What is the top speed anyone has seen in a GTM? I ask this because I keep seeing comments such as "faster than an Enzo" or similiar, but it seems to me that is unlikely since an Enzo will do approx 220 mph.
The capabilities are whatever you make them. I have no doubt that there are GTMs out there that have enough HP to go well past the 250 mark. I just two weeks ago took a GTM to over 2 Gs in a corner with DOT tires. There is no question that the GTM has certain advantages over the Enzo...like weight and potential horsepower...that will allow it to out perform an Enzo in many respects. Will your girl want to get naked on the buttery leather of the GTM...only if you put that buttery leather in YOURS!
For all intent and purposes, the limiting factor on a "stock" GTM is going to be the vavle springs in the heads !! PERIOD... end of sentence.
How is this you ask ?? Well, let me elaborate. Since your STOCK porsche transaxle ( even the G5050/52) only has gearing to accomodate 165 mph based on the top rev limited stock valve train of aproximatley 6200 rpm of the stock LS based motor... thats all your gonna get, even if you have 2000 hp and mclaren based road stability.
correction: Looks like the G5052/50 could make it to 190 mph at redline.... but you would quite literally have the engine on the edge of grenading itself which could prove fatal if something happened that locks up the driveline on you if even just for a second before you could mash the clutch in you would be history !
Even with enough hp, what about the problem with high speed stability?
I'd worry more about the tires exploding than high speed stability. Yes there is an issue, but it can be dealt with through a couple canards, a splitter, and good vehicle setup.
It's kind of academic here in the US anyway. I've owned several cars, including a Maserati, vette, as well as several cars I've built myself. Most "displays" of speed are over well before you'd reach terminal velocity. 99% of street encounters are over by about 100-120 mph when the loser realizes there's no way he can win.
If you want bragging rights for something like that, you gotta go do the Texas Mile or enter a race like the Silver State Classic Challenge. Now that looks like fun!!
I'd worry more about the tires exploding than high speed stability. Yes there is an issue, but it can be dealt with through a couple canards, a splitter, and good vehicle setup.
I also remember a thread about steering at high speeds being an issue, can't remember the specifics, basically someone thought it darted too much over 160mph? But one guy posted a reply saying "if I can't get my GTM to go 200 I'll burn it!" HAHAHA.....pretty funny.
At any rate I think it's worth bringing up...I don't have a GTM yet but I'm curious to know if it's that big of deal or not?
That was nowhere near the limits of the gtm, but it was beyond the limits of this driver and the road I was on. I wish I could say it was smooth and stable ride. I'm sure the body modifications we made are contrary to good aerodynamic principles and our car was not built for that type of activity. But I had to do it once right?
I imagine that Ferrari does wind tunnel tests on their body designs, to look for issues at higher speeds.
Not sure if anyone has been able to do that with their GTM, but I question if FFR has ever done them?
From 'Competition Car Aerodynamics' by Simon McBeath:
Vmax (mph) = cubed root of [(whp x 146,600)/(Cd x Frontal area in ft squared)]
425 Wheel hp, 18 square feet, and drag coefficient of .50 = 190.5 MPH
Seems kind of high even with a dirty Cd.
Cd of 0.55 = 184.6 MPH.
By the way, if you want to go this fast in a GTM, evidence seems to suggest that a rearward aerodynamic ballance is reqired for high speed stability. This is detailed in 'Competition Car Aerodynamics' as well as 'Race Car Aerodynamics: Designing for Speed' by Joseph Katz. At speeds this fast, understeer is a good thing, oversteer= bad.
In other words, put a good rear wing on it and the aero should be fine. Work the bump steer out and get your toe settings right and the suspension geometry should be fine. Get some good dampers and the suspension dynamics should be fine. Then go fast!
