Well I took my car out of storage this past weekend and upgraded my old tires (235's) and wheels (15in) to 285's out back and 255's in the front on 17in rims. The car handles much better with the upgade but my 1/4 times did not impove. Check the mods in my sig. Previously I ran a best of 15.78 @ 92.2 mph- G- tech times. I used to burn the peg-leged tire all through first. I can still break the new tire lose half way through first. Anyway, I thought that added traction would knock off some time but it gave me very little. My new best time is 15.71 @ 92.0 mph and that may be attributed to the cold weather 43'. So my questions are: Do my times seem right for the mods I have made?? I know that G-techs are not the most accurate measure of MPH , tending to be off by 2-3 mph. I want to be in the low 15's by the end of the summer. I have yet to do the ultimate tbi mod of grinding down the lip of the tbi, nor have I messed with the fuel pressure or timing. There has to be a few tenths hidding in there. I have seen people on this board with similar mods running much better times of 15.3's ect. So what should I be running and how can I do it. Thanks ---Sorry it was so long!!

 

just checking to see if my sig will show up.

If it doesn't here it is: 305tbi, 700r4, edelbrock intake, open element, edelbrock headers, 3in hooker cat back, aerochamber muffler, no cat, hypertech thermomaster chip, 160 stat, msd wires, cap, rotor, coil, 285 yokohama A520 rear tires, 255's front

 

a lot fo that is most likely because your new tires are taller than your old tires. that reduces your gear advantage and will slow the car down.
if you want to get larger tires it is best to match teh overall height over the new to teh old, just buy a wider tire, not a taller one.

later
tim

 

Do an LT1 cam swap and check out www.ramairbox.com A dual snorkel will give you 2 tenths and a mph for about $120 bucks. But the cam is the biggest jump. All my mods are in my signature with the best times. I will warn you that once you get low 15's you will want 14's...and once you get 14's, 13's will not seem too far....you get my point

 

15" to 17" rim change means the tire place he went to already matched up the rims to the right metric profile/wheel height for his application. Well at least a place with any brains did.

Your STOCK 15s should have been P235 60 R15
Stock wheel height is 26"

Front 17s should be P255 45 R17 correct me if I am wrong

Rear 17s should be P285 40 R17 correct me if I am wrong

The sidewall height is a percentage of the tread width in millimeters

A 235 60 R15 has a sidewall height of 141mm or 5.55"
5.55 + 5.55 + 15 = 26.1" Wheel height (Close enough for government work) Stock wheel height is 26"

A 255 45 R17 has a sidewall height of 114.75mm or 4.52"
4.52 + 4.52 + 17 = 26.04 (close enough for government work)

A 285 40 R17 had a sidewall height of 114mm of 4.49"
4.49 + 4.49 + 17 = 25.98 26" (Again close enough for government work)

Hope this helps Peter,
Good luck with whatever your trying to do with your car.
Check the tire profile and go from there. If they are different ie higher aspect ratio then NJ speeder is right you might have a taller tire than stock which is effectively giving you a numerically lower gear ratio.

I will still maintain that the slow times are dead nuts on for a stock car with chunky wheels and tires on it.
Increasing the tread width increases rolling resistance, weight transfer, and vehicle weight, all of which contribute to effect 1/4 mile performance. If you are not making the power to move those rear wheels and get the nose of the car up for weight transfer then you're not going anywhere. If you plan on drag racing then Skinnies are a MUST.

PS

When I said that 255s in the front are good for autocrossing but are useless on a drag car did anyone stop and think about why?
No, someone took it upon themselves to delete my initial reply. Maybe whoever erased my intial responce should think before making an *** of himself.

If you cannot comprehend what tire aspect ratios are then learn about it. Don't let your "moderator" status go to your head. It's obvious from ALOT of the posts and replies I read on this forum that the majority of people on here need help with the basics and the so called "experts" are just as clueless. Talk about the blind leading the blind. How can run if you don't even know how to walk?!

