Why the loss of power when hot?
05-19-2008, 05:31 PM
#1
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Why the loss of power when hot?
I've experienced this and have seen numerous posts where others have reported this happening. Car gets hot and at some point some have seen the oil light come on and others report that there is a noticeable loss of power. I have seen both. Can someone explain what happens? Thanks.
05-19-2008, 06:29 PM
#2
Team Owner
i could possibly see the oil light coming on while at idle when really hot , as the oil is thin and the pump cannot generate enoigh pressure to keep the light off.
Outside of this i would be very worried if the oil light came on
Outside of this i would be very worried if the oil light came on
05-19-2008, 06:54 PM
#3
You need to tell us more. How Hot? above 250deg F?
As Cyl temps increase power simply decreases, ever notice how well these cars run in weather below 40deg F? I live in RI and these cars love the cold dry weather. Also if you are running oil temps above 250deg F I'd back off on the throttle you start to run the risk of really overheating and even detonation issues.
Also how hot is it outside and how humid? I have driven 911s in S. FL in 98deg 100% humidity and these cars hate this weather.
Be sure your oil cooling system is working as well as the thermostats to the engine mounted cooler as well as the front fender cooler.
But in the end it is normal for these PCars to lose power in humid hot weather. Just don't hammer on the throttle if the oil temps start to rise above 250deg F
As Cyl temps increase power simply decreases, ever notice how well these cars run in weather below 40deg F? I live in RI and these cars love the cold dry weather. Also if you are running oil temps above 250deg F I'd back off on the throttle you start to run the risk of really overheating and even detonation issues.
Also how hot is it outside and how humid? I have driven 911s in S. FL in 98deg 100% humidity and these cars hate this weather.
Be sure your oil cooling system is working as well as the thermostats to the engine mounted cooler as well as the front fender cooler.
But in the end it is normal for these PCars to lose power in humid hot weather. Just don't hammer on the throttle if the oil temps start to rise above 250deg F
05-19-2008, 07:06 PM
#4
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Yes, hot as in 250+, and yes I do back off. I'm not saying it's outside temp-dependent; of course, the temp the oil gets to is, but the oil temp-power loss relationship is not. The power loss I've seen, and read about, is pretty abrupt, it's not gradual. I'm wondering about the mechanics or physics or whatever it is going on that causes that.
I do have what I would characterize as a high-temp vacuum leak. It seems like when the car is hot something opens up maybe, as the idle becomes higher. I've checked the AA regulator and it is closed, and again it seems temp-dependent. Next time that happens I plan to try shooting starter fluid around to see if I can find something but it's hard to reproduce as it happens only at the track.
I am running a Carrera cooler, and it was working well, keeping temps below the middle line, til this past weekend. Both oil lines to the cooler were hot so seems the thermostat is working ok. Wondering if I'm running lean, potentially due to whatever it is that is causing my high idle.
I do have what I would characterize as a high-temp vacuum leak. It seems like when the car is hot something opens up maybe, as the idle becomes higher. I've checked the AA regulator and it is closed, and again it seems temp-dependent. Next time that happens I plan to try shooting starter fluid around to see if I can find something but it's hard to reproduce as it happens only at the track.
I am running a Carrera cooler, and it was working well, keeping temps below the middle line, til this past weekend. Both oil lines to the cooler were hot so seems the thermostat is working ok. Wondering if I'm running lean, potentially due to whatever it is that is causing my high idle.
05-19-2008, 07:20 PM
#5
Drifting
High idle can be caused by an intake and/or vacuum leak.
After fixing that I would check the timing and mixture.
Per Bruce Anderson "250 is too damn hot !!"
John
After fixing that I would check the timing and mixture.
Per Bruce Anderson "250 is too damn hot !!"
John
05-30-2008, 01:32 AM
#7
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My shop is going to hook in a wide-band a/f meter (I think that's the right terminology) to see real-time under real conditions. We'll see what happens.
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05-30-2008, 08:51 AM
#8
AFR
RPM
WOT Switch
Air Flow Meter signal (0-5volts)
You are on the right track.
05-30-2008, 11:47 AM
#9
Rennlist Member
Why does power go down when an engine is too hot? There are two big reasons. First, much above 250F and the oil starts to break down. Your oil is what is primarily responsible for keep the bottom of the pistons cool and the piston rings lubricated enough to keep a tight seal for combustion. So if that breaks down, then some of the combustion leaks past the piston rings, reducing your power. Secondly, if the cylinder heads are too hot, then you start getting an incomplete or improper combustion, resulting in power loss.
