Part 7. The engine…strip and specification

When the big GSX1100 engine we’re using in our Katana left the factory 32 years ago, it was by contemporary standards, already a monstrous powerplant. You’d have struggled to find anything better in a road bike in 1982 and even by today’s standards, it’s far from breathless.

But, the thing with GSX1100 motors is that whilst they’re good as standard, they’re even better when they’re tuned. Amazingly so. Perfect for a racebike? We think so, which is how we ended up going down this route in the first place.

When Motocatman stripped our motor down, what we found was pretty much as expected – although with black crankcases the expectation was that the crank would have been welded from the factory – it wasn’t, and pointed to the fact along with some poor painting on the crankcases, that this motor was potentially made of components from slightly different models, not originally fitted in the EZ model we’d nicked the motor from.

The stripdown was also just in time to save the clutch from destroying the engine; it had a loose centre nut and knackered basket – any more serious running and it would have given up, catastrophically. Doubtless this fatigued combo was behind the low rpm death rattle I’d heard when I’d first picked the bike up from the seller (note – so they don’t “all do that”, just the ones that have a clutch about to explode)

Nonetheless, overall, as it had run to 95bhp on the dyno, aside from the clutch issue and the lack of correct crank it was a good motor.

Well, the engine got stripped – right down to the last nut and bolt, every single part of it came apart. Paul Boulton ( took care of this in a matter of 3 hours, including wrestling the engine from the frame.

Soda blasting – old stuff made new. A&C near Aylesbury are top people!

Just like the frame, the first stop for the cases and block of the stripped engine was Soda Blasting. Again A&C Soda Blasting near Aylesbury provided the service and we got every external part blasted back to fresh white metal.

So – stripped and cleaned, but what were we going to do to it?

The head

Well, the specification for the head was something we never got as far as having to work out for ourselves. A tuned head from the slightly later GSX1100EF model became available from Nick Pepper at Suzuki Performance Spares. The head was £800, but although that looks a salty price the spec was just about as good as it gets for an Endurance race engine.

Our head was newly built for another road racers second engine, but never fitted after a change of plan. I’m really happy we got the chance to buy it, as we’d never have had the budget to build it to this spec! It’s fresh, unused and it features;

  • 1mm oversize inlet valves
  • 1mm oversize exhaust valves
  • heavy duty valve guides
  • recut valve seats
  • Ray Debben ported

Completing the head were our camshafts, which Grumpy1260 had reground to a “mild” spec by Kent Cams, and the longer adjuster screws needed to adjust the higher lift cams, which have had the base circle removed enough to not work with the stock adjusters – Nick Pepper at Suzuki Performance Spares again providing these.


We were unsure if the head had been skimmed or not, so we did a volume test once the motor was built and got to 10.75 to 1, a sensible amount, so we left thoughts of any further skimming alone.

Remember – endurance engine, no need to be the fastest, it just needs to run and run. We were originally planning to run 12:1 compression, but backed off the idea once we’d sweated the ratio we had and had taken a little counsel from a wider range of GSXperts.


The block – Wiseco 1170

The head we’re using is from an EFE engine which  originally packed 1135cc compared to our 1074cc, and with the oversize valves in this one, the guys at Grumpy1260 made it clear we’d need to have an overbore of at least the popular 1170cc size using Wiseco’s to make all the hay possible from that specification.

So that’s what we did.

Race ready dome is great, but not as good as the little velvet bags that each piston comes in. mmm – thanks to Grumpy1260 for the supply of our 1170 Wiseco pistons.

Again the Grumpy guys sorted the engineering part of the upgrade and got the block bored, and also provided the parts. The 1170 kit comes with 4 Wiseco pistons (supplied in lovely little velvety bags!) and a head gasket that works with the slightly larger bore.

The clutch

There is a modification to the clutch backplate made by the guys at Grumpy which gets rid of the clutch nastiness I’d experienced on the motor when I’d first heard it – the rivets on the original backplate wear, the clutch cushion springs rattle and I’m assuming that once the balance starts to wander, the clutch centre nut starts to vibrate loose – mine had, so it’s a reasonable assumption.

The fix is to weld the rivets and use heavier duty cushion springs. It costs £175, but sorts the problem for once and for all.

Needless to say, as we like the idea of a functional clutch, we did it.

The Grumpy clutch mod. Get one, or rattle your engine to destruction. Simple.


The Crank

The later model GSX’s (supposedly any with black crankcases) came from the factory with welded cranks – our cases were black but our crank wasn’t welded 🙁

If I could weld at all, I’d like to weld like that. Our (second) crank. Thanks to Suzuki Performance Spares.

So, one thing we needed was to get a welded crank, thanks again to Nick at Suzuki Performance spares who had a pre-welded crank with the less favoured but perfectly adequate 492 rods (493’s are the ones that seem to be the most lusted after, I still know not why)


Heavy duty camchain, APE heavy duty tensioner blade, APE slotted cam sprockets, APE manual tensioner. Got to have them. All from Grumpy1260.

