Why cant my engine rev to 7k rpm?

[camlob]

So I have a 993 with a 4l P&C, PMO ITBs 50mm, DC 43 -112, LWF, AASCO valvetrain, Pauter rods, solid lifters, TEC3r ecu, 1 3/4 headers etc.

My tuner didnt follow John D's cam timing specs which were: Set lift at overlap to 3.2 to 3.4mm. My tuner set it at TDC intake 111 degrees and exhaust 115 degrees.

Sorry I dont know if the tuner went with this cam timing. My problem is the car only made 295rwhp on a dynapack dyno. For comparisons sake, my brothers 997 GT3 RS made 350rwhp.

We just finished tuning today and it cant make hp beyond 6.7k rpm. At 7k power drops. I bought the pauter rods to rev up to 7.5k. So I am wondering why the cut-off in hp.

I am happy with the hp increase from 267rwhp from my 3.6 because of the increase in torque. But I am dismayed on why I cant rev higher. That is the reason why I bought pauter rods.

Is it maybe I should follow John D cam timing specs? The car is still in the shop so I can have them time it to Johns specs.

Should I give it a try? Or is the cam just too mild for a 4l? I visited a 993 in Singapore and it had a 3.8l with ITBs and had a RSR profile cam. The increase in displacement in my case could make my even mild cam milder.

What do you guys think? Tnx

[JohnJL]

I think you've answered your question...check that cam timing. How do those degree measurements equate to John's linear measurements?

[Tippy]

Post in the "911 engine rebuilding" forum if answers here don't suffice.

[Bill Verburg]

Originally Posted by camlob

So I have a 993 with a 4l P&C, PMO ITBs 50mm, DC 43 -112, LWF, AASCO valvetrain, Pauter rods, solid lifters, TEC3r ecu, 1 3/4 headers etc.

My tuner didnt follow John D's cam timing specs which were: Set lift at overlap to 3.2 to 3.4mm. My tuner set it at TDC intake 111 degrees and exhaust 115 degrees.

Sorry I dont know if the tuner went with this cam timing. My problem is the car only made 295rwhp on a dynapack dyno. For comparisons sake, my brothers 997 GT3 RS made 350rwhp.

We just finished tuning today and it cant make hp beyond 6.7k rpm. At 7k power drops. I bought the pauter rods to rev up to 7.5k. So I am wondering why the cut-off in hp.

I am happy with the hp increase from 267rwhp from my 3.6 because of the increase in torque. But I am dismayed on why I cant rev higher. That is the reason why I bought pauter rods.

Is it maybe I should follow John D cam timing specs? The car is still in the shop so I can have them time it to Johns specs.

Should I give it a try? Or is the cam just too mild for a 4l? I visited a 993 in Singapore and it had a 3.8l with ITBs and had a RSR profile cam. The increase in displacement in my case could make my even mild cam milder.

What do you guys think? Tnx

where are you located?

[HawgRyder]

As a general rule of thumb...you "retard" the cam timing for higher RPM's.
If the cam timing is advanced from the split overlap position...your low end HP and torque will rise.
This is all dependent on the possible interaction of valves to pistons of course.
Long duration cams have more ability to be "tuned" for RPM performance....but...be careful that you don't kill the low end response for the high end HP ... or you might actually lower your circuit times.. (dog coming out of the corners)....LOL
Bob

[175K911]

Sounds like a nice engine setup. But I'm maybe not clear on what you're saying.

If I read your original post, you're saying the engine will wind to 7500 but it's power falls off after 6700. Is that correct? Of did I read that wrong.

You've done a lot to the engine. When the dyno pulls were being done, did they map the air-fuel ratio (AFR)? As others have said, follow the cam builders specs, especially Dougherty. Then, are you running a good AFR or is the engine too rich or too lean? The AFR will be a good indicator as to whether the PMO's are set up right.

When you get it sorted out you should have one heck of an engine!

[camlob]

JohnJL - Thanks Ill have them redo the cam timing to Johns specs.

BillV - Am based in Manila.

Tippy - The engine rebuild forum dont reply to my posts! Maybe I have to be an engine rebuilder to get a reply! LOL....

Hawgryder - Thanks for the inputs. Ill have the shop check if the timing is advanced. The previous tuner is experienced but probably has his ways. I think he should have followed Johns specs. I explained to him the situation, he said I need a bigger cam with 305 duration!

175k911 - Yes the engine will rev past 7k but hp is dropping. Peak hp is at 6.7k. Yes AFRs were 12.5-12.7 at full load. They synched the ITBs again. So it all boils down to cam timing IMO.

Thanks guys. You are a helpful bunch.

[KTL]

I agree cam timing is important to your dilemma. But it's not the only factor in the peak power location.

What size are the main venturis/throats in your PMOs? If your throats are too small, you can choke off the power peak. My 3.0L engine is currently intentionally choked with smaller than desired main venturis (to get acceptable AFR) and the power peaks at 6500 rpm per the dyno graph. When I install the larger main venturis, the power peak will shift further up the rpm range, as a result of allowing the the engine to "breathe" better. The low rpm power/torque will suffer as a result.

[camlob]

Originally Posted by KTL

I agree cam timing is important to your dilemma. But it's not the only factor in the peak power location.

What size are the main venturis/throats in your PMOs? If your throats are too small, you can choke off the power peak. My 3.0L engine is currently intentionally choked with smaller than desired main venturis (to get acceptable AFR) and the power peaks at 6500 rpm per the dyno graph. When I install the larger main venturis, the power peak will shift further up the rpm range, as a result of allowing the the engine to "breathe" better. The low rpm power/torque will suffer as a result.

I have EFI ITBs 50mm with 42lb injectors.