__________________
Build started 15 Dec. 2002
Registered June 2009, kit delivered December 2002. Levy Stage 4,347,EFI TKOII, IRS, FFR 4 into 4 pipes, Roh 17 inch wheels, Nitto 555's, 275/40/17, 245/45/17. Power steering, C&R Water to oil heat exchanger. Levy / Ron Davis big radiator. Tweeker RT, WB O2 system. Street performer seats, tracks both sides, Levy pro bump steer, SN95 front spindles, FFR LCA's, Levy / Willwood brakes.
Lomborghini titanium silver, painted by Gordon Levy.
Last edited by Bill Hughes; 09-22-2012 at 09:00 PM..
For the roadster, the data has already been done. All theoretical however I'd guess spot on to about the 200 MPH mark. I don't think a Cobra could do 250 MPH with 1,000 HP, regardless of how long the straight was. It would end up in an ugly accident north of 200 MPH. Terminal velocity for the Cobra is how big are your balls?
As said before, most vehicles have a terminal velocity beyond that of their performance abilities. With enough power and correct engineering, the super cars and hyper cars of today could hit 300 MPH. The problem you end up facing is tire life. The Bugatti Veyron's tires only last a short time at high speeds. There are Ford GT owners who have taken their rides in excess of 260 MPH in a standing mile and I believe they ran slicks.
If you have enough power, engine layout is not a problem. If you twin turbo a well built LS engine, you'll probably let off the gas with plenty of untapped ability. This is assuming that your suspension and steering is set up correctly for high speed runs. FFR did some wind tunnel testing for the GTM but I don't think they looked at speeds north of 200 MPH. If stupid fast speeds is your desire, I would look at a different vehicle that has already been tested for such speeds.
__________________
-Joe
"Autoholism: An incurable addiction medicated daily with car porn."
"It's loud, smells like gasoline, and shakes, shudders, and bucks. It makes your arms tired and your feet hot. You nearly crash about once every ten minutes. It's so damn wonderful you can’t believe it." -Automobile Mag. Oct. '04
For all intent and purposes, the limiting factor on a "stock" GTM is going to be the vavle springs in the heads !! PERIOD... end of sentence.
How is this you ask ?? Well, let me elaborate. Since your STOCK porsche transaxle ( even the G5050/52) only has gearing to accomodate 165 mph based on the top rev limited stock valve train of aproximatley 6200 rpm of the stock LS based motor... thats all your gonna get, even if you have 2000 hp and mclaren based road stability.
correction: Looks like the G5052/50 could make it to 190 mph at redline.... but you would quite literally have the engine on the edge of grenading itself which could prove fatal if something happened that locks up the driveline on you if even just for a second before you could mash the clutch in you would be history !
Presumably, if you are going to be utilizing an engine that has the kind of power to get the GTM up to,and perhaps beyond 200 MPH, then I would not expect you to be using a G50 gearbox of any type, and if you were then you would certainly have had it re-geared, and strengthened, and a limited slip differential installed into it.
If you were to use the more robust GT2 or Twin Turbo gearbox, and assuming that you have a rolling tire diameter of 27 inches, then I think you will find that you can break the 200 MPH mark with room to spare on the tach, using just the stock gearing. With a simple gear change you can further your protection of the engine even more.
As with any project you have to have the correct tools to do the job safely, otherwise bad things can, and often do happen.
I finally dynoed my new LS3 I built last weekend. @ 6500 RPM in forth gear I was at 157 MPH.
I built my motor to handle 7000 RPM, that said max power was made at 6154 RPM and I was making 425.1 HP and torque was at 362.8 at the rear wheels. With about 19% power loss because of the tranny I should be at 505 at the motor.
It blows my mind that some guys are putting 1000 HP in the GTM.
Happy Building, Ron
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GTM #66 LS3.2 525 HP Custom G-50/01 LSD kook exhaust system Aluminum Tanks
is there not a very long straight at Neubring? Shirley the pro driver in that youtube video topped out. I don't have youtube at work, but that's where I'd look first.
Ron my LS1 dynoed at 408 HP 362 Ft Lbs of torque at the rear wheels. Funny our torque numbers are identical. So according to my gear chart my G50-20 with taller 5th & 6th gears woulld give me 165 mph in 5th and 212 in 6th at 6500 rpm.
