There is nothing a normally aspirated lo-compression 318 can do to a properly built 904/998/999, that it cannot take.
The 2.74-1.54-1.00 ratios are actually wide ratios with splits of .56/.65; slightly wider than the
2.45-1.45-1.00 ratios of the standard A904, with splits of .59/.69..
But if you are limited to a very low hiway gear, they help get you moving off the line.
Typically those ratios come with a 2.20 or a 2.45 rear gear, and so the starter gear is 6.03 or 6.71
When 2.76s were the norm with a 2.45 low, the starter-gear was 6.76
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In post #1, you say you are building a big-cammed 318 but you did not mention having boosted the compression or installed a hi-stall Convertor.
I can tell you right now, that if the car still has the original hi-way gears and factory convertor, when you install that 318, you are in for a shock.
Installing a long-period cam, always , always reduces the cranking cylinder pressure, which reflects the power that the engine will make at LOW rpm.
The factory cam has an Intake Closing Angle (ICA), of about 48*, and makes about 135psi on that, at sealevel.
Typically a modest 318 cam will have an ICA of 58*, which will drop the pressure to 125, which is a loss of pressure of 7.4%; which will reduce your lowspeed performance to about 86%. This goes away as the rpm rises, but it will take to about 3500rpm, in your case. to break even, and 3500rpm is 38mph with a 2.45 x2.74 gears. I told you it was shocking. If you happen to have 2.2s, then the break-even point is perhaps 42 mph. And with 2.2s you can get 65=4500 in 2.74 first gear, just after your big cam has awakened. I told you it was shocking. With 2.45s it is a bit better, 65=5000, probably close to the power-peak of your big cam. The point is this; with either rear gear, it's gonna be one gear to 65 mph. At 55mph on the downshift from Third to Second for passing, the rpm will be about 2860@10% slip with 2.45s, or 2580@10% slip with 2.2s; so forget trying to pass in Second gear. And in First, all you get is a second and a half of screaming engine, then back into Second..
Performance off the line with either gear, with the lost low-rpm cylinder pressure, and the stock TC, will be, abysmal .
So, the questions remain;
1) are you planning on boosting the compression?
2) are you installing a higher than stock TC?
3) are you considering a more performance oriented rear gear?
If you are looking for 340 type performance, remember that the early ones made about 175 psi, ran at least 2400TCs and with the automatics always came with 3.23s unless ordered otherwise. They were advertised at 340 ftlbs at 3200rpm. Into the E70-14 tires of the day, that was about
340 x 2.45 x 3.23 =2690ftlbs.
Yours with stock compression, will be lucky to make 150 ftlbs,at the very lo stall, so;
150 x 2.74 x 2.45=1007 ; I told you it was shocking.
And I didn't even factor in the hydraulic TM (Torque Multiplication) that occurs inside the TCs.
So, the bottom line is;
You just gotta budget for a higher stall TC and some big rear gears to go with that big cam. Maybe you have, IDK.
And if it was me and I had it all apart, there is absolutely NO WAY I would put it back together at the stock compression ratio. Maybe you have considered that, IDK; you didn't say.
BTW
I have swapped a 340 cam into a lo-compression 318, twice!, with the big open-chamber X heads. Both times with a 904 and once each with 2.76s, and 2.94s; and ...... both times the car was a dog off the line, and didn't wake up, as predicted, until late in the rpm band.
And one time into a 69 Satellite 9.5 Scr(IIRC) 318, with 3.23s Which was better, but it didn't wake up off the line until we put a much higher stall TC into it.
