[opelwasp]

i just put in a getrag 240 into my gt. it runs great 1-4 but when i am in 5th and pushing it it overheats. i also top out at about 105 in 5th (4000RPM), but can hit 135 in 4th (6500RPM). what gives?

 

[kwilford]

Neither issues are surprising. At least the top speed, for sure. It is common for top speed to be less in a higher gear, especially an overdrive gear. Engine horsepower output (torque times rpm) is roughly proportional to rpm, so at 4000 rpm, you're only getting 4000/6500 (61%) as much power as at 6500 rpm. Usually the power drops off at the highest rpm, so maybe you are only getting 4/5ths as much power. So instead of getting 150 HP at 6500 rpm (which must be approximately what your engine puts out, to get even close to 135 mph), your engine is only putting out 120 HP at 4000 rpm. Since aerodynamic drag is proportional to the square of the speed, the engine runs out of power (the drag is greater than engine output) before it can get up to full rpm (and speed). And besides, if you could get to 6500 rpm in fifth gear, you would be doing 170 mph. Which would probably take upwards of 225 HP, even with the GT's small frontal area and reasonably good CD (Coefficient of Drag). I would have thought that you would get a bit better than 77% of the top speed while in fifth, but maybe you have a very high lift cam, with dual 45 DCOE's, which would cause the engine to have peak power at the higher end of the power band. To get the highest possible top speed, the engine output has to be closely matched to the gearing, so the engine peaks at just the point that aero drag "wall" gets hit. So use fourth for speed runs, and fifth for economy cruising...

As for the overheating, it's probably a related issue. Do you have a belt driven fan or an electric? If it's belt driven, then it is turning at only 61 % of the speed as in fourth gear, while the power output is probably at 75%. And thermal efficiencies are poorer at lower rpm, since the coolant flow is less, and "heat soak" (heat lost directly to the block and pistons) is greater. Usually the air flow due to the speed overcomes the reduced fan output. Do you have something blocking the air intake (either the grill or the nostril inlets)?

 

[opelwasp]

ok my engine is a 1.9, 30 over flat top pistons, 2.0 valves in a 1.9 head, 252 dur cam with .420" lift, ported intake manifold, pacesetter header, and a 38 DGAS. I also have a 3.67 rear end and 205/50R15 tires.