picking a cam/diff gear calc?

[TCSTEVE]

Hey guys, time to pick a cam. Ive gone through all the posts and cant find a neat little program which was posted here that worked out 1/4 mile times based on cars weight/HP/Rev and final ratio. Can anyone help out?
Anyway motors a 2502V, 500 holley, 2.92 lsd, 4speed manual in a TC wagon 1200kg. I'm loking for low end torque, so a pretty mild mid ranged cam would be best. Exhaust has been ported and the head is set for unleaded any advise would be great.
Cheers Steve

 

[xctasy]

1.Check ryans posts, a newbie in the US forum. He's posted the details.

2.I've posted one called Virtual Reality Drag Racing a while back. It goes through the details.

3. Bowling and Gripps site on the Tech Section links.

4. On my work HP 32s calculator, I've got a full program which allows you to optimise the diff ratio to suit. It's at work at the moment! It runs the basic formula listed by the three above, allong with David Vizards historical acceleration algorithim.

Point 1: The key is to ensure the car is travelling through the 402m, 1320 FT, 1/4-mile traps at maximum safe revs in either 2nd, 3rd or 4th gear.

Point 2:The maximum safe chage up rpm is about the peak power rpm, plus 5 to 20% , based on the type of engine you have.

Point 3:You plug in the inches of tyre height, and it gives you the ideal diff ratio.

There's a bit of judgement needed if the car is automatic, because and ideal gear ratio could result in the car cruising at 100 km/h with the converter slipping all the time. Very dangerous. In the end, you have to trade off acceleration for cruise rpm. One guy in a Aussie Street Machine mag at Keysborough diff spares taught me the best steps to optimise diffs to suit the car.

Sorray about all the details!

I'll give you the answer soon if you like, but it depends on lots of variables, Steve!

 

[TCSTEVE]

Cheers xtaxi, im new to picking a cam but i do want to keep the 2.92 to save on wheel spin and get some top end out of it.
It has gotten up to 190kph or so with the stock tired 2V just want the car to continue picking up speed from 100 to 200 kph like it's 0 to 100kph
and the torque to come on quick in the rpm.
the torque will get the light weight corty off the line then wait for the cam to peak before the motor runs out of rev, one or two gear changes should be enough to drag up to 100 on the street but i understand what your on about when it comes to gearing it for 400 metres thats where i'm lost a little. A 2.92, 200+HP and 1300 kg should get me over the 400metres at a decent rpm?
Would a 3 speed manual be better i've got one lying around and have really considered it. Gears are 1st 2.95 2nd 1.69 3rd 1.00
Any figures you can give me would be great.
Thanks Steve

 

[xctasy]

Sum up is that for most 250 Falcons, 3.23:1 gears are best. For the Cortina 250, 2.92:1 gears are best.

Why?


Other aspects rule out just using the formula noted above. Unless you go for very wild head mods, or a very high, unstreetable cam, forget getting any more than 195 hp out of the engine. Radical cams in Fords produce issues with stress on the timing chain and camshaft.

With 1200 kilograms (2645 pounds) and, hopefully, about 195 hp at 5000 rpm, and maximum torque of 245 lb-ft coming in at 3600 rpm. Peak revs are about 5400 rpm or so.

The quarter mile time would then be (with a 90 kg person), about 14.9 seconds at about 90mph if everything was geared right. Close to the lightest 5-speed manual XR6's did with a 162kw engine in 1993.

At 90 mph, the car should be geared to run through the traps at 5400 rpm or so. Past the peak power rpm by about 5% or so.

The 225/60 13 tyres most people run on Cortinas gives about 588 mm tall tyre loaded, and about 600 mm unloaded. The given 2.92:1 diff gives an rpm of 38.0 km/h for every 1000 rpm the engine turns. At 5000 rpm, thats 190 km/h actual.

At 145 km/h, that is only 3815 rpm in top, or with an realy close ratio 4-speed, thats about 5000 rpm in third. (Third is about 1.31 or so, and is there for doing 29.0 km/h for every 1000 rpm.

The 2.92:1 diff that's in it is close to ideal.

A cross-check is that at 100 km/h, the engine is turning over at 100/38 km/h = 2632 rpm. Good enough for a nice cruise. I sure wouldn't want to go for 3.23 gears.

Remember, Falcons have taller wheels. in most cases, the Falcon will work best with 3.23:1 gears, the Cortina 2.92:1. The ratio difference is 10%, the size difference in tyres is 10%.

The three speed will be worse with the 2.92:1 gears. You'd have to have a 4.11 Nissan Pintara crown wheel and pinion diff in it to make it work, and it'd be then doing 3700 rpm on the highway...way too much.

 

[TCSTEVE]

Thanks again xtaxi youve cleared up heaps, ill sleep easy tonight.
Ill go with the 2.92 diff. can get a rebuilt lsd 2.92 for it too
Looking at the figures i should be looking for a cam that has good power at 2700 to 5200rpm right? Any suggustions anyone?
Thanks Steve