NP993

Colin, do you recommend backdating the cam sprockets to the 964 type if you want to get the cam timing set perfectly, or can it be done with the 993 setup?

mongrelcat

Mike J's rebuild writeup mentions the back-dating quite a bit.
http://www.pcarworkshop.com/index.ph..._timing_Part_I

beentherebaby

Accurate cam timing is important but I've done a lot of cam testing and I've never seen 20 HP from changing the cam position as much as 8 crank degrees with both single and dual overhead cam engines.

It sounds to me like in this example, advancing the cams - which is what you are doing by increasing the opening lift, is producing better throttle response which it should at low speed but it will also chop an equal amount of power from the top end in all the dyno work I've conducted. When you open the valve sooner it has to close sooner and that hurts top end power unless you are running a cam with excess duration. There simply is no free lunch.

Obviously with accurate engine dyno testing you could determine exactly what difference this cam timing change makes.

NineMeister

I do, although even with the early cam sprockets it is still a royal pain to get the timing perfect due to the backlash in the design. From memory it is something like 2.5 degrees of play that the early system has with the pin in any one location, hence if like me you are trying to time the valves to within 0.25 degrees of each other you are in for a fight.

NineMeister

Without wishing to sound condescending, the closer you get an engine to optimum tune the more critical the valve timing is - but when I say optimal tune I am referring to overall volumetric efficiency and combustion speed & efficiency, not simply air fuel ratio and ignition set to mean best torque on a dyno. I have actually witnessed a dyno test of a 150bhp/litre Honda race engine where moving the camshaft timing (either intake cam or exhaust cam) by just 0.5 degrees from optimal lost 9hp but to be fair this engine could only be described as "super-tuned" and it is an exception. More recently I had a similar experience testing cams & fuel on a 3.8 litre 9m race engine, without changing anything other than road fuel, cams & timing on an identical engine I found around 50hp and I attribute at least 40hp of that to the camshafts. Both cams had near identical valve lifts & duration.

In respect of advancing or retarding a 911 cam, because the relationship between the intake & exhaust lobes are fixed during manufacture in changing the timing you are actually altering all 4 valve events at the same time (IVO, IVC, EVO, EVC), whereas in an ideal world you would alter just one to determine what the engine needs. I hence have to totally agree with you that running any 911 cam set with 3 sub-optimal timing points does not help performance. I suppose the real secret is then knowing how to work out which one is right!

Bill Verburg

the lash and slop due to the tensioners is the issue, you can always get the timing holes where you want by jumping a few teeth on the cam sprocket due to the odd # of teeth used, the goal is to get both sides as close to the same spec as is possible

It certainly ain't easy, right Mike?

chris walrod

I can relate to what Colin touches upon, slop in the pin bores etc. Once you think its spot on, you snug the bolt, recheck, all good, then final torque and its off !! Can be very frustrating indeed!!

bart1

Once again, Colin has spoken. "Listen to the NineMeister, you must."

Thanks for giving me one more thing to worry about...

matt777

So would it be reasonable to expect a much reduced power loss on a fairly stock 993 engine then?

It seems that the earlier pinned set-up isn't ideal either. In my previous life/career, we would just drill and pin the both once they were in position. Removal may be required to do that here. It looks like a good application for the use of an offset bushing. I haven't seen any mention of the procedure for determining/verifying TDC. I assume this is done using a dial indicator and fixture before the heads are installed? This is all very interesting and informative!

crg53

To verify TDC, you can use a "piston stop" and degree wheel, or a digital read out one, like Colin uses.

I find this thread vey interesting and informative, and if possible would like to ask Colin to elaborate on his thoughts for the best procedure for cam timing, it seems you have a better way to get closest to optimum.

Thanks

Juha G

So the OP is saying there is 20hp to gain from a stock engine with just timing? I don't buy it.

If it was badly off, sure. But I don't believe the factory timing could be so bad.
A couple of hp, maybe. 20...no way!

The method I have allways used (on BMW 6 cylinder engines) is to first accurately determine TDC with a gauge. Then find the valve open/close angles.
I think everything beyond that falls into the accuracy of the cam gear itself (play in chain etc.).
I have yet to time a boxer but I'm sure that day will come too...

matt777

If a guy wants to spend some money...

NineMeister

I'll see if I can dig out the old dyno plots .....

NineMeister

I don't time cams, I set valve timing events, but since no cams made are totally accurate the secret is about getting the best compromise between all 4 timing events on each side of the engine - to do this you need to measure the full intake and exhaust valve lift profile then find the best fit to the events that you are looking for. I do it with a digital system, find TDC with a piston stop, measure every single lobe (yes, every valve!!) and work it all out from there.

clubsport1

I think there is a little confusion here, you do not so much have the potential to gain 20bhp on a stock engine, but re discover the bhp that may be missing from a stock engine if the timing does slip for some reason.