Lotus Elan and Lotus Elan Plus 2 Forums

after removing my engine in chasing a couple of coolant and oil leaks, I was surprised to find that I have what appears to be a big valve high compression head! (this is a late 66 elan coupe, non SE)
There's an N marked in front of No1 plug, and the thickness is the same as the big valve specs.
I haven't measured the valves yet as I need a valve compressor to get them out.

The cams though appear to be standard, with no ring at the front. base circle is 120 and lobe lift looks to be around .34 (8.82mm?)

I've attached some pics, including one of the intake which looks like someone has had a bit of a grind, but maybe this is what they look like standard? I trawled google for pics but didn't come up with an answer. Maybe one of you can help me out.

There is quite a bit of wear on the cam bearings and bearing surface on the cam, so I'm wondering if It's an opportunity to replace with cams better suited to the head. I also have freshly rebuilt carbs ready to go and could jet them to suit as well.

pretty interesting and a nice surprise!

Thread disappoints...

I'll be honest Steve it was clickbait

Looks like a Sprint head to me... Has that intake valve been kissing the piston? (maybe to encourage head )

You have my sympathy for being different



With most of our cars being over 50 years old and most of us not being the original owners it's hardly surprising that we uncover some unexpected modifications or quaint engineering (bodges) from time to time. Most of the accepted wisdom states that the so-called big valve head was just a marketing exercise and that the increase in compression alone made most of the power increase over so called standard. The SE engines were rated at 120bhp with standard valves.... big valve= big deal!

For those naysayers who do not believe in the effectiveness of the larger valves, the increase in power of the big valve head is commensurate with the proportional increase in valve area of the larger valves which is about 4-5%. The rest of the power increase was as a result of different cam profiles and jetting changes

David
36/7988

msd1107 wrote:For those naysayers who do not believe in the effectiveness of the larger valves, the increase in power of the big valve head is commensurate with the proportional increase in valve area of the larger valves which is about 4-5%. The rest of the power increase was as a result of different cam profiles and jetting changes

David
36/7988

Apart from those who disagree with accepted wisdom of course!

does the intake normally look all ground out?

I don't think the valves have been hitting, i think it's just some shine from the flash.

dgym wrote:does the intake normally look all ground out?

Mine didn't originally but I hand worked it with a file to match the Weber, so it now looks a bit like yours.

msd1107 wrote:For those naysayers who do not believe in the effectiveness of the larger valves, the increase in power of the big valve head is commensurate with the proportional increase in valve area of the larger valves which is about 4-5%. The rest of the power increase was as a result of different cam profiles and jetting changes

David
36/7988

The power increase in the big valve head engines was due to a number of small modifications

1. Larger inlet valve area as described above
2. High cam valve lift at 0.36 versus 0.35 for SE engines
3. Tubular 4:2:1 exhaust manifold versus cast for SE engines
4. Higher compression ratio by shaving the head 40 thou
5. Machining changes around the inlet valve throat that matched the larger inlet valves
6. Larger 33mm carb chokes
7. Changed ignition advance curve

The electric versus water pump mounted fan and alternator versus dynamo also reduced power losses but as Lotus were always vague about the test standard for their quoted hp it is not possible to tell if they also counted this

cheers
Rohan

thanks for all the replies guys.

My question now is,
If I were to fit sprint cams to this head, do I need to beef up the springs or any other things? Will the standard timing gear be ok?
will the bottom end be ok?
I can change the carb jetting to the sprint spec very easily.

OR, are there any other fun cams I could fit without having to change too much?
cheers,
-Jim

The standard springs are fine for the slightly higher lift sprint cams.

There are a number of alternate short duration higher lift cams that produce substantial more power and torque but these require a replacement of the valves, spring, and maybe followers to accommodate the higher lift in the rather limited space in the twin cam head. if your valves and springs and followers are at the end of their life then the rebuild for a high lift cam costs about the same as standard rebuild.

Then you get tempted to fully utilise the capability of these cams to produce even more power above the 6500 rpm limit of your standard bottom end .... its a slippery slope after that

If your going to be at Richards for the Feb LCV get together i will be there and we can talk about the options

cheers
Rohan

Yes Rohan I've put the date in my phone so I can't forget.
Looking forward to picking your brains in person!

So it looks like you have an high compression head with the 'N' stamping and 'S/E' cams judging by the lift.

What diameter chokes are in the carbs?

the carbs were a bit confusing when I got the car, one being a Dellorto DHLA 40E, and the other a 40 (no prefix).
both have the same jettings but the E has an extra progression hole.
Main choke is 30.
main jet, air corrector and idle jet, also the pump jet are as per big valve spec, the main emulsion tubes are to standard spec.

I had since found a matching pair of 40E's and was in the process of fitting them. I had started by simply swapping the original jets into the new carbs as I knew the car ran with those jets, but was having trouble getting idle UNTIL I unscrewed the Idle screws WAY out beyond the advised amount of turns. Im guessing this was because the idle jet holders were 7850.3=very lean. this was before I even knew about the head being big valve.

It was about then that i heard that coolant dripping on hot engine and smelt that same smell and realized I needed to do something so it was engine out and here I am.

I do now have extra 32 chokes that came with the E's that I bought so I'll be able to try them when everything goes back together.