Isuzu G-series Twin Cam

Been a long time coming, finally have the engine in the car so though I should start sharing with the guy's that have helped.

Slow start but sorted the starter motor so I could get a custom set of pipes made.

The high torque starter with the straight up and down arrangement has really opened up the room to fit the extractors.

Good info. I've been using reduction starters for years but of course I have Belletts, Florians & Wasps with the olde G161-OHV engine (else G200Ws) with their leftside startermotors and those vehicles don't have the same clearance problems as Geminis.
Apart from only drawing about 140A instead of nearly 250A which is much kinder to the battery, the Jackaroo/Rodeo 4ZE1 etc reduction starter will still crank acceptably with a battery voltage of 5.2V! (That was when my cold G200W fired. IMO that was unbelievable and it was the day I scrapped my sequential CDI build despite cracking a simple and cheap circuit.) As to ignition below 6V - I've also been using RG dizzies for years (Gemini/GZ diizzies fitted with early RB Gemini reluctor guts & igCoil with shaft-cut & gear to suit the non-GZ G-series engines) though igniting below 6V still blew me away... shit man, even I only design for 8V-16V operation!).
The reduction starter is also far superior for cranking off railway crossings & busy streets (maybe if you run out of fuel since Isuzus engines are too reliable to breakdown) or out of bad bogs.
Meanwhile you can sell OEM LHS GW or non-GZ engine starters for a small fortune to the unknowing or clearance challenged Gemini owners whilst picking up used reductions for ~$50 from wreckers.
FYI - There are inline reduction for Geminis (that are smaller than the OEM non-reduction starter) but use those with metal gears - a place in Castlemaine Victoria used to have them (~$200 each last decade).


My understanding is that fitting Gemini diesel crossmembers (to Geminis!) means better clearance though engine to the LHS engine bay may still not be as good as the aforementioned older Isuzus.

BTW - if you have the upright oil filter assemblies (from 117s), I'll gladly swap with Gemini/Piazza forward down types. I even have one with an oil cooler tap.
Unfortunately we all want the Gemini/Piazza sump except for Florian/Wasp owners that want the 117 "steering box" type sump.


One tip - try to use the original G200W exhaust manifold - there ain't much better. You need a good extractor designer (that understands the Isuzu twncam GW to equal or better.

Another tip - if you want low down torque (which is what I recommend for these engines - why use an old 8-valve for high RPM power?) then a 2:1 extractor length from the st'd manifold puts peak torque at about 3.000 RPM which is great for road use - ie, peak torque (peak fuel efficiency) at 3,000 RPM.
That - and having suitable fuel (eg, twin Webers/Delortos) and inlet manifolds - is all a 400Nm G200W needs. IE - mine was 300Nm @ 2k & 383Nm @ 3k RPM. That effectively means TWICE the vehicle acceleration - at least to 3k RPM (but torque is reasonably flat above 3kRPM - ie, 270Nm (100kW) @ 6k RPM.


PS - I saw recently on another forum that people seemed to question 400Nm "standard" G200Ws. I'm not sure on their motives since the build is relatively well known and very similar to the G180W tunes that the HDT and Peter Brock used. Maybe it's another case of the new not knowing the old? But some of these people also think or thought that Bellett GT-Rs have "PR91W" chassis numbers (and are only white or orange with long taillights); that breaking a steering column pin means loss of steering; and that an oil pump determine oil pressure etc etc so Dog knows how they think or research!
You should find various independent references to similar performance on this site but similar refs appear on other Gemini etc sites.

Good pick, it is a Gemini. The reduction starter is a custom from the Castlemaine guys ( a bit more than 200 bucks though). I do have a genuine Gemini twin cam starter but wanted more clearance. As you said being able to "wind" you way out of trouble is useful with a good starter, I would also love to take another step in the future with a high compression and high advance style build so a good starter is a must.

I deliberately stuck with the standard Gemini cross member as I wanted to keep the flexibility of going back to a standard engine if I could ever bring myself to move the car on. They are getting older and more valuable, slowly and I do have a great 2 litre SOHC that would do a job for me still. The plan is tarmac rally for the car and still be registered.

