Hemi GX140 build

65ShelbyClone

Well-Known Member
#1
I'm starting this so I don't clog-up others' with my own stuff.

I've had this GX140 for about 25 years and it was always used for experimenting. It was disassembled 10+ years ago and I thought it got thrown out, but low and behold I found the parts! The original head was ported, cut, and modified into scrap, so what now?

Enter the hemi Predator head. Stock 140 valves are 24/22mm; the hemi's are 27/25 and the ports are larger to boot. By my estimation, with valve springs it could move the 140's power peak to over 7000rpm without porting. So I got one. It will be styled after the second-gen Chrysler Hemi in orange with a black valve cover.

Anyone else thinking about this should know that the compression ratio will only be about 6.8:1 and there will be no quench because the piston is so small.




I milled the letters off of the valve cover so it wouldn't mess up the wrinkle finish.



Some more mockup for better throttle response.
 

delray

Well-Known Member
#2
nice, yes you better start your own thread. don't want to piss off any old farts on here. am kind of surprised that is the second only engine I have seen painted up like a hemi Chrysler....cool. well with 6.8 compression you can run 87 octane all day.:rolleyes: cool if you could find a charger minibike chassis without a motor and paint it factory blue they came in. maybe some redline tires too.
without looking up the bore and stroke? what is it on a 140 motor? not a honda guy...sorry.
do I see a belt clutch. are you going to run it?
 
#3
Very cool! Too bad someone doesn’t make an aftermarket tin with ‘HEMI’ stamped in it.

Dumb question... what would prevent facing off the head to up the compression on a GX?
 
#4
Btw, I don’t think you’ll mind me polluting g your thread with this ... I love that pic you posted! Even with the gas mask on, you can see the “what the...” look on the driver’s face! Talk about perfect timing!
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#8
I've always loved that vintage pic. I used to have the issue of HR it was in. I picked up one of my favorite lines based on that photo, for lads with poorly thought out prospective builds. "That motor will be wicked, but rough on tires. Rod bolts go right through em" Nitro is wild stuff. We fixed a pallet of TF billet heads that had shoved exhaust valves out the spark plug hole

LOVE the Gen 2 Hemi colors! Firepower was the script on my Gen 1 50's Chrysler Hemis.

Valves pretty close to the bore on that 140?
 

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#9
I've always loved that vintage pic. I used to have the issue of HR it was in. I picked up one of my favorite lines based on that photo, for lads with poorly thought out prospective builds. "That motor will be wicked, but rough on tires. Rod bolts go right through em" Nitro is wild stuff. We fixed a pallet of TF billet heads that had shoved exhaust valves out the spark plug hole

LOVE the Gen 2 Hemi colors! Firepower was the script on my Gen 1 50's Chrysler Hemis.

Valves pretty close to the bore on that 140?
Nice pics! The cross ram remjnds me of an old friend’s 413 Wedge, except those were more extreme. Probably the craziest factory intake I’ve ever seen!
(Not my friend’s 413):
 

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65ShelbyClone

Well-Known Member
#10
Yep. Pissed on will get a person hurt.
Piston in the wind can do that too. Means the motor came unglued.

cool if you could find a charger minibike chassis without a motor and paint it factory blue they came in. maybe some redline tires too. without looking up the bore and stroke? what is it on a 140 motor? not a honda guy...sorry.
do I see a belt clutch. are you going to run it?
I do have a Zebra frame that needs some reconstruction. It's at least from the right '68-72 time period. If only I could find a piecrust slick about 15-18" tall. BxS on the GX140 is 64 x 45mm (2.520 x 1.772"). It's effectively a smallbore GX160, but there were a lot of other differences. You do see a belt and a clutch, but it's not a belt clutch nor a clutch belt. ;)

Very cool! Too bad someone doesn’t make an aftermarket tin with ‘HEMI’ stamped in it.
You read my mind.

Dumb question... what would prevent facing off the head to up the compression on a GX?
That's a good question actually. Valve seats get in the way. It looks like maybe 0.100" is possible before hitting them, but the chamber is huge to begin with so lots of milling still won't get it comparable to a 14cc inline head. Kart racers complain about having to weld it to do that which bring its own set of complications. It's also possible to get a tall piston with a thick dome that could be machined to clear the head with a lot of popup.

