Race Engine Forum UK

Why seemingly de-tune a car? Well, I can see the theory behind it, as I said, it's a luxury saloon car, not a performance car, admittedly It does seem a backward step though on paper.

Martin.

That's enough debate.

Back to fact. What are the valve sizes please?

GC

Evodelta you said:

"series 3 Themas did have smaller intake valves (7.2mm smaller) and ports than the other 16v heads in the F/L range, matched with a tiny (.36A/R) T3 Turbo it was done to create more low down torque and less lag, they managed 205 Bhp*. The exhaust valves are 1mm larger at 29.5mm."

To my knowledge this is correct Guy as I have heard of people using Thema S3 exhaust valves for the 1mm upgrade too.

Well, it's all very interesting, well done, thanks for bringing something really new (to me anyhow) into this domain.

GC

So Guy, any definative answer on this?

I have had seven S3 Themas now and not knowing really plagues my mind! I always came to the conclusion that this was not good for performance thats why I used a Thema S3 Variable Induction head on my modified 329BHP Thema, it was the same as an Integrale head but with bigger exhaust ports.

By looking at Thema S3 Turbo Dyno runs (attached) perhaps someone on this forum will be able to see if smaller valves actually helped in some way compared to the Integrale 16v?

Well, OK, you'd like views, but this is GC Q&A, not 'General discussion'.

Since you are clearly very au-fait with the results from this unusual type, and certainly pretty competent as far as I can see - I wish to hear your detailed appraisal of the effects of the large ex valve/small inlet before I remark on it. I can put forward design justifications, cause/effect, sure, but with most of what I write I try to base it on experience from actually working on the material.

But here is a head where I have no experience to call on - although naturally as an automotive engineer and race engine builder that is long way from saying I don't understand the design philosophy behind it and the potential effects of doing the valve layout that way.

Other members please refrain from remarking at this time unless you have direct personal experience of this head.

Please keep it concise, bullet points would serve well.

Thanks,

GC

Well Guy I don¢ž¢t know that much really. I have heard that the smaller inlet valves combined with high lift cams supposedly give a better spread of torque and more power. However to contradict this apparently the 2.0 16v 147 now has smaller inlet valves that reduce torque and power for sake of emissions - now 150 hp and 133 lbft instead of 155 hp and 138 lbft. Nevertheless technically and theoretically I am lost would appreciate your knowledge on this one.

OK, so we have nominally 27mm inlet valves and 29mm ex.

Can you give me some more data please?

I'd like to know the following if you can do it:

1. Inlet port nominal measurements horizontal and vertical at entry
2. Splitter diameter in inlet port
3. Ex port measurements at exit horiz nad vert
4. Factory (OE) cam timing (with clearance for measurement, 0.45mm I think) and lift.
5. OE max boost and max power/torque with rpm.

Please quote the actual data, not simply stating 'same as some other engine', otherwise I'll have to look it up myself, thanks.

Then I'll do my best to summarise the design philosophy behind that unusual head layout and what it's capable of.

GC

Hi Guy.

I am sorry I do not have the S3 Thema Turbo manual for this information, perhaps someone else on this forum does?

Hi,

I also have a S3 Thema turbo, and yes, the inlet valves are smaller than the exhaust valves.

Inlet : 28mm
Exhaust: 29.5mm

Factory figures : 205 bhp @ 5750 rpm
30.4 kg/m @ 3750 rpm
Max boost is 1.1bar, dropping to about 0.8~0.9bar at high rpm.

I've rebuild the engine lately using a Fiat Coupe head (but I took the 29.5mm exhaust valves from the Thema head) and I can easily say that the low-down torque was better with the smaller inlet valves.
Off-boost feels much torquier. (spelling?)

I remmember the inlet timing : opens: 0 deg BTDC
closes: 43 deg ABDC

I don't remmember the exhaust timing (I think its 43-0) but I will check it when I get home from work.


Boaz.

That is very interesting Boaz.

I always wondered if it was a bad idea moving to the bigger inlet valves?

What power are you now making and do you have a R/R graph?

Guy: Does this shed any more light on design philosophy please?

I don't know how much power I make, but I still didn't finish the project. I am too busy with my studies and work.

Actually, not too much to do. I only need to put in the big turbocharger and build a new downpipe for it.


Using the head with the big valves was a good choice, so don't worry about it. The head with the smaller valves might be better if you keep the engine un-modified, but for big numbers you need the bigger valves.

