61 engine rebuild. What to do?

[Steven B.]

Re: 61 engine rebuild. What to do?

In the case of my engine, an original 283 CI, 245 HP, the compression went to 9.0:1 but the other mods increased HP. You can lower the CR and the other mods can increase the HP. I did the same with my '77 L-82.

As for the oringnal HP being 315 and accurate, years ago I read a dyno chart which concluded the 283 and 327 engine HP ratings were not overstated. I will try to find that chart and put it on the TDB.

Steve

[Duke W.]

Re: 61 engine rebuild. What to do?

"Other modifications" can raise HP despite lower compression, but both combine to reduce low end torque, which is not the way to go with a road engine. Lower CR also increases fuel consumption - not a good deal at today's fuel prices.

I have SAE gross data from a recent lab dyno test of a totally OE rebuilt 327/340 - everything OE spec, including a new Duntov cam, no head work other than basic valve reseating, OE pistons. The only "modification was the .040" overbore

295 SAE gross HP @ 5500

Peak torque was 343 lb-ft @ 3800, which almost equals the OE 344 lb-ft rating at 4000, but is still a little short considering the nearly 7 cubic inches gained from the overbore.

An OE engine will not come close to the claimed SAE gross HP without head massaging!

You can take most of the claimed SAE gross numbers from that era and drop them about 10 percent to get a reasonably correct number for a production engine.

Then you have to lop off another 10-20 percent to arrive at a reasonably good estimate of SAE net. This can be a big wild card because it depends a lot on accessory and exhaust configuration.

The conversion to SAE net air density, alone, knocks 4.5 percent off SAE gross, and the rest of the loss is front end accessories and exhaust systems.

The good news is that the clutch fan consumes less than 2 HP if the clutch does not tighten, but about 15 if it does. Also, the 2.5" C2 exhaust system is very efficient - at least with small blocks, and my estimate is that SAE net on most C2 small blocks (assumng the fan clutch does not tighten) is only in the range of 10-12 percent less than honest SAE gross.

The 283/245 OE specified CR is only 9.5, and this is no problem with today's premium unleaded. The '77 L-82 has a specified CR of 9:1, and it was designed to operate on regular unleaded 91 RON fuel of the era , which is essentially the same as today's 87 PON. Why on earth would you want to further lower the CR?

My advice to owners of '71-up engines with low CRs for regular unleaded is to raise the CR to the limit of today's premium unleaded. You will pay about 5 percent more with every fillup, but fuel consumption will be lowered and equal amount, so the increased torque/power across the rev range is "free".

Duke