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What will it take to do it? I have two '96 C4's, one has an LT-1 with an automatic, the other has an LT-4 with a six speed stick. From what I've gathered online the LT-1 car can do close to 160 mph and the LT-4 car can do close to 170 mph. The LT-4 car would be the one that I'd like to try to make go 200 mph because it has a better trans and differential than the LT-1 car. I'm thinking that an engine built with low compression and with a high boost supercharger putting out 600-700 horsepower should hopefully give me enough power to hit the 200 mph goal or at least get real close to it. Are any suspension mods needed? The car has been upgraded with C5 brakes by the way, hopefully those will be adequate enough to stop the car from that kind of speed. I've had the car up to 150 mph so far and it seemed to be handling it with ease. Any suggestions on how to make a C4 go 200 mph (short of strapping a rocket engine to it) would be greatly appreciated.
My experience with high speed and braking is to do your high speed **** in an area that doesn't require you to use brakes. Using the brakes at those speeds can be interesting if it's not something you do frequently
Coasting back down to earth is much safer than using the brakes anything north of 130 imo. above 130 it is always amazing to me how fast the car slows down just from the force of the wind
Aside from that, look at this. It was posted on the forum some time ago. Don't know if that's wheel horsepowers or not, would make more sense if it was. My goal is in yellow. Time to print a new one.... Lol
Theoretically you could go 200 mph with 350 horsepower. It would probably just take forever to get there. Obviously your gearing needs to be correct too.
Also if your car is lowered, it's drag coefficient should be less which would require even less power to get to those speeds.
Last edited by Pwnage1337; Sep 2, 2019 at 02:39 PM.
Shoot, Callaway hit 250+ mph with the Sledgehammer in the late 80's. That was a 900hp twin turbo. There's a 270mph C4 that raced at Bonneville that made something like 900hp. But, it was ballasted up to 6,000lbs, solid rear axle with no travel, 1/2" travel limiters in front, full aero nose. The reason for the weight and suspension mods were because the salt is so rough.
With something like 500+hp and a ground hugging front air dam and the right gearing, 200 should be doable on asphalt. Probably want to stiffen your shocks up.
That sounds like a fun goal, do you have a place in which to achieve this goal? Texas mile, salt flats? The amount of space you have is going to make a difference in what kind of power you need.
In my experience with boosted engines, top speed runs put an incredible amount of stress on the motor and your tune will have to be very conservative to keep the motor together. It can be done but probably not on a stockish motor. I would try to make power with a big inch short block and big heads, then look into a blower if you need it. Torque numbers are your friend with this type of build, it takes a lot of torque to push through the air at those speeds.
Last edited by thurman_merman; Sep 2, 2019 at 03:18 PM.
Torque has absolutely nothing to do with the question of top speed. Horsepower and drag are the only two figures that matter at all. We had a thread on this a while back. The chart pictured in pwng1337's post is correct for aero drag, if it has the correct drag coefficient and frontal areas for a C4. Once we know the drag force a car has at a certain speed, it's easy peasy to determine the power required to equalize a certain amount of drag. The formula is for tractive effort (the total linear propulsive force at the tires' contact patches) is:
TE = (375*P)/V, where TE is tractive effort in lbf, P is horsepower, and V is mph.
or, to get the power required to generate a certain tractive effort:
(TE*V)/(375)=P
So if a C4 has a total of 659.3 lbf of resistance (aerodynamic drag) at 200 mph, then the formula tells us it would require 351.6 hp to reach 200 mph - exactly what the chart also says. Keep in mind this is power at the contact patches - it does not take into account all the frictional losses in the drive train or tires. I don't know what that number is: I think it's far more dependent on velocity of all the moving parts than the overly-simplistic 15% number commonly used to convert from wheel hp to crank hp. It's probably safe to assume that 400hp would get the job done.
Some things to note here:
Everything is based on power, not torque. The car doesn't give a damn whether the 400hp comes from an engine spinning 9000rpm with little torque or an engine spinning at 2000rpm with huge torque. It only cares that the engine is making 400hp at 200mph.
For a given horsepower, tractive effort goes down as velocity goes up. There is no way to trick this fact out of existence. Aerodynamic drag goes up with the square of the difference, such that doubling of the speed quadruples the aero drag. But doubling the speed also halves the tractive effort available for a certain amount of power, so the power required to reach a certain velocity goes up as the cube of the speed difference. That is, doubling the speed actually requires eight times the power.
Gearing is a real issue here. A stock 96 C4 6sp has a 3.45 rear axle ratio and a 0.75:1 5th gear. It redlines at 188mph (6400rpm) in 5th gear. 6th gear is heartbreaker at 0.5:1, and therefore it redlines at 282mph. Assuming the engine makes peak power near redline (like the LT4 does), obviously the car as equipped cannot muster anything close to peak power at 200mph. If the engine makes peak power at 6100rpm, then changing the final drive to 3.09 would allow the car to hit exactly 200mph in fifth gear at peak power. Different C4s with different transmission and axle gearing (and different peak power rpm) need to adjust.
Last edited by MatthewMiller; Sep 2, 2019 at 06:02 PM.
If that's true than I will stand corrected. My experience comes from a car I had that would run to right around peak torque in an overdrive gear then fall on its face. It could have been something else causing that to happen, it definitely wasn't lack of horsepower.
If that's true than I will stand corrected. My experience comes from a car I had that would run to right around peak torque in an overdrive gear then fall on its face. It could have been something else causing that to happen, it definitely wasn't lack of horsepower.
