Speed in first gear?

Ant · #1 (**permalink**) 03-13-2005, 03:15 AM

I am in the process of ordering a custom built racing transmission, and 5th gear will be 1.1, no problem with 2nd/3rd/4th as when I establish 1st gear I will space them evenly!

What is a good first gear ratio, considering that the car shouldn't weigh over 1000kg, and still be street driven at times, also running a 9.2" alloy blocked 358 Nascar type engine bored/stroked to 380 cu in, running to 7500rpm.

I have used the gear charts eg Smokemup.com entered my data diff ratios 3.27 and 3.08 rear end ratio on 25.6" diameter tires.

I dont want 75mph in first as I had with a 2.32 first gear Toploader on 3.07 at 25.5" rubber, or have a first gear that finishes at 45mph also!
What I am coming up with is a 60mph first at 7000rpm.

mr0077 · #2 (**permalink**) 03-13-2005, 06:53 AM

Ant, if this is purely for performance (and cruise is secondary), a simple approach would be to establish the desired top speed, and divide that by your desired speed in first (I agree, 60 is a good choice). If you pick 165 mph tops, you end up with 165 top speed/60 first gear = 2.75:1 first gear ratio, with something like a 3.45:1 rear end ratio.

If the cruising rpm is more important, work it backwards from the desired cruise speed/rpm, for example 70 mph/2500...

Ant · #3 (**permalink**) 03-13-2005, 02:40 PM

mr0077

This setup is purely for performance and cruising is secondary! Another point I will have to find out when the engine is built, is the cruising speed at 2500rpm = 58mph 3.27 ratio and = 62mph 3.08 ratio on 25.6" tires.
The 2500rpm maybe a bit low in the revs for a 7500rpm SBF, whereas 2800-3000rpm might be better, and some say well you can change down to fourth gear if you have to keep it on the camshaft!

Also with this car I have gone away from originality as far as tire size I love the look of the 26.5" tire but I have bitten the bullit and gone for 17"x12" rears etc.

BevanWright · #4 (**permalink**) 03-13-2005, 03:01 PM

Sounds like you should have kept the Gulf car Ant?

Excaliber · #5 (**permalink**) 03-13-2005, 03:19 PM

The original Comp cars ran something like a 3.73 or so and the CLOSE ratio top loader where 1st gear was 2.32. Figure a 26.5" tall tire for back then. 1st equals 64 mph at 7000 rpm.

With a WIDE ratio, 1st is 2.78 equals 53 mph at 7K.

My experience has been with the reasonably light weight cars traction is a major issue. A 3.70 rear gear is pretty low and that WILL fry the tires at will. Without slicks, your only kidding yourself, the car would actually be FASTER accellerating with a TALLER rear gear to "tame" wheel spin!

LOT's of replica owners have stated time after time their ET 1/4 mile times improve by short shifting to 2nd, or even starting in 2nd because 1st is simply "useless".

That high speed in first your talking about is a result of spinning the engine to 7000 rpm, a lofty number! I'd lean toward what they used "back in the day" for gearing myself.

My ERA gearing is such that 1st is good for 65 mph at 6000-6500 rpm. Yet I can easily run in the 11's at the 1/4 mile.

Ant · #6 (**permalink**) 03-13-2005, 03:52 PM

Excaliber,

That makes sense, I am trying to balance that thinking with practical use, as I dont want to slip the clutch on take off with a high first gear, and bear in mind this is a five speed 1.1 trans, and not a four speed where the gears needed to be spaced!

I am not far away from your figures only 5 mph with 60 mph at 7000rpm, and 7000 is a lofty figure, I agree, what I am trying to do is be able to street drive it away from the lights etc so first gear needs to be practical, now that sounds irish when I say the car is purely for racing etc, but this car will see street miles as I want to be able to enjoy it when I feel like instead of going an hours drive to the track!

Bevan,
Its all happening the Ram SC from the UK, and an alloy SBF and associated parts almost on the way from the US, the engine to be built by Grahame Morrison, Blenheim, going to be a real over bore 358 type 380cubes setup trying to stay on 10.1, in case I have to limp home on 96 octane, still not sure totally on alloy blocks, Grahame has some reservations on them!
What speed does the Seanz have in first or have you ever worked out the ratios? Going to run same wheels as your car from Arrow, and either 3.27, 3.08 gearing!

BevanWright · #7 (**permalink**) 03-13-2005, 04:19 PM

Hi Ant,

All sounding good then. When will Ram deliver?

It would be good to be able to run pump gas - thats the one drawback I find with my car.

