To Big Valve Or Not That Is The Question

[berni29]

Hi All

I have the opportunity of fitting a rebuilt Big Valve engine or Standard engine to my plus 2 rebuild. I have never driven a big valve engined car and am not sure if there is an appreciable difference. Also given that the fuel economy at low speeds (which is mostly how I willl be driving) is worse is it worth fitting? I have read many of the period road tests and some report much worse fuel economy.

Does anyone have any experience of this.

Many thanks

Berni

[types26/36]

Berni,
I would have thought the big valve engine would have been the better option purely from a point of power, the plus two's I have driven were no rockets due to the extra weight I assume, I am of course ignoring the economy factor which I consider to be academic if you drive a Lotus (this could change as the petrol price rises while we speak)....may be we will all be driving "city cars" in the not to distant future.
Rgds Brian.

[david.g.chapman]

Hi,

I am having big valves fitted to my Stromberg head as I write. All the valves and seats were to be changed anyway, so I though - what the heck. Let's do it!

I won't know the fuel consumption for a few weeks yet - but will publish this if you like.

I think the standard Weber/Dellorto head gives 20-25 MPG, and the Stromberg 20 to up to 35 MPG if you tread carefully.

I would have thought you would lop a few percent off for the big valve variants.

Dave Chapman.

[JACKJABBA]

Some of the reports I have read, say that the big valve head doesn't realy give that much more performance, and another that it was a sales gimmick. I would look at the cost involved. Do you have the correct carbs and distributor for the engine, if not then is the small increase in power worth the extra outlay for other parts. If you get an increase of 6% is it worth the effort.

[steveww]

The Big Valve head is only slightly bigger on the inlet valve - 1.528 vs 1.563 = 0.035 or 0.889mm difference. Hmm not that much is it :unsure:

If you are having head work done anyway then go for really big valves - 1.625 inlet / 1.4 exhaust.

The maximum power an engine can produce is roughly governed by the size of the inlet valves. Total area of inlet valves (mm2) divided by 30:

Standard: 158bhp
Big Valve: 165bhp
Really big valves: 178bhp

As most people want a tractable road car they don't want to go too wild on the cam say no more than 290 duration with a CR of 10.5:1. With out going to a steel bottom end 6500rpm is about it and you should end up with 130 - 140 bhp at the most. This is still below what the rule of thumb says is possible for the standard valve size. Really big valves are only of any use on full race engines.

If you have a Stromberg head installed at the moment and then go over to Weber / Dellorto then you will see a drop in mpg as the Weber/Dellorto carbs are not as fuel efficient.

[types26/36]

Its generally agreed that the valves alone (inlets only are increased by approx 1mm) do not account for much increase in power, the extra HP comes from a raised compression (40 thou off the head) and different cams, some reports say that the heads were ported but I've never seen any big difference also the carbs were rejetted ..........of course the "BIG VALVE" inscription on the cam cover MUST account for at least 10HP. :rolleyes:
Rgds Brian

[gjz30075]

I think what everyone is saying is that there are far too many variables to simply say whether or not big valves make a difference. Maybe it will on the dyno but not at the seat of the pants. If you have the choice between the two engines, pick the big valve one only because, and make sure you check, you'll be getting a lip sealed, big main capped, meatier block type of engine for future upgrades. But again, after 40 yrs or so, many things could have been changed. I think, more importantly though, you should look for the later factory upgrades in the whole motor rather than simply looking for big or std inlet valve. Just my .02.

Greg Z.

[berni29]

Hi

Thank you all for the responses.

I came by the big valve engine when I bought a rolling chassis recently. It has nearly new webber carbs and all of the ancillaries (except for the alternator). I already have two standard engines. One from my crashed car which went well, but gave a puff of blue smoke on startup, and one from my derelict (but low mileage) rebuild candidate.

Looks like I am still undecided!

Thanks again

Berni

[1964 S1]

I've been monkeying with Elans for 30 years and have decided a puff of blue smoke on startup is a good sign, for the car, not the environment. It means you've still got enough oil to get everywhere in the engine. The best runners I've ever had puff a bit... If it's a good runner otherwise, I'd use that engine. Did you run 20w 50 oil in it?

[type36lotus]

Is 20w-50 goodness of badness? I use 20w-50 in my S3 Coupe'

[berni29]

Hi

From memory I did use something like a 20/50 mineral. It was classed as an oil for older engines. If I use the big valve I will use a 15/50 part synthetic I think.

Berni

[1964 S1]

I've been running Castrol GTX 20 / 50 in my Lotuses for years and years. I don't do a whole lot of winter driving tho...