115 replies to this topic

[Cooperman]

The Imp engine was excellent in basic design. It is a long time ago that I last did any work on one, but the production version had a problem with the head gasket and it was not unusual for the water pump to fail if the engine was revved a bit.

 

However, the concept of an all-alloy 875 cc engine with an OHC and an over-square bore to stroke ratio made if efficient. The 998 version used wills-rings as head to block seal and these seemed to work OK. It was possible to take the Imp engine out to almost 1100 cc (from memory) and with twin 38 DCOE Weber carbs it was certainly powerful. Rootes had a lot of race and rally success with the 998 Imp and I recall doing a couple of rallies in ex-works 998's.

 

Technically the Imp was ahead of the Mini, but manufacturing quality was 'iffy' at best and the suspension was no match for the Mini's superb rubber cone springs. Militant union activity at the Bathgate plant didn't help and the distance from the Rootes base in Coventry was not ideal. The Imp Sport was a good market competitor for the 998 Cooper. Oh, and how that little engine would rev!

Edited by Cooperman, 15 July 2019 - 06:06 PM.

[mini13]

I've done a bit with imps, and want aware of any glaring design issues ( with the Engine), the main problem always used to be people not keeping antifreeze/ inhibitor in them so the head/ block eroded killing the gasket.

[Cooperman]

The head gasket issue with the Imp engine was over-emphasised by Mini enthusiasts, but there were a few problems, as stated, mainly by poor maintenance.

 

I recall a friend of a friend who had an Imp and who came to me saying the head gasket had gone and could I help in part-exchange for money (a good incentive!). My reaction was that it would need the head skimming, but the owner said he had to drive to Glasgow from Hitchin a few days later in it. It was with reluctance I said I would do my best, but that I would not guarantee anything unless the head was skimmed. He was willing to take the risk, so I whipped the head off, sanded down both block & head surface with emery cloth on a flat pad. The head was re-fitted and bit of extra torque applied to the head nuts (or were they bolts?). It still had the same head gasket and was working well 18 months later.

 

The pity is that BMC didn't see the writing on the wall with the Imp and realise the way engine design for production cars was going. An alloy engine in the Mini by about 1967 would have worked well.

[Mite]

Outside of the A-Series evolving differently from its conception, BMC really needed to follow an linear / evolutionary route to replacing its engines as practiced by Renault and Nissan.

 

Renault - Billancourt > Cléon-Fonte > Energy (possibly D-Type too) > K-Type

 

 

Nissan - licensed built BMC A-Series (not sure which models used said engine) > Nissan E engine > Nissan A engine (later renamed 2nd generation E engine upon featuring OHC) > Nissan MA (as well as Nissan CA/etc) > Nissan CG > Nissan CR

 

 

BMC did look at an all-alloy A-Series a few times during the 1950s and 1960s in an attempt to reduce the weight from about 120kg-ish (in 1275cc form?), though not sure how the weight savings of the former in all-alloy would have compared to other small all-alloy engines like the 875cc Imp OHC (176lb / 79.8kg) and 598-848cc Reliant OHV* (136lb / 62.6kg-ish - allegedly the latter was a down sized Chinese copy of the 803cc Standard Eight engine and even featured an 8-port head).

 

For another comparison the unbuilt and arguably unviable 750-998cc 9X engine was said to have weighed 200lb / 90.7kg. 

 

As for the Imp engine, read (in both the Henshaw and Mowat-Brown books on the Imp) the lowest capacity version was originally planned to displace 800cc prior to being uprated to 850-875cc with the max displacement of the original Imp engine being 948cc (the 998cc was apparently not a full production engine while Chrysler cost-cutting meant the 948cc version became 928cc in the Sunbeam). Meanwhile had the Imp been a success (or Rootes avoided most of the problems during its expansion plan around the time of the Imp's development, etc), a taller-block version of the Imp engine was considered displacing around 998-1148cc with the latter displacement used in rally Imps.

 

*- In Reliant Three-Wheelers: The Complete Story, the non-all-alloy prototype version of the Reliant OHV was said to be 50-60lb / 22.6-27.2kg heavier than the old Austin 7-derived side-valve engine prior to becoming an all-alloy unit. Also there was an unrealised plan to in the 1970s to develop a 60 hp OHC version of the all-alloy Reliant OHV engine (when BRM approached Reliant about building such an engine) originally intended for a four-wheeled Bond Bug known as the Reliant BRM (page 10-13) engine, a project which ended upon Owen Organisation ended its financial support of BRM.