In either of these two cases, you are doing damage to your engine and something should be done to correct the problem. In a good running engine, you should not lose power when it gets a little hotter than usual. In fact, since an internal combustion engine is really just a heat-engine of 80% efficiency and a 20% by-product of torque, and engines job is to produce as much heat as possible. Theoretically, the more heat - the more power it can make. So for this reason, engine builders are trying to keep as much heat in the combustion chambers and inside the exhaust system as possible. The only caveat to all this heat in and around the engine is to make sure of two things:
1.) The heat has a good escape route
2.) Air coming into the intake is as cool as possible
In either of these two cases, you are doing damage to your engine and something should be done to correct the problem. In a good running engine, you should not lose power when it gets a little hotter than usual. In fact, since an internal combustion engine is really just a heat-engine of 80% efficiency and a 20% by-product of torque, and engines job is to produce as much heat as possible. Theoretically, the more heat - the more power it can make. So for this reason, engine builders are trying to keep as much heat in the combustion chambers and inside the exhaust system as possible. The only caveat to all this heat in and around the engine is to make sure of two things:
1.) The heat has a good escape route
2.) Air coming into the intake is as cool as possible
05-30-2008, 12:11 PM
#10
Also as mixture leans heat increases, not sure why maybe someone can explain this. But this is why air plane pilots try to save fuel by leaning the engine from the **** pit but they do this while watching cyl head temps. The pilot will lean the mixture as much as possible but keep the cyl head temps at a reasonable level. So, if you have a lean running condition your engine heats quickly. Just something I learned from private pilots.
Why does power go down when an engine is too hot? There are two big reasons. First, much above 250F and the oil starts to break down. Your oil is what is primarily responsible for keep the bottom of the pistons cool and the piston rings lubricated enough to keep a tight seal for combustion. So if that breaks down, then some of the combustion leaks past the piston rings, reducing your power. Secondly, if the cylinder heads are too hot, then you start getting an incomplete or improper combustion, resulting in power loss.
In either of these two cases, you are doing damage to your engine and something should be done to correct the problem. In a good running engine, you should not lose power when it gets a little hotter than usual. In fact, since an internal combustion engine is really just a heat-engine of 80% efficiency and a 20% by-product of torque, and engines job is to produce as much heat as possible. Theoretically, the more heat - the more power it can make. So for this reason, engine builders are trying to keep as much heat in the combustion chambers and inside the exhaust system as possible. The only caveat to all this heat in and around the engine is to make sure of two things:
1.) The heat has a good escape route
2.) Air coming into the intake is as cool as possible
In either of these two cases, you are doing damage to your engine and something should be done to correct the problem. In a good running engine, you should not lose power when it gets a little hotter than usual. In fact, since an internal combustion engine is really just a heat-engine of 80% efficiency and a 20% by-product of torque, and engines job is to produce as much heat as possible. Theoretically, the more heat - the more power it can make. So for this reason, engine builders are trying to keep as much heat in the combustion chambers and inside the exhaust system as possible. The only caveat to all this heat in and around the engine is to make sure of two things:
1.) The heat has a good escape route
2.) Air coming into the intake is as cool as possible
05-30-2008, 09:05 PM
#11
Rennlist Member
Here's what I did to bring my temps down. Worked very well.
1988 Carrera oil cooler 244 F thermo-switch
replacement with a lower temp one
The factory thermo-switch mounted on the Carrera oil cooler on my 88 Carrera was rated at 244 F. I obtained the info on putting a lower temp unit in from another Rennlister a year or so ago. It was about using a BMW radiator auxiliary fan switch that worked very well for him. I bought one from Pelican and put it in the car. The factory wiring harness has a single wire with a round spade type connector going to the single pole thermo-switch. I had two options (1) cut the factory wiring and solder in a flat spade connector, or (2) make a small 3-inch wire coupler with a round spade on one end and a flat spade connector on the other end. I chose option 2 so I didn’t mess up the factory wiring. I put a double layer of shrink wrap over the connection to make it rigid so I wouldn’t have to go back in there for a while. The BMW auxiliary fan switch is a two prong, Red Top, 99 degree Celsius switch (equates to 210.8 F) made by FAE, P/N 61-31-1-364-273-M323 and costs $13.75 from Pelican. Your local BMW dealer may have one, but cost may be higher.