Happy to discuss further on the Oil filter mount swap. I am keen for part to stay in circulation for people who love the cars they came out in. Will PM you latter on with photos. I have a few engines collected now and I know that I have one Upright oil filter assembly. Sumps, not sure but they look like Gemini ones to me although one of the engines I picked up is an EFI G200W out of a 117 Coupe.

I have a couple of the cast iron manifolds but as I have stuck with the Gemini Petrol Cross member I can't use them. The outlet and the engine mount don't fit in the same space. That is why I have a mate that can make exhausts making a new set of extractors. Long, long conversation, I have some knowledge of inlet manifold length tuning and design but I must admit I am still not sure on the 4:1 or the 4:2:1 debate. I have read your previous advice and is still being discussed, space might answer the debate for us.

I have 45mm dellortos I am building for the engine and I agree that high RPM is not the way to go for a street car or a standard version of these engines. I must say though it is almost impossible to find people who understand this engine. I have an old race engineer friend who has built the BDA cosworth twin cams and the 2TG Toyota twincams and only a couple of G200Ws and he even acknowledges the uniqueness.

I grew up in a mechanics workshop and I learnt early on to listen to all and then filter. Mostly I trust experience and the the dyno. I have great faith and expectations for the G200W.

Thanks for the advice. I hope to keep the discussions alive as others have, there is a lot about these engines that needs to be recorded and I can't read Japanese!

In keeping with low down torque I'd keep the 4:2:1. 4:1 is for high RPM and for that I'd suggest another engine - maybe a Mitsubishi 4G63(?) but out of a Hyundai - add $1k cams ex-USA ($2k in Aus - not that I know what the cams are) which apparently boosts it to 500HP.
Of course if you want to stick with Isuzu then the G200W is a good option.

All G200Ws were EFI. The 117 G200W should have a winged sump. It's rear has clearance for steering-box linkages.

Maybe you extractor builder can copy the manifold dimension off the OEM manifold. (I've been meaning to measure the manifold port lengths... maybe next millennium?)


I presume you have the G200WE Engine Manual PDF which is in Japanese? (Maybe symons.net.au-manuals still has a copy.) I find it of limited value except for clearances & specs etc (though I did love its ignitor circuit - a cute (negative slope) zero-crossing detector - not that that is relevant when using RB Gemini etc ignitors).

I'd be wary re GW advice. I've seen it all - Celica cams, Ford buckets, plus the usual oldskool crap like crimping oil lines for oil pressure since we all know more power needs higher oil pressure. (The latter is total crap and is proven by higher performance engines that manage to survive; crimping oil lines DECREASES oil pressure and delivery!)
And I often hear how "unreliable" these engine are... LOL. Of course that's AFTER people have worked on them.
And some of the rebuilds I've seen - whether the ~$5k of work to one G200W whose head I use as a door stop or the "hot" G161W I bought which - let's just say I use its block as a boat anchor (radius reliefs removed from the crank and hence no oil pressure and failing big ends; 1,000 km old pistons seizing because someone linished the piston skirts for balancing but didn't round off the edges; blah blah.
These days people should be wiser - but they ain't. I still hear of people fitting bigger injectors to improve performance (without EFI mods); not washing bores with soapy water; fitting resistors or voltage regulators in MAP or AFM lines to richen mixtures (the same circuits are used by those wanting lean mixtures!); and a gamut of other incredulous things.

The GW twincams are quite unique. I baulked when my Guru said the G200W needs twin 45s. For a 2L? But Guru said the G200W should have twin 48s or even larger but that the added expense wasn't worth the performance. (This was back when both EFIs & Webers were far more expensive.)
It was Guru that told me what to do for my street torquer. I got the twin DCOE45 Webers choked & jetted as specified and specific limited edition inlet manifolds and a new 2:1 2" exhaust.
At first I thought the dyno tests were a bit low until I realised its rear-wheel nature - eg I was told the mods would yield 150HP (but with ~400Nm torque!!) and I only had 135HP - the same as the std G200W - but then factor in transmission losses and Guru's HP was conservative.
One amusement was that the dyno tuners - a local mob renowned for their race engines - thought their dyno was bung... 300Nm @ 2k RPM from a then 20 year old 2L engine? Come on!! But the dyno proved accurate.
In retrospect I understand them. Though I appreciated my Florian's ability to leave almost anything for dead I didn't appreciate the significance of doubling the torque (with no "engine" mods; and especially at half the OEM peak torque RPM!!) until some years later. If someone told me that I'd be very skeptical - in fact some still seem to be despite that they should know better. (Like I said, it's similar to the old HDT builds.)