Math was never my stongest subject but if you have a 19% power loss and the result 425.1 wouldn't the motor HP actually be around 525? 525 X 81% = 425.25
Unless you engine dyno, then put right in and chassis dyno again, you really have no way of knowing what the % drop is. I have heard dyno guys say anything from 10% all the way up to 20% in driveline losses. Seems to me this number is more determined by the desires of the customer hitting a particular HP number than actual science, so be aware. The only numbers I would actually trust will be expensive to get, and even then, there can be variances between the engine and chassis dynos.
I know it's an estimation on transmission parasitic drag. I've heard from 12 to 19% from several people.
I am running G50/52 4th and 5th gears and the Dyno guy said that the numbers are very conservative. I didn't want an all out HP tune. I used one of those computer calculators and at 7000 RPM with what I have it came out at 204 MPH.
I'll never see it. If I did I would have to buy some Depends. LOL
If you are ever up in the Sacramento area Mark Romans is the man with HP tuner. He was doing stuff I've never seen before. he had to remap my throttle body to my gas peddle because I have a larger T/B.
Mark is a GM trouble shooter. When the dealership can't figure it out they fly him out.
Ron
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GTM #66 LS3.2 525 HP Custom G-50/01 LSD kook exhaust system Aluminum Tanks
The original author wanted to know if the GTM was faster than a 220 mph enzo . The FFR intended stock LS1 in a GTM will not best 200 with a stock porshe transmission. Gears are rediculously expensive at 1000 dollars each plus shipping plus installation. Yes you can MODIFY a volkswagon beetle to best 200 mph... but the stock valve train with the stock G50 drivetrain as intended from FFR isnt going to get it done.
The reason you max out on power at 6200 rpm is because your valves have reached their limit and start floating at that speed. If only you could ramp them down some when you reach that rpm so that the rotating assembly can keep increasing without that problem. I once saw a LS1 camaro in a parade with a 6300 rpm rev limiter two floats behind us and the dumba$$ was quite literally pushing in the clutch and bouncing it off the rev limiter every 50-100 feet for the crowd.... well he maybe did that 20 times for about a mile and one time the car didnt keep running. We jumped off and helped push him into a gas station and we found out a few days later he broke a valve spring, the retainer jumped the keeper and the valve fell into the engine and chewed up that head/cylinder/and piston. So you might think you will be on the edge of 200 mph with the ls1 behind a G50, but Im telling you thats not gonna be just a quick rev to 200 mph ... the engine is gonna have to push itself there in 5th gear slowly. And if you think your gonna push it past 6200 rpm losing power with the valves floating... well just make sure you take some good pictures cause I would like to see it.
I have read through several go-karting forums from california that the porsche transmission dynos alot like a front wheel drive car dynos.. from the 10-12 percent loss range. A rear wheel driven front engine car with a stick shift will lose 15-18 percent .... and a rear wheel front engine driven automatic transmission car can lose up to 20-25 %...
Don't forget about the cam. The cam has limits also. The whole valve train has to be built for it.
If you are talking about Enzo speed you can't honestly compare " stock" build to a Enzo. A big factor is the builders skill level.
Can it be done, sure it can.
Happy Building, Ron
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GTM #66 LS3.2 525 HP Custom G-50/01 LSD kook exhaust system Aluminum Tanks
When it comes to stability of rockets it's all about the center of gravity, CG, and the center of pressure, CP. The center of gravity is the pivot point for the rocket. Think of a weather vane, and the CG is the axis it spins about. The center of pressure is the point were the silhouette of the rocket would balance if it was cut out of a uniform material like cardboard. There is an equal area on both sides to make it balance. The goal is to make sure the CP is always behind the CG. This means that when the rocket is disturbed and isn't pointing in the direction of travel, it will return to the proper direction. This happens because the portion behind the CG has more area than the portion in front so it creates more force from the wind and returns the rocket to straight ahead. This same thing applies to cars even though they have tires that create friction and resist this. I've heard of guys adding weight to the trunk of their Bonneville Salt Flat cars to get more traction. The problem is this moves the CG backward and makes the car more and more unstable. The end result if this continues is eventually a spin. The car is essentially more stable traveling backward than forward.
