I think you have to change pistons, too. The pre-HE used dished pistons, the HE used domed pistons.

Hopefully someone will come along to validate or invalidate my memory

Cheers
DD

 

HE pistons with the early 1971-1980 flatheads will probably give you something like 23-1 compression ratio.

In short you are right you’ll need to use early pistons with the early head. Or go for custom pistons.
The first batch of engines had the word Jaguar cast into the valve cover. Later ones had a decal over the flat spot where Jaguar was cast. Those first ones had 9.0-1 compression no matter where they went. The ones with decals were 7.8-1 if shipped to America and 9.0-1 for the rest of the world.
From mid 1980 to mid 1981 English bound engines had 10.0-1 compression.

If you are going all out racing Cosworth builds a light forged piston worth additional horsepower over the factory pistons. While many people will sell you alternatives you’ll need to be very careful about specing them out.

 

11.5 cosworth piston forged!
pic of the 11.5 to1 comp. ratio forged piston for PreHE by Cosworth UK, cost donno!
pic shows it looks convex but thats camra angle, it is actually a flat top piston , the small bowl in middle is just below the sparkplug, i suppose to keep pressure in the middle so as to not tilt the piston and increase drag friction,(a lot of engineering in that piston)!
ron

 

Considering how light it is compared to the weight of my forged Aries and how sophisticated it is, it is simply a must have.

 

Heads and pistons. But do not expect the results to be the same. The earliest pre HE’s had a terrible carb set up that cost 20 horsepower over the later fuel injected engines. Later Fuel injected engines got better and better fuel injection until the advent of the HE.

The HE head was designed first to meet California upcoming smog laws and 2nd to improve fuel efficiency. There was nothing to improve power.
It improved mileage for a tiny bit but the other improvements made at the same time did most of the fuel mileage improvements. Going from either a 3:30 or 3:07 final drive to a 2:88 helped the most.
Plus the slightly earlier switch from the Borg Warner to theGM Turbo 400 transmission got almost the rest of the way.
improvements to the fuel injection brought it the last bit.

Fuel injection for the HE is on the edge of too lean if used in the earlier Flathead. It was set for the stratified charge used in the HE and will normally lead to burned valves if used on the Flathead unless the mixture is adjusted to the rich side.

Racing the HE’s unless mixture is dealt with will lead to blown head gaskets, and likely burned valves.

One final point. British and continental cars got decent compression 9.0-1 except the last year of the Flathead which went to 10.0-1
Most American engines were 7.8-1 pistons with a tiny handful of the earliest V12’s the exception. Those will be recognized by Jaguar cast into the valve covers rather than the decal that says Jaguar. Those are 9.0-1

 

Hi All,
What compression would my Australian delivered 1978 fuel injected Daimler Double Six have?
Is that the last iteration of the pre-HE engine?
What would be it’s horsepower?
regards
Al

 

I’d assume the same as England 9.0-1 with 289 hp. The last iteration of the “Flathead” was 1980 with 10.0-1 compression and 299 horsepower. ( one year only)

 

The famed hot rod variant.

I think I found and posted the VIN breaks for the cars having this little-known and desirable engine. I recall that production actually ran less than one full year, from mid-1980 into spring of 1981. I'll try to verify that and/or find the info

Cheers
DD