OT - Exhaust back pressure

[KurtM]

Looking at a 1967 Ford F-250 4x4 with built 390. Guy says the motor has 500+ hp with about 2k miles and is hooked up to Flow Master exhaust. Says during the winter months truck runs fine and does not overheat but during the summer trucks runs fine except when at a stop light and starts to overheat. Claims he was told the pipe between the headers and mufflers is too small and is causing motor to heat up due to engine can't expell heat through exhaust because of not enough back pressure. Has a custom built 4 core huge radiator, no indication of head gasket problem and fan clutch works.

Thanks,
Kurt
95 525ia

[repenttokyo]

why doesn't he change the pipe if it's so simple?

[Ross]

********. How big a pipe does he expect you to believe is necessary at idle?

[KurtM]

Need to get him to change out pipe and test prior to buying. This truck is clean for a '67, guy has owned for 20 years and for a California truck no rust except where bed seams together and at that only minor surface rust. Interior is fair, door panels are good, seat and dash pad need replaced. Truck will be used for pulling daughters horse trailer now and then and for pulling a camper.

Kurt
95 525ia

[TC535i]

It HEATS UP because of backpressure at idle? Sounds like another issue... I wouldn't believe that at all.

[Dr. evil]

is too small and is causing motor to heat up due to engine can't expell heat through exhaust because of not enough back pressure

if the exhaust is too restrictive i would think that there is too much back pressure. causing reduced exhaust gas scavinging for the cylinders

sounds to me like a fan clutch, or something cooling related

[Qube]

Uh, what is it? A two stroke engine?

[rob101]

if the exhaust is too restrictive i would think that there is too much back pressure. causing reduced exhaust gas scavinging for the cylinders

sounds to me like a fan clutch, or something cooling related
fan issue, forget his exhaust BS. he sounds like a moron. exhausts are not cooling systems

[Macv]

fan issue, forget his exhaust BS. he sounds like a moron. exhausts are not cooling systems


Definitely! No backpressure causing overheating doesn't make sense.

[Jon K]

Ehh you guys haven't experienced a clogged cat.

Whether this is the issue or not, I am not sure, but restrictive exhaust can raise operating temperature on an engine.

[rob101]

Ehh you guys haven't experienced a clogged cat.

Whether this is the issue or not, I am not sure, but restrictive exhaust can raise operating temperature on an engine.
but only at a stop light?
i mean come on, the engine is idling.

[Chris'91'525i]

Ehh you guys haven't experienced a clogged cat.
Whether this is the issue or not, I am not sure, but restrictive exhaust can raise operating temperature on an engine.

1967 models had no Cats.
1975 and earlier Calif. vehicles are smog exempt.

Could be something as simple as a radiator cap not letting the system build pressure, a Rad. hose collapsing or thermostat sticking in the closed position.

Should not be anything a decent mechanic could diagnosis simply.

If it is truly putting out 500 HP, thats engine has some serious high compression. If everything is not set up correctly it won't take much to over heat it.

[Jon K]

No I realize it doesn't HAVE cats, but I am equating small cross pipe to clogged cats, it does tend to heat up.

[Ferret]

No I realize it doesn't HAVE cats, but I am equating small cross pipe to clogged cats, it does tend to heat up.

IIRC a contstriction in an exhaust can also cause reflections back up into the manifold and cause all kinds of problems. Decreases engine efficiency badly too...

[bsell]

Looking at a 1967 Ford F-250 4x4 with built 390. Guy says the motor has 500+ hp with about 2k miles and is hooked up to Flow Master exhaust. Says during the winter months truck runs fine and does not overheat but during the summer trucks runs fine except when at a stop light and starts to overheat. Claims he was told the pipe between the headers and mufflers is too small and is causing motor to heat up due to engine can't expell heat through exhaust because of not enough back pressure. Has a custom built 4 core huge radiator, no indication of head gasket problem and fan clutch works.

Thanks,
Kurt
95 525ia

Please define 'built' for us.

My main concern is if the cylinder walls are too thin. If during the 'build' job, the cylinders got bored out to their minimum thickness and then he added in some high compression pistons and dialed in too much ignition advance at idle and I bet the water temps climb like a rocket.

Also what about the heads and head gaskets? What kind of heads and what was done to them? Did he massage the combustion chambers and thinned out the metal between the fire and the water jackets?

What's up with the water pump? Is it possible that the pump is running too slow at idle due to underdrive pulleys?

I tell you I would rather have an old worn out, running, original engine over the crap work that some folks call 'built.' Get a parts/build list from the guy and talk to the shop that built the engine to see what went down.

Good luck,

Brian