Jump to content

Twin SU’s with CP Cam


Recommended Posts

Having built a couple of engines with Lucas PI and a CP cam I have always been pleased with the performance. I now have a TR6 with twin SU’s and Kent hi torque cam (TH12/6). It really lacks power and I want to rectify that. Can I simply fit a CP cam and play around with timing and the SU’s to get a reasonable improvement? The option is to fit Lucas PI otherwise, as I have most of the parts already. It has a 9.5 compression head which has been ported and polished a little.
 

Car is how I bought it so it could just need using and tuning, but I do feel it lacks power.

Thoughts and suggestions would be much appreciated.

Thanks,

 

Colin

Link to post
Share on other sites
3 hours ago, colin3511 said:

Having built a couple of engines with Lucas PI and a CP cam I have always been pleased with the performance. I now have a TR6 with twin SU’s and Kent hi torque cam (TH12/6). It really lacks power and I want to rectify that. Can I simply fit a CP cam and play around with timing and the SU’s to get a reasonable improvement? The option is to fit Lucas PI otherwise, as I have most of the parts already. It has a 9.5 compression head which has been ported and polished a little.
 

Car is how I bought it so it could just need using and tuning, but I do feel it lacks power.

Thoughts and suggestions would be much appreciated.

Thanks,

 

Colin

Do you know what needles you have in the SUs? Has it been setup properly on a rolling road?

Link to post
Share on other sites
32 minutes ago, DRD said:

Do you know what needles you have in the SUs? Has it been setup properly on a rolling road?

 According to my old SAH book, you should be using two BAE needles with an extractor manifold along with a GT system.

As you have fitted Kent camshaft, can you please advise what the timing is?  SAH recommend their 37-63, 74-28 fast-road 83 camshaft, with this type of conversion with 280 degree duration.

Please note, I have no experience of this conversion for SUs.  The information I give is taken from the SAH book.

Bruce

Link to post
Share on other sites
2 hours ago, astontr6 said:

 According to my old SAH book, you should be using two BAE needles with an extractor manifold along with a GT system.

As you have fitted Kent camshaft, can you please advise what the timing is?  SAH recommend their 37-63, 74-28 fast-road 83 camshaft, with this type of conversion with 280 degree duration.

Please note, I have no experience of this conversion for SUs.  The information I give is taken from the SAH book.

Bruce

It depends if there are other mods, the OP mentioned the head had been done. My rebuilt engine ended up with BBA needles as BAE were too lean.

Link to post
Share on other sites

To clarify it is a TH12-6 cam. 270 duration, 31/59 67/23.

No experience of carburettors so best option appears to be Lucas PI. If that is the consensus, does anybody have the pedestal drive gear (drives oil pump and metering unit) willing to sell to me?

Thanks,

Colin

Link to post
Share on other sites

I had that setup on a Saloon engine many years ago. 

It went fine once revving but was flat as a pancake below 1750 rpm and just awful in traffic.    I changed the cam to the late carb 307621  cam.  The engine went very well then pulling from low revs smoothly.   

Peter W

Link to post
Share on other sites
3 hours ago, Andy Moltu said:

My experience with my 6 which had lost its PI before I bought it was that the cam doesn’t really suite twin carbs.

Transformed by re-fitting the PI.  Other cams seem to work better with twin carbs including the CR cam.

I've seen others with similar experiences from the CP cam. I suspect that the CP cam doesn't work well with SU carbs because it has quite early opening of the inlet valve

With Lucas PI, each cylinder has an individual inlet tract. At low revs, when the inlet valves open, back pulses can interfere in a twin SU manifold, causing rough running. It's the same or worse with EFi setups that use a single throttle body on a big plenum.

With Lucas PI or triple Webers, one choke and butterfly per cylinder avoids this interference at low revs.

The CR cam doesn't open the valves as early, so works better with twin SUs.

Nigel

Link to post
Share on other sites
18 minutes ago, DaveR said:

BAE needles with yellow springs should improve things. 

Don't fit the standard 2500TC Saloon BDB needles as these are far too weak for your set-up.

Dave

+1

One further point to note is that breathing through twin SUs, this engine must have an airbox and filter(s). Without an airbox, Chris Witor - the guru for Triumph's big saloons - states that the mixture is so weak as to make 2500TC engine almost undriveable. I've experienced this with the 2.5 litre saloon engine in my GT6 which runs twin HS6 SUs. Fitting an airbox and filter absolutely transformed the performance.

If your SUs are running open without an airbox, try that first.

Nigel

Link to post
Share on other sites

Fitted with standard air box but also modified to connect to the PI airbox and filter too.

DRD - don’t get me wrong, it’s okay but compared to a PI it lacks power. Will playing with the jets and springs improve things or is PI or Weber’s with a CP cam the best solution?

Thanks,

Link to post
Share on other sites
2 hours ago, colin3511 said:

Fitted with standard air box but also modified to connect to the PI airbox and filter too.

DRD - don’t get me wrong, it’s okay but compared to a PI it lacks power. Will playing with the jets and springs improve things or is PI or Weber’s with a CP cam the best solution?

Thanks,

Playing with jets and springs won't make much difference unless they are worn. However if the needles are incorrect then proper calibration on a RR should make a good improvement assuming everything else is OK. 

My 6 runs on SU's and went from 70hp on BAG needles after its engine rebuild, to 145hp on the correct BBA ones.

Link to post
Share on other sites
39 minutes ago, DRD said:

Playing with jets and springs won't make much difference unless they are worn. However if the needles are incorrect then proper calibration on a RR should make a good improvement assuming everything else is OK. 

My 6 runs on SU's and went from 70hp on BAG needles after its engine rebuild, to 145hp on the correct BBA ones.

Slight drift here.

Where was the RR you used?

Peter W

Link to post
Share on other sites
12 hours ago, BlueTR3A-5EKT said:

Thanks for the info, it was where I was considering going.

Peter W

A real guru on SU's and Webers. I think he's taken a break for the time being as he's shielding due to Covid.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.