Edited by Mite, 16 July 2019 - 02:18 AM.

[mab01uk]

The advanced for its time, lightweight all alloy SOHC Imp engine (ex-Coventry Climax) was fitted to the Mini's transverse fwd gearbox by several Mini racers in the 1960's and 70's due to its high reving ability and tuning potential in the 1 litre class. One of the most famous Mini racers to use this hybrid set-up was Ginger Marshall who had many successes racing a Mini Van/Traveller in the Modified Special Saloon class during the 1970's.

1970s Special Saloon car racing with almost unlimited mods allowed the Imp engine which was far more technically advanced than the old A series and could be tuned to give reliably more power hence the Mini-Imp engined hybrids. I spent many weekends at the circuits in the 70s watching Ginger Marshalls Mini-Imp challenge the Hillman Imps domination of the class. Eventually both the Minis and Imps developed into Maquire type space frames with fibreglass bodies and Ginger Marshall unfortunately put a Reliant Kitten body on his!

Some more Mini-Imp details here:
http://www.startline...1/terrapin2.htm
Quote:
"At that time it was fitted with Allan's Imp engine / Mini gearbox conversion. "It was eminently suitable for hillclimbing where the car runs for just a few minutes, and usually not much more than a minute in anger. Allan had bolted a thick alloy plate onto the bottom of the Imp engine and then cut the gearbox away from a Mini engine. The plate formed a flange which bolted onto the the gearbox. This was fine for hillclimbing but was less than oil tight when we ran a race distance. However, "Ginger" Marshall had, during the 1980's run a Reliant Kitten special saloon with an Imp/Mini Hybrid."
"Ginger's brother was the mechanic and I spoke to him. They had had 5 special blocks cast which included the block itself and the adaptor to fit the Mini box. That gave a relatively reliable oil tight seal. As luck would have it, one of the people who had bought an engine from the Marshalls advertised it a week or two later in Autosport, so I bought it and fitted it, which solved a lot of problems."

 

The Imp Engine in detail here:
http://www.imps4ever...tech/engine.htm

[mab01uk]

Another 'period' Mini engine transplant below which was based on the Imp engine.......shame few of them survive today as they were so much more interesting than today's Honda Vtec conversions!


I remember this Mini-Imp well and recently discovered this thread with some photos on the Ten Tenths Forum:-
http://tentenths.com...ad.php?t=122899



Monty Guildford's Mini racer fitted with an Imp engine, he finished it in 1973 after an 18 month build.
The engine was built and tuned by George Bevan but the gear box was designed by my Monty and built by Peter Wise of Goodland engineering of Tonbridge Kent. The crank was a one off. Note also the exhaust coming out of the nearside A panel!

[Cooperman]

in the mid-'90's my son had a 1967 Cooper 998 historic rally car into which we put a fully prepared 970 'S' engine with SCCR gearbox and 4.1:1 FDR. It was a good engine with a 286 cam, very light steel flywheel, twin HS4's, plus all the usual stuff including lightened rockers to allow higher revs. It revved to around 7500+ rpm and I had a class win on an historic rally, the East Anglian Classic.

 

However, when my son did an all-tarmac historic rally in it he was surprised to find himself losing about 2 seconds per mile to a 998 Imp. At one service point I wandered over to look at this Imp ad it was a well-prepared car with twin 38 or 40 DCOE Webers. It was a flyer and won the class with my son 2nd in class. My 1275 'S' could have beaten the Imp with ease, but it was not in that class of up-to-1000 cc.

 

The Imp was very competitive in rallying. Colin Malkin won the Motoring News Championship in one and Rosemary Smith won the Tulip Rally outright in one - not bad for a girl!

[Earwax]

Nice thread and some excellent research and knowledge shown from enthusiasts,, thanks for sharing. 

 

After hearing mention of the differing engines etc in development and of the Imp , Does anyone know, how or why Fiat Arbaths were allowed to race with their boots open? For motor companies and teams spending a reasonable amount of quid on R and D, the poor old aerodynamics of the mini didn't get much of a look in. or did it and i am ignorant of these details?    ( different times of course... but I saw a very good documentation of wind tunnel testing of the LandRover Discovery,, TD4????( model unsure) which aimed at blending the bends and changing the pressure dynamics of that brick like shape)

[mab01uk]

Nice thread and some excellent research and knowledge shown from enthusiasts,, thanks for sharing. 

