Innocenti/De Tomaso 1275 Engine specs.

[Chipper]

Does anyone happen to know how they managed to get around 75 bhp from the De Tomaso Innocenti Mini 120's 1275cc A-series engine?

I gather they had an SU HS6 carb fitted, for starters, and perhaps it was based on the Cooper 'S', judging by the 11-stud cylinder head...

I found these extracts from online, which states:
"...the 71bhp (74 on 1978 models) of the de Tomaso.

In fact, the latter is to all intents and purposes in Cooper ‘S’ tune, though it uses a single 1.25in SU in place of the S’s twin 1.25s, and has an ordinary 1275 camshaft, advanced by 4 degrees, rather than the old S’s wider-overlap grind. What’s more, these pokey units are not specially breathed upon by some Italian tuning wizard, but are actually produced in the UK and then shipped out to the Italian manufacturer, which makes you wonder what’s to stop BL from slotting them into the 1275GT; excuses about ‘rationalisation’ begin to sound a bit hollow…"

"by Mini Mad Swede » Thu Dec 27, 2018 10:48 pm
There's a fact fault there. The Detomaso never had a single 1.25" SU. It had a single 1.75" HS6, on an alloy inlet that I've never seen fitted on any other A-series.

As for the camshaft that was advanced 4 degrees, thats most likely the same camshaft that was fitted to Allegro 1275's."

Sources:
https://www.aronline.co.uk/cars/innocenti/mini-90-120/

http://mk1-forum.net/viewtopic.php?f=5&t=24118

[philthehill]

The one major difference in the engine components were the conrods.
The Innocenti engines were fitted with upgraded conrods and can be identified by the horizontal bar between the outside webs of the rod.

innocenti con rod.jpg (112.05 KiB) Viewed 1368 times

The 1275cc 'A' Series camshaft can be advanced by at least 3 degrees from standard without problems and gives a slight upgrade in performance.
All other mods must be taken into consideration when advancing the camshaft timing though.

See the link below regarding the 940 head fitted to the 1275cc Innocenti Mini:-

http://www.somerfordmini.co.uk/eshop/in ... age&id=118

The HS 6 was a special tuning option and was available in the UK.

[Chipper]

Thanks for the info, Phil. Certainly sounds like they hit upon a good use of various BL parts bin components...

How does one actually advance the camshaft timing, without it having a Vernier adjustment like some aftermarket setups?

[philthehill]

The easiest way is to recut the woodruff key slot in either the cam or crank sprocket or use an offset woodruff key.

Here is a kit that allows camshaft timing variance without having a vernier set as per usual style. Note the multiple crankshaft sprocket slots.

https://www.ebay.co.uk/itm/Classic-Aust ... Swc~tbR9gF

A quick calculation shows that if you advance the camshaft gear 1 tooth you can achieve 4.5 degrees advance at the cam.

Cam gear 40 teeth. Crankshaft gear 20 teeth.

360 degrees divided by 40 teeth = 9 degrees for each tooth.

As the camshaft turns at half crank speed therefore...…………..9 degrees divided by 2 (half speed) = 4.5 degrees relative to the crankshaft.

Therefore it is possible to advance the camshaft by 4.5 degrees with standard engine/cam set up without incurring any problems.

[Chipper]

Have you tried this, and would you advise it? I have a 1275cc MG Midget engine, which as far as I know has the stock internals.

[philthehill]

I have not tried it as I have a full vernier kit with Kent Cams race cam fitted and the cam is set to the Kent specification.
Vizard talks about the additional 4 degrees of advance in the 'Tuning The 'A' Series' book, page 265 in the section relating to cams and cam timing.
See also chart Fig 11.2 for cam details as set out on page 265.

[Chipper]

Interesting, I'll check it out - thanks.

[liammonty]

If I’m looking at the correct section in Vizard’s book, that section talks about the effect on when full lift is reached relative to TDC by advancing or retarding the cam by 4 degrees, but not on the actual effect it has on power or torque. Figure 11.2 gives the timing details of various cams, but again, does not discuss the effect of altering the cam timing. The section does state, though, that the cam used in 1275 A+ engines was essentially the same as that in earlier applications (1098, as well as 1275 I believe), but with the entire profile advanced by 4 degrees which resulted in better running. So he says that advancing your cam timing by 4 degrees will make an improvement if your 1275 has the earlier cam, as doing this will mimic the later improved A+ version. I suspect the Innocenti engines were fitted with the older pre-A+ cam profile but that the engineers had already learned that this advance would be of benefit.

You’ll note too that later in the section, Vizard states that due to poor tolerances in the standard set up, standard cam timing can vary considerably between engines that are ‘the same’ and also that different engine specs may require a given cam timing in at a figure different from that recommended by the manufacturer.

In general terms, my understanding is that advancing the timing a few degrees brings peak power and torque lower down the Rev range, while retarding it leads to peak figures being developed higher up the Rev range.

[Chipper]

Interesting stuff. Presumably, this could all be done by simply removing the timing chain and rotating the cam by one tooth in a particular direction, then re-assembling - with no need to remove the cam?

[liammonty]

Interesting stuff. Presumably, this could all be done by simply removing the timing chain and rotating the cam by one tooth in a particular direction, then re-assembling - with no need to remove the cam?

From Phil's calculation above, it seems that you would be able to do it roughly, yes. Got to be worth a go during lockdown! The only trouble is the road testing afterwards. I repaired a faulty vacuum line to the EGR valve on our Transit motorhome 3 weeks ago and i'm desperate to see what difference it's made to performance, but I know that even if I use it to go for essential provisions on my once-weekly outing, I'm going to get pulled over immediately, so I've got to wait

[philthehill]

One more difference between the BMC/BL 1275cc and the Innocenti/De Tomaso 1275cc engine is that the duplex (not simplex) timing sprockets are steel as per the Cooper S. Pt No: AEA696 camshaft sprocket. Pt No: AEA695 crankshaft sprocket.

[Chipper]

Interesting stuff. Presumably, this could all be done by simply removing the timing chain and rotating the cam by one tooth in a particular direction, then re-assembling - with no need to remove the cam?

From Phil's calculation above, it seems that you would be able to do it roughly, yes. Got to be worth a go during lockdown!

Which direction would the cam need to be rotated in to achieve the desired advance? I presume, with the engine running clockwise, it would need to be turned one tooth anti-clockwise, is that correct?

[philthehill]

The camshaft sprocket needs to be turned clockwise to advance the camshaft timing.

[Chipper]

Ah, thanks for that, Phil. I'm glad some-one's on the case! Might give it a go sometime...