2.0L race engine - help required......

[redrobin911]

HI - I'm having problems with my 65 2.0l race engine and cannot get it to run properly and are there any engine guru's who can point me in the right direction before I go completely mad!!

Engine runs 906 pistons and 906 cams with std heads that have had some flowing work done. car runs on Solex carbs. Ignition is electronic but have got a fairly decent advance curve allowing 30-35 degrees of advance 6000 rpm +. Cam timing has been checked and double (treble) checked. We've tried running the carbs with the vacuum enrichment circuits and also without - current readings are with the circuits in use. Other notable mods are we're fuelling the carbs directly (i.e. no floats) and returning unused fuel to the tank. Also the air intakes are slightly taller than std.

We've been running on the dyno - see some typical results below. Peak power is c. 170BHP we are jetting rich - with an AF of around 12.5:1 and will look at running it leaner possibly at 13:1 to 13.5:1 but don't want to go too lean for obvious reasons. The engine sounds good until it hits 6750 rpm when the harmonics seems to change and the dyno results show that it struggles to get to 8000rpm. The engine "feels" that it could go higher but wont. We've noticed that from 6750 rpm and upwards that the power curve fluctuates ie bounces up / down by as much as 5 - 10 bhp and as you can see the power falls off the cliff at 8100 and the engine simply will not rev any higher. Also notice there was a dip in power around 500 - 5500 rpm but we ironed that out with different air correction jets, but it did introduce a flat spot (running too lean) at 4250rpm - so still some fiddling to be done in this transition area.

I had understood that these engine should rev freely up to 8500 rpm and mine struggles to get to 8000rpm. I know we might need to start playing around further with air correction and main jets and might need to experiment with different emulsion tubes to get just right but fundamentally I want to be able to get it to rev out around 8500 rpm then work back to get the fuelling correct over the mid range.

Question then is - what could be stopping it from revving up to 8500? Jetting only or a combination of cam timing, ignition timing and jetting? Is there one area that I should concentrate on first or do I need to look elsewhere - i.e. is valve bouncing - if so what to do?

I suppose I could just be getting maximum power from what I've got? But the engine feels like its more to give and cant give it - help please!

The lines on the graph are confusing - 4 different colours are four different runs - the lower of the curves is the torque curve.

[majordad]

I suppose it still retains a rotor arm ( with it's built in rev cut off ) ? Possible cause ?

Have you the right cams ? Brian Powley had mad ones in his TR with a similiar engine.

Should you not be using titanium rods at those revs ?

[redrobin911]

I suppose it still retains a rotor arm ( with it's built in rev cut off ) ? Possible cause ?

Have you the right cams ? Brian Powley had mad ones in his TR with a similiar engine.

Should you not be using titanium rods at those revs ?

Yes rotor arm no cut out, 906 cams which are OK - could be right re the rods as are polished but std. still should rev over 8k?

[Nick Moss]

I would suspect the cams, I have had 2 race motors built over the last few years, one was a 2.5ST with supposedly RSR cams, it only reved to 6500, the next was a 3.5 with RSR cams from a different source, it revs happily way past 7500.

[redrobin911]

I would suspect the cams, I have had 2 race motors built over the last few years, one was a 2.5ST with supposedly RSR cams, it only reved to 6500, the next was a 3.5 with RSR cams from a different source, it revs happily way past 7500.

Hi Nick - thanks for this -I have a spare set of new 906 cams still boxed that I might try - came from Mr Barrie.

[fetuhoe]

Robin,

What is the overlap you have the cam timing set at? It should be 6.8mm at TDC with 0.1mm valve clearance

The Ti rods really have no effect on the ability of the engine to rev but will influence the inertia of the rotating assembly so it would run up more quickly and produce less radial load on the bearings . I don't think they would generally be eligible on a Period F Appendix K car.

Do you have a measurement of the peak to base circle 'height' of the cam as a basic guide to the profile. Inlet should be 37.6 and Exhaust should be 36.6.

Lobe separation angle should be 96 degrees.

Ignition timing is a bit of an unknown as 906 engines are Twin Plug so the data published may not work too well.

If your cams are not correctly ground and the overlap is wrong then the impact on horsepower will be significant.

We bought some NEW 'S' Cams a few years ago from a well known cam grinder.

We installed them in a 204 BHP engine 3.0 SC along with SSI exhaust and Sport Silencer.

We fitted carbs and 10.5:1 compression pistons at the same time.

