BBC Peak Torque & Horsepower RPM

[Chuck S.]

Re: like I said above, 049 heads are GREAT!

Mark, thanks for your comments. I believe he's going to use the engine as-is, so your news on the heads is good news, but he won't get full advantage of your porting recommendation.

He's mostly interested in bolt-on mods (requires no expertise on his part and he doesn't have to farm it out), so he is reluctant to get into any internal work. Judging from the cleanliness under the valve covers (clean bare iron/steel with a clean oil traces), it appears to be essentially a fresh engine rebuild that has been run-in. We believe the "builder" changed the cam to something "hotter", but have no idea about the specs. There will probably be no good assessment of the engine's potential until the project can be driven.

One interesting observation on the stamp pad...no VIN derivative stamp in the usual location. This may confirm that the engine started life as a marine engine. Curious...I also didn't see any significant broach marks on the pad. I'm going to look again next time I'm over there. Maybe it's a restamp...uhmmm...Nah.

Thanks, everyone for your advice and suggestions.

[Clem Z.]

if it was a marine engine it will have

brass core plugs. also most marine engine will have rust in the water jackets unless it was used with a heat exchanger.

[Mark #28455]

The cam is really important.

Many of these engines are rebuilt with flat top pistons for about 8:1 compression. If that's the case, you absolutely need to run a short duration camshaft or else your dynamic compression ratio will be too low and the low to midrange power will be pathetic (aka: soggy bottom end). If it has flat top pistons and the cam is an aftermarket one listed as more than 268 degrees duration or so (about 220 at .050), you'll find out.

I use a flexible shaft light I bought at Sears (looks like a Christmas tree light bulb on the end of a 10" coat hanger), put it in through the spark plug hole and you can see the piston top (if the engine is out of the car).

Torque is king on the street - with flat top pistons, shoot for about 214/220 duration at .050 with about 114 lobe centers and you'll be very impressed with the torque from 1500 to 5000 RPM even through the stock exhaust system. Hmmm... sounds almost exactly like the stock 390 HP cam - just bump the lift a little...

Good luck,
Mark

[Chuck S.]

It Has Domed Pistons...Believe CR 10.5-11! *NM*

[Mark #28455]

Re: It Has Domed Pistons...Believe CR 10.5-11!

The stock 1971 LS6 used a small dome piston - about 0.100" for 9:1 compression ratio. The stock type 1970 LS6 had a larger dome piston - about 0.300" which with the open chamber heads will provide about 10:1 to 10.3:1 compression. You can run the 10.3:1 compression on pump gas, but it will usually require a cam with duration similar to the factory solid lifter cam - approx 240 to 245 degrees at .050 or so. There are many factory solid lifter BB engines running on pump 93 octane gas - you probably won't be able to run 36 to 38 degrees advance, but a total of 34 degrees mechanical advance will probably work OK.

The old "LS6 crate engines" had this combination and while they ran OK, the new 454 HO is actually a better street motor with essentially the same peak power, lower compression that easily runs on pretty much ANY gas, and much better lower RPM performance due to its shorter cam duration.

Mark