Faster more !

[futureproof]

Any one got any thoughts on this.
I have SBC to TH350 auto to 2:88 LSD. 18 inch wheels 295s
Best 1/4 mile 13.6.

Car takes off like a rocket, very little spin, beats anything over 1000ft but then I start to run out of umph up to 100mph terminal speed. If I could get the terminal speed higher I am of the opinion the time would go down.

Ideally I need a longer 2 and definitely 3rd gear.

Can you mess with the gears in a TH350 box, (I am aware the 3rd is 1rev in, 1 rev out) or is there a slightly longer jag LSD. Loosing the LSD would not be a good idea as going in a straight line in this application is good

There must be a list of jag diffs on the Cobra Club, but I am still finding my way around the new site.
Ignore engine tuning here, that is sort of ok.

Cheers.

[craggle]

As far as I know all the jag ratios are lower than the one you currently have.

2.88:1
3.01:1
3.31:1
3.54:1

Craig.

[craggle]

Just for information the gearbox is a 3 speed auto with ratios of
First Gear - 2.48:1
Second Gear - 1.48:1
Third Gear - 1.00:1
Reverse - 2.07:1

Do you know what torque converter you have? You can get high stall torque converters but not 100% sure what the advantages are?

If you have 295/35 18 tyres then these do 772 revolutions per mile. Multiply that by your diff ratio of 2.88 gives 2223 revolutions of the prop shaft per mile so in 3rd gear your engine is also doing 2223rpm per mile which equates to one mile per minute or 60 miles per hour.
To do 100mph your revs should be 3705rpm so the engine should still be pulling strong at that range. Not really sure what you can do to improve things.

Correct my maths if I'm wrong, it is late at night!

Craig.

[FatBoy]

I may be wrong, but it seems to me that the final drive ratio is too high rather than too low and that the revs need to be higher.

Paul

[clive a brown]

Hi, Could I ask what's your 60 foot time?

cheers clive

[cobracol]

Yes think 2.88 is as low (or high depending on how you look at it) as you can go, fitting a higher diff will give you better take off speed, if you can keep the traction, but will run out of puff at an even lower terminal speed, are you shifting manually? or letting the gearbox decide?, suprised you are not getting the power well past the 100mph , is your engine breathing properly?, and getting full throttle, as it should pull strongly to 5k rpm, if breathing and fueling ok, my mate took his auto camaro to santapod last week and managed 15.2 = 1/4, worse in auto, and its a 5.7 inj and a lot of extra weight, although the gearbox is 4 speed, maybe that is an answer, but most run tricked up 3 speeds anyway, if everything is ok perhaps you need to post on the drag racing forums, such as rods and sods to gain some info

[futureproof]

OK - this weekend was slower than average, may be the humid air. Normally get into the 13s, best 13.6. But looking for that extra 1/2 second fries your brain and is great fun.
on a 14.0s run RT = .65, 60' 2.06, 330' 5.80, 1/8 8.93 78.72 MPH, 1000' 11.69 88.9mph and 1/4 14.0 94.07mph.
I got beaten by a turboed 106 Peugeot, took him ages to get off the line but came smoking past at 1000' and terminal speed of 107mph (13.8s). I have suspected I need more after 1000' but if ever proof was needed being over taken by a Peugeot was proof

I normally shift from 2nd to 3rd but it seems to make little difference to the time.

Dont get me wrong, the car is nice and not too much of an animal, but i have got into this 1/4 mile thing and before i spend on the engine, which is dialed in, running a new Eldebrock and electronic ignition, irridium plugs etc. Mild cams, rollers etc.

No idea what is in the g/b.
Any one got a decent link to a drag forum ?
Cheers for the thoughts

[Lloyd Barnes]

Interesting post. Sounds like the drag racing bug is burrowing its way under your skin? Some random thoughts underneath!

2 seconds 60ft time is about right (ie pretty good) for a manual box Cobra on street tyres. Controlling spin off the line is very hard to do in such a powerful and light car without slicks.

I would say if you have a standard autobox in there, and you say you get little spin, its actually doing a damn good job as a kind of traction control! You can change the torque converter for a higher stall unit. It will basically let the engine rev up more, getting further into its powerband, before it lets the engine hook up to the transmission. However you may then find you start to spin off the line! and so the next step then would be a set of rear slicks (although you can't run them in the Castrol Challenge, street legal cars only)... and so on! The downside to a higher stall converter is you'd get the same characteristics on the road. Depends what you want to spend and how far you are prepared to let street driveability slide to improve 1/4 mile times.

What state of tune is your 350 Chevy in? I managed 12.34/114mph with approx 450bhp & 420flt/lbs 383 Chevy, Clive was about the same & I know Colin was running in the high 11's with a 500hp+ LS motor. If your 350 is fairly standard low 13's are probably somewhere near?

As I understand it the TH350 came as standard with 2.52/1.52/1.0 ratios? I'm pretty sure you can get 2.75 first gear ratios, no idea of costs though. With a 2.88 rear diff it sounds to me like you're very low geared/set up for crusing. My Dax was running a fairly normal 3.54 rear end and a TKO500 which has a 3.27 first gear (the TKO600 has a 2.87 first, partly why I chose the 500 for more oomph off the line). With a 2.88 and 2.52 first you are much higher geared. An LSD equipped 3.54 diff would be an interesting swap.

Fascinating area drag racing if you like to get into the detailed engineering aspects of the car. Enjoy

[Sidecarbod]

My thoughts for what they are worth….

I think that you should enter the data for your car into a gear ratio calculator, there are loads on line, I suspect that with your current diff your car is geared for a top speed at peak revs of about 180 MPH! You will need a hell of a lot of BHP to push a Cobra with ‘brick aero dynamics’ up to that sort of speed, so in other words your car is over geared for the power that you have which will probably start to manifest its self as poor acceleration over 100-110 MPH.

The good news for you is that changing the diff ratio has a very small affect on first gear, a little more on second gear, etc, etc. It has the most affect on top gear.

Really you need to be realistic with what top speed your engine can push your car to, as a VERY rough guess 300 BHP is probably good for a true 145-150 MPH. (Cue a load of replies “I’ve seen 150 MPH with 230 BHP!!!”) I think that there is a rule that states to double the speed of a car you will need four times the power so you can see that even getting an extra 10MPH on top of your max speed needs quite a bit more power. This is why in my humble the final drive ratio is critical if you want to get the most out of your motor. (By final drive ratio I mean the diff and the gearbox ratio combined). Some people aren’t bothered by this and will over gear their car in order to keep the cruise revs down and the MPG good. I guess which way you go is up to you!

The reason that I believe the above is that I’ve spent hours pondering different gearbox and diff ratios before swapping out my gearbox. My old box in theory should have allowed my car to hit 160 at 6k RPM, there was no way my engine would push the car to that speed, my new box will allow the car to go to 150, that’s quick enough for me anyway!.

AJMHO!

[Sidecarbod]

Edit to the above!

I see that your auto box has a 1:1 ratio, maybe your diff is not quitre so over geared then, I was assuming that your top gear was around 0.8:1, Even so your diff might have geared your car for too high a top speed for the BHP that you have. (Not that I know what BHP you have)