Alloy blocks - there must be a lot of them in use by now, almost all new motors are alloy (as in new cars), cant be too bad eh?

I have never worked out the ratios, but well over 100kph in first - maybe I will have to take it out for a run to see?

BevanWright · #8 (**permalink**) 03-13-2005, 04:21 PM

ps - the arrow wheels - I dont reckon the design i have is great for cooling - you should consider the minilite style. If I had the cash - I would get these, they would look good in 17 inch on a cobra.

Ant · #9 (**permalink**) 03-13-2005, 07:13 PM

Bevan,

You may be right with the cooling of your wheels, I still want the original look Cobra wheels even though originality isnt a criteria with most of the car!

18/4/05 it should be finished then another 40days on the water and here about the end of May!

Will be at least 130 bhp down on your car, but this car should be quite light!

BevanWright · #10 (**permalink**) 03-13-2005, 08:04 PM

and it will have the disability of a Ford engine too.

So why arnt you using your wheels you had before?

BevanWright · #11 (**permalink**) 03-13-2005, 08:06 PM

My car has more power now by the way. When it was rebuilt the heads were flowed, and a bigger cam went in. Not sure why I did this - it had too much before!

Fred Douglass · #12 (**permalink**) 03-13-2005, 08:09 PM

......and I have only looked at the guts of the tranny (cleaning and changing oil) but it APPEARS that 1st and Reverse are identical (could this be?)---which means, yup, I could go 62 in reverse. Is this a good idea? Hooo-hah!

BevanWright · #13 (**permalink**) 03-13-2005, 08:13 PM

That is quite possible. Some Autos have the same ratio, so why not manuals.

The ride at 62 in reverse would be way more exciting than going forwards at that speed I guess!

Would that make your car a rear engined front wheel drive (with forklift stled rear wheel steering)?

Fred Douglass · #14 (**permalink**) 03-13-2005, 08:27 PM

Bevan,

I belive your #2 and 3 above are true...!!!

Ant · #15 (**permalink**) 03-13-2005, 09:27 PM

Fred,

What rpm are you doing at 62mph?

Bevan,

I sold the Trigos as I am going to 17" same size as yours as I need different backspacing offset on the front of the Ram, as it has a wider track, plus going 13" rotors etc to up the brake capabilities.

I have done the sums on Ford and Chev, they work out about the same, and a crate engine would be a bit cheaper but this engine is going to have bits like Jessel ceramic cam follower and rockers, starts getting a bit expensive with dry sump etc.
I am still not convinced I need an alloy block as the weight saved in my 1000bhp rated transmission (magnesium case 80Ibs) is 50Ibs over the 130Ib T56 I was going to use, thats within 10Ib of the 60Ib saving with the alloy block!

Fred Douglass · #16 (**permalink**) 03-14-2005, 07:10 AM

...the "First-Reverse being equal" was a joke.

As to the r.p.m.s, I hate to enter the (American) 5th Amendment or plead bufoonery....BUT I didn't pay too much attention to the r.p.m.s other than it was in the neighborhood of 6900+. My redline in the 351 Cleveland is c. 7000. I am stroking the engine at present, so it is not in the car. The transmission is a Tremec 5-speed. Car is the work of one Ron Butler---a Kiwi!

You all in the Antipodals wouldn't have any information on Australian/Kiwi mods on the 351C do you? If you know how I can get my hands on any such info---including heads--just e-mail me. I would appreciate it muchly.

Chow and Carrion,

Ant · #17 (**permalink**) 03-14-2005, 02:24 PM

Fred,

Theres is a few guy around who love the Cleveland, I know of one who dynoed his 377cuin at 530bhp and that wasnt big compression in a replica Dick Johnson bathhurst Falcon.

Will email any information I come across to you- no problem.

Anthony · #18 (**permalink**) 03-18-2005, 09:33 PM

To do 60 mph in first @ 7000 rpms, you want the 1st gear X rear gear multiplication to be about 9.2 to 9.4

Ant · #19 (**permalink**) 03-19-2005, 12:43 AM

Anthony,

I dont know about those maths, all I know is 50mph at 7000 in first gear is to low and will just be useless and fry up, and 75mph at 7000 is to higher in first gear - so I guess somewhere in the middle and I have come up with 58mph @ 7000 on a 3.27 with 25.6" diameter rubber.

Other things to consider are cruising speed with 3.27 gearing is 3000 rpm = 70mph, quite low, but in NZ the roads are crap, and I am building the car to get off its arse and motor to 160mph @ 6800rpm - that should do it!