Take a look at this picture of my car at an autocross (Ignore the blue tape. The parking lot had a ton of rocks on it!) I printed it out and glued it to some foam board. The leftmost line is the CP of the car in basic trim which I found by balancing it on pencil. The middle line, CP2, is after I added the sweet white "wing" on the back using some tape. The rightmost line is the CG, or the pivot point of the car based on a 60/40 rear bias. As you can see, there is more area in front of the CG so the car is going to want to "fly backwards" if disturbed at speed. The wing helps, but the CP is still in front of the CG.
My guess is this has played a part in those incidents where GTMs have lost control at high speed. I'd be interested to hear what the race guys think of this theory.
Based on your calculations, it would seem fitting to add some aerodynamic support
up front such as the splitter, and perhaps some modification to the radiator
inlet, and hood vents.
I've read some material on high speed design where keeping air from passing
under the car, and inside of the nose helps maitain stability. This is
probably part of the reason why FFR redesigned the front end of the GTM.
Back to the original question:
How important is top speed to GTM owners, or future buyers?
When, and where will you ever use 160+ MPH?
(aside from off-road race entries)
I'd like to see these supercars that boast 180+ MPH do it without falling apart,
or the need to modify the body.
I'm not saying it's impossible, I'm just recalling the texts, and video trials
where crews were masking the front end of the car to prevent the hood
from fluttering/ripping, wipers from flying off, and body panels from shattering.
Several OEM sport/super cars benefitted from taping air inlets, headlight
pockets (more to reduce drag), and lowering the ride height. Some cars
didn't make the cut, and needed 5th gear to hit their highest speed as
opposed to 6th gear because the engine wasn't powerful enough to overcome
drag.
__________________ "Welcome to the Internet where everyone's a mechanic and owns a 10 second car." Proud new owner of GTM#105, Aug. 27/11. Toronto, Ontario.
I'd like to see these supercars that boast 180+ MPH do it without falling apart,
or the need to modify the body.
This happens a couple times a year in different parts of the country. A lot of the "super" cars make it out to these events and the nicely modded ones do 200+ without falling apart...
__________________
GTM build in progress
first start with TT's Video
First drive with TT's :) Video
I have so much fun driving and banging gears on a winding road on a beautiful sunny day. That's where this car comes alive! Going in a strait line "top speed" or running down a drag strip, that in it's self doesn't do the car justice. It's so much much more than that.
Not to mention that the car is a penile extension LOL
For me it's not all about top speed. It's how you get there. I have never been in a car that turns more heads or takes turns so well.
Happy Building, Ron
__________________
GTM #66 LS3.2 525 HP Custom G-50/01 LSD kook exhaust system Aluminum Tanks
Somewhere near Pensacola, florida theres an old runway where once a year they have cars come out for a texas mile. I have been invited twice to take the GTM vs my friends GTR. I just got my castor corrected as far back as humanly possible, I just got me gearbox from Erik back with the tallest 4th and 5th gears possible without grinding the case to make them fit in there, and a double 9" clutch.... so maybe I can find out next one they have.
Somewhere near Pensacola, florida theres an old runway where once a year they have cars come out for a texas mile. I have been invited twice to take the GTM vs my friends GTR. I just got my castor corrected as far back as humanly possible, I just got me gearbox from Erik back with the tallest 4th and 5th gears possible without grinding the case to make them fit in there, and a double 9" clutch.... so maybe I can find out next one they have.
Somewhere near Pensacola, florida theres an old runway where once a year they have cars come out for a texas mile. I have been invited twice to take the GTM vs my friends GTR. I just got my castor corrected as far back as humanly possible, I just got me gearbox from Erik back with the tallest 4th and 5th gears possible without grinding the case to make them fit in there, and a double 9" clutch.... so maybe I can find out next one they have.
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