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Small bore power

David Vizard looks at a way of getting Imp power
without going the one litre way and
tests a few carburettor combinations at the same time

Twin Dellorto 40 DHLA carburettors gave extremely good power
with the small capacity engine peeking over 67 bhp

It has often been said that one of the first moves with tuning an lmp engine is to go from the 875cc unit to the 1000cc one by means of the wet liner. Although I would agree that there is no substitute for cubic inches, this does seem to me like its selling the 875 unit short. Even in 875 form the Imp engine will produce some pretty respectable figures and yet remain flexible enough to use on the road. If you have a small capacity version which has potentialy many thousands of miles left in it then it would seem to me a worthwhile exercise to use up at least the major part of that wear life before you trade it in for a big bore job.
Here I would like to pose a question which I am sure many Imp owners would dearly love to know the answer to: Can we get the sort of power output that the 1000cc engine in its standard form gets by utilising tuning equipment other than going by the l000cc route? In other words can we bolt on some goodies to give the same hp? Well that, my friends, is one of the objects of this exercise to establish whether or not this can be done.

twin 1½" SUs
constant depression carbs
single 40 DHLA
twin 40 DHLA
BHP tables:
cam timing variations
carb changes

I suppose what really started the ball rolling was a conversation I had with Andy Chessman of Greetham Engineering at Edgwick Road, Coventry. To cut a long story short Andy informed me that he had a head and cam combination which increased both torque and bhp throughout the normally used rev range. The head itself was done with economy in mind as much as any power increase that it may achieve. The inlet ports were slightly enlarged and reshaped, but they were left fairly rough so as to aid fuel build-up. The inlet valve remained at the standard Imp Sport size. The exhaust ports on the other hand were enlarged quite considerably over the standard and the exhaust valve itself was increased from 1.06" to 1.125". that is a 12% increase in area. The general idea of this is to cut down pumping losses on the exhaust stroke. This will make a small contribution to bhp but more important it will also help the economy.

Now onto the cam: this particular one was designated GE2. It had a 0.3125" lift and timing which went something like this: 28-58-66-20 (exact figures of course depend on what you line the cam in at, as we shall see later). The next move of course, was to build up an engine to try this head and cam combination on. A newly bored block was acquired together with a set of Hepolite pistons, part number 18542/KR.

A reground crank was used together with Powermax bearings, part numbers RB4417LC and RM3328LC for big ends and mains respectively. The crank, rods, pistons and flywneel were given a balance job just to add that final touch of smoothness. At this point the Greetham Engineering head was fitted using a Reinz gasket rather than the standard type. The GE2 cam was then installed and correctly shimmed.
Now came the moment when the cam was to be timed in. One of the things we were going to try on the dyno was to mess about with the cam timing to see where it gave the best power and to see how much difference altering the timing made. So to start with the cam was timed in so that the inlet valve was open 0.060in when number One was at TDC and just starting its induction stroke. This position is retarded from where we finally expected it to be, but it would at least show how much difference incorrect cam timing could make. So that we could set the camshaft to various positions we did, of course, use a multihole timing sprocket available from Chrysler Competition Department, under the part number CTS1034. One other point before I forget we also used the Chryster cam carrier which has extra clearance grooves cut in the tappet bores to clear the ends of the cam lobes on higher lift cams. The part number for that is CTS1035. This provides clearance for cams up to 0.360" lift.

On completion, the engine was dropped onto an engine brake and set to run at light load at 3.000rpm until it was run in. When this was completed it was given a complete service and check over and from here on the power testing could begin in earnest.


The first job of course was to find the optimum cam timing. First of all the engine was set up to give the maximum possible power with the cam timed to give 0.060in of lift at TDC on number One cylinder just as it was starting its induction stroke. With this cam timing, you will see, the best power we could obtain gave actually less bhp than a standard engine. With the camshaft set to 0.080in the situation looked more promising. It did in fact show a slight improvement over the output of the standard engine. Still this was not as good as it could be.
The correct cam timing for this particular camshaft is in fact 0.110in. At 0.100in power output improved still further. At 0.112in setting though it did give the best bhp, 112 being as close to the required 110 as we could get with a multi-hole sprocket. At 0.120in the power was back off and of course we didn't like to go any further than this because the valves must be coming very close to the pistons at this setting and they had not been notched for any extra clearance

If you now compare the output achieved with the cam set correctly in the standard engine the whole thing looks a bit more promising. There is an increase in power throughout the rev range over that given by the standard 875, but compared with the 1000 the bhp down the lower end of the rev range with this GE2 cammed engine is down. However once we get up around the 5.500rpm mark the GE2 cam begins to pay off and in fact produces a slightly superior peak output than the 1000. Along with this the cammed 875 engine has about 1.000 more useful rpm than a standard 1000cc unit.

From these figures then, it looks like we are able to achieve half of our objective. We can match and even surpass the power output of a one litre engine, but we still fall short as far as peak torque is concerned.

