Camshaft options for 2.5 engines??

[Matt George]

Afternoon all,

I've got a few queries regarding camshafts, and hopefully some of you can help shed some lights on the merits of different profiles etc.

After rebuilding and 'hotting up' a 2500TC motor to go into my saloon, my dad has now been bitten by the modifying bug, and is considering some changes to his standard ex-US TR6.

We junked the 307621 cam that was originally in my 2500 unit, in favour of a 308778 PI cam, so obviously I now have the former going spare. So my question is this - would the 307621 be any better in a CC TR6 than the 311399 cam it has in at the moment?

I've checked the cam information chart on Chris Witor's website, which gives the following details: 307621 is 10/50 and.215", while 311399 is 18/58 and .240". I'm no techie, but hopefully some of you guys that are can help with this one?  :)

Cheers,
Matt

[Richard B]

Well it would be an improvement, has his TR6 got the emission specced head and exhaust manifold?  ??)

If so I would dump those asap!   :)

Replace with a UK 2.5S head or a PI head and exhaust manifold of your choice.

[TimW]

Richard_B wrote:

Well it would be an improvement, has his TR6 got the emission specced head and exhaust manifold?  ??)

If so I would dump those asap!   :)

Replace with a UK 2.5S head or a PI head and exhaust manifold of your choice.

or a late GT6 head ;) ;)

[Matt George]

Richard_B wrote:

Well it would be an improvement, has his TR6 got the emission specced head and exhaust manifold?  ??)

If so I would dump those asap!   :)

Replace with a UK 2.5S head or a PI head and exhaust manifold of your choice.

Hi Richard,

The car is all original I believe. The inlet manifold looks the same as the long branch version on my car, while the exhaust looks standard TR6 to me. Skimming the current head is an option.

Forgive my ignorance, but please could you elaborate on exactly why the potential replacement cam would be an improvement?

Thanks,

Matt

[GT6 M]

I've checked the cam information chart on Chris Witor's website, which gives the following details: 307621 is 10/50 and.215", while 311399 is 18/58 and .240". I'm no techie, but hopefully some of you guys that are can help with this one?,   you say

you ask

Forgive my ignorance, but please could you elaborate on exactly why the potential replacement cam would be an improvement?

the answer

the 10 50  refers to the timing figs, so you got 10+50=60+180=240 degs of duration
wid a cam  lift of 215 thou.

other is
18 58 =76+180=250, and 250 thou cam lift

PI/GT is 25 65 so 270 and 232 ,

the more duration the higher it will rev to make power
Im very happy with the output of a PI/GT cam, so try and get one of them.

M

[Nick Jones]

To put it very crudely, the longer the valves are open for and the further they open, the more air gets in the engine and the more power you get,

The catch is that there is only a limited amount of time for the valve to be open. ie piston needs to be traveling in the right direction to be sucking or blowing, so if you leave the valves open too long then you can get the some of the air you just sucked in being blown back out again.  That effect is much worse at low rpm as at high rpm/air flows the air's own momentum helps stop this happening.  You can then also increase the amount of "overlap" where the inlet and exhaust valve are briefly open together so that the momentum of the departing exhaust gas can produce a "suck through" effect on the inlet.

Short duration cams are good at trapping all the gas at low rpm as they don't keep the valves open long enough to allow any charge to be lost to reverse flow.  This helps give a good steady idle, strong low-end torque and lower emissions.  They loose out to longer duration cams at higher rpm by not exploiting the extra flow potential available from the extra open time and gas momentum.

Very long duration cams (290º+) are poor at trapping gas at low rpm due to lots of reverse flow.  They will have a rough, loping idle, relatively poor low down torque and poor emissions at low rpm (though not necessarily at high rpm).  Once the revs come up though, they produce alot more torque due to the extra valve open time and gas momentum packing more charge into the cylinders.

In between are the various compromises attempting to give the best blend of the above characteristics.

