14.56 to 15.03 1/4mi??

[pantheroc]

I ran 15.03's last night, but I ran 14.56 in November 2005. I have a dilema and need help determining where I possibly lost 0.5 seconds. I'm personally thinking I need a new tune to compliment the PHP CAI.

Here are differences in car status between Nov and yesterday:

14.56 (Nov 2005)
K&N drop in filter.
Rear tires were same size as front (tire place put wrong ones on rear).

15.03 (May 2005)
PHP CAI.
Rear tires correct size.

Other notes of interest:
Temperature was around 54 degrees both times.
The engine oil is due for it's 3k change, was ~1k in during Nov. run.
The tranny fluid is still orig at 35k, soon to be flushed.
I've been mufflerless since October 2005.
DR performance tune since June 2005.

[Breadfan]

The smaller diameter tires gave you an effectively higher gear ratio.

Now that you have stock size again your ratio is back to stock. This will account for some of that time.

Also, how consistent are you at the track? For me on street tires a few 10ths variation is certainly not uncommon. Mix that in with different tire sizes and I think you got your difference.

How's your traction at the track?

[Smokie]

Post the timeslips, PHP air box makes A/F ratio rich, you lose power there.

[Paul T. Casey]

Mufflerless may reducer torque.
May need a new tune for increased airflow (PHP)
Traction issues
Temp. was the same, but was engine temp?
Lastly, even with the same temp. what were the weather conditions? At FFW in Atlanta this year, qualifying day was cloudy, humid, low barametric pressure with temps in the low 60's. I ran about a half second off my normal times. On race day, it was sunny, crisp, and high barometric pressure. Temps were in the upper 70's. I was back within a tenth of where I normally run. Another issue could be fuel quality. If it's a little low, the car will pull timing so as not to knock. This will also slow you down. I had this recently, and cured it with some race gas. I was running about 1/2 second slower than normal. Put in 4 gallons of race fuel, and even with the extra weight of the fuel, I was back where I belonged.

[Breadfan]

I ran the numbers for you.

Stock tire in the rear is 28.6". If you had the fronts on the rear that means diamater was 27.3".

If you have stock gears (3.55) you can run the numbers to get your new effective ratio.

(Original diamter * ratio) / new diameter

or

28.6 * 3.55 / 27.3

= 3.71

So you effectively gave yourself about 3.71 gears so putting the fronts on the back was like installing a set of 3.73 gears.

Depending on how the trans shift points reacted to this you probably ended up faster due to this.

[BLACKMARAUDER04]

How Much Gas Or Extra Weight Did The Car Have?

[pantheroc]

Thanks for the responses!

In Nov I was consistant within .08, May within 0.06.
Traction seemed good. It didn't seem like I warmed them up like in Nov.
Engine temp was normal both times.
The barometric pressures are unknown.
Don't remember gas level first time, I think maybe a few gallons more in Nov because I filled it up and drove it 55 miles to track ~7/8ths. Yesterday I had ~5/8ths.
Weight, ~5lbs more junk in back seat yesterday.

I'll try paying more attention to the RPMs while brake torqueing on the line, warming tires and some of your notes.

Here's timeslips: http://www.mercurymarauder.net/showcase/browseimages.php?c=3&userid=2544

[CRUZTAKER]

Ok...I had to open several windows to compare...and here is my uneducated guess:

The timeslips from both years are very similar in regards to the performance from the start. It half way and further down the track that the changes become apparent. However...half a second difference isn't all that much. Particularly with only a handfull of sample runs for the comparison.

I'm thinking weather. You know air...humidity and such. Perhaps fuel...and/or the mix of what fuel you had with that days weather. Could have even been the difference of a full versus 1/4 tank of gas.

I am NOT thinking tune yet. You would need more data...more timeslips to compare.

I have had a bit of brick wall as well in regards to bettering times from last year. But...I haven't had enough runs to compare yet either.

[Rider90]

Big lunch? Go to Capers often? :)