GTD performance figures

I've recently been given a copy of "cartest", a simple computer performance simulation package.. it's quite old so some may have seen/used it before. I only see it as a bit of fun, but the results are reasonably accurate, if somewhat optimistic! & it does give you an option to easily compare different changes & the effects they have on performance....

Basically, you punch in the cars's weight, power, gearing, drag co & a few other items & you can then 'simulate' the cars performance.

For fun, I've typed in the details for my stock GTD. I've estimated the power at 225bhp @ 5750 & 200 lb/ft @ 4000, a co of 0.35, weight 2600 lbs & got the following results;

0-60 in 6.2 secs
0-100 in 15.3 secs
standing quarter 14.5 @ 97.2mph
top speed 146mph

Lowering the gearing (radically) by trying a 5:1 diff gives virtually no increase in performance!

however, If I take 600lbs off the weight, the performance is well up, even with the same engine;

0-60 in 5.2 secs
0-100 in 12.4 secs
standing quarter 13.5 @ 104.7mph
top speed 147mph

Using the stock weight (2600 lbs) but adding 100bhp & 100lb/ft (325 bhp & 300lb/ft) gives a stonking improvement;

0-60 in 4.7 secs
0-100 in 10.3 secs
standing quarter 12.7 @ 113.5mph
top speed 149mph

an interesting comparison! anyone got any 'actual' figures I can use to test the programs accuracy?
 
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Hi Julian

Go to the Star is the Car bit on the club site and you can use the info on my car there. It should weigh the same as yours as it is also standard body and chassis. There are real results you can compare with.

I would be interested to see what you come up with. Let me know if you need more info.

See you at Brighton and Longleat?

Malcolm
 
Hi Julian,
Interesting reading - I've recorded a 0-60 of 3.7 secs on one occasion and regularly manage sub 4.2 secs, although that is on softish slicks. (In Roy's GTD40 Mk1)

The car has a lightweight chassis, lightweight body panels and an all alloy 348 with a running weight of approx 1020kg.

I would be interested in what your program gives for a power output of say 500Bhp/450ftlbs @ 6000rpm.

Does the program ask for any details re gearing or tyres used?

Fastest official qtr thus far is a 12.3 @ 128 although we have also seen 12.1 @131 on one occasion.

regards

Paul Thompson
grin.gif


[ August 20, 2002: Message edited by: Paul Thompson ]
 
Regarding gearing, the program asks for individual ratios for each gear, and final drive ratio. I've used my stock R30 ratios & FD. It takes account for transmission lsses in each gear and this can be adjusted.

Also it asks for compression ratio so I've guessed mine at 8.5:1.

Malcolm, for your car I've used 350 bhp @ 6000 (redline) & 300 lb/ft with a 10:1 compression ratio, gear ratios as per R30 & 2600 lbs weight & got these results;

0-60 in 4.6 secs
0-100 in 10.0 secs
standing quarter 12.6 @ 114.7mph
top speed 149mph

Paul, for your car I've used the same ratios, 500 Bhp/450ftlbs @ 6000rpm, guessed at 10:1 cr & weight of 2244. It gives the following results;

0-60 in 3.5 secs
0-100 in 7.0 secs
standing quarter 11.2 @ 129.6mph
top speed 144mph (?)

Altho it's an old program (DOS based!) there are loads of parameters you can adjust & given time, I'm sure the variables could be sorted to give more accurate results. Looking at pauls *actual* times compared to *simulated* times, it's not that far out. Possibly raising the drag co or maybe transmission losses in gear may bring the times down to a more accurate figure..

The program gives loads of other info, graphs, etc.. & you can fiddle with all sorts of stuff!! If anone wants a copy of the software I may be able to email a zipped up copy, but I only have a slow modem & computers ain't my best skill!
 
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Interesting Julian

My best ever 1/4 mile was at 12.6* although I have gone quicker than 114. However at Brighton if you get a following wind your times and speed are better. A force 5 SW straight in off the sea will give you a big improvement. Obviously the computer does not have wind, sorry I mean allow for wind!

0-100 seems a bit optimistic for my car but hey I shall stick with 10 secs as you are a mate! Sounds good if nothing else. Maybe telemetry will prove us right or wrong soon. If car stays together long enough.

Top speed is low. I have done 155 so far and bottled out. Unlikely to test that again as that is a bit toooo quick for comfort. I suspect that the gear ratios you have assumed are not exact but as we didn't supply you with the correct ones.... I know Andrew has a spread sheet of ratios somewhere.

I would be interested in a copy of the software as MS-DOS is at my level of computing. Swap for a simple suspension programme?

Malcolm
 
In terms of 1/4 mile launch, there are some chassis dynamics that I am not real sure of that seem to work against rear engine low cg cars with stiff suspension compared to typical drag cars. This has a small effect on MPH (at end of 1/4) and large effect on 60 foot times and ET. With the low cg the weight transfer to the rear is small, but there is already a rear weight bias. Intuitavely it has to do with how the car reacts and transfers weight on the tire. If you ever see cars do this right it is amazing. Take a current NHRA Pro Stock Car for example. They have ~1400 HP hold the car on a 6800 RPM rev limiter with throttle wide open. Drop the clutch hard at 6800 RPM (shift at ~9300) and the car just goes.

