View Full Version : Gearing and top speed
RetroRacing
10-27-2015, 07:16 PM
So, we have two RA boxes, 4.44 gears. That tops us out at 150ish. Does this car go faster than that with 300ish hp at 1700lbs? ( before anyone asks, yes we are pretty sure of the weight complete)
Do we need to go 4.11's?
Rasmus
10-27-2015, 09:25 PM
Top speed isn't determined by weight, as long as distance isn't a factor.
Top speed is a function of frontal area, aero drag, tire drag, and horsepower. Rough estimates 300 whp will get you to about 190ish mph. 300 bhp will get you to about 180ish mph.
I used this calculator (http://warped.org/vw/calc.html) Using the Lotus Elise for input.
And here's the funny thing about frontal area and aero drag, to go 200mph you'll need 400bhp. 210mph: 470bhp. 220mph: 550bhp.
RetroRacing
10-28-2015, 12:03 AM
Your right about the weight of course, but it does effect how quickly you accelerate so in a way, it does effect top speed at a fixed distance. We have trimmed the car out significantly, flat bottomed the whole car and ducted the front rad to direct the air over the car
Looks like we need some new gears...
billjr212
10-28-2015, 09:29 AM
The top speed of a Lotus 311 (race version) is 174 mph. That is with 450 hp and presumably better aerodynamics. Seems that would be the best base point to an estimate for the 818R.
http://www.lotuscars.com/3eleven/
Evan78
10-28-2015, 03:56 PM
Are you planning some land speed racing?
Plebeian
10-28-2015, 10:54 PM
Rasmus, that calculator seems way off to me. Per my recollection, the 505 hp Z06 Corvette was well documented to top out at 198 mph, yet that calculator is stating it takes far less to go 200 mph.
Plebeian
10-28-2015, 11:00 PM
Does this car go faster than that with 300ish hp at 1700lbs? ( before anyone asks, yes we are pretty sure of the weight complete)
1700 lbs with driver and fuel? Is that 300 rwhp?
C.Plavan
10-29-2015, 09:13 AM
I would not want to go over 150mph in an open cockpit car built in my garage. :)
Pearldrummer7
10-29-2015, 09:55 AM
What tracks do you need to go over 150 mph on? I have a 4.44 final drive and figure my rev-limited top speed is a little over 150. I haven't hit that (or too close to it) and can't imagine "needing" to
RetroRacing
10-29-2015, 10:25 AM
As for tracks, PIR without the chicane front straight, Thunderhill end of main straight, The Ridge front straight and Spokane. Our tri-rotor IMSA car was hitting 165-170 at the last endure at Portland, so I think we need to be close to that, but asking if it's even possible.
I can see keeping the 4.44's in the car for Thunderhill, but we are running a .82 ish 5th gear, so at 7500 rpm we are at 153mph. If we take it to the stock 8250 rev limit (really don't want to) we get into the right range 165-170, but because of the 4.44's our launch onto the main straight should be sweet, so I see us running out of gears before the braking zone.
We should be close to 300 hp at the wheels. Using a 4 lb battery, a 15lb clutch/flywheel assemble, UL rotors that are 7/8" thick, lighter header, no center console, etc took over 150lbs out. Still not done, CF body may be in the works for at least the top panels before testing.
We already go 150ish with cars from the 1980's that we build in our shop....fun is not always smart I guess.
Sgt.Gator
11-02-2015, 07:12 PM
Retro my LGT 5 speed 4.11 was just hitting 158 mph at Spokane at 6500 rpm. That is with less HP and 50% more weight than an 818!
If you switch to an LGT 5 speed 3.90 that would give you 169 mph at 6500 rpm.