The 307621 is 10/50 and the shortest duration/lowest lift of the lot.  It's an emissions cam and has nothing to commend it but a smooth idle.  It's used in the 2500S and late 2000s and probably in later US export models

306785 is 18/58 and another common one used in mainly non-sporting or emissions-compromised pretend-sporting applications.  Better than the above...... a bit.  Earlier 2000s used this one. (and Mk 4 spit)

311399 is also 18/58 but has a bit more lift as an attempt to claw back some lost grunt.  Used in late PIs and late UK Tr6s (CR).  Strong low end but runs out of puff at 4500rpm.  Also called the 125 bhp cam (in a 2.5)

308778 is 25/65 and at 270º is a pretty sporty cam for a 60s OEM offering. Vit/GT6 Mk2 and Mk1/earlier mk2 PI use these to good effect.
Gives a good compromise between docile, torquey low end while still managing a strong mid-range and pretty fair top end.  Makes 132 bhp (ish) in the PI application and 104 bhp in the 2L engines though with careful engine preparation, a well modded head and the right induction and exhaust, up to 140 bhp is possible from the 2L using this cam.

307689 is 35/65 and the "king" of the OE cams.  As used in TR5/early6 (CP) and much transplanted into PI saloons.  Also used to good effect in carburetted 2.5s and 2.0s though possibly with a bit of character at the bottom end.  Supposed to give 150bhp though 140 - 145 probably more realistic.  Not easy to find in good condition these days.  Newman do a near copy which apparently works well.

I like the 308778 myself and have them in both 2.5 and 2.0 engines, both injected.  The 2L makes about 120 bhp.  The 2.5 is as yet untested but seems to go well  :)  If this cam has a fault it is that it could do with a little more lift, especially on the inlet.  This can be achieved with roller rockers or I think Chris Witor does a re-profiled one with a bit more lift.  Original ones in good condition are getting scarce.

Cheers

Nick

[Richard B]

Nick thanks for that, a great summary.

The bhp confusion on some of the cams is to do with DIN & pre-DIN measurements

"TR6 produce 150 bhp (110 kW) (145 hp DIN) at model introduction.
Later the non-US TR6 variant was detuned to 125 bhp (93 kW)"

The later TR6 cam is not recorded anywhere as a pre-DIN value (tha I can find). From what I have gathered, the 132bhp PI Saloon cam comes in around 127bhp if measured DIN.

So the later PI cam (311399 18 - 58 240 thou lift) is supposed to be not that much worse than the earlier PI saloon cam (308778 25 - 65 230 thou lift); but as is more asthmatic as the revs rise .

[CRAJ]

Richard_B wrote:

So the later PI cam (308778) is supposed to be not that much worse than the earlier PI saloon cam (311399); but as is more asthmatic as the revs rise.  

Sorry Richard, you have put the cam numbers the wrong way round, the early cam is 308778.

Colin.

[Matt George]

Thanks to all have contributed here, certainly improved my understanding of cams!

I don't think we'll actually do the swap, was more a case of thinking out loud and wondering which of the cams we have 'in stock' so to speak was the better bet.

Even sticking with the '125' version, surely a head skim to remove the US-spec low compression and then maybe a set of HS6s would offer a reasonable power boost? Car already has a twin sports exhaust system on the standard manifold, so a mild improvement over stock.

Matt

[piman]

Hello Matt,

forgive me if you already know this, but if you do a cam swap, also move the cam followers keeping them in the correct order.

Alec

[Matt George]

Hi Alec,

Thanks for the tip  :)

Matt

[andy thompson]

Richard_B wrote:

Nick thanks for that, a great summary.

The bhp confusion on some of the cams is to do with DIN & pre-DIN measurements

"TR6 produce 150 bhp (110 kW) (145 hp DIN) at model introduction.
Later the non-US TR6 variant was detuned to 125 bhp (93 kW)"

The later TR6 cam is not recorded anywhere as a pre-DIN value (tha I can find). From what I have gathered, the 132bhp PI Saloon cam comes in around 127bhp if measured DIN.

So the later PI cam (311399) is supposed to be not that much worse than the earlier PI saloon cam (308778); but as is more asthmatic as the revs rise.  

I think the 150 was rated 142bhp DIN in Germany

[Richard B]

CRAJ wrote:

Sorry Richard, you have put the cam numbers the wrong way round, the early cam is 308778.
Colin.

B0JJ0CK5!       :B


No I have'nt, well not anymore  :X  


ps useful things TARDIS

[Richard B]

lagerzok wrote:

I think the 150 was rated 142bhp DIN in Germany

Happy to be corrected Andy, do you know if the GDR version of the TR6 was the same as the UK version?  :-/  I should add I took that quote from Wikipedia, so it was bound to be wrong.  :B

IIRC the DIN version includes alternator and water pump, whereas the ore-DIN version was ran with the fanbelt removed.

[andy thompson]

Pretty sure the 69-72 euro spec TR6 was identical to UK - although most likely assembled in Belgium