North wrote:

---

I actually did a test about 15 years ago with a bonestock pontiac 350 2bbl (in a 71 LeMans I believe). We ran it on a chassis dyno with a factory spec single exhaust (2" head pipe, single stock muffler and 2.25" tailpipe) and it put 145 horse to the wheels. Then we installed a factory correct dual exhaust (2" front to back with stock mufflers and resonators). HP rose to 157 with no other changes made. Btw, flipping the factory air cleaner lid added another 4 horses

I would imagine a 4bbl engine would benefit more than a 2 bbl since peak hp was at higher rpm where increased flow capacity of duals would be more beneficial. Most GM 4 bbl engines in the early 70's were rated as gaining 25 horse net with duals and most 2 bbl's gained 15 net horses. Mine only gaining 12 makes sense since I was measuring hp at the wheel.

Don't forget that net horsepower is still measured at the flywheel.

SAE Gross horsepower (SAE J1995)... stock engine without exhaust pipes, alternator, emission pumps, water pump, air cleaner etc. Some car companies used open headers during test, others used stock manifolds. Power was measured at flywheel

SAE Net Horsepower (SAE J1349)... stock engine with Air cleaner, exhaust system, water pump, alternator, emission pumps. However it does not consider things like power steering pumps or other optional draws. Power also measured at flywheel. The drop from gross to net hp was typically about 15-20% on vintage cars with dual exhaust and 30-35% with single exhaust

Drivewheel horsepower (no specific standard) is typically measured in a car fully equipped as in the net horsepower above but measured at the wheel after transmission, driveshaft and differential losses. Typically this also means things like Power steering and AC (turned off) compressordrag is also lost. typically the drop in vintage cars from SAE net is about 10-15% for manual transmissions and 15-20% with automatics.

---

The 2BBL vs 4BBL comparison is relatively easy with these, GM did it for you, for a SBC design engine anyway, but the results will be similar with others. In 1969 model year the 2BBL L65 is the same engine as the 4BBL LM1 except for the carb and intake. The LM1 had its peak SAE gross hp fiddled with by GM but not in Australia:

L65: 250hp@4800rpm, 345ftlb@3200rpm.

LM1: 255hp@4800rpm, 365ftlb@3200rpm (GM North American rating).

LM1: 275hp@4800rpm, 365ftlb@3200rpm (GMH Australia rating).

As a comparison both GM and GMH rated the 1968 L48 the same as for most SBC design engines used here: 300hp@4800rpm, 380ftlb@3200rpm

I think it is pretty fair given that most of GMH's SBC engine rating matched the North American rating except for the LM1, that 275hp is probably the true LM1 SAE gross figure. The Australian LM1 was a Tonawanda engine, identical to the US version but with a different bend on one exhaust manifold and a slightly different distributor curve (as most of GMH's engines did) as the fuel here was of slightly different characteristics. They were 388 blocks, 441 heads and 184 intake with a standard Quadrajet.

ABC123 wrote:

---

Not to be defeated, I took the centre out of a Ford window clip and was back in business 

---

 Good for you!

... but now you have a F**D part on a Pontiac!