I imagine the design meeting was something like:

1. Inlet valve size can be reduced - with high enough boost there will be adequate combustion/exhaust gas energy passed by the ex valve event to drive the turbine.
2. Reducing the inlet valve size and having short duration cams will reduce the reverse-flow thru the inlet valve at lower rpm and improve the emissions and off-boost torque response.
3. Cross scavenge (cylinder purging) will be reduced by smaller inlet valve but there is an excess of this anyway and the cooling effect is adversely reducing the exhaust gas temperature on overlap and extending the cat light-up time.
4. Cylinder fill will be adequate with the right boost level even with a smaller valve. There may be a reduction in torque in some areas compared with a contemporary inlet valve layout.
5. Design BHP is achievable as per laid-down design constraints.
6. There is no cost-reduction benefit.


Was it successful? Well, without knowing exactly what was said it seems to work well enough, though for high power tuning the first thing to go would be the small inlet valves. All in all one I'd cast it as one of those 'wacky' ideas that never caught on, bit like Volvos with headlights on all day and car engines with twin spark plugs. Sorry to appear so dismissive about it.

GC

Hello…

I’m a little stuck and would really appreciate some advice on my cam setup on this rather unusual twin cam!

Here are the technicalities to aid your answer:

The Thema S3 Turbo timing is:

Inlet opens: 0 deg BTDC
Inlet closes: 43 deg ABDC (223 duration)

The exhaust cam is identical (43-0).

The Thema cams have 8.6mm lift.

The inlet valves were smaller then the exhaust.

Inlet: 28mm
Exhaust: 29.5mm

My Thema S3 Turbo now uses a Tipo inlet cam and standard exhaust cam with a big valve head. The turbo boost characteristics are now identical to before as I'm using a smaller faster spooling turbo.

My problem is that the car feels less responsive low down (below 3000 rpm). Forum member 'Boaz' has noticed the same thing. Is this anything to do with the Tipo cams 9.77mm lift and 290 duration as a pose to the 8.6mm and 223 duration of the standard inlet cam or the new inlet valve sizes?

Would the standard cams be best as there is zero overlap this way?

Advice much appreciated.

Since there are 2 of you with the same problem, let’s assume that we can ignore things like fuelling, ignition curve, detonation etc and all the hundreds of things that can upset an engine’s low-end torque response.

By big valve head I presume you mean just big inlet valves?
I suppose by 'faster spool-up' turbo you mean one with a smaller turbine or A/R ratio. The first thing I'd say is OK, but doing that can lead to back-pressure problems as the load and rpm increases, because the ex gas cannot get thru the turbine as fast and the in-cylinder condition (of high pressure ex gas trapped) tends to cause reverse flow of both inlet mixture (back out of the inlet, fighting the incoming charge) and ex gas too. I suppose it is unlikely that this is a back-pressure problem in the ex port and turbine entry itself. (FWIW the converse is that big bits at the turbine end give low back pressure and the power to run up big compressors and generate high boost, but they don’t work well at lower engine speeds because of their higher inertia)

Some important turbo-tuning points!

Doing mods to OE turbo units. There are only 3 successful ways to do it:
1. Copy a proven setup precisely from someone else with proven good reliable results.
2. Have a turbo mapping specialist evaluate all the changes by careful analysis before you change anything.
3. Do one mod at a time - as I would if going thru an engine tuning & faultfinding regime.

Unfortunately it seems to me you have not gone about it in any of those ways! So importantly you’ve no idea where you are running on the compressor flow map. Everyone does it (mainly due to the inaccessibility of (1) and (2) above and the GCRE site is, after all, here to try and educate and coach and that's fine by me. However, be aware that running a turbo in an inefficient zone is really bad news for power, and if you haven’t heard about surge and choke, (I figure you do know but you won’t be the only one reading this post) you’ve got some research to do.

Now, you've changed the inlet valves, inlet cam and turbo (or at least turbine/housing) and the engine is 'soggy' at the lower end. This is not surprising. The bigger inlet valves would not in themselves cause a problem, just give more flow (albeit either way!), because you likely have the same & contemporary valve size ratio that other engines have, not the weird setup from the production S3. A normally aspirated engine would work fine with those mods and have more torque pretty-well everywhere, yours should, but it doesn’t, because it’s got a turbocharger bolted to the ex header and that is a totally different animal. The normally aspirated engine invariably has more lift on the ex valve than a turbo unit. I’ve done many turbocharged competition engines with fully-ported head, n/a inlet cam and std turbo ex cam and got great results.

If you have read the driving characteristics of new setup accurately poor torque low-down I suggest points straightway to some exhaust gas related problem. I’m sorry about all the ‘ifs’ ‘buts’ and ‘maybes’ in my posts but that’s how it is!

I need to pick on the things you have modified to work out what might be going wrong. I’m pretty sure the problem will be the ex cam timing or the turbo itself, with your feedback we’ll see. Let us assume at this time that the new turbo is not a complete mismatch (that in itself is a big 'if' but we'll ignore it for a moment in the hunt for a simple solution). Let’s examine how the cam timing (influencing lift at tdc and thus overlap) could be the problem. I stress –could!
It's nothing to do with the duration of either cam per-se. The latter influences many things but it can be ignored in this context. I’m talking about when the valves open and close relative to each other.