 

Thanks everyone for taking the time to reply.

---Frmula, You were pretty much right on with my tire profile. Front and back are (40 r17) so tire hight should not be a factor, Right?? I was able to read your fist post before it was erased and I am not sure as to why it was removed. I respect and appreciate any and all information that you or anyone is willing to give me. One can never learn too much!!!

So my times seem right for the mods I've made? I guess I was hoping for too much.
Thanks again!!!
P.S. My stock runs were all around 16.3 @ 86mph.

 

One thing no one mentioned is to change the gears to 3.42s or 3.73s and get a posi to go with it. This should help lower your ET.

Steve

 

What altitude are you at??

Those times seem pretty slow if you ask me.......

Yes, the traction out back will help you, but the traction up front will hurt you. Wider, "sticky'er" front tires are harder to push and will slow you down.

My car with all mods listed in sig., EXCEPT the holley projection setup, G-tech'ed a 15.2 with a 7.0 0-60 (sorry don't remeber mph for the 1/4). I would think you should be running at least that with your mods (although I don't know if you have adjusted fuel pressure, opened up spark plug gaps, advanced timing or other little things, but still...)

Make sure to first cover the basics, give your car a great tuneup! Then toss open the spark plugs to around 45, set base timing at about 2 (I found with the h-tech it pings at 4), richen mixture (if neccessary), do all free mods to TB, run it with 1/2 tank (any less and no traction) at night in the cold and I'm sure you'll be in the low 15's!

Good luck!

JT

 

Street tire to street tire, a simple tire swap most likely won't improve your times any, even if it is going from 235 to 285 on the rears. That's a 50 millimeter difference. I is, and it's not that much of a difference.

Anyway, like NJSpeeder said, some if it is the difference in tire diameter.

I really can't see what's holding you back. I actually have about what you do as far as mods on my TBI car and it runs 15.1@89mph. It may be a tuning issue, or something in there is not performing up to par. It has a 26 inch overall tire diameter with 2.73 gears. What is your overall tire diameter? That's all that's really standing out... If you don't know or don't really want to go out and measure, then what's the second number on your tire size? eg P285/Rxx-V17 blabedeeblah blah..?

 

Thanks again guys for the replys.
I guess what is really puzzling me is that my trap speed and e.t. seem way off from each other. For example JZA' car ran 15.1 @ 89mph. My car produces a fairly similar high trap speed of 92mph but produces a high 15.7ish E.T. compared to JZA's low 15.1. I was told that the trap speed was the true measure of a engines power because E.T.'s are subject to traction, reaction, ect.. So I thought that my high E.T's had to be the result of poor traction since reaction time is not a factor while useing a G-tech. Altitude should not be a factor, I'm in Detroit and I don't think it is any more than a hunderd or so feet above sea level, and my tire diameter is close enough to 26in, that is not a factor. There are no vac. leaks, compression is good, trans is good. What could be the cause of such a high E.T. when I also have a fairly high trap speed??? Ahhh cars, gotta love them!!!
I will try highering the fuel pressure and advancing the timing to 2' and see what happens.

 

92 MPH is good for 13.70 in a 3700 race weight car!!!

60' would be 1.81
Flywheel HP of 297 (I doubt it with the mods you have AND on a STOCK 305)

Get to a track and see what the car really does, and throw that G-tech away it's way off!!

 

I have had accurate results from my g-tech with my brothers 99 WS6 transam. The numbers we obtain seem true to the advertised hp of 320. When running his car, (which is bone stock) in the 1/4 we get results of 13.46 @ 103.4mph. 0 to 60 in 5.33 sec. These times are right on with Motor Trends and Car and Drivers reviews. So even if lets say the Mph are off by as much as 4mph, I still would have a trap speed of 88mph which is still enough to be in the low 15 range. I understand that going to the track is the only true accurate measure, only I still have few months to test and tune before my track opens. peace----