6.3 vs 7.2

[Steve Payne]

The compression and cam profile were the 2 single biggest things that affected the power output. This was done to reduce emissions.

You also have to consider the petrol that was available then and now. The high compression engines could run on 5 star ( 100 octane ) if needed and the lead help supress pinking as well.

These days with 95 octane being the norm this must be taken into account when thinking about raising the power output of these engines.

10 to 1 is about the maximum most modern engines will take with a carb and distributor.

A wild cam will give you a good increase in power outputs but with a poor idle and make the car terrible in traffic and when cold.

Steve

[CrAzYMoPaRGuY]

Jensen 383 vs 440 is a strange comparison- the 383 Interceptors had early non-emissions compressions for the most part, the later 440s that are rated at 8.2 to 1 compression are often 7.8 - 7.9 to 1 static compression when actually torn down and measured. A mild mannered cam kept cylinder pressure up but killed off whatever performance was left.
The SP 440 had the six pack pistons, this should have performed much better than a single four barrel 440 version. Higher compression and a more aggressive camshaft, but the exhaust manifolds used are killers for power.

It's always the PACKAGE theory IMO. You can up a 440's compression to 10:1, throw a large hydraulic cam in it, a good intake, and a performance carb, but the car won't respond as it SHOULD unless the exhaust is factored in to the modifications. 500hp won't like truck exhaust manifolds and 1 3/4" exhaust pipe.

My next Interceptor will be for performance, bigger cubes, aggressive power, and the whole drivetrain to match.
(SOMEBODY has to give Ming a run for his money!)

[Kerry Moore]

The 7.2 when it was first fitted in 1972 was rated at 330bhp and 425lbs/ft, the same as the 6.3. Around the end of 1973 the way engines were measured was changed and the 7.2 was then rated at 280bhp and 380lbs/ft.

This was exactly the same engine but appeared to have lost power as the readings were lower. It wasn't, it was just a different method of measuring the engines output. An early 6.3 on the new system would have would also have read 280bhp and 380lbs/ft.

The very last cars in 1976 were rated at 220bhp, and this was due to the amount of anti smog equipment fitted.

The difference between the 6.3 and 7.2 cars is compression. The heads, valves, cams, etc, etc are all the same, but the early 6.3's were 10:1, the 7.2's were 8.2:1. The loss of power from low compression was made up for with the extra 900cc's.

The main difference is the 7.2 has a deeper block (about 3/4" or 20mm) and is a long stroke engine. This means it doesn't rev as quickly as a 6.3, but tends to be torquier.