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Thread: Poor Performance When Hot

  1. #1
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    Poor Performance When Hot

    My motor has always seemed like it looses 20 to maybe 25% power when not only when it's hot, but as the motor gets hot when the outside temp is hot. As I was writing I remembered one of the only things it's had from almost the start it the MSD distributor, so I suppose I can check the timing when it's really hot. When I was at the track last it was hot outside. I drove it just over 1 mile and when almost all the way there, I nailed it and thought, yep she's running good. On the track I made 3 test and tune runs in a row. Each time I lost more and more power. I let it cool with the hood up for about and hour and It was better for 1 run but that's it, the second run it was a dog (in my eyes anyway).
    I do have a cold air intake. Besides the distributor, I'm at a loss. Almost everything else has been changed at one point or another. I know all cars loose power when hot, but this is way more than what I would consider normal. Even my girlfriend can feel it and comments before I even say anything (on the road).
    Any suggestions?
    Tony

    1955 Bel Air Sport Coupe

  2. #2
    Registered Member NickP's Avatar
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    So, what was the weather stations report? I don't mean on the TV BTW. Elevation? Location?? Pics of the "cold air intake system"? The variables are mountainous. Eliminating engine internal issues, the rest mentioned can have a profound negative impact if not addressed. What's the engine cooling system consist of, fans, shroud etc.?

  3. #3
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    Just how many tenths are we talking, not unusual to have to adjust the dial on a bracket car when you found robin. This is also why most 10.90 Super Stock cars can go 10.20 un stopped. I have always said if the temp outside is comfortable to you, your car will like it too.

  4. #4
    Registered Member chevynut's Avatar
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    How do you know it's losing power? Does your ET go up? How are you estimating a 20-25% loss?

    An internal combustion engine is more efficient when running hot. We can't run them extremely hot due to the materials and fluids used and their limitations. Fuel atomizes better in a hot engine. So the engine internal temperature isn't affecting your power unless it's causing detonation. Modern cars tend to run hotter so they are more efficient than older cars.

    Colder air has more oxygen molecules per unit volume, so you can add more fuel and get more power. On a cold day, your engine is ingesting cooler air so it will produce more power, assuming you're getting the cold air to the intake. It's normal to lose power on a hot day but probably nowhere near 20-25%. It's related to air density.

    I'm wondering if you're still suffering from symptoms of boiling fuel. Do you still have the bypass hooked up? If so, it could be returning hot fuel to the fuel tank, raising the fuel temperature so it's easier to boil in the lines and carb. Have you monitored your fuel pressure when you experience this power loss? Where is your ignition coil mounted? Is it getting hot? As the temperature of the coil windings increases, the electrical resistance increases. Also the insulation on the wires could be breaking down or leaking current. However, I think a fuel delivery issue is more likely.
    Last edited by chevynut; 06-12-2018 at 07:29 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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    56 Chevy 2-door BelAir sedan
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  5. #5
    Registered Member chevynut's Avatar
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    What ignition system are you running? Are you using a knock sensor?
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  6. #6
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    Need a lot more information. Is the engine missing or stumbling? What's the coolant temperature when it runs best and what is it when worst? Are you making any attempt to cool the engine between passes?

    Seems to me that the weather isn't what's causing this, if it was the weather the first pass would be off too, unless there was a wild temperature swing, or a weather front moved through.

  7. #7
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    The loss in performance is so great that I can feel it on or off the track. I'm not talking times because as I've said many times already, the track is just a track being run by some guys with no money, there is no electronics, no tree, nothing but a track and lights that run off a generator. This has been going on for 7 years, long before I've ever driven on a drag strip. It's easy to feel it on the road. If you felt the difference from a cold day to a hot day you just may understand better. The coolant temp is probably down to 180 in the winter and it runs 190 in the summer, I don't go out much when it's over 90F since I have no a/c. There is no missing or stumbling. It's not at all like what it used to do when the fuel boiled. It's just feels like a dog when hot and has an extra 100 HP when it's cold. It really is a huge difference. Like I said in the original post, yes it runs a little better if I let it sit for an hour with the hood up, but that doesn't last long.
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    Tony

    1955 Bel Air Sport Coupe

  8. #8
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    I can tell you my best ets have been on passes were I pulled on track at 150/160 degrees and launched at no more than 170. Usually my best ETs have came in the late fall. Plus the loss of 2 tenths is enough to make a car feel like a turd.

  9. #9
    Registered Member chevynut's Avatar
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    As I said before, an IC engine is most efficient when it's hot. Your problem has nothing to do with coolant temperature imo. The amount of work done is greatest when there's a large difference between the engine temperature and ambient. I personally doubt you're seeing a 100HP difference.

    That said, you WILL lose power if the air is hot and humid. Hot air is less dense, and humid air is even less dense. Density is also affected by barometric pressure. But all of those things are relatively minor.

    If you assume a baseline of 70F and 50% RH, then raise the temperature to 90F and 50% RH the air density drops to about 96.1%. At 70F and 80%RH it's about 99.6%. At 90F and 80% it's about 95.5%. So you can see temperature is a bigger factor.

    You should see no more than about a 4-5% drop in HP due to temperature and humidity. At 400 HP that's 20HP.

    If you think you're losing more than that, you have something else going on. I didn't see an answer as to whether you're using a knock sensor or not. At higher temperatures your engine will detonate easier. If there any evidence of that? Can you hear any pinging or is the car too loud? I wonder if you have too much timing advance. If it's not timing, it is probably fuel delivery. Or it's only a normal 20HP drop

    Here's an article that may be of interest....

    http://www.onallcylinders.com/2013/0...er-conditions/
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  10. #10
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    It's more like when hot, it runs like it did when I had a c/r of about 7:1 (1978 smog motor) and a smaller cam, instead of now close to 10.5:1 with a larger roller cam (not counting when it's not too hot) Maybe even more of a difference.
    I don't know about the hotter the engine the better, that's great for efficiency, but not for HP.
    No knock sensor. No knock or ping. Nothing to hook a knock sensor up to!
    Tony

    1955 Bel Air Sport Coupe

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