Timing the Greetham Engineering GE2 cam with Competitions Dept. multi-hole timing sprocket
The Greetham Engineering GE2 cam was timed using Chrysler Competitions multi-hole timing sprocket.

Removing the fan

Having got the best of what we had so far, a few little experiments seemed to be in order. First of all the fan was removed to see just how much difference this would make to the power output of the engine. Up to 4000rpm the difference was insignificant. At 5000rpm the fanless engine showed two brake more. At 6000 it showed three more; al 6750 it showed 3.8 and by the time it got to 8000 the difference was 5.3bhp. Adding extra radiators or some means of keeping the engine cool whilst disposing of the fan means more performance.

Removing the air filters

With the fan refitted the next thing we tried was to see if the power went up on open intakes. Doing this would establish whether or not the Imp engine could do with any more carburation. With the air filters removed from the 1.25in Stromberg CDs, the midrange torque and power came up quite substantially although the top end did not improve as much as expected, in fact peak power remained virtually unchanged. These figures suggested though that the head and cam combination could do better if the outside ancilliaries such as carburation and exhaust would allow it.

1½ SU

Our next move was to try twin 1½" SUs on open intake on this engine to see if they were significantly different to the performance given by the Strombergs.
A comparison of the figures will show that if anything, the SUs had a slight edge over the Strombergs. But the difference was generally marginal. Certainly making the change from Strombergs to SUs would not be a worthwhile modification since you are likely to spend out a lot of money on SUs for a very, very small gain. One thing that running on open intakes did prove however, is that the engine could do with a little more breathing, so 1t was fair to assume that a better induction system, plus possibly a better exhaust would pay off dividends in increased power output.

Twin 1┬╝in. SU carburettors gave barely any improvement
Twin 1¼in. SU carburettors gave very little improvement over the standard Imp Sport Stromberg CDs.

Constant depression vs. fixed choke

The next carburattor set up that I tried was at the suggestion of Andy Chessman and this was a single DHLA 40 Dellorto on a combined inlet and exhaust manifold. Previous experience with a single twin choke carb on a four cylinder engine has produced results which I really wouldn't bother to write home about. A rough analysis of the situation is that with a single 40 or 45 on a tour cylinder motor, you can (depending on the way you couple the cylinders) get adverse shock waves which upset a fixed choke carburettor when it is having to contend with unevenly spaced pulses. This can cause a mixture bias, making one of each pair of cylinders fed, richer than the [...] , I don't want to give you the impression that this happens in every case because it doesn't; it seems to depend on the type of cam you use, manifold and exhaust system etc. Indeed there must be numerous variables which affect it. However there is a very real possibility that it can happen and the result of putting a single 40 on can produce a lot less performance than you would expect. The reason that this bad carburation doesn't happen with constant depression carburettors like the SUs or Strombergs, is that these carburettors, at part throtle, very effectively damp out the major proportion of the shock waves. Its all very well theorising, but as the saying goes the proof of the pudding etc.

A single Dellorto

With this in mind I then tested a single 40 DHLA on a combined inlet and exhaust manifold. Such a manifold is available in the Coventry area of course from Greetham Engineering. It's also sold by such establishments as Team Hartwell in Bournemouth, Janspeed, and a few other reputable manifold manufacturers.

With the new manifold and carburation bolted up and the silencer fitted, we began our testing, and after a while we began to get the feel of what the engine required in the way of chokes, jets, emulsion tubes etc., and as soon as we got on the right track the power began to come up. A quick glance down the power figures will quickly reveal that over most of the rev range a highly respectable power curve was achieved.
At 2500rpm the power was considerably down, probably for the reason I have just explained. However, unlike some motors on a sing1e 40, it displayed no vices at 2500rpm on full throttle. Sometimes spitting back through the carburettor can occur and general very uneven running. This was not experienced. By the time we reached 3000rpm the power was beginning to pick up. From 3500rpm upwards the power curve produced was very good. Indeed on this set-up we achieved the highest torque of any carburettor set-up that we tried. By the way - the power goes on climbing at the top end. It's fairly obvious the exhaust is having an effect on the output as well as the type of carburettor used. The fact thatl the engine had a useful power curve from as low as 2000rpm round to 7750rpm on this single 40 set-up shows that the cam was able to cater for the engine's requirements over a very wide rev range, but even more relevant is the fact that with this head, cam and carburetion set-up you will achieve more performance than just doing a straight one litre engine, but admittedly you will have to use a lot more revs to achieve that performance. If you fancy using an identical cam, head and carburation set up as we used, here are the jet settings for the Dellorto we found worked best:

These were used, together with 50 idle jets and 50 pump jets. 1f you change anything from what we had on our test engine then the chances are that these carburettor settings will not be the ones to use.

single Dellorto 40 DHLA carb  gave reasonably good power output
The single Dellorto 40 DHLA carb was a rather unusual fitting for the Imp but gave reasonably good power output.