The M Factory gearing calculator. http://calc.teammfactory.com/
3.166 / 1.882 / 1.296 / 0.972 / 0.738 / 4.111FD - '05-'06 USDM Legacy 2.5GT
3.166 / 1.882 / 1.296 / 0.972 / 0.738 / 3.900FD - '07-'09 USDM Legacy 2.5GT
CHOTIS BILL
11-03-2015, 09:56 AM
I would not want to go over 150mph in an open cockpit car built in my garage. :)
I have gone over 150mph in an open cockpit car not only built in my garage but one I designed myself. If you get it mechanically balanced and aero balanced with the proper air flow around the driver there isn’t that much difference from going 130 or 140 except the fence posts go by a bit quicker and the steering gets a bit heavier from an increase in downforce. :)
Bill Lomenick
RetroRacing
11-03-2015, 03:23 PM
Sgt. Gator, we have raced in a few enduro's with you (black RX7's) and you are a rocket down the straights! I like the big gears found in the RA boxes, as well as the closer ratios, but do now realize we will need to change final drive. Can you put a 4.11 or a 3.9 in a 1999 RA box? Anyone,,,,,,anyone,,,,Bueler,,,Bueler,,,,
Real time recon
11-04-2015, 09:50 AM
A thousand light years ago.I blew a 427 @ 140 MPH. And it wasn't fun OR cheap....1968 Corvette 427, 4 on the floor Roadster convertible
I found this: http://www.wallaceracing.com/Calculators.htm several years ago. LOTS of good automotive calculators (tire size, gear ratio, speed at RPM etc.)
Ray
Sgt.Gator
11-04-2015, 02:23 PM
Sgt. Gator, we have raced in a few enduro's with you (black RX7's) and you are a rocket down the straights! I like the big gears found in the RA boxes, as well as the closer ratios, but do now realize we will need to change final drive. Can you put a 4.11 or a 3.9 in a 1999 RA box? Anyone,,,,,,anyone,,,,Bueler,,,Bueler,,,,
Yep I've seen you blow by me in the corners many times! :)
The LGT 5 speed is as strong or possibly stronger than an RA tranny. See http://www.rallispec.com/gea_syn_rst1140_t.html
It would be cheaper to buy a used LGT tranny with the final drive you want. The close ratio RA gears are going to mean a lot of shifting which could actually hurt your acceleration times. The powerband on the turbo Suburu engine is plenty wide enough that the close ratio may be counter productive. It's not like an RX7.
Here's the RA gears to compare to the LGT ratios posted above. Gear Ratios: 3.083 / 2.062 / 1.545 / 1.151 / 0.825
You can use the MFactory calculator posted above to compare the two side by side.
A used LGT 5 speed is about $500 to $750. They are always for sale on the Legacy GT Forum because of guys swapping in the 6 speed tranny, or available from wrecking yards. Let me know if you want one and I'll keep an eye out on the LGT forum and ask in the PacNW subforum if anyone is selling one. Heck I have one sitting in my garage from my 6 speed swap waiting to go in my future 818 build, but with Chad selling his 818R I may not need it.....:confused:
FWIW, everyone I know who has the Moore's Performance Blast Plates swears by them, despite the controversy that they really don't do anything to strengthen the case and gear weakness issue.
RetroRacing
11-04-2015, 02:43 PM
I see your point...need to get out of the no torque mind set.
C.Plavan
11-08-2015, 12:06 PM
Yep I've seen you blow by me in the corners many times! :)
The LGT 5 speed is as strong or possibly stronger than an RA tranny. See http://www.rallispec.com/gea_syn_rst1140_t.html
It would be cheaper to buy a used LGT tranny with the final drive you want. The close ratio RA gears are going to mean a lot of shifting which could actually hurt your acceleration times. The powerband on the turbo Suburu engine is plenty wide enough that the close ratio may be counter productive. It's not like an RX7.
Here's the RA gears to compare to the LGT ratios posted above. Gear Ratios: 3.083 / 2.062 / 1.545 / 1.151 / 0.825
You can use the MFactory calculator posted above to compare the two side by side.
A used LGT 5 speed is about $500 to $750. They are always for sale on the Legacy GT Forum because of guys swapping in the 6 speed tranny, or available from wrecking yards. Let me know if you want one and I'll keep an eye out on the LGT forum and ask in the PacNW subforum if anyone is selling one. Heck I have one sitting in my garage from my 6 speed swap waiting to go in my future 818 build, but with Chad selling his 818R I may not need it.....:confused:
FWIW, everyone I know who has the Moore's Performance Blast Plates swears by them, despite the controversy that they really don't do anything to strengthen the case and gear weakness issue.
I have a good spare LGT trans that can come with the car...... :P Or I can sell it to Retro.
Real time recon
11-08-2015, 04:24 PM
So, we have two RA boxes, 4.44 gears. That tops us out at 150ish. Does this car go faster than that with 300ish hp at 1700lbs? ( before anyone asks, yes we are pretty sure of the weight complete)
Do we need to go 4.11's?
Found this.It also has a calculator Within >> http://mustangforums.com/forum/4-6l-general-discussion/643258-what-rpm-s-at-60mph-with-3-73-a.html