This isn't getting used, but one like it could be made to work with a custom rod.


Valves pretty close to the bore on that 140?
I looked into that before getting the head and they're actually not. I think a 28.5 intake would fit fine.
 
#11
Wow. Roomier than I'd thought. Full bore is 3.2cc/mm so even at diminishing returns, a .100 cut would help a lot, I'd think. BY mentioned -4cc from .070-.080
SB Mopar seats get skimmed and leave valve below deck all the time, and you can always cut deeper to get the 45 back if it gets too close.

I was looking at domed pistons for a hemi, and saw some tall/blank one in the Asian scooter market.
 

delray

Well-Known Member
#12
zebra frame will still be a good candidate for your engine build...

how much over lap does the hemi head chamber have on that small bore. will you have to use the hemi gasket or something in between? for example gx 160/bore gasket or larger? kind of sounds like you will have alot of unwanted cc's. nice if you can find a way to tighten up things..

whats going on with the small flywheel? with that on it should spin up fast. just wonder if you might get a unwanted vibration(might?) what kind of clutch or drive unit are you running on it.
I really do have the bike for that engine. unfortunately it's all original and staying that way.
 

65ShelbyClone

Well-Known Member
#14
What are you suggesting?



I was reluctant to post that because I'm not sure when or if it will happen, but it's fun to look at. I have a jug of liquid supercharger here and might try that first.

how much over lap does the hemi head chamber have on that small bore. will you have to use the hemi gasket or something in between? for example gx 160/bore gasket or larger?
There's only about a mile. This is the 212 gasket:


A stock GX140 gasket is nearly the same. I think the gasket bore is huge because Honda might have used the same part on the 160 (68mm bore) and 200. A smaller one probably won't seal with the stock hemi head because the fire ring will only be supported on the cylinder side.

whats going on with the small flywheel? with that on it should spin up fast. just wonder if you might get a unwanted vibration(might?).
It's small, it has fins, it was free, it was easy to retaper, and spins ~8000rpm in the original application. I expect it to kick and snort pretty bad, especially with a pull starter. o_O The 140 has a smaller taper anyway, so not many options outside of a custom part.

what kind of clutch or drive unit are you running on it.
Probably a DIY manual clutch of some sort.

Fun fact: the hemi heads come stock with a three-angle cut on the seats.
 

65ShelbyClone

Well-Known Member
#17
1.) I didn't realize the 140 had the smaller taper like a 120.
2.) Cheap, light options, with curved timing and lighting output, are just a backplate away.
1.) Well, I think it does. Others on 4cycle have mentioned it and I can't find GX160/200 dimensions to check. The 140 crank isn't forged either, just cast.
2.) I thought about doing that with a bike flywheel and stator to get a charging coil.

1.) Does that come as a fuel additive?
2.) How safe is it on the fuel pump?
1.) I suppose diluting it to less than 50% would technically make it an "additive."
2.) Methanol precautions apply.
 

delray

Well-Known Member
#18
wonder how well that extra over lap effects the the flow in the chamber? it sure can't help any. nice if you could of close up that area.
 
#19
1.) 1.) I suppose diluting it to less than 50% would technically make it an "additive."
2.) Methanol precautions apply.
I assume you are talking about the real deal, nitro, and using methanol... correct?

What sort of percentage (nitro) can you get away with in these small engines, and what is the main limiting factor?
 

65ShelbyClone

Well-Known Member
#20
wonder how well that extra over lap effects the the flow in the chamber? it sure can't help any. nice if you could of close up that area.
True, but I'm not going to worry about it because the short-turn is an especially weak point with this head anyway. I have a spare that will get massaged while this one is in use and there will be a comparison along the way.

In the picture of the blower there are some 26lb springs visible. I installed those yesterday and my thumbs are still reminding me.

1.) I assume you are talking about the real deal, nitro, and using methanol... correct?
2.) What sort of percentage (nitro) can you get away with in these small engines, and what is the main limiting factor?
1.) Correctomundo.
2.) I expect to find out. Realistically the limit is probably whatever methanol power level is splitting the engine blocks currently.


Don't buy expensive methanol like that in the photo just to burn; it was expired stock from an analytical lab at work and destined for disposal. Why waste it, amiright?
 

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