Doesn’t Camellia sound like a Sexually transmitted disease, or is that just me? Anyway, they make great camchains (I hope), so don’t take the piss.

Primary drive gears

A conundrum.

Standard PDG’s = helical cut, see pic of clutch basket above (upside = smoother transmission, downside = axial backlash that can’t always handle big power and higher workload without catastrophic failure)

OR – the Upgrade – straight cut PDG’s (upside = can handle higher power and higher workloads without the axial backlash reliability issue)

BUT- We didn’t have the £1,000+ spare to make this mod.

Whether we should have added these or not is a hard one to juggle with – opinions amongst tuners varied, and naturally, like anyone building an engine that is meant to last as well as be fast it’s easy to recommend a modification that all but guarantees reliability. But, at a price, and that was a price we just couldn’t stretch to.

But, on the upside one common thread amongst the GSX experienced was that the threshold for adding them is probably 150bhp or so – we’re (probably) going to be 10% or more under that.

Also in favour of us not needing them is a similar big GSX engine built by Grumpy1260 and used at the Classic TT last year which made it home high up the rankings on the very helical gears we’re worried about – that’s the TT – surely a reasonable test of their ability to do the job asked of them?!

Helical primary drive gear – boo. Adequate though. We hope.
Robinsons Suzuki – seals, pins, bolts, nuts all from stock for a 32 year old bike. Awesome.
Goodies for our motor – all from Grumpy1260 aside from head and oil filter cover, which is a genuine Lucky7 item.
Wiseco piston kit – the deal that never stops giving – gaskets included.
An oil filter cover CNC’d with a logo that I designed for Lucky7 a few years back. This makes me happy. It’s the little things…


Checked over, deemed perfect, put back in. Simple, and necessarily cheap too.


I plumped for the DYNA-S ignition based on recommendation from the guys at Grumpy1260 – Gary (or Grumpy to give him his full name) has been running a Dyna-S ignition on his ET since the week after Jesus was crucified, ok, maybe not quite that, but it’s been working perfectly for an amount of time that ‘s unfeasibly long. What we need in Endurance are simple systems that work – the Dyna-S may not be as flashy as the Dyna 2000 systems, but for what we need – perfect!

These systems retain the standard mechanical advance/retard unit and are really just a simple change to the pickups behind the right hand cover.

We also sensibly picked up a Dyna rev limiter from the Grumpy boys to work with the Dyna-S system, as otherwise a missed gear could see all the lovely oversize valves we’ve got in our head making an unscheduled appearance in the outside world! No no no, we don’t want that.

High flow Oil gears

GS750 engines feature a higher ratio for the gears that drive the oil pump, which are a straight fit  – essential in a tuned GSX1100 motor? Everyone says so, everyone.

The problem with that, is that the now well known GS750 oil pump gears modification has seen stock of said OE gears on both the new and used markets dwindle to a trickle.

Good news for us though was that Star racing make gears to do the same job, and I was lucky in timing my need for gears to perfection, with Suzuki Performance spares getting them in stock almost to the hour we needed them in the motor.


We’re precluded from using flatsides by the rules, leaving just the one sensible option – Keihin roundslide CR’s.

These are sex in metal. Carbs rock, I’ve caught this strange lustful desire to look at carbs from a few of my mates, Steve and Scottie both like a good perving over a carb and although I never used to feel any sense of arousal when looking at the things that simply mix fuel and air, that’s not the case any more.

I have had plenty of bits and bobs done by perfectly well by Allen’s Performance, so my first (and last) port of call for the carb shopping was them.


Steve the owner is also a racer, and I’m sure he won’t me saying, a fully paid up member of the carburettor anorak club. A good bloke to engage on the subject of the carbs for our old Kat.

It seemed from my research that tuned air-cooled Suzuki 1100 motors could wear carbs any size from 33 up to 40mm plus, but considering everything, including the availability, I plumped for 35’s. This will give us a decent mid-range and driveability, whilst maybe sacrificing a little top end – remember, it’s an Endurance engine. Widespread power = good, peaky power = bad.

We also have the fuel consumption equation to consider, so a carb at the smaller end of the available spectrum makes a difference to how much fuel we have to carry, how heavy the bike is and of course how fast our refuel stops are. It all needs thinking about, endurance is not about just being fast on track.

How fast does it go mister?

I dunno yet, it hasn’t been on the dyno – but when it has I’ll let you know!

I reckon….within a sniff of 130bhp on MSG Racing’s dyno. Pure guesswork based on common sense, wishful thinking and a bit of need.

This 130bhp at MSG Racing could be as much as 143bhp on at least one other Dyno that we have a benchmark on, and would be mid 130’s on most others. MSG’s dyno is low reading, which won’t make you happy about your power figures until you get to the track, then you realise your fewer ponies are bigger and stronger than the other boys!