noob question: low profile tires on a 1/4 mile?

[181]

I planned to purchase a close ratio transmission for my car but so far, things did not go as I planned. Now I´m still stuck with my VW Thing with Rancho Pro Street transmission, 3.78/2.06/1.26/0.89 4.12RP, engine is 2276 with Web 86C (I know), 11:1 CR, Steve Tims Stage 2 + heads, 1 3/4 CPS Python, 48DRLAs with 42mm vents.

On the street I ride on 205/65*15 Pirellis, I tried M+H DOTs on the strip but without close ratio it was a pure debacle. I´m thinking if going low profile for next race would help. Something along 195/55x15, to keep the 3rd gear revs up? I know it is a huge compromise but I´m looking for options for my Saturday race. Close ratio is on the list!

Thank you!

[Zach Gomulka]

What RPM do you shift at? Crossing the line in 3rd? What is your MPH?

[181]

I think I shifted to early, say 5500rpm, crossed the line at 3rd gear, final speed approx 140 kmh. Too slow, too heavy.
I would use some basic advices and training I think:-)

[richie]

When you dynoed it what rpm was peak power at? I would be thinking you should be shifting about 7200-7500rpm, if you did then the drop would be
1st-2nd  7200rpm would be 4000rpm in 2nd
             7500rpm would be 4100rpm

2nd - 3rd 7200rpm would be 4400rpm
              7500rpm would be 4600rpm

3rd-4th  7200 rpm would be 5100rpm[ this is 105mph]
             7500rpm would be 5300rpm [this is 109mph]

So realistically you are not going to shift into 4th gear, this was all calculated with your 205/65/15 street tyres. The drop from 1st to 2nd is going to kill it whatever tyre you use, I would have the shift light at 7400rpm and run it up to about 7800rpm in 1st gear only so it doesn't die when you shift into 2nd, then shift at 7600rpm from 2nd to 3rd

Hopefully that all make sense?

The dot drag radials don't really change the rpm drops much maybe 50rpm, they will make 1st harder to launch with being a little taller[26inches compared to 25.5inches] but not to much difference, a 195/55 being  23.44inches might help it leave the line if it doesn't just spin them all the way through 1st

cheers Richie

[Zach Gomulka]

I think I shifted to early, say 5500rpm, crossed the line at 3rd gear, final speed approx 140 kmh. Too slow, too heavy.
I would use some basic advices and training I think:-)

I'd say right now you need seat time more than anything. Richie has some great advice there.

[glenn]

I wrote a Excel spreadsheet that calculates gear ratios and tire size. It then creates a graph. Download it and plug in your numbers.

http://www.glenn-ring.com/tech/tranny_calc.xls

[modnrod]

This is just an option to both reduce your tyre slip and also drop your street gearing for more torque multiplication.

If you used Yokohama Advan 195/50-15 (very sticky road race tyres) then it's like changing your diff ratio from 3.875 down to 4.356 for overall gearing. Using Glenns gearing chart (that's a good one too, thank you Glenn!), that would have you crossing the line at 140kph at about 6300rpm, instead of your current 5600 in 3rd.

Other options are slicks in 22x8"-13 or 23.5x8"-13, but you would need 13"rims, and ideally drop the nose too. These 13" slicks have similar sidewall height, and therefore wrinkle and grip, and a more usual 26x8"-15.
 

As said, seat time, lots of it, it's very addictive! Have fun.

[181]

so today I tried my times with a GPS app on my mobile phone. I don´t know how exact it is but I got the following:

1st run:
60: 3.458@67.15kph
330: 6.51@99.8 kph
1/8 mi: 10.484@120.64kph
1/4mi: 15.525@123.39kph

2nd run:
1st run:
60: 3.534@74.33kph
330: 6.562@100.61 kph
1/8 mi: 9.481@117.23kph
1/4mi: 15.446@121.77kph

3rd run:
60: 3.476@64.28kph
330: 6.523@101.11 kph
1/8 mi: 9.426@120.22kph
1/4mi: 15.489@136.08kph

 

that was unprepped abandoned road, on my pretty 205/65 street tires on 4 lug steel sprintstars. Tire pressure at 1.5MPa, ambient temp 20°C, engine is adjusted, valves set, carbs synced perfectly, advance at 32°BTDC without pinging. I can shed some (say 50-60lbs) weight by removing doors, tools, spare tire etc from my Thing..I can get some few HP by removing fanbelt. I wonder if I can gain some leverage with my ERCOs, which currently wear street tires in 195/55x15. I would like to get into high 14s befoire Saturday race:-) I also got a fair bit of tire slip, I launched somewhere at 3500-4000rpm ( I was too nervous to watch closely:-D

[alex d]

what did you do differently on the 3rd run? speed is a lot better in that one

[181]

I dropped tire pressure even more. I think first two were on 1.9MPa. Third one on 1.5MPa.

[richie]

Something weird there, you gain over a second from the 1st run to the 2nd and 3rd at the 1/8th but no real gain at the 1/4, and the 330 is about the same on all three runs

[181]

as I said I use an android app in my phone, so who know how exact this is...

[Torben Alstrup]

I think you have several issues.. An engine like that should be in the 180 hp range, with 250ish Nm torque, which again should send even a heavy Thing down the quarter in something like 13,5 - 13,7.

From what I have experienced. 1. The venturies are on the large side. 40 mm would be more appropriate. What do you mean with "I know" about the 86C cam? The Python kills power above approx 6500 rpm. But your engine has stopped making power about there anyway, so its not so bad.  On unprepped pavement you may need to reduce tire pressure even more, down to 16-18 psi to get maximum grip. The launch rpm is on the low side. you should leave at about 4500 - 4800 rpm.

Some things for you to try.

Swopping to lownprofile tyres will of course change the mph points at shifts. Do you feel the engine is bogging when you shift to third ?

T