Won't rev above 4500rpm!

[North Cali Autocrosser]

I recently rebuilt the engine on my car. I put in a bigger cam and boosted the fuel pressure. The car runs great and makes good power but it won't rev above 4500 rpm, it pulls hard until that point then hits a wall. I need help.

[TheRealWave]

Is the fuel filter cloged? Peddel going all the way down? ;)

[North Cali Autocrosser]

I've got plenty of fuel, and my pedal is mashed to the floor.

[67-03 RIP]

What kind of lift on the cam? We talking bigger cam, or BIGGGER cam? When you did the rebuild did you put in new valve springs?

Ron

[Slow-N-ugly]

Check the throttle linkage under the hood, my buddy had the same problem (older carbed car though), the pedal was floored but there was sloop in the cable.

[StockV6Cam]

It's probably your heads. The heads on these 60 degree v6's are horrible. You can tell just by my dyno sheets that they die at around 4500 rpms.

my dyno sheet

[North Cali Autocrosser]

The cam isn't that much bigger than stock, 204/214 with .423 lift, I reused the stock springs and rockers. Linkages are all ok. She has plenty of power but when it gets up to 4500 she won't go anymore. Before the rebuild it would do this at about 5700.

[North Cali Autocrosser]

I found out that once I hit 4500 if I let off the pedal some the car will take off again and get above 4500rpm. Could it be that the MAP sensor isn't compensating for the new cam?

[Ex-5.0]

When the MAF sensor went bad in my '93 Ford (sorry guys, but its a fun car), it acted the same way. At that time it was under warranty and the dealer had a hard time diagnosing it. They did all the fuel stuff (filter, check pump, clean fuel injectors) then decided that the cat. converters were plugged so I got a new H-pipe with 4 converters and it still was'nt fixed. Finally, they replaced the MAF sensor and all was right again. (It was all warranty so it did'nt cost me anything.)

What probably happening is that the new cam is letting the engine breathe a lot more air at high rpm and the resulting MAF sensor output is outside of the computer's map so it doesn't add any more fuel. At 4500rpm the mixture just goes lean so the motor won't pull. I think the key to this is that it pulls above 4500 when you back out of it. That probably puts the airflow into the motor back into the range where the computer can handle it.

Valve float could also be a problem with stock springs and a big cam but since it pulls higher with part throttle that rules out valve float because float would tend to be an absolute limit on how high rpm the motor could make power.

I think you will need some computer work to take into account the new cam.

[North Cali Autocrosser]

Thanks for the help guys, hopefully I'll get this worked out soon.