RV8 cam timing - need a quick answer..!!

[domV8]

Hey guys,

Need a quick answer on this -

I have the TVR Power TVR835 cam in my 4.6 RV8 - the successor to the original TVR435 cam.

The power is dropping off a touch early, and I want to correct this - because I think something is not right.

The timing for the cam is supposed to be 111.25 (BTDC?) - and when I asked the RR mapper to check the timing, he said it was "within a couple of degrees of this"...

So my question is - how exact does the cam timing have to be..?
Could being "a couple of degrees out" have a noticeable effect on the power delivery characteristics..?

Or would it have to be more than a couple of degrees out to have a noticeable effect..?

The fact that timing was stated with "points" of a degree, (ie. 111.25) suggests that the timing requires spot-on accuracy - but I don't know enough about this area to make a call...

Can anyone enlighten me..?



Many thanks,


Dom

Edited by domV8 on Friday 11th March 09:54

[jr6yam]

From my, admittedly limited, knowledge of the subject I think new
cam's are timed a couple of degrees out to account for chain stretch

[domV8]

Changed cam about 3000 miles ago. New chain etc

Does this mean that the delivery characteristics should be the same if it is timed to within a couple of degrees - or is it likely to make a difference...?

The torque is dropping off after ~4000rpm - but I have just added 44mm inlet manifold/shorter 44mm bellmouths/71inch plenum, so expected the torque delivery to be shifter further up the RPM range...

Others with same cam have different delivery characteristics than mine (ie. more torque higher up) - therefore thinking either cam timing is too far out..?

Anyone got any other suggestions - or further info about the timing?


Thanks,


Dom

[Pupp]

If 'it's a couple of degrees out' actually means it's a tooth (or two) retarded, I'd be more concerned for my exhaust valves than for a bit of lost top end torque. If non-adjustable timing gear is used, then it is certainly possible for the cam to have been ground other than on the nose of whatever timing figure is specced (ie for the keyway to be out in relation to the lobes). Always worth properly dialling them in, essential on higher lifts/longer durations

[domV8]

Timed in with a vernier.
Cut-outs machined into pistons.

Timing was checked by measuring piston height at blah blah (can't remember exact details) - does this sound right..?

When you say essential for higher-lift cams - what tolerences are we talking..?

Thanks,


Dom

[clive f]

you may have done this, but when timing in a cam you need to know exact tdc, and have a 360 degree gauge fitted on the front of the crankshaft/pulley, once you have tdc you can turn the engine over until you reach the required btdc for the cam.
at the same time you need a dial guage on top of no1 inlet lifter so that on maximum lift your are the correct bdtc angle given for your cam.


I timed in a cam once on my griff 500 and made a bit of a hash of it, from memory it needed to be 108 deg but ended up at 111,(confirmed this on rechecking) the result on the dyno showed bhp at 329 but torque was down to 240, it should have been around 300bhp and 340 ft/lbs torque.
after chatting to Jamie at castlerace he told me that when racing they sometimes used to change cam timing for different circuits, being able to move the timing up to 5degrees either side of the given setting,before having problems with hitting the exhaust valves, to either raise or lower bhp and torque.

it may be that your timing could be a bit out?

Edited by clive f on Friday 11th March 13:38

[domV8]

spend said:

If its a 4.6 & the power is dropping off at higher levels, possibly not the cam but your expecting one of the ones with crap heads+induction to make the same as one of the better ones with BV heads?

Hi Spend - got BV heads, valve diameters measured & confirmed at engine rebuild...