We set the cam timing fired it up and ran it in carefully.

The performance was quite disappointing so we starting changing venturis, jets etc, etc.

On a Rolling Road the AFR readings were all OK - a bit rich at idle but spot on from 2000 to 6750rpm.

Power was a very disappointing 178 BHP.

We spent around a week changing everything we could think of, checking timing and adjusting carbs - all to no effect.

In the end we fitted up 2 x dial gauges and mapped the cams.

We had around 1.1mm of lift at the overlap settings and not the 5mm we had anticipated. The cam manufacturer had ground the correct inlet and exhaust profiles but at a lobe centre angle of 110 degrees and not 98 degrees. This doesn't become clear until you measure both cams at the same time.

We returned the cams fitted replacements which measured with the correct lobe centre angle and tried again. - 252 BHP.

When we fit new cams we now always map both cams and check lobe separation angles.

Valve bounce could be an issue and if so the engine will stutter at its rev limit and sound as if it hits a rev limiter.

You really need to check spring clearance at maximum lift and the nose force on the spring.

Do you know what springs you are running. Standard 911 springs could coil bind slightly with a 906 cam and I would recommend Eibach 20444.212 as these will work with standard retainers. There is a 'standard' Eibach spring also available but this may not be strong enough for the job.

AVL Shrick also make some very good springs but the ID is slightly different to the stock spring and it needs to work with an Shrick spring seat and retainer.

[redrobin911]

HI again

A little further on and now with some data -see below - I'll need to recheck the data where the exhuast valve closes as some data seems to be missing / wrong - I'm hoping there is enough here for some pointers. we'll also get the timing data from No. 4 clynder to compare with cylinder 1.

I found a spreadsheet on pelican parts where someone had degreed a early 911S cam and just added my data to compare the two. Our data was taken with 004" or 0.1mm clearance for refernce.

The overlap point of the S cam seems to be about 15 BTDC our cams around 22 BTDC.

Does this data look like a 906 cam? - also if it is should we retard the timing and give it a run or ?? From the data I have it's obvious its not an S cam.

Again to remind you my engine struggles to rev and max power is at 5500rpm

Thanks in advance......

[Bruce M]

I know nothing.... but a quick google suggests the 906 cam has very similar lift for both inlet & exhaust lobes.

http://www.drcamshafts.com/911profiles.htm

Unless that's a misprint?

[fetuhoe]

Early S cams have the following characteristics: Inlet Valve Lift 11.3mm Exhaust Valve Lift 10.3 and Lobe Centre Angle is 98 degrees.

This changed very slightly on the 1969S when Exhaust Valve Lift increased to around 10.45mm.

The 906 Cam seems to be similar with Inlet Lift at 11.6mm (as opposed to the 11.7 suggested by John Dougherty) and 10.6mm on the Exhaust. The Lobe Centre Angle Reduces to 96 degrees. This information comes from Porsche literature

There is a substantial difference the timing of these cams and from your graph this is not apparent.

It is convention with 911 cam to specify the 'Overlap' in terms of Inlet Valve Lift at 360 degrees - this is what is referred to as the overlap position in the spec books.

The Inlet Valve Lift for an S Cam is nominally 5.2mm at this location and 6.8mm for the 906 cam.

It is also convention to draw these curves starting from 0 degrees which is TDC on the compression stroke.

By superimposing the two cams profiles on the same axis without reference to the zero position is tricky.

If you fit known S cams into an engine and make the measurement then you will see the correct comparison.

If you 'shuffle' the axis on your spreadsheet so that inlet lift at 360 degrees (zero on your graph) is 6.8mm and assume that you have a 906 cam and then shuffle the S profile to 5.2mm Intake lift to the same position you should see the timing differences.

For Comparison:

S cam 38-50-40-20 at 0.1mm clearance

906 cam 104-104-100-80 at 0.1mm clearance

I must say that an early engine with Solex Carbs and S cams will typically produce around 180BHP and will struggle to rev beyond 8000 rpm. The last one we had gave 178 BHP at 7800rpm and this engine had 69S Heads and Pistons.

If you have a cam out of the engine we have a carrier and head all set up to measure profiles and I would be happy to do make the measurements.

If they are factory 906 cams then the cam nut should be a different size and thread to a standard cam but if they are aftermarket this will not be relevant.

If you do decide you have S cams then setting the lift to 5.0mm will help the bottom end slightly and setting to 5.4mm will improve top end compared to the nominal 5.2mm.