Car engines are marginal at best in a airplane. First they have too short of a crankshaft end, no drive washer either and many comes with a short threaded crank end and a clutch adapter tip too. Then there is the head which has too many cooling fins and needs to be machined down, but how much do you take off the top though? Also the carburetor is all wrong, with a push pull throttle on them that is difficult to impossible to use in a airplane. Finally, the car engines might rev up high, but they only do so for a few seconds not a long time, thus with a little tiny prop on them so it'll turn 38,000 RPMs, won't give it any quick airspeed in a airplane, the take off roll will be about 500 yards long, so allow for a long runway and the plane may stall out in the turns too.

Ultimate speed is the slope soaring speedster planes. no engines, no motors, just gravity and air and wind. They set all the eye popping amazing speed records. Engines actually slow them down too much.

But at present Jett engines are the way to go. Nelson used to be "IT" but he quit making engines though, but his engines are still really good. Years ago it was Rossi, but the fastest Rossi's were "tuned" to run even faster too. Picco and NovaRossi are still good, but aren't as good as Jett engines still.  One could check out the Pylon racing engines as well, they have had a number of those do really well for speed too. Many years ago before Jett was around, I used to run some Rossi engines and the full throttle high speed low to the ground fly byes were always fun to do. That was running a Rossi .60 on a .40 size plane with a thin wing.

But all out racing engines, only have two throttle settings, off and full throttle. If they have a carburetor on them at all, it is for killing the engine after the heat is over. The engines are tuned to be all out racing, no compromises, so tractability is not going to be there. For racing and speed they are the kings, but for sport flying they are really bad. it is like trying to drive a Daytona 500 race car on city streets during rush hour, or trying to drive a 1200 hp nitro-fueler superchargered dragster on a city street with a 30 mph speed limit.

Now with Jett engines you can get all out racing engines or their not quite so hot sport Jett engines that have more tractability to them so they behave better and have a throttle that works.