Austarion

I'm heading out to the track tonight. I know it's late notice, but hey...

The car feels different with the new pipework / bov / intercooler / much larger wheels (14's to 17's competely changed my rpms).
It's odd, but some times its feels slower than how I remembered it 3 months ago... (Or I'm just use to the power now)

Anyhow the track will prove one way or the other.

damn.. wish i had seen this post earlier.. I was just 10 minutes down the road from there. How'd you go?

Unfortunately my suspisions were confirmed.

Tonight Best was a 14.810s @ 91.40mph (2.084s 60')
Previous Best was 14.573s @ 95.65mph (2.191s 60')
More traction with the Yokahama 255/40/17's... But much slower trap speed. Major concern as this is indicating a lost in power of approx 25hp! (I need it back)

what rims did it have on it b4?
and how are the intercooler pipes different?

Possible causes from what has been said:
- Larger (ie. heavier?) wheels/tyres overall may be contributing to a higher drivetrain power loss.

- 255 tyres at the rear (and was it 225 at the front?) is probably alot wider than you were running on the 14's, the extra rolling resistance may be a factor, negating any initial traction gain.

- Larger tyre diameter changing your effective gearing which would reduce your acceleration rate.

Other factors to consider:
- Any recent addition of weight in general?, Stereo and cabling, battery box and cabling, carpeting, spare wheel, spare tools, McDonalds drive through wrappers?

- different fuel qualities/brands? fuel load(weight issue again)?

- Differences in atmospheric conditions between events? was the air cold and dense on the first event compared to hot and thin this time?

Any number of these factors could combine to cause what is a relatively minor change in your timeslip. Lastly and not to offend, but maybe you just didn't nail the driving this time.

Your engine spec is close to what I'm looking at doing, your times give me a good indication of the results I could expect, thanks for posting Tomsun

jk wrote:Possible causes from what has been said:
- Larger (ie. heavier?) wheels/tyres overall may be contributing to a higher drivetrain power loss.

- 255 tyres at the rear (and was it 225 at the front?) is probably alot wider than you were running on the 14's, the extra rolling resistance may be a factor, negating any initial traction gain.

- Larger tyre diameter changing your effective gearing which would reduce your acceleration rate.

Other factors to consider:
- Any recent addition of weight in general?, Stereo and cabling, battery box and cabling, carpeting, spare wheel, spare tools, McDonalds drive through wrappers?

- different fuel qualities/brands? fuel load(weight issue again)?

- Differences in atmospheric conditions between events? was the air cold and dense on the first event compared to hot and thin this time?

Any number of these factors could combine to cause what is a relatively minor change in your timeslip. Lastly and not to offend, but maybe you just didn't nail the driving this time.

Your engine spec is close to what I'm looking at doing, your times give me a good indication of the results I could expect, thanks for posting Tomsun

WHEELS:
I agree, the larger wheels could be a factor. Gearing, weight and rolling resistance. I still have the 14's that I could prove it one way on the other but would have to go to the track again.

WEIGHT:
I have added a larger battery and the bodykit.

FUEL:
Same (1/2 tank), same fuel (Caltex 98)

CONDITIONS:
I'll have to check, but not too different from memory.

DRIVING:
The biggest factor, especially in a manual. But with the better 60ft time, I can rule that out. I did note the it didn't feel like it was running well in the higher RPM's. Might be struggling for air with the new air box. Could be a restriction. (Still using the same K&N filter)

HP:
With the big drop in trap speed I still think it is a HP issue as well. I'm going to take it back to JEM tuning and do a power run, check the AF ratios and take it from there.

TOMSUN wrote: WHEELS:
I agree, the larger wheels could be a factor. Gearing, weight and rolling resistance. I still have the 14's that I could prove it one way on the other but would have to go to the track again.

I reckon it's the larger rolling diameter of the wheels that has raised your time. Engine has to work that bit harder to get everything moving. Like riding a bike in a lower gear, easy to get going but you run out of top end, higher gear is hard to get going but top end is better. At the dragstrip, you aren't using your top end speed, you need to get there as quick as you can.

Stand a 17 and a 14 side by side and see if there is a height difference. Then do some mathematicals and you can work out the difference in rolling diameter (Or get a bit of string and wrap around each one and measure the difference) As mentioned above, bigger diameter will lower your acceleration.

Or next time (if there is one) make alternate runs with both sets of rears and see what the difference is.

Agree with Al, one of my old cars went from 14.7 to 13.9 on the same day by change of rear wheels. Set 1 17inch had 255 tyres, set two 17inch 225 tyres and lightweight volk rims, rolling radius was way different. I did a number of runs with both so not a one off miracle. I was trying to see how quick a bog standard s15 Silvia could go, 13.6 was my answer.

I prefer smaller rims with taller sidewalls for drag racing as you can have a much lower pressure in the tyre, + our track is shit when not prepared.

you may be oversimplifying the meaning of the differing trap speeds Tomsun, if all other factors and components were identical between runs then it would be more relevant to blame horsepower.

or you could just be down on power, would be worth a power run on a dyno if it bothers you.

if you notice, most people at the drags use smaller driven wheels and higher profile tyres, and they let the pressure drop a bit.. so the wheels you use are definately a big factor.

Damn.. i wanna go down now and see what I can do.. but perhaps I should get a shift light first.

Different airpressure/temp/moist. will make a differens...
...but I'll put my cents on the different rolling diameter...
What about added (McD-) weigt to the driver? :D
What gear are you in when passing the line?

Booked it in at JEM on Monday for a couple of power runs...

fugazi wrote: What about added (McD-) weigt to the driver? :D
What gear are you in when passing the line?

85kg (inc. Large Quarter Pounder) :D

Ok and I had to buy a helmet at the track $110 as my old one was 15 yrs old (even though it is immaculate)

Funny I can't really remember but I think I was about 6000rpm in 3rd?
On the last run I was shifting a bit earlier as it didn't feel right as only managed 88mph and had just shifted into 4th.

Who wants to go halve on a set of slicks? :D

fugazi wrote:What gear are you in when passing the line?

I've heard that ideally you should be at the top of the rpm range in 4th gear when crossing the finish? Is this true?

thrash wrote:

fugazi wrote:What gear are you in when passing the line?

I've heard that ideally you should be at the top of the rpm range in 4th gear when crossing the finish? Is this true?

yes

TOMSUN wrote:

thrash wrote:I've heard that ideally you should be at the top of the rpm range in 4th gear when crossing the finish? Is this true?

yes

Wouldn't that be close to 125mph (200km/h)?