171e Webcam timing & piston valve clearance

[flatspin911]

I am building a 3.2 short stroke (using JE 10.5:1 98mm pistons) on a 3L case and want to use a Webcam 171e cam. The purspose is a track only car. I will be using Weber 46's.

The timing spec from Webcam for the 171e cam is 6.2 to 6.5mm at overlap.

I've installed the cam and checked Piston to Valve clearance.
The exaust valve to piston clearance range from
1.0mm to 1.5mm Cyl 1-3
With timing set to 6.5mm
I am using racing valve springs but I am still concerned this is very close.

I can advance the cam to 7mm and get 1.5 to 2mm of clearance
and advancing the cam to 7.5mm gets me 2+mm of clearance on exhaust.
At this point the intakes are between 1.5 and 2mm.

So my question is about tradeoffs.
I can make this cam work but at 1mm advance over max recommended.
My other option is to stay with the cam spec of advance and then shim up the cyclinders. But I would give up CR and not sure I want to do that.

What am I giving up with this amount of advance?

[Steve@Rennsport]

It sounds like you may need to machine a valve pocket or two to get some additional clearance for overrev protection unless you are using Ti valves.

Remember, advancing and retarding cam timing affects the powerband and this is a consideration when deciding where to set things at. If you wish to turn the engine hard, I would not advance the valve timing just to gain valve clearance.

10.5:1 is not much static compression with those cams so this will be a tradeoff.

FWIW, when we build engines like these, we have JE make the pistons to our own specs (compression and dome heights) and then we finish machine the valve pockets as necessary in each application.

[flatspin911]

Steve, thank you for your response, I appreciate it.

My thoughts are that this is a track car and advancing the timing will move the power band lower. I did some modelling with Dyno2000 and advancing the timing by 2 degrees moves the torque curve slightly left but does not seem to affect the area under the curve. This might give a little more pull out of the corners sacrificing the top end a bit. By doing this I get 2mm+ clearance on all exhaust valves. I am inclinded to try this before tearing it down and sending the pistons to a machine shop to cut them 1mm.

You mentioned 10.5:1 is not much static compression with those cams. Can you expand on this a bit. Do you mean that with the large overlap of these cams I can run a higher compression? What will I give up here?

[Steve@Rennsport]

Originally Posted by flatspin911

You mentioned 10.5:1 is not much static compression with those cams. Can you expand on this a bit. Do you mean that with the large overlap of these cams I can run a higher compression? What will I give up here?

Dynamic cylinder pressures are affected by valve timing and when one uses longer duration (or narrow lobe center) camshafts, one needs more static compression for acceptable off-peak torque as well as competitive power.

Engines must be configured as matched components and specifications otherwise overall performance will not meet expectation.

In your example, 10.5:1 CR would be appropriate for an aggressive street engine or an endurance race engine with 120/104 cams as that would make decent power and still run well on 92 octane fuel (with twin-ignition, of course). For an engine with 171/149 cams, I'd be at 11.3:1 or better to prevent severe torque loss below 5000 RPM.

[flatspin911]

I hear you on the matched engine components, still working on learning that....

120/104 cams is my other option.
So I took your advice and I put the 120/104 cam into the simulator and its a nicer wider/flatter torque curve. The HP curve looks actually a bit higher and also flatter.

Looks like the 120/104 cam is going in! Also solves my piston / valve clearance issue.
I'll save the 171e for when I get some higher compression pistons!

Thanks Steve again I appreciate your feedback.

The ports on the heads have been opened up to 40mm from the stock 39 Intake and 35 exaust. Any comments on the affect of the opened ports on this configuration

[Steve@Rennsport]

Originally Posted by flatspin911

I hear you on the matched engine components, still working on learning that....

120/104 cams is my other option.
So I took your advice and I put the 120/104 cam into the simulator and its a nicer wider/flatter torque curve. The HP curve looks actually a bit higher and also flatter.

Looks like the 120/104 cam is going in! Also solves my piston / valve clearance issue.
I'll save the 171e for when I get some higher compression pistons!

Thanks Steve again I appreciate your feedback.

The ports on the heads have been opened up to 40mm from the stock 39 Intake and 35 exaust. Any comments on the affect of the opened ports on this configuration

I think you'll be much happier with those cams,....

Its not difficult to negatively affect head flow with simple enlargement of the ports and I'd offer that these stock heads support up to 330HP in a 3.0 to 3.2 litre engine without touching the ports.

Caveat Emptor on making any changes without being able to check them on a flowbench.

[flatspin911]

So I installed the 120/104 cam on the left bank.
The cam specs are

IVO (BTDC) = 32 degrees
IVC (ABDC) = 52
EVO(BBDC) = 44
EVC(ATDC) = 24
Lobe centre = 100
Intake lift @ 0.050 = 0.476
Exhaust lift@ 0.05 = 0.450
Webcam calls for a timing setting of 5.2mm intake with .1mm lash at overlap.

When I measure the intake its 5.2mm at overlap but the exhaust is around 3.2mm looks like the cam is really 4 degrees advanced (I have a degree wheel on the crank pulley and a dial guage on both intake and exhaust valves). I also noticed that the lift is
Intake lift = 0.464" vs 0.476
Exhaust lift = 0.430" vs 0.450

So are they really telling me that I should install it with 4 degrees advance? Or do I have a rocker arm ratio problem or cam base circle reduced such that it doesn't give me the full specified lift and then I have an offset when setting timing at overlap?

When I do a Dyno2000 plot with 4 degrees advance it brings the torque lower with very little affect to the top end.

Perhaps I will go with this setting.

[flatspin911]

What would be the right header for this setup?
1 5/8" (38mm ID)
or
1 3/4" (41mm ID)

For tack only application?