The higher inlet lift isn't a problem as such, but the inlet valve opening position and consequent lift around TDC may well be. You should be aware that the cam timing figures are not the true ones the engine sees, so to speak, because they are derived (and stated) by using a wider-than-actual running clearance, I think 0.8mm, whereas the true running clearances are, say 0.4mm in and 0.45 ex. So with ex cam timing or 43/0 it will actually have some lift at tdc, but it won’t be much. However the stated cam timing doesn’t affect the full-lift position of the cams so let’s look at that.

You did not tell me the new inlet cam timing (I mean the full lift degrees) but we know the old cam of 0/43 deg was set (via the OE pulley, which all cams are) FL 111.5 deg after top dead centre ( inlet full lift). If you changed over to a n/a cam on the same pulley you'd get the same timing, FL 111.5 deg ATDC. That is quite late for an inlet cam and I definitely would not go any further retarded, because at low revs you will already have worse reverse flow past the (bigger) inlet valve after bottom-center. Once the engine speed builds up and the inlet momentum builds up (esp under boost) the engine can capture that extra flow, but at low speed it is one reason competition cams can give a rather poor torque response.

Is the bigger valve, with that 111.5 deg cam timing causing a significant reverse flow problem? Well it wouldn’t on an n/a engine, but here, maybe, read on...

The ex timing is the same degrees as inlet but of course 111.5 deg before tdc. The ex cam has very little lift at tdc; allowing for the true cam-bucket running clearance it opens somewhere before 43 deg before bottom-centre and shuts just after tdc. So what you've got is a virtually closed ex valve and an inlet valve open, dunno, maybe 1.5mm or so at tdc and though it's certainly true that inlet and ex will be open together for a small number of degrees just before tdc on the ex stroke (the inlet is opening up and ex is shutting down) you won't get much cylinder purging and cross-scavenge. Those latter things are important; engines depend on good purging (it all goes out of the ex port under the pumping action of the piston) and good cross scavenging (the inlet charge forces it out).

On a turbocharged engine the low CR means the clearance volume is pretty big. That volume can retain a lot of ex gas (one reason high compression engines give more torque is the smaller Vc). So instead of the cylinder being nice and clear of residuals at the end of the ex stroke, it’s contaminated. Even if the gas pressure there is low enough (ie: not above atmospheric) not to flow into the inlet tract (because there is no overpressure to stop it) it is still going to contaminate/block the incoming charge. This may be the root of the problem, though I do emphasise that at this time we are ignoring the match of the turbo entirely.

The solution may be to increase the overlap still further (the bigger inlet valve has done part of the job by giving us a bigger valve curtain area).
I would not advance the inlet cam timing to give even more lift at tdc than it’s already got (that would be mean moving FL from 111.5 to 110-106 deg or so, and you might run into piston-valve clearance issues). But I might be tempted however to change the ex cam timing from 111.5 BTDC to say 108 or 106 deg – opening later and closing later. This will increase its lift at tdc and there is a sporting chance that purging and cross-scavenge might start to work.
That said, if you don’t dry-build you won’t know how close the ex valve comes to the piston, so it’s a bit of a risk. And you’ll definitely need dti, protractor etc to do that (outlined in one of the skills sections in GC V/W) and adjustable timing pulleys.

The other thing you could do, and this is what I would do, is leave the new turbo and return to the original S3 inlet cam and see what happens. Or keep the new inlet cam and go back to the old turbo. What we’re really short of here is critical feedback by which I mean ‘what really happens’ if we do just one step.

The whole problem with tuning turbocharged engines is the effect every single change has on the exhaust port/turbine state. I can illustrate this. You want faster spool-up so you fit a smaller turbine. Smaller turbine has less inertia, easy to get going, that’s true in principle. Spooling-up spins the compressor faster so technically you can get close to the surge line pretty quick – low mass flow due to low engine speed but a lot of manifold pressure (I don’t know at what throttle/rpm your engine develops positive boost of course so don’t read me as ‘hyper-critical’ there, I’m just remarking on what can happen). You want more charge mass in the cylinder so you fit a higher lift cam and big valves. Well, provision has to be allowed for the much greater volume of exhaust gas to get out. And we’ve reduced the flow thru the turbine and kept the same ex valve, same ex cam and not improved the ex flow characteristic with anything, well, basic, like porting or seat mods. Not going to help any, if you follow my logic.


Hope it makes useful reading, I hope I’m not way off the mark with reading your setup and my response to...

GC