One problem was twisting gearbox output shafts (I'd typically use Gemini 5-speed boxes with 0.782 5th with ZZR etc bells) but I intend future G200Ws to use MUA boxes (4ZE1 Rodeos etc). I didn't do burn outs but my drag technique was off the line in 1st and then straight into 5th (usually because that was sufficient to keep up with opponents before finally flooring it once they backed off).
I also used a heavy duty Rodeo clutch but will use Exedy clutches in the future. (Remember - a "clutch is torque" - not power.) And I'll go hydraulic - those Gemini etc clutch cables broke too often.


BTW - despite using the older points dizzy I used to get ~10L/100km - 27mpg normally and 24mpg when leadfooting around town. In retrospect that too is bluddy amazing considering the performance but pea torque is best fuel efficiency.
Future G200Ws will have RG dizzies (of course!) though I intend to go EFI (Delco/Delphi) so I can omit the reluctor thereby making RG dizzy construction easy - a straight substitution of parts with no drilling or extra cable exit.

Yeah the advice I got from the exhaust guy was pretty much the same. 4:2:1 probably 1 5/8" primaries around 19 inches long (will measure a cast iron one when I get the chance out of interest) and "long" 26 to 29inch secondaries probably 2".

There is no doubt about a GW I want to build a strong car that was like a ZZR in its day.

I do have one winged sump so the rest are Gemini which is handy, I have also got a copy of the manual and you are right the clearances and sizes are useful.

Yeah the advice has been interesting, I plan to use an internally standard G200W to start with and once I have a good feel for it work on a new one. When I say standard I do have a set of vernier cam gears coming which typically help get the cams in the correct spot which has always helped in the past.

We have talked before about the capability of the head and there is no doubt the standard one can breathe. I have a set of 45mm Dellortos sorted as they are a fair bit cheaper than the 48's and I am after torque and drivability on the street in the first instance. Sounds like the same path as your original.

I have a "close ratio" box out of a 117 coupe I think bolted in nicely at the moment. Wasn't aware of the output shaft problem, I only really run poor tyres or on dirt in the old Gemini so I never had issues might have to look into that further as I intend to run R types or slicks depending on the situation. Yeah looking at Exedy myself, forgot about the broken clutch cables, got good at changing them on the side of the road though. Might be a bit tough in the Gemini with twin carbs.

I have almost finished the dizzy. It is a GZ body with a electronic points replacement kit, cut down shaft and the gear off a busted GW early model electronic distributor. I started down that path as I knew how to modify the GZ ones well.

I know there has been a lot of debate about camshafts too on the forum but is there a known profile that works with the standard engine? I have a lead on some clever guys in NZ that will do a custom profile based an extensive list of dimensions and characteristic of a particular engine set up was considering seeing what they can offer.

Photos of the intake manifold and the distributor will follow once I have them back from the engineer.

Oh no - not a bluddy Pertronix points replacement kit? I've been caning them for years - even before the RG dizzy... way before.

If you go for the torque build I wouldn't worry too much about verniers etc, the st'd timing marks should do. Sure, later you might get into that but I reckon you'll be very happy with the 300-400Nm torque for a while. I actually tried downtuning my Florian after a while as it proved a bit too powerful but that might change with LSDs & a lower ratio diff (it was probably 3.9 but might have been st'd 4.1 but I'd like to try a ratio of about 3.5).

Great that you have Delortos. They usually pull a few more horsies than Webers but getting jets etc used to be a problem. Plus I don't have the Delorto tune. I don't know if there is some conversion table to match Delortos to Weber emulsion tube etc specs.


There is a common cam grind that boosts the HP from ~150HP to 220-230HP but that requires forged pistons since st'd G200W pistons only handle (or are only rated for) 180HP, however a few have said st'd pistons were fine for 230HP.
To get the full ~220HP a slight port job is also required.