Big power loss when warm

[C0V3R]

Ive been noticing this a bit more lately since I've been on a few longer drives. When I first take the car out its really crisp for the first 10-15 minutes, and then progressively looses power as it warms up. After an hour or more so it seems I loose in the vicinity of 15-20% of the power, maybe more, and that its less inclined to kick down also.

My only current thoughts have something to do with heat soak, but I still dont understand why it should loose so much power. The temp guage is well into the low end (not even 1/4 of the way).

If you can think of any causes/solutions let me know!

Anyway, my setup is as follows:
289w
Weiand 7515 intake
Plenum divider and phenolic carb spacer
edelbrock 1406 600cfm
MSD 6a + pro billet diz.
K&N filter
c4 tranny

 

[1969mach1351]

My stock 351W did something similar. From about 5-10 minutes after startup it would be really "crisp" off the line and would lose traction when not even trying but once the thermostat opened then it would run like normal. Instead of losing power like you explain I looked at as gaining power for about 5 minutes. I had similar setup as you too, aluminum intake, Holley carb, MSD, and K&N. I tried using a 165* thermostat to keep it in the cooler range but didn't help.

 

[zookeeper]

Since you didn't mention that it was missing or cutting out, I'd have to guess that it's running a tad rich. The reason I'm saying that is that engines need more gas when cold, but as the engine warms, it's more efficient, and therefore needs less fuel to make crisp, responsive power. Start by verifying this by removing the plugs after a longish (30 minutes or so) drive just how you normally drive the car. Let it cool to lessen the chance of burning yourself, then remove the plugs and see how they look and smell. A strong smell of gasoline and/or black, wet or sooty looking will let you know for sure. This works for any engine related problem, reading spark plugs is the end-all lie detector for your engine.

 

[zookeeper]

My stock 351W did something similar. From about 5-10 minutes after startup it would be really "crisp" off the line and would lose traction when not even trying but once the thermostat opened then it would run like normal. Instead of losing power like you explain I looked at as gaining power for about 5 minutes. I had similar setup as you too, aluminum intake, Holley carb, MSD, and K&N. I tried using a 165* thermostat to keep it in the cooler range but didn't help.

This is poor reasoning, since all internal combustion engines are most efficient at higher temps. This is why newer cars run at 190-200 degrees, because that's where they get the most power out of the least amount of fuel. That's also why truckers cover the radiator openings on cooler days, cold engines never get up to temp they always make poor power, not to mention possible engine damage since the pistons haven't reached the needed temp, they are loose in the bores. Another way to isolate the problem is to swap in a one heat range hotter plug and see if the power comes back. If this really wakes up the motor, it's too rich. But that doesn't mean you should simply leave the hot plugs in, it's just a quick and dirty way of knowing if you have more gas than you can burn.

 

[Ozsum67]

Hense why I mentioned a rich carb-choke condition. If you are truely into reading plugs, then you need to shut it down at full speed, let it coast somewhere safe, or trailer it home, and read them then. Plug conditions change as engine rpm changes. That's why NASCAR shuts them down on the back straight away and coasts them to the pit to do the reading.

 

[brianj5600]

I'm thinking you may be a little on the rich side. A rich car runs good cold, a kind of choke condition. As it warms up, it needs less fuel.

I am also a big fan of cold air induction. For every 11*f the intake air goes up, you loose 1% hp.

 

[zookeeper]

Hense why I mentioned a rich carb-choke condition. If you are truely into reading plugs, then you need to shut it down at full speed, let it coast somewhere safe, or trailer it home, and read them then. Plug conditions change as engine rpm changes. That's why NASCAR shuts them down on the back straight away and coasts them to the pit to do the reading.

Race cars are meant to run at wide open throttle, therefore they do "plug chops" at wide open throttle, same as any race vehicle. But street cars are different, and we don't need a jeweler's loupe (like race tuner's use) to see a horribly tuned motor. We aren't trying to get that last 3 or 4 horsepower, we are trying to get the tuning headed in the right direction, therefore a 15 mile drive (at normal speed) will do do just fine. In fact if we were like race tuners, we would use a bore scope to see into the cylinder to read the piston tops, which is more accurate, but we don't need to be that anal about it.