 

After hearing mention of the differing engines etc in development and of the Imp , Does anyone know, how or why Fiat Arbaths were allowed to race with their boots open? For motor companies and teams spending a reasonable amount of quid on R and D, the poor old aerodynamics of the mini didn't get much of a look in. or did it and i am ignorant of these details?    ( different times of course... but I saw a very good documentation of wind tunnel testing of the LandRover Discovery,, TD4????( model unsure) which aimed at blending the bends and changing the pressure dynamics of that brick like shape)

 

Dave Vizards 'Mini Tech News' magazine from March 1982 on Mini aerodynamics may be of interest below (available long before all the current Mini magazines appeared in 1990's, also note me listed as 'Roving Reporter' )

I have yet to find the more in-depth technical report on Mini aerodynamics that I have stored away somewhere in the loft.........

 

'Mini Aerodynamics' article and diagrams below by Peter Davney

 

 

 

 

 

 

 

 

 

The rest of the Mini Aerodynamics thread is here:-

http://www.theminifo...i-aerodynamics/

Edited by mab01uk, 15 July 2019 - 10:00 PM.

[DeadSquare]

Nice thread and some excellent research and knowledge shown from enthusiasts,, thanks for sharing. 

 

After hearing mention of the differing engines etc in development and of the Imp , Does anyone know, how or why Fiat Arbaths were allowed to race with their boots open? For motor companies and teams spending a reasonable amount of quid on R and D, the poor old aerodynamics of the mini didn't get much of a look in. or did it and i am ignorant of these details?    ( different times of course... but I saw a very good documentation of wind tunnel testing of the LandRover Discovery,, TD4????( model unsure) which aimed at blending the bends and changing the pressure dynamics of that brick like shape)

 

 

"Does anyone know, how or why Fiat Abarths were allowed to race with their boots open ?"

 

 

The homologation regulations forbid almost any modification of the body work;  the idea was that the car in the showroom should look just like the race car, but Abarth had a very small engine bay and needed to cut a hole in the boot to fit the Webbers.

 

They came to the conclusion that there was no reason why a showroom car should not be displayed with the boot open, and when the scrutineers read the rules, there was nothing to say that Abarth were wrong.

[Mite]

Apparently the Fiat 100 Series engine only about 10-20kg lighter than the A-Series, however it did feature an alloy cylinder head whereas such an arrangement was considered yet discarding during the A-Series's development. Not sure what else of value the Fiat unit had which could have been carried over to the A-Series.

[Earwax]

Great article mab01uk, thanks for finding it and thanks to Deadsquare for the abarth info......and rule interpretation.

[mab01uk]

An Experimental Alloy A Series, Made By BMC
"In the late 1950's BMC realised that their "Workhorse" engine the A series could do with a bit of a face lift. To this end they commissioned The British Aluminium Co. to investigate the possibilities of making an all Alloy A Series. This project was overseen by George Harriman with input from other BMC worthies including Issigonis. By 1963 "BA" had produced a number of these blocks in varying configurations, with blocks actually running in at least 1 Riley Elf and the project looked like it was going to go somewhere."

 

More details and photos in links below:-
http://mk1-performan...mental_dept.htm

http://mk1-performan...lock-report.pdf

http://mk1-performan...loy_block_2.JPG

Edited by mab01uk, 18 July 2019 - 11:46 AM.

[hazpalmer14]

You can still get alloy 5 port heads can't you? I know Paul S uses on turbominis forum uses one because the alloy alters the properties when it comes to compression ratios and boost etc

[mab01uk]

You can still get alloy 5 port heads can't you? I know Paul S uses on turbominis forum uses one because the alloy alters the properties when it comes to compression ratios and boost etc

 

HEAD UNLEADED 5 PORT 940 ALLOY ROAD / RALLY

Mini Spares own 5 port std type alloy head based on the 12G1805 "S" 940 casting with fully modified porting for fast road /rally use.

https://www.minispar...CE.aspx|Back to

 

ALLOY 5 PORT RACE HEAD 37x31mm

Mini Spares large valve alloy race ported and gas flowed head has race quality valves with special beryllium valve seats and special colsibro guides similar to those used on 8 port. Valves are spaced further apart to eradicate heat transfer causing cracking as found on old original cast heads. Weight approx 5.7kg.

https://www.minispar...AHT347RACE.aspx

Edited by mab01uk, 18 July 2019 - 06:17 PM.