Losing power during a race...what's happening?

[Ho Hum 74]

Folks,

Recently I started racing my 74 2.7 MFI 911. It's a blast but I've been really frustrated by a problem that shows up about half way into the race. This weekend I qualified 1st in class twice and pulled an entire front straight lead on my nearest competitor when my engine started losing power. This is the third time it has happened and I cannot figure it out.

Basically what happens is that the car starts off great with full horse-power and then starts to miss at high rpm (above 6000, motor revs to 7,300). This is generally as the oil temps are in the 210 degree range. I have a very large cooler up front and temps never get higher than this. The situation gets progressively worse to the point where it feels like the car has 50% or less power and seriously bogs down. At this point all my fellow competitors catch up. But I was still able to hold onto 2nd in both races.

After the race once the car cools down to 180 degrees and the motor is back to it's usual perky self….strong as ever. I can qualify in the morning session (car sitting overnight) and practically run 20 minute session without any issues. As the day progresses and the temps increase it gets worse.

When this first happened, I replaced the coil thinking that it was unable to charge properly and I had a very old coil anyway. Obviously it is either fuel or spark. But it also backfires a bit when this happens so I'm thinking that fuel is making its way into the cylinders, sitting there and when the spark finally arrives is detonating. I'm concentrating on the spark.

I have not pulled my plugs yet (one race old), I have points and an MSD box. I have brand new plug wires as well. This has never happened prior to this year and the car has been very reliable otherwise. My MSD box is mounted to the firewall and actually sits on the engine sound pad. The only thing that I can think of is that I had removed the heat exchangers to replace an exhaust gasket and due to time, had not replaced the heater ducts to the flapper boxes. Could this be heating up the firewall and making the MSD box do something funky?

Any thoughts on how I could approach this issue? It's impossible to duplicate unless at the track so hard to determine if a change has had an effect.

Thanks
Tristan

[Bill Verburg]

Oil temps does not tell the whole heat story. Sounds like the heads may be over heating causing detonation, audible or not.

More air to the heads, better gas, more oil cooling

[jmz]

I've heard the MSD doesn't like heat. You might have something with that thought.

[Eagledriver]

Tristan,

One possible explaination would have to do with your heat exchangers. With the ducts open there is little pressure inside the heat exchangers, this will cause 2 problems. The first is that less air will flow into the MFI thermostat causing the engine to run richer and richer as it gets hot. The second problem is that you are bleeding off more air from the fan shroud and pissing it out of the exhangers, this leaves less air for cooling the cylinders and heads.

-Andy

[Ho Hum 74]

Andy, I think that certainly is part of it because I have never seen my car get to 210. It is always just above the 180 mark.

Thanks
Tristan

[304065]

I bet you a Warsteiner (size LARGE) that what instead is happening is that the left heat exchanger is blowing hot air onto the fuel lines that run from the tunnel up through the engine compartment. OK, it's a return system, but the lines are still steel and can get hot, which might be vaporizing the fuel. The fact that it's occuring around the boiling point of water makes me think it's a heat/fuel issue, concur with Bill about better gas.

[PorscheGuy79]

Tristan, I tired sending you a PM on an unrelated subject but your box is full.

[Ho Hum 74]

John,

Nice thought. I will defenitely re-connect those ducts and then wack myself in the hear with that Warsteiner!

Matt,

Try twhitehead@deloitte.com thanks!

Cheers,
T

[Plavan]

Tristan,
My race car did the exact same thing. Answer a few questions
1.) What fuel cell are you using?
2.) Are you using Super Unleaded?
3.) How old/new is your fuel cell?
4.) stock fuel pump?
5.) Does the car seem to run better when it has a full cell of gas?

I have a slight feeling you have the same problem I had.

[Jay Auskin]

Tristan, I'm going to bet it's fuel related. I see your car is a '74. Do you have an MFI fuel transfer pump or 'something else'? Also, it could be a hot temp boiling issue. Can you find a way to further restrict the return flow?

Lastly, and this is easy to check....how clean is your fuel tank? Pull out the fuel sender and see if there's crap in the tank. It might not be temperature, but it might be fuel starvation from crap on the inlet screen. My car had this problem, and it sounded like it was running rich and backfiring. It was starving, and then would 'pop' when fuel would start up again. I run a 2.4T with higher compression 2.2T pistons. I hit ~235F on the track running hard at Blackhawk. I had no issues with heat and fuel since I had my tank cleaned and sealed. Good Luck!

[Tyson Schmidt]

How's your fuel tank ventilation?

It wouldn't be the first, or even second time I've seen a fuel tank trying to implode because the evaporative system for the fuek tank had been messed with, and there was no venting for the tank.

So the fuel pump draws a vacuum on the tank to the point that it can't deliver fuel. It'll only happen at the track, because that's the only time that much fuel is consumed that quickly. During street driving, it has time to draw air past tiny air leaks at the cap, etc.

[cstreit]

I talked about this with Tristan for a while and I think John and/or Tyson might be onto something. I was wondering if t was a fuel boiling issue, but never heard of it happening in the MFI pump. However the fuel line idea holds water for sure!

...and Tysons idea of it being related to tank venting is good too, might not be heat at all...

[Ho Hum 74]

Folks,

Thank you very much for the thoughtful replies. I think what I'll do is the following:

1. Re-connect my heater hoses
2. Replace fuel filter
3. Check tank screens
4. Check the tank ventilation

Regards,
Tristan

[Plavan]

Do you have a fuel cell or tank?

[Ho Hum 74]

Tank

[Plavan]

My fuel cell went bad and was clogging everything..... thats why I lost power.
Just follow what other suggested. You will chase that gremlin down.

[Carl83911]

I was reading an article related to power loss at high heat and high rpms. it pointed to a faulty CD box or MSD. just my .02 cents.

[Ho Hum 74]

Update:

Was at the track this weekend in 95 degree ambient temps. Perfect for my car! I had reconnected the heat exchangers as mentioned above.

Well, no dice. The car ran for 15 minutes and as oil temps got to 220 it then felt again like it was starved for fuel.

Here's where we are zoning in on the problem: We think it's a fuel pressure issue. My fuel pump is supplying the MFI pump with 6 psi static. Everything I've read now says the MFI supply should be 14.2 psi.

I guess the supply pumps in MFI engines are different. Remember, my car was a 74 911. The PO probably kept the old pump.

I will change the fuel pump, filter and look at all the lines.

Any other suggestions?

Thanks
Tristan

[MotoSook]

Other than the possibility that the sypply pump has started to degrade, what other things may have changed in your car from last year?

[Rot 911]

Did you check the possibility, as previously suggested, of a plugged vent line to the gas tank? Easiest way to check is when you get the fuel starvation problem is to pull off and unscrew the gas tank. A "wooshing" sound would confirm a plugged vent.