Based on what I have heard the power to weight of a GT40 should produce much lower ET's (not necessarily MPH) if it were not for this launch situation.
 
Malcolm,

Actually the computer does have wind (tee hee!) You can tap in a head or tailwind, along with air temp, relative humidity, road level gradient... loads of stuff!! The ratios I used are 3.36, 2.05, 1.38, 1.06, 0.82 & 3.89 FD. However, different raios don't seem to affect performance much, untill you go too high & then the car hasn't got the power to pull.

If I were to use a 5.5:1 diff in my car in would have a theoretical top speed of 104 mph & 0-100 is reach in 14.9 sec. this is opposed to 15.6 with a 3.89:1 diff.... I'll email you the zipped up version later so you can have a play! your suspension would be most welcome!

CCX33911

The program asks for % of weight on the front wheels & I've guessed at 35% for the GTD.. if anyone has a more accurate f/r bias figure I'd be interested to know! As you say, these cars aren't really set up for standing quarters, but it's an interesting point of comparison...
 
Hello again,

Thanks for the data Julian - where did you source the program?

The 0-100 for Roy's GTD is also quite close as the 3.7s to 60mph continued to 100mph in 7.6s. It was from the startline at Longleat hill climb last year.

Malc's quite right about Brighton, with a head wind, no records are going to be broken! but with a tailwind? well...

Roys recorded 172+Mph down a runway and could no doubt have gone faster but as we only allowed 400M to slow down (some of which was a bend) he'd probably be pushing his luck!

regards

Paul Thompson
 
thanks for that CCX33911,

Interestingly the change in weight distribution improves the results slightly, 0-100 is down by 0.2 sec, & standing quarter down by 0.3 but with the same terminal speed.

Paul, If you'd like a copy of the program, email me direct & I'll send you a copy.. It's alot of fun if nothing else!
 
Julian,

Since it is a simple DOS program, is the
source code accessible. If so, I might be
interested in a copy so I can use it to code a
java or C based version, complete with
drop boxes and GUI. I can also than submit
to those engineers here as to improvements
in the theory applied. May take a while
since it will be purely from a hobbyist's
perspective, but I do have a programming
and engineering background (which means
plenty of physics and math to boot).

Ian
 
Ian,

you appear to be talking in an alien language! I'm not really a computer expert, but I've found the program is pretty good & I understand there is an improved windows based version available, but you have to buy it.. try a search on "cartest" using google.

If you want a copy of the version I'm using, mail me offline & I'll email it to you. If you know how, could it be posted onto a website, so anyone can get it? I'm happy to send out copies, but with my slow modem I'd much rather not do multiple copies???
 
Weight disribution on a GTD is usually pretty close 55/56% rear - 22/45% front. My car is right on 55/45. I have the same gear ratios you used and the weight is close with driver and 1/2 a load of fuel. On my P-7, 6 year old rock hard 275-55/15 (27") tires I turned 13 flat in the 1/4 at 103. This is with a 320hp engine,
 
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Guest

Guest
Well, we have the Brighton Speed Trials coming up on September 14th. How about people commenting on what they expect the entrants to acheive to see if it matches theory!

Tonight I phone up the scrutineers to see if my windscreen will prevent me from taking the 40, but I can confirm that the damage incurred at the track day was just a trashed spigot bearing. Really trashed! But no collateral damage thank goodness. Just getting some material to make a non roller bearing instead. If only there was an easy way to get the old spigot bearing out....

Malcolm
 
hi malcolm,
pack the spigot bearing full with wheel grease,take a shaft approx the same size as the gearbox input shaft ,place in the hole and smack with a hammer. Hydraulics does the rest. Be prepared to get covered in grease but it saves hours of work!
smile.gif

cheers dave
 
G

Guest

Guest
Tried that before, and you do get covered in grease but the bloody thing didn't budge. My mate watching thought it very funny! Thanks for the tip though.

My thought on the next spigot installation is actually to drill and thread two holes either side of the main central hole. This way you can bolt on a puller in the future. Therefore if you haven't yet fitted this bit to your car, consider this as an option.

Malcolm
 
Malcolm,

On my boat, I have used a special puller to
extract water pump impellers. It has very small arms, and can fit in very tight spaces.
Something similar might work for you. Search for marine engine water pump impeller pullers. They aren't cheap, but will save a
boatload of effort, if the arms will fit through the spigot bearing hole.
You shouldn't give up on the grease and hammer technique just yet. Close tolerance on the shaft and bearing hole is critical.
The grease will only compress so far and then voila'. Maybe a bigger hammer?
Best of luck!


Bill

[ August 29, 2002: Message edited by: Bill Bayard ]
 
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Guest

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I have borrowed a Blind Bearing Puller from my local tool shop. I am not sure who sponsors who as I seem to get great deals all the time but still seem to spend too much money with them! Anyway this is a loan and looks just like you describe above. Please let it work! If I can I am going to sneak into the garage tonight to give it a go....

Malcolm
 
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