This would be my first "major" mod so I have a few questions...


This is all based on my 88 IROC 305 TPI 700R4 car with 2.73 rear end and the std 16" IROC wheels and tyres (to be changed to wider 17@ Torque Thurst IIs)...

The car is stock except for K&N's, a TB Aerofoil, descreened MAF, aftermarket exhaust (decat) and soon headers of some sort (ones that will get rid of the emissions junk)...
Apart from that it is STOCK.

I'm considering changing the rear end from 2.73 posi to 3.42 posi.


If someone can confirm my maths, or more to the point the theory behind the maths I'd appreciate it...


I'm doing 1,700rpm at 70mph with 2.73 rear gears, if I divide that by 2.73, then multiply by 3.42 (proposed new gears) I get just over 2,100rpm.

So my highway cruising should still be great.

Does that sound right? (give or take 100rpm)


Taking the maths 1 stage further...

Obviously the top speed of the car will be reduced.
As the car has peak power at about 4,500rpm the current thoretical top speed (ie no wind resistance) would be:

1,700rpm / 70mph = 24.28rpm per 1mph
So...
4,500rpm / 24.28 = 185mph.

Say switching to 3.42 gears increase the rpms at 70mph to 2,150 (see previous post)...

2,150rpm / 70mph = 30.71rpm per 1mph
So...
4,500rpm / 30.71 = 146mph.


Now obviously these are theoretical, with wind resistance and the fact the car would be in overdrive it wouldn't have the power to push the car up to those speeds. Although with the 3.42s it stands more chance of powering itself to 140mph than with the 2.73's I would imagine...


As I wouldn't be drag racing in Overdrive, just regular Drive...
Checking the manual it's a 0.7x multiplier.

Which means the cars top speed in normal Drive with the current 2.73 gears is 129.5mph (185x0.7)

So with the 3.42 gears it would be 102mph (146x0.7).


Does all this sound right?
If it is it sounds to me like the best gears for my car in its current state for drag racing (as I can't imagine I would be crossing the line at over 100mph) while still giving good gas mileage at 70mph (cos the rpms aren't too high)...


I have no idea what parts I need or where to get them from as I have no idea exactly what rear end the car has other than it is 2.73 with posi on an 88 IROC...


Would I see much improved acceleration?
Because the car won't be able to go as fast in 1st and 2nd so it would jave to change up, is that not counter productive?

Also, when I change the wheels should I stay with the same overall wheel/tyre diameter?

 

3.42's on a tpi with a 700r4 is a good choice for all around performance, good top end, good acceleration.

u have 3.23's and my top end is great and all but i dont really need to go that fast, i rather have better acceleration.

 

Okay, Focus, I've got a question, or two, for you. I can't tell from your info whether you have a ten-bolt or 9-bolt rear axle. If you don't know, you might want to go outside, get down on the ground, and count the number of bolts.

Although there is an occasional difference of opinion, if you have a ten-bolt (that's what I've got), many members prefer a 9-bolt. The 9-bolt is stronger than the ten. Now, with that said, keep in mind that the parts for that 9-bolt are going to be pretty pricey.

Something else to consider. My ten-bolt has these iron calipers that aren't very good (if you have the iron calps, and you've had to replace the pads, you know what I mean). When I swap in the 9-bolt, I'm also going to replace the iron calps with a nice set of PBRs.

Let me know.

 

I will have to go and count the bolts as I have no idea...
I have drums on the back if it helps...

If I end up needing to change the entire rear end I will go for a Ford 9" rear as I plan on handing over a pile of cash to BadAss Racing Engines within the next 18 months for a killer SBC, so if it involves spending quite a few $$$ just to change the ratios I'll do it proper with that engine in mind.

Hopefully for now I can just spend about $300 at most for the bits to change the ratio in what I've got and then pay whatever the UK specialists want for fitting it...


I'll count the bolts when I get home and let you know...

 

A 305 auto car wouldn't have a 9 bolt assuming it's the stock rearend.

With the average price of a gear swap I'd say it would be well worth it to snag the 3.42s... I've been running 3.42s for quite some time in both my thirdgens, I think it's a great all around ratio to go with.

 

You can't go wrong with the 3.42 and you should love the added feel of your car. Check out the calculations page on here and all of your math will be solved for you. Remember, your tires size has a big effect on whatever gear combo you go with.

 

Focus,

I just made the calculation on my pc.

2.73's at 70mph I get to be at 1750rpm
3.42's at 70mph I get to be at 2200rpm

Top speed with 4500rpm limit
2.73's 180mph
3.42's 143mph

Have you thought about 3.23's insted of the 3.42's? Reason I'm asking this is that I'm currently in the UK also and I have no traction with the 2.73's let alone a set of 3.42's. The road surface is in a word crap here.

I'm thinking about going the 3.23 route myself and fixing the posi to aid in traction.

btw, how fast have you had the car so far?


Robert

 

In light of what you want to do you certainly can't go wrong with a Ford 9 inch. However, there is a guy on this board, Miles, who has a 427 with a 9 bolt. I've been told the 9 bolt is good to about 400 horsepower. That's better than what our 10 bolts can take.

 

I didn't have time to count the bolts last night as I didn't finish work until 10pm...

Off the line the car varies, and not necessarily in line with the weather conditions.

Sometimes it peels off the line, sometimes it bogs a little and sometimes it really launches perfectly and hauls...

Obviously it will lose traction bad with 3.42s but I am planning on a set of TT2s with 275 tyres which will help a little and the other bits I need such as SFCs and the rest of the bits are supposed to help if I am understanding correctly.

I've not taken it to Santa Pod yet, or gotten around to connecting up my GTech unit but I have had it off the clock (115mph clock) in third, I suspect I hit the rev limiter at around 120mph in third.

Hopefully I have a 10 bolt then as I rang Thunder Racing and they said they had a kit for the 10 bolt series 2 carrier (I assume that means a carrier for a 2.xx gear) with a thicker gear so I could use 3.43, they wanted about 300 dollars for the gears and install kit.

 

I'm pretty sure Ray is right. In all likelihood, you've got a ten bolt.

 

You've got a 10-bolt. Don't **** around with 3.42's. 3.73's will put a bigger smile on your face and make better use of the power (or lack thereof) that a 305 is making. If you had a 350 making 400+hp, then I'd say go with 3.42's. Who gives a rats *** about top speed? Do you do a lot of top end freeway runs? No? Then worry about getting off the line quicker. First will be a challenge, but second and third will REALLY wake up with 3.73's.

My .02

 

I know what you're saying, 3.73 seems to be the most popular choice.

But I do a lot of highway cruising at 70-75mph and gas here is $7.00 per gallon so I gotta be careful with rpm (and thus fuel consumption) at those cruising speeds...

 

$7.00 PER ****ING GALLON?!?!?!@!#qdfau4i3r11`#?>?

Thats ricockulous. Disregard my previous post. Ditch the 2.73's in favor of some mileage master 2.56's. BUY NOW!

 

My first 85 Z28 had 3.23's and my current 85 IROC-Z has 3.42's. The 3.42's are way more fun...... A lot of your traction problems could be related to your tires or maybe a lack of positraction....

Now if you have a 350, forget traction with anything less than drag slicks and a tacky track........ BTW, I've gotten 24.5 mpg with my ZZ4 equipped 350 going over a mountain range......