using motorcraft SP447A, and have tried Autolite 104 with the same results.
I’m using good wires, the MSD 6Al, and MSD Blaster coil. Prior to that I was using an ACCEL coil.

 

[AeroCoupe]

When you pull the valve covers another thing to look for I are the rocker tips making good contact with the valve tips.

You didn’t mention excessive valve train noise at idle to I guessing no vent push rods or loose rockers.

This is out there but before you pull springs do a leak down test on each cylinder.

 

[Noobz347]

Ignition Stuff:

Right now I have NGK TR55 gapped at .045
I was using motorcraft SP447A, and have tried Autolite 104 with the same results.
I’m using good wires, the MSD 6Al, and MSD Blaster coil. Prior to that I was using an ACCEL coil.

Can you pull the MSD out of the mix? (not the coil)

--and--

What is your coil saturation at 4000 RPM? (may need to rig something up to check voltage)

 

[Jarhead67]

When you pull the valve covers another thing to look for I are the rocker tips making good contact with the valve tips.

You didn’t mention excessive valve train noise at idle to I guessing no vent push rods or loose rockers.

This is out there but before you pull springs do a leak down test on each cylinder.

There’s no noise at idle so I don’t think anything’s bent. When I pull the valve covers I’ll physically check everything. I’m going to pull some springs and get them checked.

Leak down test is a good idea, I’ll have to borrow a rig at part store.

 

[Jarhead67]

Ignition Stuff:




Can you pull the MSD out of the mix? (not the coil)

--and--

What is your coil saturation at 4000 RPM? (may need to rig something up to check voltage)

Yeah I can, that’s a good idea. I’ve got the intake off and some springs off right now so I can’t test the results of removing the 6AL.

 

[Mcmahst]

It doesn’t act like it’s a mechanical issue. I also suspect the MSD box. Those misfires at load, could you hear it out of the exhaust or intake; was it a set of big bangs, or just muted sounds, like a rev limiter would sound? Good luck.

 

[2000xp8]

INSTEAD OF JUST CHANGING SPRINGS, I THINK I’LL CHANGE HEADS.

If these are gt40 iron heads, i agree with this. A 331 with those heads is a total waste.
Now should they float that at that low of an rpm? I'm not sure, someone more versed with cams and springs will have to answer that.
I had an engine way back that floated stock head springs with an E cam, it's an unpleasant feeling and sound, much like the rev limiter sound you are speaking of. I should have never done it in the first place.

 

[Noobz347]

I second that emotion.

 

[Mcmahst]

Yea, but there’s no problem and then one day, 4k limit because the valve springs woke up and decided they no longer like that specific RPM. I doubt that is more likely than an ignition issue. Occam's razor people.