The *entire* intake/exhaust system is now "opened up" - thats why I am thinking it may be the cam..:

AFM removed
ACT smoothbore intake & 90-elbow (all heat-wrapped)
71mm plenum
44mm aluminium trumpets
44mm intake manifold (5l manifold)
Thermal gasket
TVR BV heads
S/S ex.manifolds (JE Engineering IIRC)
S/S Y-piece (with cat)
S/S swan neck
S/S exhaust


Obviously head porting is a bit of a lottery - but I would expect the torque to last longer on the RPM range given the current set-up...?

clive f said:

I timed in a cam once on my griff 500 and made a bit of a hash of it, from memory it needed to be 108 deg but ended up at 111,(confirmed this on rechecking) the result on the dyno showed bhp at 329 but torque was down to 240, it should have been around 300bhp and 340 ft/lbs torque.
after chatting to Jamie at castlerace he told me that when racing they sometimes used to change cam timing for different circuits, being able to move the timing up to 5degrees either side of the given setting,before having problems with hitting the exhaust valves, to either raise or lower bhp and torque.

it may be that your timing could be a bit out?

So 3 degrees made a load of difference - this is what I was wondering...

I *believe* that TDC was measured using the method you describe - but I think I'm definitely going to have to get the cam timing re-checked...


Many thanks,


Dom

[domV8]

spend said:

There are loads of myths about TDC, spawned from the earlier engines with an adjustable timing pointer. Later engines have the keyway cut in line with the crank journal for pots 1&2, and the timing pointer aligns perfectly with the odd bank bore. It's pretty foolproof unless the crank is twisted or the keyway cut incorrectly. If you ever strip an engine and look at these things it is a perfect example of KISS IMHO, neat + logical + minimalistic + elegant. Quite likely to give better results than sloppy folks / pistons / big ends / mains so just extend the line off the pulley and line it up with the centre of the top of the bore (equidistant between the head bolt holes) to check your dead stop method.

OP might just be that the vernier has slipped, seems like a lot of folks have problems tightening them up?

Thank you Spend - I shall use all this info when getting it checked.
Much appreciated.


Dom

[rev-erend]

111 degrees - nice and mild then.

Even with an adjustable vernier - I found it impossible to get spot on. The within a few (2) degrees comment is about right.

My vernier is by cloyes.

Think I set mine to 106 I think but got a crazy M256 and was taming it down a little for the road (should be 104).

[domV8]

rev-erend said:

111 degrees - nice and mild then.

Can you elaborate on that..? Sounds interesting...

Dom

[domV8]

Because I asked him to check it (having given him the degrees it *should* have been) when the car was being converted to run the Emerald.


Dom

[domV8]

The real answer is - I don't know.

I got the feeling he thought this was within acceptable limits of the correct timing (hence his statement of "within a couple of degrees"..?).

However - from Clives post above - looks like even 3 degrees can have a marked effect on some cams.

So perhaps his supposition (I am guessing here) that a couple of degrees would make little difference is not correct in the case of certain RV8 cams...?

I think it will be interesting to get it on the dyno, and try various different cam timings, and see the effect on BHP vs. Torque - and try to get the best mix...

I will posting the curves over the next 48hrs.



Dom

[Pupp]

domV8 said:

I think it will be interesting to get it on the dyno, and try various different cam timings, and see the effect on BHP vs. Torque - and try to get the best mix...

I will posting the curves over the next 48hrs.



Dom

You're really going to dyo it, tear it down, redial the cam timing, build it up, dyno again and repeat several times over, allowing for cool down, fluids etc? Ermmm, can I have custard with that?

[spend]

Thats how they do the can timing in Nascar AIUI.

[v8 racing]

before you start chasing your tail! there has been no mention of what bhp your engine is making? with the upgrades and cam you are using i would expect to see around the 265 bhp mark and peek bhp at around 5300 rpm, the reality its doms cam speak to him, i am sure he would be happy to help you.

[domV8]

v8 racing said:

before you start chasing your tail! there has been no mention of what bhp your engine is making? with the upgrades and cam you are using i would expect to see around the 265 bhp mark and peek bhp at around 5300 rpm, the reality its doms cam speak to him, i am sure he would be happy to help you.

Spot on with the peak RPM

Power graph comparisons