+2 Twin Cam

[david.g.chapman]

Help!

I did a compression test on my +2 last December, after a slow loss in performance. Sure enough, cylinders 1 and 4 were down to 90 psi, and 2,3 were at 150psi.

After getting the head off, I found the exhaust valves burnt on 1 and 4, so ground in all the valves until they sealed again. However, the valves on 1 and 4 were getting thin at the edge, but I eventually got a grey ring over at least half of the valve seat. A few checks with some classic car collegues seemed to show that this was all right.

I re-assembled the head, and set up the valve clearences OK, though the shims were down to 58 thou - pushing it a bit, but if 60 thou is in the range, what the heck.

Compression was restored to 150 psi on all cylinders, and as I have a low compression head I left it at that.

800 miles later, and guess what! Cylinder 4 is down to 100 psi!!!! (others OK)

I even richened the mixture to cool down the engine a bit, and it has not overheated to my knowledge in this time.

I assume that the valve has burnt again (have not done a wet test yet).

Any ideas to stop this re-occuring? New valves this time?

Dave Chapman

[steveww]

If you are using unleaded fuel which is the deafult these days then it sounds like you are getting value seat regression (VSR). Looks like you will have to have new seats anyway so off with the head for new seats and guides

IIRC QED do a complete rebuild for about ?950 - ouch

[david.g.chapman]

Thanks for that.

Some more info.

I use 95 RON unleaded, but add Castol Valvemaster (the type that does not raise the octane rating). I have no pinking problems.

Strange that cylinders 2 and 3 have always been OK.

The valves were burnt on one side only. When I finished grinding, there was a full width seat on one side of the valve, and a half width seat on the other side (Valves for cylinders 1 and 4).

I wonder if I need full width all the way round to get the right cooling?

I just can't bear thinking about putting a "new" head back on and the same thing happening.

Dave.

[1964 S1]

Are valves supposed to rotate under normal operation? If it's "burnt on one side" is that nearest the spark plug, does the head/flow look like the valve has been holding that one position? Does the car get constant highway high rpm use?

[1964 S1]

uh, I just re-read your 1st post, were the old valves re-installed? If yes, I'd say you reburnt a burnt valve. The other thing that comes to mind is timing, but just the 1 and 4 cylinders? I dunno... exhaust manifold flow??

[david.g.chapman]

I had a chat with Burton Engineering about this (a local Essex specialist in Twin cams).

They say that with RON 95 unleaded the valves get hotter than with old leaded petrol (true), and that even with the Castrol Valvemaster added, there is a chance that in the hottest cylinder(s) the exhaust valve can stop rotating (as it normally should, I think).

There is then a progressive breakdown of the valve and its seat in one place, leading to my trouble. Their answer is to have unleaded valve seat inserts fitted.

Now I am not so sure about this. I thought Castrol Valvemaster was one of the best additives you could get. Are Burton's just trying to get business, or is this true?

All I want to do is stop the valves burning - I am inclined to just fit new valves instead of trying to re-use old ones (even though they did seal for a while), and stick to the Valvemaster.

Any thoughts?

Dave.

[rgh0]

David

I dont think the valves rotate in any normal circumstances. The are very positively linked to the retainer via the collets and the frictional loads between the retainer and springs would stop any rotation I believe. The bucket rotates but not the valve
I dont believe valve rotation or lack of it is your problem IMHO.


The valves burning can be caused by 2 things

1. Sealing area getting too hot
Mixture, ignition timing, and inadequate cooling of the valve and seat all can contribute to the valve seat and valve face getting too hot. Worn valve guides can also contribute as a significant amount of cooling of the valve occurs via the stem to guide contact area. The cooling water flow to 1 and 4 exhaust valve area is less than to 2 and 3 due to the layout of the cooling passages and location of the offtake from the head between cylinders 2 and 3. Some head gaskets also make this worse depending on their sizing of the various holes for water flow.


2. Valve geometry or material.
If the valve or seat material is inadequate it can burn in what is a "normal" environment for an exhaust valve. The lack of a protective layer of lead oxided on the valve and seat with unleaded fuel means this is more of a problem as fresh metal is continually being exposed to oxidation. Valve seat recession with unleaded fuel is just slow oxidation and wear of the seat and valve. Your just getting faster oxidation. The Additives like valve master help but are not as good as the lead oxide layer from leaded fuels.

If the valve head is to thin due to being reground too many times then the lip of the valve will run hotter than normal as the metal thickness required to conduct head way from the edges has been reduced.


Hard to diagnose your particular problem without seeing and measuring the valves and seats and head but my guess would be a combination of worn guides, seats and valves all contributing to pushing the valve over the limit. If valves are old then these materials are also not as good as modern stainless steels for exhaust valves

A complete new set of top quality copper alloy guides, hard seats and stainless valves costs about A$800 to A$900 here or about 300 to 400 pounds in the UK for the components plus the cost of machine shop time to install. Compared to the hassle of removing and replacing the head I would go for a rebuild and get it right paticularily as your valve clearances are down to the minimum shim thickness. I would then check the cooling passages and head gasket being used and use one that has the smaller water passage holes between the cylinders so it forces more flow through the ends of the head to the middle and thus cools the 1 and 4 exhaust valve areas better.

