Isuzu G-series Twin Cam

Been toying with the idea of building a DOHC for a while now, and seeing as i should have my S15 sold and my Turbo Gemini running within a couple of months, im planning on building something abit pokier for my TE sedan. You know what they say, once you have power its hard to go back...

If or when i do this, i want to get this completely right and dispel the myths that DOHC's cant make power or are useless. Which means research research research. Ill start by listing what i want to do and by listing the "rumoured" mods ive heard of being done.

The plan is to run either a G180w or G200w in naturally aspirated form with a fuel and ignition ecu, some sort of reprofiling of the cams, headwork etc etc. Ive been looking at quad throttle bodies and have come to the conclusion the most economically viable and easiest to mount are those off a late model motorbike. I can post a link which shows in great detail the sizes of butterflys, injectors, motorbike type etc they come off.

From my guess work/theory, id be looking for atleast a 45mm butterfly. Yes/No/go bigger?

Headwork is a given - head ports are pretty large in standard form and it goes without saying that the manifolds will be modified to match - my query is in what particular area to modify the head to improve flow. Ive heard that due to the angle of the valves and the lift of the cams, larger valves are unfeasible. Ive also heard of shit about raising the inlet port path - this reeks of cost. Any ideas?

Cam profile - any ideas and what to go with? Keep in mind the car is to be daily driven so although i can put up with a bit of lumpiness and doughiness down low, it cant be ridiculously undriveable and reasonable economy is a factor [id be happy with 13L/100]. Rev limit can be in excess of 8500 - remember thought that i dont want it too lumpy.

Distributor will have to be a CAS type to be able to fire coil-on-plug style with LS1 V8 Coils. Any ideas in which direction to head here?

Compression ratio increase would be a nice thing however for the cost of forged pistons [1300] id like to avoid this. I HAVE [which is why ive mentioned this] heard of a particular engine using Toyota 2T/3T pistons. Can anyone specify or speculate if this is likely to increase the CR via higher crown height, and if/or it would need alteration to prevent valve contact.

Extractor design, due to the lack of aftermarket equiptment will have to be made. Ive noticed most AUS-modified DOHC's run 4-2-1 systems similar to SOHC Geminis whilst most JAP DOHC's run tuned length 4-1's. Can anyone point me in the right direction here - again i can live with a lack of low down torque as long as top end performance benifits.

The aim is 140-150rwhp which i feel is not unrealistic.

Open to discussion, at the very least it could yield good information for others.

Nick-

very feasible. the standard internalls are alot stronger than gemini internals so i wouldn't worry about the toyota pistons. standard comp is about 9 something to 1. so you could deck the block abit and get it upto 10.5:1 or 11:1. throttles would be bigger than 40mm less than 50mm what ever you can get your hands on (my inlet ports were 42mm and my inlet manifold and carbies where about the same). get a 3 angle valve job ported head. cam's will cost abit because there is two and cam bearings are pricey (try and save the old ones). you can get get offset dowels to adjust the standard cam's will save you heaps and any bigger cam's might make it less enjoyable as a daily. electronic ignition 2.5inch exhaust with extractors. and you'll easily make 150hp at the wheels. but the upside is you'll have plenty of torque.
if you want to spend a bit more offset grind the crank bore it out 40thou lighten and balance and put some bigger cam's and you'd be making plenty of power.

when you assemble it take special care setting uo the chain tension. they sound great when these motor's are put together right but one i had, the chain had stretched and there tensioner couldn't take the play out of it and it sounded ordinary.

Sounds like engine will be pretty nice. Certainly the power is possible. I understand these engines are good for 80kw standard, so thats almost 110hp with no mods. I think most of these engines getting around arnt setup properly and are under performers. There was a street legal one of these in Adelaide on ebay last year with 280rwkw (boosted of course). As for tips on mods, ive tried to do mine exactly right with the efi, but its completely stock. Will be good to see how you go with yours.

