Head swapping compression ratio options

Hello, I’m new to the forum and am curious as to the compression ratio possibilities from head swapping. I would like to run e85 in a v12 swapped streetrod.

The people that will know will need more information regarding which engine you have pre HE or HE (high efficiency) on one of the engines the C/r is determined by the dish in the pistons as the head (s) are flat.

I’m going to be receiving a complete engine from a 1976 XJ12C and an unidentified short block, heads and random parts. I read somewhere years ago that flat heads on HE gives 13:1 ?

You read wrong, but welcome

A 76 is flat head pre-HE. So any head swap will either make no difference (since all pre HE are the same flat ‘Heron’ head engines with in-piston chamber), or will reduce the compression, since the HE has small offset chamber designed for flat top pistons.

Flat head on an HE engine is too high CR (15:1 depending on US or Euro pistons and 5.3 or 6.0) HE heads on a pre-HE engine are too low. For simple power find a six liter engine. For traditional ‘tuning’ use a pre HE and lots of money.

Personally, I think you are getting way ahead of yourself, as do most people who dream of ways to tune an engine they hardly know. Any stock V12 is perfectly capable of putting you in jail or on a mortuary slab if used hard. How much excitement do you want from a street rod?

In your shoes I would use it stock for a year and save for a manual transmission instead of trying to mess wirh parts swapping.


Thanks for the wise advice! I definitely like the manual trans idea. I have E85 fantasies because there’s a pump about two miles from my house!

13:1, up? E85 wont be enough to protect against likely detonation.

I have those fantasies as well…
I believe that with a set of different injectors that would provide more flow it could be feasible.

Don’t put flat heads on an HE it’ll knock like a diesel.

As Peter said significant power improvement involves significant cash injection on a Carb V12 engine.

Heck, it’d probably run on diesel!

Yes ,not for long though!

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Christopher, there were officially only two CR’s available in the pre HE engines, A 9.0:1 for most markets and a 7.8:1 for crap fuel markets. There were rumors of a 10:1 engine that was available, but never seen one.
The original HE engines were 12.5:1 CR, but this had to be lowered to 11.5:1 for the US market so as to be able to run on unleaded.
Putting pre HE heads on the HE engine will give you in excess of 25:1 CR. Will only run on “Rocket Fuel”.

25:1??? Do you have the figures for that?

There was a 10:1, but for one year only, 1981 I think. It has the distinction of having Digital P EFI, which according to Bywater is what facilitated the increase in CR. The fueling from the D Jetronic used on the 7.8:1 and 9:1 engines wasn’t accurate enough to support 10:1.

Bywater also opined that the 10:1 engine was the best of the bunch.

Kirbert, many of my customers on upgrading their V12’s with my 6L kit, have experienced up to 20% improvement in fuel economy with either 10:1 or 11:1 CR’s. Which is what I would expect from the over fuelling that the D Jetronic does on the 5.3L.

YES Paul, have a full set of comparative figures on all the V12 combinations…