Hints on cam settings for 'chassis dyno' testing

[emil]

Hi Guy and others,
Out of curiosity and in order to learn more about setting up an engine I have made a reservation at a local engine builder/tuner for a session on his rolling road. Obviously you can spend many hours on a dyno, and this will only be from morning to lunch, but I'm sure it will be interesting all the same.

In my planning I have been considering various parameters to adjust and I started to look into the cam settings. Being a student at the time I built the engine I didn't feel I could afford dyno time so I simply set the cams at 110 FL, did a dry build, and let them be.
When I recently started looking over my old measurements I noted that the exh lift @ TDC is quite low (1.14 mm) and began to wonder if it would be worth testing if there are any potential performance gains in setting the exhaust at, say 105 or even lower (100 deg would result in 2.3 mm lift @ TDC).
If you would consider this a good idea I will obviously have to lift the head and do a valve/piston clearance map before dynoing.

In order to give you a better view of the current cam settings I have attached a *.xls document with cam lift curves for the current settings as well as general data, such as lift @ TDC etc. I made the measurements a while back but I believe they are pretty accurate. One thing that struck me as odd was the short actual duration of my (supposed?) 1608 exh camshaft. -At 0.4 mm clearance the duration is nearly 300 degrees, but due to slow valve opening the duration is only approx 240 degrees at 0.7 mm clearance.

The engine specs are:
Fiat 132 2L (in a 124 Spider). It has dual IDF40's w 36 mm chokes on waffle manifold. Pittatore 310 degree inlet cam (nominal lift 10.73 mm), original Fiat 1608 exhaust cam (both timed at 110 deg). Pistons and head (ported, flowed, using the instructions from Guy's book and homepage) from 1800 124 Coup’‚©. Everything is balanced, lightened, etc. C/R 10.8:1 approx.
Original cast 4-2 exh manifold and downpipe, single straight through muffler, 2 1/4 inch exhaust. It also has a SAAB 9000 Turbo oil cooler, and 72 deg C water thermostat. Unmodified distributor from a 1608 cc 124 BC. Set at 7 deg static timing, 33 deg max.

If you had the time to say a few words in this matter, and perhaps come with some useful hints I would be very grateful!
Many thanks from Sweden,
Emil

[Guy Croft]

Emil, no-one is going to read that download.

Open in Excel, zoom to 100%, adjust font to 12, drag to full screen size and then copy and paste into MS Paint, and save as JPEG and post an attachment, OK? Like I do with flow curves.

Then I'll take a look at it.

Look forward to it, regards,

GC

[petert]

It's a bit hard to get a full picture of the intake cam without the duration @ 0.050", but I'd be more inclined to at least run it at 106-107 deg. with that CR. You'd need to verify valve/piston clearance however, as you'd probably push the lift @ TDC closer to 0.130" and beyond. If it's a running more than 250 deg. @ 0.050", I'd be more inclined to run 104-106 deg.

I don't know anything about the exhuast port flow of that head, but I'd be inclined to leave the exhaust cam at 110 deg. to begin with.

I have no expertise in these engines but that's my two cents worth.

[emil]

Guy and Peter,
Thanks for your suggestions. I hope the following attachments will be better. I have added actual durations at 1.27mm (0.05") and 0.2mm (0.008") lift.
You can find these numbers in the table below.
If there is anything else that you would like to know about the engine don't hesitate to let me know.
All 'actual' numbers are stated with 0.4mm (0.016") clearance. Please note the big difference in lift @ TDC..
Best regards,
Emil

[petert]

Given that you've got 248 deg @ 0.050" on the intake, I'll stick with my original recommendation of 106 deg. LCA as a starting point. Your CR is probably not high enough to run 110 deg. effectively, but then again, I really don't know a lot about your engine and I'm generalising. Another example is the Z900/1000 Kawasaki (8V twin cam). They're very poplular as drag bike engines in the States, but they all seem to favour 110/110 regardless of the grind. Have a look at http://forums.dragbike.com/

Another example is the BDA Lotus. They seem to be happier at 102 deg. (or less).

[Guy Croft]

Hi Emil,

Thanks for your pm, I commend your interest in the subject and this is what I have to say.

You can alter the cam timing if, as you know, you have mapped the proximity of the valves to the pistons beforehand and know that the valves have adequate clearance to the valve reliefs. The closest I would tolerate with any valve spring is 80 thou" vertically (2mm) and more is safer.

Insofar as cam timing is concerned, 110 crank deg full lift (FL) is good all-round setting but recent work I have been involved with suggests that narrow lobe separation angles (ie: the sum of the full-lift positions, eg 110 plus 110 = 220 crank deg) may tend to give a superior peak rpm response, in other words, the engine gives more power and holds its top-end torque better (flatter torque curve) ie: the area under the torque curve is superior. So, 102 deg might well be more powerful overall than FL 110 deg timing. I emphasise 'might', because nothing is proven till tested, and I simply do not have a sufficiently quantifiable back-to-back test (on the same engine) to compare.

(For those who are not familiar with cam timing FL 110 deg means the inlet valve is at full lift 110 crank degrees after top-dead-centre on the inlet stroke, the ex valve is at FL 110 crank degrees before tdc on the exhaust stroke.)

Now here is the ‹Å“catch¢ž¢.. The exhaust manifold and inlet tract (including rampipes, the only thing you can realistically change on a dyno test) - if perfected for one FL setting, will just not be right for another. Which is by of saying that you cannot 'optimise' (perfect) the torque curve on the dyno with only set of each of those components. All you can do is 'experiment' with cam timing, at each stage perfecting the mixture and ignition timing for those settings, because they may have differing demands, ‹Å“keep your fingers crossed¢ž¢ for an acceptable result, and hope you don't bend a valve! I have done it, with different length inlet tracts and rampipes too, it¢ž¢s not much fun, trust me.

Above all, remember that TC valves can hit each other at high tdc lift, not just pistons, and the map of piston to valve behaviour MUST be done between 20 deg before tdc ex to 20 deg after inlet, taking into account the position of the piston at each degree point.

If I were you, I¢ž¢d leave the cam timing well alone, but if you try this, let us know how you get on.

GC