View Full Version : 3.8 SC vs 5.0 bench racing myself(stock cars, fast guys may not be interested)

10-08-2009, 10:13 PM
Not trying to start a big debate here, just trying to get some thoughts off you guys after getting some experience with both.
The facts:
1989 XR7 auto completely stock- 16.12 @ 86.37mph
1989 XR7 manual removed air silencer and cats, magnaflow resonator, K&N- 15.25 @ 89.8mph
1993 Turtlebird LX 5.0- 16.54 @ 84.19mph

Most things I've read say stock for stock the SC takes the 5.0 with the presumtion that this is because the supercharged engine is more powerful, period. I'm thinking there are other reasons.
So far both 5.0 MN12's I have owned, a 1991 XR7 and this 1993 LX, have had autos(of course) and 3.08 gears whereas almost all of the SC autos came with 3.27's. I'm thinking that is roughly .1-.2 seconds on an ET and about 1/2 a car length in real world terms.

My second observation is that the auto in the supercharged XR7 would run all the way up to redline prior to shifting into the next gear. The LX would shift at 4500rpm. The run listed for the LX was held in first until 5,000rpm but then shifted out of 2nd at 4500rpm, crossing the line at about 3,500rpm:p. The best run leaving it in D the whole time with it also shifting out of 1st at 4,500rpm was 16.62 with most in the 16.7x's and one in the 16.80's. So I'm thinking with a similar shifting trans that's another .1-.2 seconds off the ET.

So right there, anecdotally, it's a wash.

Some variables:
Advantage LX- runs were probably 20F cooler, no traction concerns, true dual exhaust to stock mufflers, weight savings due to severly peeling paint.:p
Advantage XR7- launched at 2,000rpm or so compared to a conservitave 1,500rpm for the LX, freshly rebuilt trans, LX loaded with a trunk full of tools/luggage for a weekend of travel.

There you have it, just for conversation sake, I have V6 and I have V8 and I like them both.

10-09-2009, 12:44 AM
your auto sc numbers should be mid 15's

10-09-2009, 10:07 AM
your auto sc numbers should be mid 15's
Ok, let's say 15.5 for the sc. The 5.0 was 16.5, deduct .2 for gears, .2 for shift point, .2 for 200lbs in trunk, .2 for more agressive launch rpm, and .2 for a good complete cooldown after driving to the track( I forgot about that).
Any of those completely off base? That would put it right there.

10-09-2009, 10:08 AM
I run a 15.2 with my sc and in a cool day I run a 14.8 that is may best time

10-09-2009, 10:52 AM
The gears would make a difference but ...
The LX is lighter (trunk junk excluded) and still goes slower. I think it's all about the greater power of the SC engine.

I don't know if the transmissions would come with different speed governors between the SC and LX, though I suppose it's possible. But there are two variables you haven't ruled out ... the first is wear, since the XR7 has a fresh rebuild. The second is TV adjustment. Maybe they are adjusted differently.

10-09-2009, 11:18 AM
I hadn't thought about the tv adjustment. After burning up my first auto as long as it's not slipping I've called that adjusted.:eek:
The power ratings and my butt dyno are really what got me thinking about the gears when I realized they were different and then the shift points. The early sc's were only rated 10hp higher which isn't that much, of course the sc's curve is probably much better.

10-09-2009, 12:04 PM
Although the engines output the same peak power. The SC has a very nice Torque curve with lots of it for longer period of time. Thats whats propelling the car. Dont look at peak HP for a comparison. ;)

10-09-2009, 12:24 PM
Exactly, that's what I meant by the curve being better, the size of the area under the curve I'm sure is much larger than on the 5.0. More power, sooner.

XR7 Dave
10-09-2009, 12:56 PM
To be fast a 5.0 LX needs 3.55 gears or better. Lack of torque kills the 5.0 but it's pretty easy to shave almost a second off one.

10-13-2009, 02:39 PM
To be fast a 5.0 LX needs 3.55 gears or better. Lack of torque kills the 5.0 but it's pretty easy to shave almost a second off one.

What if u supercharged it

10-14-2009, 03:52 PM
The difference is the SC was made to perform well. When you look at what they started with, a 140HP V6....they really made the SC version haul.

The 5.0L, when you look at what they started with..........they did nothing. They just dropped the 5.0L in a uninspiring T-bird. The result was it moved better than a n/a V6. That's about it. If they wanted it to perform, it would have receieved a different stall convertor, tuned AOD and final drive upgrade, which is all it really needs to jump into the mid/low 15's.

10-14-2009, 06:28 PM
That's what I was trying to say, I think, only you were able to do it coherently.:p