The thing is that although power keeps building for some range after the torque peak, it doesn't build as fast as the need for power to keep increasing speed. This comes back to the fact that as speed increases, the requirement for the power to push that speed increases with the cube of the proportion difference. So just to make up some semi-realistic numbers, if your engine has a 4000rpm torque peak and a 5000rpm power peak, it might make 25% more power at 5000rpm (the power peak) than it does at 4000rpm (the torque peak). That's a 1:1 increase in speed and power: you are going 25% faster and making 25% more power. But to actually push the car 25% faster takes 95% more power (125%^3). So although your engine is making more power, it's not making enough additional power to push the extra speed.
However, in your car's case, if you had lowered the overall gear ratio so that it was approaching peak power rpm at top speed instead of peak torque, it would have gone to a faster top speed. Another way to think about this is to imagine a car with a constantly variable transmission (CVT). At any road speed, it can run at any rpm. It will always achieve the fastest road speed when running at peak power rpm. Likewise, it will always accelerate the hardest at peak power rpm.
I get the idea but it doesn't make sense to me in my particular case. Car made 450whp@6200/425wtq@5100 and would fall on its face at 5k in 5th at ~135mph. There should have been plenty of power to push it up to redline (6400) at that speed don't you think? I never put much thought into it except for as torque started to fall it couldn't push against the wind anymore. Car was a 3000lb rx7 btw.
Last edited by thurman_merman; Sep 2, 2019 at 10:44 PM.
After a quick Google search I see that it widely accepted that max speed is a product of hp so I do stand corrected on the subject.
That notwithstanding, you had 412hp available at the wheels at 5100rpm. RX7s (of all generations) should be pretty low-drag cars, with good drag doefficients and small frontal area. So at only 135mph, there should have been no problem accelerating right on up to redline.
Last edited by MatthewMiller; Sep 3, 2019 at 01:31 AM.
I scared the hell out of myself with my 1968 C3 and its 427 engine. I use to like to see how fast the car would go on runways at small airports. At over 140 mph the front end of my C3 gets very light feeling and I suspect it is actually "lifting". The 427 is making over 500 hp and 550 lbs. torque and it winds right up in any gear. The Power is there to go fast, I just don't have the gearing to do it. The car is a convertible and came with a 3.36 rear end was my "limiting factor" according to Comp Cams. They kept telling me to change the rear to a 3.70 at least and I wouldn't do it.
I would be very careful driving at high speeds like these, you need top move the speed up gradually so you don't get surprised. The C4 may have a different front end and may ride better at high speeds but take your time and "learn the car" again because at speeds she is another whole different beast to contend with. Speed makes everything happen REAL fast and you need to be prepared.
Going to Red line in my Fourth gear would be possible if I didn't start to float at 135-140. Maybe it is time to put the 1978 pace car's front spoiler on the C3 again. That or just go slower....
I get the idea but it doesn't make sense to me in my particular case. Car made 450whp@6200/425wtq@5100 and would fall on its face at 5k in 5th at ~135mph. There should have been plenty of power to push it up to redline (6400) at that speed don't you think? I never put much thought into it except for as torque started to fall it couldn't push against the wind anymore. Car was a 3000lb rx7 btw.
Originally Posted by MatthewMiller
That notwithstanding, you had 412hp available at the wheels at 5100rpm. RX7s (of all generations) should be pretty low-drag cars, with good drag doefficients and small frontal area. So at only 135mph, there should have been no problem accelerating right on up to redline.
Yea. Something wrong with that car for sure. I am ~3050lbs with 420+ at the wheels. 4th goes to 150mph and 5th goes to a theoretical 235mph w/ a 0.64 5th gear and 3.45 final. I haven't pushed her past 160 but even with the tall gear she is still easily pulling.
things happen fast at that speed and you have no control if something happens you are dog meat lol
Done a few top speed runs and both times (ZR-1 and my Camino) around 150 got a quick reminder how stupid it was.
1/4 mi type stuff for me these days I want to live another day...it takes one side gust, fluid leak, rabbit running out to turn things nasty.
If not then what other engine mods besides the supercharger will need to be done to make enough power to make it happen? Does that blower make enough boost (8psi) or should a higher boost blower be used and if so, how much more boost would it need?
Last edited by TheGreek!; Sep 5, 2019 at 08:45 AM.
If not then what other engine mods besides the supercharger will need to be done to make enough power to make it happen? Does that blower make enough boost (8psi) or should a higher boost blower be used and if so, how much more boost would it need?
I can't see the link at the moment (blocked at work!), but my guess is 8psi on an LT4 can make enough power to do it as long as it's got enough fuel (injectors) and a proper tune. However, go back and re-read my last bullet point in post #6. The stock gearing is a real issue if you really want 200mph. You'll basically be limited to 188mph with the stock gears. So that will have to be addressed. The downside is that changing to a 3.09 gear will hurt the lower-speed performance and around-town experience.
I've topped out the 415 in 5th about 10X. That reads 200 mph on the speedo. That's around 700 hp from my LT5 which is lowered an inch, 3.73s out back and redline raised up to 8000 rpm. Car is very stable. Also done it in my TT C5 on high boost (about 900 hp). The speedo gains as fast as the tach rises in 4th and 5th, and marches right past 200 mph at the top of 5th. I need about a mile less in the C5 with the extra hp on tap. 150 mph is completely different from 100 mph, and 200 mph is equally different from 150 mph. Looking far out onto the horizon is an understatement. I took my Dad out with my in the ZR-1 on one run, and after we had peaked and were coasting down, he pointed to a side road (on our closed section of property) as it approached and said we could take a left there. I looked at him, and said we are doing 155 mph!!!!!! We aren't making any 'left turns' for another mile or so!!!!!! Even as a passenger, after 200, 155 seems like you're toodling along at walking pace.
200 mph C4?
