Lotus Elan

Is 11.5/1 CR to high for US premium gas

PostPost by: holywood3645 » Wed Oct 17, 2007 8:59 pm

Hi all, is an 11.5/1 CR to high for US premium 92/93 (R+M)/2
octane pump gas.
Are the thicker Head gaskets availiable that could help reduce CR?
I have heard of some solid copper head gaskets...

For clarfication purposes
European pumps deliver 95 (RON) as "regular", equivalent to 90-91 US

User avatar
Coveted Fifth Gear
Coveted Fifth Gear
Posts: 1072
Joined: 07 Oct 2003

PostPost by: bill308 » Wed Oct 17, 2007 11:27 pm

Hi James.

Good question. I'd be very leery of any hemi head, road CR exceeding 10.5:1. The best available pump gas available here in CT, USA (North East, USA) is 93, (R+M)/2.

The way I understand it, a hemi head, like the twin cam, is a fast burn combustion chamber. To slow down the flame front, one needs a high octane gas to avoid detonation. Risers on the piston increase CR but may also slow down the flame front due to geometry, a bit of a trade off. Still, exceed 10.5:1 with reluctance.

If pinging is apparent under high load conditions (full throttle and relatively low rpm), one can always dial back the ignition timing. To do so however, reduces torque and hp. It is my opinion, that excessive CR ,offset by reduced ignition advance, verses lower CR with more advanced (near book values) ignition timing, is a net looser. Others may have a different opinion.


Fourth Gear
Fourth Gear
Posts: 781
Joined: 27 May 2004
Location: Windsor, CT USA

PostPost by: hatman » Thu Oct 18, 2007 9:02 am

Keep your high CR (and potentially high engine output) and just add Tetraboost to push the octane rating up to whatever level's needed for the engine. Easy. :D
Third Gear
Third Gear
Posts: 407
Joined: 05 Oct 2004

PostPost by: elancoupe » Thu Oct 18, 2007 9:47 am

At the time of my build, I was looking a the same issue. My pistons needed decked for compression. My builder (a well known Lotus race builder) advised me that 10.5:1 was about the limit for a street driven, pump gas motor.

14 years later, I only have good things to report on the results. Pinging has never been a problem.
Fourth Gear
Fourth Gear
Posts: 977
Joined: 11 Sep 2003
Location: Spam-on-Rye

PostPost by: rgh0 » Sat Oct 20, 2007 3:37 am

11.5 is probably to high without a fully mapped fuel injection / ignition system and a knock sensor.

10.5 is the limit I would use. In Australia you can get modern composite style head gaskets in a range of thicknesses to help get the right compression ratio. The Elan Factory can supply them.

User avatar
Coveted Fifth Gear
Coveted Fifth Gear
Posts: 7511
Joined: 22 Sep 2003
Location: Melbourne, Australia

Total Online:

Users browsing this forum: No registered users and 7 guests