Twin Dellortos

If one Dellorto was good, two should be better. I think that's a fair assumption to make, since inevitably one carburettor barrel per cylinder produces the best results. Using twin 40 DHLA Dellortos was in fact the basis of our next test. For this, we mounted these carburettors on a Derringlon combined inlet and exhaust manifold. If I remember my facts correctly, Stuart Derrington informed me that this was the manifold that they originally made for Fraser Imps and it is still reckoned to be one of the best manifolds around as far as power production is concerned. I was originally a little worried about the large bore pipes on this exhaust system, principally because the system was originally designed for 1000cc racing engines rather than 875cc road engines. However, my fears proved totally unfounded as this manifold plus the twin 40s turned our unit into a veritable screamer, as you can see from the power figures.
When looking at these you must bear in mind that the exhaust is still going through a silencer and we still have the fan on and the higher rpm we go, the larger amount of bhp is dissipated into it. Let's take our peak power at 7500rpm. With the fan on it produces 67½bhp, but at these rpm close on 5bhp is being sapped up by the fan. Minus the fan, the engine will produce 72.5bhp and at 8000rpm it will produce 69bhp. As far as the torque output is concerned the 875cc engine made 54.3lbs/ft of torque at 6000rpm against 57.1 lbs/ft of torque for a 1000cc engine at 4500rpm. So you can see torque-wise it's getting pretty close, although it occurs at 1500 revs higher.
Carburettor settings for the Dellortos proved anything but what we expected. On an engine of this size we had really reckoned on choke sizes around 28mms and main jets around 95 - 100. This proved not to be the case. After much checking out with various alternatives of jets and chokes we arrived al the best setting to be 30mm chokes, 115 main jets, number 6 emulsion tube and 180 air correctors. This together with 35-40 pump jets and 40 idle jets.

Twin Dellorto 40 DHLA carburettors gave  power peaking over 67 bhp
Twin Dellorto 40 DHLA carburettors gave extremely good power with the small capacity engine peaking over 67 bhp


Well, that brings us to the end of our dyno test, but just let's analyse what we have achieved: first of all the engine is giving close on 80bhp per litre and is still thoroughly useable on the road. In the final stage of tune we have a power output way in excess of that given by a standard 1000cc Sport engine although, admittedly, it will have cost us a little more to do it.
Against the price factor we can weigh the fact that all the parts that have been fitted to this engine can be considered as bolt-on bits, so if you have a really good 875cc unit, you can get this power without even removing the engine from the car. Doing it this way you can go and use up the wear life of the 875cc engine and when it comes to the time of an overhaul, pull the motor and then convert it to one litre. With all these same parts fitted to a big engine you should see in the region of 80bhp and that, my friends, should make for a very quick Imp.


The installation provided a few of its own minor problems, namely that there was inadequate clearance for the forward carb trumpet air filter. It fouled the chassis member and the remedy was brought about with the aid of a hammer judiciously aimed! The second problem was the silencer: one was used on the test bed so all figures relate to a silenced engine, but the test bed silencer was much too large to be fitted in the car.
This is in fact a problem with the competition manifold we were using. For rallying where noise regulations have to be met, the Competitions Department rearmounted silencer kit, part number CTS1024 should be used. On our particular motor (which was for use in a road car though, a trip to my local Exhaust Specialists (look them up in the Yellow Pages) brought favourable results. The silencer fitted was a Bainbridge ST32 (from a 1965 Sunbeam Rapier). This provided acceptable noise output so long as you didn't pile on the revs too much in a built-up area.

Std Imp SportGE2 Cam Plus Modded Head
Cam Timing Variations
RPM 875cc 1000cc 0.060in 0.080in 0.100in 0.112in

Camshaft timing

875cc Engines with Cam (GE2)
Head and Carb Changes
RPM 2x
CD 125


Bernard Porter obtained permission from Cars & Car Conversions, allowing The Imp Club to reproduce this article (among others). It was reproduced in Impressions vol. 3 (1983), no. 8/9 (Sept./Oct.).

David Vizard is a well respected engine tuner who has written many articles and several books, including 'Tuning BL's A-Series Engine' which anyone interested in tuning any kind of engine might want to read.
Tuning BL's A-Series Engine, 2nd Edition / David Vizard, Inter Auto Book Co., Haynes
ISBN 0-85429-414-7
Lots of valuable engine info, backed up with flow bench and dyno testing.

How To Build Horsepower / by David Vizard.
How expert David Vizard builds horsepower in virtually any engine. Includes: shortblocks, cylinder heads, cams, induction systems, ignition systems, carburetion, headers, exhaust, and much more! Tricks, tips, and techniques the DIYer can use in his own garage to gain big horsepower improvements in his race or road engine.

David Vizard was Contributing Technical Editor to CCC in 1970

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File version: 29 Sep. 2013
File start: 20 Apr. 1997