I've written elsewhere about 220+HP G200Ws by merely adding twin 45DCOEs (F-2? emulsions), L1 cams and a slight port polish.
New pistons are required for above 180HP.
If you have a 2:1 extractor length of about 24-26" (from the standard exhaust manifold) into a 2" system, the torque should be well over 300Nm from 2000 RPM.

Pete, im not to worried about torque - im used to driving peaky 4cyl's. Hell, my "torquey" SR20DET only gets going once theres 4000rpm on board, and fook me does it tear yourballs off when it gets there lol.

Id rather go 4-1 if its of benifit to top end power.

Also another thing Pete, have you ever had L1 cams running in a DOHC to know how lumpy/much of a bitch it is? Only "heavily" cammed car ive driven was a mates Datsun 240K Sedan with an EFI L28, Autronic ECU, Datrally74 cam etc etc. Made 135rwkw, idled at 1300 and was only lumpy on low throttle openings under 2500rpm - something that wouldnt really cause dramas on a Gem considering how short most of the gears are.

Ive been keeping my eye out for a good, cheap DOHC in need of a rebuild but nothing so far

antus wrote:I think most of these engines getting around arnt setup properly and are under performers

Yeah im of the same belief as you dude. let us know how you go with your setup, any information is good information.

if your going all out ive got a bare 1.8 block and head no valves or springs and the crank needs grinding.and ive also got a crapper that i found out has been sitting in the rain for 5 years with the head off it looks like the pistons are rusted into the block but i can probably russle up all the bits you might need to come to wollongong and pick out all the bits.

If you can lob me a pm with what state each motor is in, how complete they are, what they need, what version they are [ie oil filter location] etc with a price then come tax time we might be talking turkey.

Also, heres the quad throttle info for those interested.

http://msefi.com/viewtopic.php?t=6627

I've been told that while L1 cams are way too lumpy for sensible road use, in G-DOHCs they are quite acceptable.
I suspect that "220HP" G200W in Tassie had the popular DCOE45 & L1 job (though 48DCOEs & Delortos are better still).

disregard this post i wasnt paying attention.

if u need parts let me know...got lots of blocks/heads/cranks/pistons here

want serious stuff? i got a prepared g200 block here, with a bullshit head(bought from a engine builder was his "frankenstein motor") the ports hve been cleaned up and the chambers WELDED up and clearenced for the valves for monster comp

the cams have huge lift and require flat tops to run it, this would b a unstreetable motor tho

farrrk with quadsor webers would sound hot!

cheers

GeminiCoupe wrote:

antus wrote:I think most of these engines getting around arnt setup properly and are under performers

Yeah im of the same belief as you dude. let us know how you go with your setup, any information is good information.

75rwkw
Dyno sheet here:
http://www.symons.net.au/gemi/gemitwincam_small.jpg

Im allready considering what a cam could do for it now

What sort of mods besides the Kalmaker? Fuel/Ign or fuel only, what dizzy etc. Standard exhaust manifold or extractors?

Would be fun with that sort of power

No mods. Dizzy is a camira dizzy cut through at the bottom and welded on to the other half of the original dizzy. Has a spot for the ignition module that way. Its fuel & ignition, standard exhaust manifold, 2" exhaust. Stock as a rock, just custom fit to the car and the ecu. Has all sensors, idle air controller, map, o2, speed, temp, tps, a surge tank down the back on bosch fuel pump. Camira injectors. I really didnt expect it to rev out to 7000rpm, but according to the dyno sheet, it did. I was expecting more like 5200 or 5500.

yeah they rev pretty well. the zzr dash has the redline set at 7000 rpm.
If you are going to mod it the first thing i would be doing is upgrading the throttle body its the most restrictive part of the inlet tract. How did it sound?

You're taking me for a spin sometime antus and yeah, I've heard of worked ones revving like crazy - 8000rpm+...

Dynovidorban