Whether you can get away with just new valves depends on the condition of the seats and guides and this needs detailed measuring to tell if they can be reused with the new valves.

Rohan

[steveww]

As you are going to lift the head anyway just bite the bullet and go for a complete rebuild with new valves, guides, seats etc.

Otherwise you will have a lot of driving to do between these petrol stations
<a href='http://www.leadedpetrol.co.uk/' target='_blank'>http://www.leadedpetrol.co.uk/</a>

[david.g.chapman]

Thanks for your replies - brilliant!

Although there are improvements with fitting new valves, (the old valves stems were 1 thou undersize) I can't be certain of fixing the problem.

Rohan and Steve have tipped the balance, so its off to QED. They say they can get 140 BHP out of a Stromberg head. We will see...

Dave.

[steveww]

so its off to QED. They say they can get 140 BHP out of a Stromberg head

Hmm 140bhp I was always told that 130 was it for a Stromberg head. As my S4 currently has a Stromberg head this is interesting. I was considering fitting a Weber head to get about 140bhp, looks like this might not be required now. What do they do to get 140bhp? More importantly what do they charge?

[david.g.chapman]

Steve,

140 BHP from a Stromberg head. QED said this to me at Donnington last year, and I was suprised. However, I gave them a ring the other day to talk about replacement valve seats, and they seemed to back-pedal a bit when I asked about this possibility.

Since then I have taken the head to Dave Wills, a local Lotus engine specialist. He is going to sort my head out, and in conversation he said that the most he has seen from a Stromberg head is 150 BHP!

The engine was go cammy though, as to be almost undriveable on the road. Its that pesky siamesing of the inlet ports, probably.

Dave.

[steveww]

150bhp from a 1558cc engine on carbs and dizzy is going to be cammy no matter what carb it uses. You are getting 100bhp per liter, modern engines manage this with variable valve timing, fuel injection, variable inlet tracts and a load of electronics. I am booked in on a dyno day soon so it will be interesting to see what I get from my pretty standard Stromberg twinc. I am hoping for about 120bhp <_<

BTW: Found that Holbay to a cam for the twinc with 288 duration 0.389" lift. Sounds about ideal for a road engine and should squeeze out a few more bhp. Just waiting for them to send me full details.

[rgh0]

How "cammy" an engine is depends on a lot of factors apart from the cam. And you dont really need all the modern wiz bang gear to get a good powerful and tractable engine, though it certainly helps.

My Elan has a very very tractable 165 HP out of 1600 cc with a 300 degree .410 lift cam. Easier off the line than a standard car despite 45DCOE's and 36 mm chokes.

The secret in getting this tractability is having the carbs tuned properly with a good transition from idle to the main power circuit and a high compression ratio and matched ignition timing curve. In my case the compression ratio is 12.5 running Avgas. The higher compression ratio compensates for loss of compression due to overlap at low revs and thus avoids the cammy characteristics of to much cam in a low compression engine. To low overlap in a high compression engine and I would have too much compression at low revs and have to retard the ignition to avoid detonation.

A road twin cam with 10.5 compression ratio needs a cam of less than 290 degress to get similar tractability and you still have to spend the time on the dyno or playing with lots of jets to get the carbs right. The Holbay cam sounds good if you dont want to go to the expense of new high lift valve gear as standard valve gear should do this .39 lift.

The problem with a Stromberg engined car is that you need to keep the duration even shorter due to the siamiesed ports and a limit of about 280 degrees is my guess. However maximum lift will still help at whatever duration you choose. How much lift can you get into a 280 degree cam I dont know but .410 should be possible and at this lift and duration you should have about 140 to 145 hp at 6500 rpm and 10.5 compression. However I dont know anyone who makes a cam like this as most of the manufacturers have to much duration for this lift. The best I can see are typically .38 or .39 at 280 degrees and I would guess this produces about a 135 Hp engine.

Perhaps some of the people who have developed stromberg engines and dynoed them can comment ?

Rohan

[rgh0]

One other thought I forgot to mention

MG racers here use what they call a "scatter cam" in their engines to help overcome the limits of siamesed ports. This has different timing for each of the adjacent cylinders so they do not affect each other so much. This enables them to run a longer duration cam with bette results. It certainly seems to work as they are getting around 180hp plus out of 1860 cc B series engines with the 5 port head running a single 45 DCOE.

Has anyone tried this approach with a Stromberg engined Twin cam ?

Rohan

[steveww]

You would have to have a scatter cam custom made for the Lotus twinc as I have never seen any mention of these by any of the cam suppliers.

So with the Stromberg head you would go for no more than 280 duration. I know the shared inlet port causes problems but I do not know the reasons why. I understand that at certain parts of the cycle both cylinders are drawing mixture from the carb. Assuming the carb has enough flow capacity why is this a problem?

As it happens I have a Weber head sitting in the back of the garage and a pair of DCOE 40. Looks like I will be shipping the head off to QED only problem is it needs the works I will have to save up the pennies.