DragStuff

So the new race car goes through the traps at 8,400 rpm's and we would like it to be about 7,800.
My question is what are the pros and cons of either switching from the 31 10.5 W tires we have now to a 33 10.5 W tire to make the 5.29 gear we have look like a 4.90 something , or changing out the gear set for a higher gear.
The tire change would be a easier fix and quicker but is there any disadvantages or advantages to this over a gear set change ?
Thanks for any input ,Jim.

Presently, the car advances 18.4 inches with every driveshaft revolution. With the tire change, it would be 19.6 inches; gear change, 19.9 inches. Either would get you close to the rpm you want (about 7900 with the tires or 7800 with the gears).

I would expect better 60s with the tire change.
http://www.racetec.cc/shope

Thanks for the info. That is what I came up with also . I just wanted to check to make sure.
Is there any other things to consider when increasing tire size. Such as handeling affects ,traction, or ladder bar adjustments ?
Why would just a taller tire change make the 60 foot times better ? The last testing we did we had a 1.28 60 .
Here are some pictures of the build if interested. http://www.hubgarage.com/mygarage/Tunne ... cles/16417

Thanks for your help ,Jim.

projim64 wrote: Why would just a taller tire change make the 60 foot times better ?

Just a little longer tire patch, which means a bit more contact area, which, hopefully, will improve the 60.
http://www.racetec.cc/shope

Some updates and questions . So we tried a 5.13 gear set and took out the 5.29 set . The car still runs the same numbers 9.06 at 147 mph and the trap RPM is still at about 8,400 RPM. Any suggestions ? We would really like to see a 8 second pass and are wondering if going to a taller gear set say a 5.00 or a 4.86 will get us there or send us in the other direction. The trap speed is what we are worried about. The engine builder says the heads probably top out at about 7,800 RPM . Am I thinking right that the extra 600 RPM that we are spinning at the end is wasted and that the higher gear set will bring that down into the engines power curve. Here is the 9.06 pass at Cecil County Drag Way.Thanks for any advice ,Jim.
http://www.youtube.com/watch?v=Pr6Qo5kR ... r_embedded

projim64 wrote:The engine builder says the heads probably top out at about 7,800 RPM .

If the power peak is at 7800 rpm, you certainly do NOT want to be turning that speed as you go through the traps. That extra 600 rpm sounds pretty ideal. You're trying to get the maximum area under the power curve and you don't get it when you clip at the peak.

This assumes, of course, the right information from your engine builder. If you go taller with the gears and the trap speed picks up, he was obviously a bit optimistic on the power peak rpm.
http://www.racetec.cc/shope

So do you have any suggestions on what way to go next ?

projim64 wrote:So do you have any suggestions on what way to go next ?

Any change in gearing would say that you're second guessing your engine builder. If he's correct (and he should know), you're right where you should be on gearing. Let's face it, you might not get into the 8s without changes other than gearing.

You haven't mentioned your suspension setup. If you haven't done anything about equalizing rear tire loadings during launch, you can make some changes that will easily get you into the 8s.
http://www.racetec.cc/shope

The 5.29 was a center section that we got cheap and tried it out to see what it would do. We run with the 422 Motorsports team from Oaks .Pa .The engine builder is Rich Lamont and his apprentice Jay Zolco .We are trying to knock the trap RPM down to reduce engine maintenance . The car has a ladder bar rear suspension with coil overs and coil overs on the front with stock a arms. Here are some links to there team and the build of the 65 Comet. Thanks for your help ,Jim.
http://www.422motorsports.com/
http://www.cometcentral.com/forum/viewt ... 17139e867f
http://www.hubgarage.com/mygarage/Tunne ... cles/16417

projim64 wrote:We are trying to knock the trap RPM down to reduce engine maintenance .

What's your shift rpm? Isn't it over 7800?

With your adjustable front coilovers, it would be easy to cancel most of the driveshaft torque effect with different rate springs across the front. See my site for details.
http://www.racetec.cc/shope

He is shifting at 7,400 RPM's .

if i have this right you put less gear in it and it ran the same?? if that is the case the engine is making more power than you expected..in short it needs less gear to do more work.load it with more gear 4.88sand it will carry it to more mph et is often found running past peak hp.

Thanks for the info . We have a center that we can through in that has 4.86 gears in it , that we would like to try. Jim.

as said above...always run as tall a tire u can stuff in it! more traction is a plus!!!

572 BBC wrote:as said above...always run as tall a tire u can stuff in it! more traction is a plus!!!

yes for the taller tires.

I found switching to a tire that grows more helps at both ends. You get a lower gear at launch, but as mph goes up the taller tire raises your overall gear at the top end, keeping the motor in its happy place (making power), and keeping a good amount of load on the motor.