Charleston2012

I currently have a C4 with ZO7 suspension and offset bushings running 2.25 neg front, 1.75 neg rear and BFG KD 275 front and 315 BFG KD rear.
I recently got a set of 03 OEM ZO6 wheels, and wanted to put 275 Kumho 710 up front and 295 Kumhos on these wheels for autocrossing.
Is it a worth wile investment? Will I notice a difference even though the rears are a a little smaller?

rustyguns

if they are new, should give you a full second

Race Prepared

The Z06 wheels are significantly lighter, reducing unsprung weight & improving turn-in rate and suspension reaction. The abs might be slightly affected but should not be too noticeable. You will find that if you run an agressive brake pad/rotor package that with the lighter wheel/tire combo that later braking and improved trail braking characteristics are enhanced. As for reduction in rear tire size, no significant change in handing should be noticed. I have run 305 Hoosiers (VRL) on my '90 with great success. You might have to purchase 1/4" spacers due to the difference in off-set between the Z wheels & stock off-set. Consider a 550 lb VBP front spring in the future for improved handling...

RAREBLU1

Go with the 710's BIG difference

B-ras

But is there a particular reason you wouldn't go 295 front/315 rear, the common 710 sizes for C5Z wheels?

Charleston2012

Budget

Solofast

With 315's on the back with his setup it would push like a pig...

We took my Z06 wheels 295/17's and 315/18's and put them on a C4 and it was miserable. Been there done that ...

Bill Dearborn

The Kumho's will work better for you. You will have to settle for a 285 or 305 width in the rear since there isn't a 295. With the 285 the rear tire diameter will be 24.8 which would give a little extra gear ratio while the 305 would be almost equal in diameter to the current 315/17 that you are running. I suspect putting the Kumho's on your current wheels will work better. If you are running GM OEM Grand Sport wheels the wheel weights may not be that much different. Even better would be to run 315/35/17s at all 4 corners.

By the way I ran C4 Grand Sport wheels on my 03 Z whenever I autocrossed it or took it to the track. I only used the Z wheels for the street.

Bill

Charleston2012

I think the 275 front and 285 rear is the best I can do, unless I can swap someones 18x10.5 ZO6 wheels for my 17x9.5 so I can truly square up the tires. Then again, it will look kinda wierd, as the front tires will look bigger due to 40 series vs 30.
Money is tight, and the wheels were very very cheap due to a friend just wanting to offload them, as he put some cooler wheels on his 03 Z

Solofast

You might try swapping the 18's from the rear to somebody who has a C5Z for his fronts. Then you would have four 17's that work just fine on the car.

That's the setup that Cashmo had on his C4 and it obviously worked well.

Charleston2012

Good Call

RAREBLU1

I've got 315's square setup all the way around. It's hard to keep the heat in the rears, I have to spray the front to keepem cool. But I've had sets of Kuhmos XP's and Falken Azenis both soft street tires and they don't come close to the 710's, you'll gain 2sec in a 60sec course.

Charleston2012

That makes me feel good as I am running same times as GTR's and some C6 Z51's with hoosiers (seasoned drivers too)

Rob31

What do you mean spray the front ?

grantar2

I currently run either 275/40/17 all round or 295/35/17 all round. Have run 295's rear, 275's front and was fine. The short answer to your question is even if the 710's have 1/2 their life on them they will be an improvement over the best street tire. And by the way they were worth between 3 & 5 seconds for me on the same course.

I wasn't sure I would like them but so I went out to buy used. I had 3 sets of wheels so it wasn't a problem. I was given tires that had some life, a lot of life but old, and I bought some for cheap. Got 6 for a $120. All have turned out fine. If you can get some used ones give them a try. Someone said there are 295's available, that is incorrect you can get 295/35/17's for a C4. I have 9" wheels so that's what I run.

Here is the key, run the fronts at 28lbs and rears at 24lbs. Try to hit it exact, it makes all the difference in the world. If its warm where your running use a spray and wet them down to cool.

I have an advantage in my division as I have not only multi time regional champions but national champions as well. Most are moving from C4's to C5's but they are more than happy to share knowledge, set up, spare parts and tires. The tire pressure seemed counter intuitive but it works.

You will love the Kumho's. I suggest you consider my strategy, see if you can get some good 50 run left used tires, and get some experience. You can't imagine how much better the car handels (unless you hit the marbles). Have a set of used tires for practice days, and a set of new V710's for your race days.

Also watch your local boards, I was able to get Kumho's for 75 cents on the dollar by using one of the regional guys' Kumho bucks. He didn't need tires and wanted the cash.

Anyway get the tires you will love them, but do run the pressure as noted 28 front, 24 rear.

grantar2

Go to your local big box hardware store and buy a 1 or 2 gallon sparyer, like you would use to spary for weeds. It will have a pump handle and sprayer. Fill it with water, and between runs after you adjust your tire pressue, spray the tires from top down to cool them. Saves the tire and really adds performance. Caution do not get your brakes.

Thats what he means by spray the fronts, although I usually have to do both front and rear.

grantar2

If you have 9.5 you can run 295's no problem, I do it on 9". In fact you can run 315's.

Charleston2012

Thanks for all the help. I will look for some tires here soon. I would like to swap the 18x10.5's zo6 wheels for the 17x9.5's

RAREBLU1

Yup thats what I'm talkin about mine get real grease felling on the 3rd run if I don't cool them down. I think the reason my fronts are gettin hot is I need more camber in the rear.

I'm still playin with the presser but right now I'm runnin 28front 26rear, they seem to like lower for me. This is my first year on them and still exploring the limits.

grantar2

I know it sounds hard to believe, but if you have an acurate tire guage try 24 on the rears. I don't know if your region has practices before a championship events, but if they do go do it. You can practice set up changes. I found the difference between 25 and 24, if I hadn't test it repeatedly I would believe it, but the difference is like being on rails. I won my first championship (Novice) against 11 other cars running 275's. I have used up my 295's before I had gotten counciled on the tire pressure. I have to find out exactly what the car's prior owner did, he was a 3 time champion, but I have a 1990 Z51, with Koni single adjustables. The car was set up by Dick Gulstrands org as the prior owner ran with the Gulstrand Racing Association here in So. Cal. I am currently running 1.75 -camber front, and 1.85 left, 1.95 right - camber I have to look up what I put in as far as neg and pos toe.