You have to remove the flex lines attached to the oil cooler and the hard lines. Be careful when loosening the mounting bolts on the oil cooler. There is a lot of oil in there and any movement will cause it to spill on you. Once you have enough oil out of the cooler to remove it, carefully lower the cooler to the ground and dump the rest of the oil out of the cooler into a drain pan. Then remove the factory switch from the cooler. You may opt to cut the factory wire and install a flat spade female connector because the BMW part has two male connectors on it (see below). Install the BMW switch with a new gasket and tighten. It doesn’t matter which male connector you plug the wire onto. You then need to make a ground wire that will connect to the other male spade on the switch. I used brown wire just like the factory so there is no doubt as to what it is for. I made my ground wire about 8” long and put a female connector on one end and a connector with an eyelet on the other end. How long you make the ground wire is up to you. Make sure the eyelet is big enough to have a 10mm head bolt go through it, because on top of the oil cooler beside the new switch there is a 10mm head bolt you can use to mount the ground wire. Once all the electrical is done, remount the cooler, reinstall the flex lines, and start the car and let it come up to operating temperature. Once the hard lines get hot, check for any leaks at the flex and hard lines. If you need to change the flex lines, now’s the time to do it. If no leaks are detected, let the engine run and monitor the heat gauge in the car. Make note of the temperature when the oil cooler fan motor comes on. Depending on gauge calibrations, it should come on at about 210-215 F. I wanted my hook-up to be automatic so that’s why I did mine this way. Hope this helps explain what I did. Let me know if you need more.
1988 Carrera oil cooler 244 F thermo-switch
replacement with a lower temp one
The factory thermo-switch mounted on the Carrera oil cooler on my 88 Carrera was rated at 244 F. I obtained the info on putting a lower temp unit in from another Rennlister a year or so ago. It was about using a BMW radiator auxiliary fan switch that worked very well for him. I bought one from Pelican and put it in the car. The factory wiring harness has a single wire with a round spade type connector going to the single pole thermo-switch. I had two options (1) cut the factory wiring and solder in a flat spade connector, or (2) make a small 3-inch wire coupler with a round spade on one end and a flat spade connector on the other end. I chose option 2 so I didn’t mess up the factory wiring. I put a double layer of shrink wrap over the connection to make it rigid so I wouldn’t have to go back in there for a while. The BMW auxiliary fan switch is a two prong, Red Top, 99 degree Celsius switch (equates to 210.8 F) made by FAE, P/N 61-31-1-364-273-M323 and costs $13.75 from Pelican. Your local BMW dealer may have one, but cost may be higher.
You have to remove the flex lines attached to the oil cooler and the hard lines. Be careful when loosening the mounting bolts on the oil cooler. There is a lot of oil in there and any movement will cause it to spill on you. Once you have enough oil out of the cooler to remove it, carefully lower the cooler to the ground and dump the rest of the oil out of the cooler into a drain pan. Then remove the factory switch from the cooler. You may opt to cut the factory wire and install a flat spade female connector because the BMW part has two male connectors on it (see below). Install the BMW switch with a new gasket and tighten. It doesn’t matter which male connector you plug the wire onto. You then need to make a ground wire that will connect to the other male spade on the switch. I used brown wire just like the factory so there is no doubt as to what it is for. I made my ground wire about 8” long and put a female connector on one end and a connector with an eyelet on the other end. How long you make the ground wire is up to you. Make sure the eyelet is big enough to have a 10mm head bolt go through it, because on top of the oil cooler beside the new switch there is a 10mm head bolt you can use to mount the ground wire. Once all the electrical is done, remount the cooler, reinstall the flex lines, and start the car and let it come up to operating temperature. Once the hard lines get hot, check for any leaks at the flex and hard lines. If you need to change the flex lines, now’s the time to do it. If no leaks are detected, let the engine run and monitor the heat gauge in the car. Make note of the temperature when the oil cooler fan motor comes on. Depending on gauge calibrations, it should come on at about 210-215 F. I wanted my hook-up to be automatic so that’s why I did mine this way. Hope this helps explain what I did. Let me know if you need more.
05-31-2008, 10:30 AM
#12
Rennlist Member
Also as mixture leans heat increases, not sure why maybe someone can explain this. But this is why air plane pilots try to save fuel by leaning the engine from the **** pit but they do this while watching cyl head temps. The pilot will lean the mixture as much as possible but keep the cyl head temps at a reasonable level. So, if you have a lean running condition your engine heats quickly. Just something I learned from private pilots.
Formula 1 race engines actually have a feature now where they can adjust (using a computer) the fuel ratio while driving. They use it to conserve gasoline to give them a couple extra laps to avoid a pit stop and put them higher in the race lead. But the downside is that they make a little less power when running lean.
The downsides to running lean with our cars are sometimes holes in the pistons or melted valves. This can and does happen to people who don't fix a lean-burn condition.
05-31-2008, 02:06 PM
#13
Rennlist Member
I removed my rear wiper ('89) and made use of console switch to control fan. Granted , this is a "manual" approach, but certainly has proven it's usefulness. One of the better mods ever done on my car....... I also installed fresh foam around the aux cooler, as I found the original dry and somewhat rotted,..now, all the air passes thru the cooler...
Best,
Best,
05-31-2008, 11:04 PM
#14
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Thanks for all the input. RE: the foam around the cooler, what type of foam did you use? I don't have any "insulation" around mine to force the air only through the cooler. I was thinking maybe that pool noodle material would be perfect though not sure if it can withstand the heat.