Engine stumbles after repeated runs down the drag strip

[JimNolan]

Guys,
   You all probably know I've been chasing a problem with my car seemingly running out of gas on the top end after repeated trips down the drag strip. The guys that race all the time may be able to enlighten me on why that is. The hood is always raised after each run and my temp gauge is never over 180 degrees. Last weekend I found out definitely you couldn't run my engine repeated times with about 30 minutes between runs. I metal gas line has got fire sleeve on it also all the way from the fuel pump to the carburetor. It's not this engine just doing this either, my 390 I had in it before did the same thing. enlighten me on this. And, it seems to be worse when the temp is around 90-105 degrees ambient temp. Thanks, Jim

[jayb]

Jim, what kind of fuel are you using?  Is this an ethanol blend pump gas, or non-oxygenated pump gas, race gas, or?  Your problem kind of sounds like a fuel boiling problem.  Today's pump gas is designed for EFI systems rather than carburetors.  In an EFI application, the fuel is pressurized right up to the injector at 45 psi or so, which raises the boiling point significantly.  In a carb, once the fuel enters the float bowl it is no longer pressurized and will often boil.  I've had fuel boiling in carbs when the outside temperature of the carburetor is only around 150 degrees as measured with one of those infrared thermometers. 

If it is a fuel boiling problem, then the car is stumbling at the top end due to too much gas, rather than too little.  Boiling fuel will foam out of the carb vents and flood the engine.  Feels just like you are running out of gas. 

Non-oxygenated fuel is better in terms of boiling point, and race gas is better yet.  I think it might be worth putting five gallons of 110 octane leaded race gas in your car next time you are at the track, and see if you have the same problem.  If you do, it would at least eliminate the fuel itself as a potential problem, and if you don't, then you can take steps to correct the issue with pump gas.

[machoneman]

2X to trying non-ethanol laced race gas. A wood or plastic carb spacer may also help keep some of the heat the carb soaks up between runs off the fuel bowls. 

http://compare.ebay.com/like/180855459145?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar

[JimNolan]

Jay and Machoneman,
   I use 87 Octane gas from the cheapest place I can get it. When I designed the engine I purposely chose compression, quench and cam to get a low DCR. I'd like to try racing gas anyway. I'll run the tank lower next time before I take it racing and try buying some racing gas. Thanks for the possible explanation. Jim
PS. I do use a 1/4" hard rubber carb spacer made for the 750 Holley.

[thatdarncat]

You might want to check the responses to this post from a little while ago. Similar issue except on the street.

http://fepower.net/simplemachinesforum/index.php?topic=1262.0

I'll add a couple comments. I've never had a fuel boiling issue in my regular bracket car using race gas. I have had problems at the strip with one of my other cars using pump gas.

I'll also add, years ago when I was doing some testing with my bracket car and wanted to measure underhood temps to gain some insight as to how it might affect my et's I purchased a radio shack remote thermometer and had the readout inside the car. It didn't really matter how long I let the car cool with the hood up between rounds, as soon as I closed the hood and started the motor the underhood temp rose within a matter of just a few minutes.

[drdano]

Is there any fuel additive that Jim could add to the tank that would perhaps help raise the boiling point of the gas?  He said he is running 87 octane, would 91 have a higher boiling point?

[ScotiaFE]

Jay and Machoneman,
   I use 87 Octane gas from the cheapest place I can get it.

I use the crapyest gas I can get and go drag racing and my car runs like crap. hmmm....

[JimNolan]

Guys,
Sorry for not answering. I don't get email notifications of posts for some reason. Anyway. I love ScotiaFE's reply. He hit it on the head. You've got to remember though that I drive this car every day. Until this year I've driven my car to the racetrack even if it's been 900 miles away. That was Gulfport. This is the first year that I put it on a trailer to take it to the racetrack, and that's been 250 and 150 miles away respectfully. I can control what's in the tank better if I tow the car. AND I WILL NEVER RUN CHEAP GAS AGAIN IF I'M DRAG RACING. Running the car like I did at Columbus has pretty well indicated what my problem is, so I can eliminate that problem hopefully. Thanks guys. Jim

[Jim Comet]

I am interested to see if the race gas solves your problem, I hope it does. If not what kind of coil are you running? Is it in a place that gets airflow? Have you checked the voltage to it. How about when it is hot as heat creates resistance and possible voltage drops. Just throwing ideas out. Jim

[bartlett]

soon as you change fuel you change your tune ... My car goes from spot on to pig rich when I go from pump sa 92 to fleet farm non oxy .... just saying ..