USA Cylinder Heads

[Steve Knight]

Why was it that BL decided to re-cast a slightly different cylinder head for the USA export models - no one seems to know. UK 2500cc engines used twin Strombergs or SU on Saloons from 1963 - 76, many more than P.I. cars. So it was not to be able to fit carburettors. BL did not have any other application for these USA heads - it was just for USA.

 

I assume the spec of USA heads are; all pairs of inlets closer together by about 1/8 inch each. i.e. ports: 2+3 6+7 10+11. ( exhaust remained the same ) I do not think inlet breathing would have been compromised by this new design. Could it have been to give more available space for the exhaust ports - which in my opinion are too small and cannot easily be enlarged on UK spec heads. Steve Knight.

Edited August 8, 2009 by Steve Knight

[Alec Pringle]

Hi Steve,

 

the priority for North America was emission control - hence the head redesign specifically for the USA.

 

Cheers,

 

Alec

[foster461]

Hi Steve,

 

the priority for North America was emission control - hence the head redesign specifically for the USA.

 

Cheers,

 

Alec

 

Alec, this is an interesting question. Is it obvious why increasing the spacing between the inlet ports aided with emissions ?. Was this done to accommodate a different manifold design or is the opposite true and the head change aided emissions and the manifold was modified to suit ?.

 

Stan

[Alec Pringle]

Hi Stan,

 

I don't know the order of priorities, head and manifold that is. Back in about '77 or '78 I asked some of the guys I knew at Triumph about the logic, as in why was the USA head spec different. Emission control was the answer, pure and simple. Like you, I wondered which was the major influence - head or inlet manifold. There wasn't a clear answer, my impression was that head and manifold were two equal and necessary sides of the same coin, as opposed to being a cart and horse with specific hitching order. That may or may not be a correct impression, I stand to be corrected.

 

Perhaps someone better informed, Graham Robson for example, might be able to offer a more coherent engineering explanation ?

 

Cheers,

 

Alec

[Bald Rick]

Steve, Alec, Stan

 

Not all USA cars had the narrow port head - 516323. It was fitted to the TR250 and the CC cars up to CC67893.

 

Cars from CC75001 (don't ask what happened to the ones in between!) and CF cars to 25777 had the 218227 head. This head was made from the same casting (312388) as some of the CR and CP cars and some of the later 2.5 PIs. These heads all had the wider spaced ports.

 

So it ain't that simple!

 

This info can be found on the Technicalities CD.

 

Tony

[Mk1PI]

Was the smaller inlets combined with the mild cam in the TR250 / early federal Tr6's an attempt to salvage some low speed torque? I realise the primary consideration was emmissions but maybe the engineers wanted to still make the best of a bad job.

The early (Mk1 GT6 etc) 2000 engines with their small inlet runners have beautiful low end torque and it is an accepted fact that the Tr5 style Mk2 head was a compromise for torque on the 2000 engine... maybe the TR250 was some sort of halfway compromise?

 

The later commonisation of UK and US heads would have been primarily a cost saving

[Steve Knight]

I am not sure the US heads had smaller inlets; just spaced closer together. I doubt this would have any effect on emmisions. Efficient combustion burning is the key, better carburation, better exhaust ? ignition improvments, milder cams ......... so no one knows. It seemed like a pointless excercise as they went back to UK heads in later production.

[Richard71]

Just a thought here.....

If the inlet ports are closer together, that would mean they're slightly further away from the exhaust ports?

That gives you more meat between the exhaust & inlet ports and possibly less heat transferred to the inducted air = better combustion......maybe?

 

Richard.

[Guest ntc]

I am not sure the US heads had smaller inlets; just spaced closer together. I doubt this would have any effect on emmisions. Efficient combustion burning is the key, better carburation, better exhaust ? ignition improvments, milder cams ......... so no one knows. It seemed like a pointless excercise as they went back to UK heads in later production.

 

Maybe the fuel entering the chamber was cooler ?

Edited August 11, 2009 by ntc