Aussie made 4 speed with ratios of
3.32-2.00-1.43-1.00, which might make things marginally worse,
Yeah, I know, this post is gonna be a rambler, but you'll figure it out. For the short version just read the last paragraph.
The splits in your Aussie trans are;
.60-.715-.70 , which are very similar to the American-made , Commando box (which is what I run on the street) with 3.09-1.92-1.40-1.00 with splits of
.62-.73-.714
I don't see a big deal
See; the actual ratio numbers are less important to you, than are the splits; the closer together the 2-3 is for you, the more time you can keep your Low-C 318 on the top of the power curve, and so, that would be the fast way around the track. After that is established, you can move the shift up or down with rear gearing.
When stuck with a certain set of splits, they determine your powerband requirement.
For instance, your 2-3 split is .715.. So whatever you rev it to in Second, you can expect the Rs to fall to .715 of that, going into third.
So, if you shift at 6000, you can expect 4290. and your powerband requirement is 6000 less 4290=
1710 rpm .
But if you shift at 5500, you can expect the Rs to drop to 3933, for a requirement of
1568.
Knowing this, you can select the LSA that best spans this requirement.There is no good reason to stretch a 5000 rpm cam to 6000 rpm; the power has long ago fallen to less than if you had shifted at the proper place.
Imo, it is better to have more power at the lower rpm, coming out of the corner, where you really need to get the engine back up on the cam, than to over-rev third gear at the top. But that would kindof depend on the ratio of slow turns to long straights. Only you know.
In any case, you can now figure out exactly at what rpm your engine needs to be,with total overall gearing you have, and cam it to put the power where it needs to be, for the most number of times.
If you got a lotta high-speed sweepers, and your cam peaks at 4800, but you are reving to 6000; that's messed up; either re-gear it, or Up-shift!
But if you got a lot of slow corners and you're coming out at 3000, on a 5700 rpm power-peak; that's messed up; either re-gear, or down-shift!
Keep the engine on the cam.
If your powerband needs to be 1568, then you gotta keep the Rs between 3933 and 5500, plus a lil overrun of say 300 so the outshift would be 5800.
If you need a powerpeak of 5500 this is gonna require a cam of about 240@.050.. Which is mighty big for an 8/1 engine. Depending on the cam's design,at 3933 rpm it's probably gonna be starting into the soft zone. I wouldn't do it, with anything but a fast ramp solid-lifter design.
To keep the power from falling away too rapidly at your stated rpms of 3000 to 3500, with an 8/1 engine, is gonna require a cam, with a very early closing intake valve. But it's gonna have to snap open to get the intake flowing. They don't make cams like that. So you have a choice; Cam for the slow-corners, or cam for the long straights.
Or re-gear the combo to prevent getting so far off the cam in the first place. Which, since you're not really using 4th at all, and way over-running 3rd anyway; it seems a no-brainer to me, to drop that 3.15 like it's on fire,lol.
Ok, now; IMO you already have a reasonably good .050 cam to work with, it just has really really excessivly long slow ramps on it. It is spec'd at 268/276... but that spec is at .008 tappet rise. IIRC; it's a solid 8 degrees bigger when measured as a Comp-Cam is, at .006 tappet rise . But at .008 tappet rise, with nominally 1.5 ratio rocker arms, the valves are still open; 1.5x.008=.012 .. That is killing your pressure at slow rpms say 3000 to 3500,lol.
But it gets worse that intake valve, seat to seat has an actual duration of over 300 degrees.... and the engine cannot start to build compression until that dang intake finally stops leaking.
At 6000rpm, this is not a big deal, because the time for the intake charge to pass thru that slit, is cut in half compared to at 3000.
A trick I have used is to put solid lifters on that Mopar cam. Then I can tell it when to close, by varying the lash.But the lash eats up the lift, so in compensation, I installed 1.6 arms, for about 6.67% more lift than the nominal 1.5 stamped ones. This is plenty compensation. Now you can set the actual closing intake point to 268 actual degrees, instead of over 300, and at lower rpms, the engine sees that as a smaller cam, much smaller, and so the pressure rises, and with it, the low-rpm power. But at high rpm, it's still a 268 cam,lol, cuz you made it so when you set the lash.
The truth is, I wouldn't use that Mopar cam in your application. The low-Compression engine cannot really afford it, and it's gonna cost more money to install adjustable valve gear, plus solid lifters, than to just select a fast-ramp cam in the first place. That 340 cam by it's gentle-ramps is designed to last the life of the engine. A fast-ramp design might be a lot harder on the valve gear, so when you feel the power go away, something has probably wore out. Be prepared so you don't have to miss too many meets.
Last paragraph, lol.
However,
I'm not ready to throw that 268 Mopar cam under the bus just yet. I would fix the gearing first. If you get your slow corners up closer to 4000rpm, those long ramps are gonna become waaaaay less important. There might be an additional 30 or more horsepower at 4000, than at 3250.
Yeah, I know, this post is a rambler, but you'll figure it out.
