Hemi GX140 build

That wasn't on nitro was it? Or was it? Been following this thread since I'm the other guy with the Chrysler Hemi painted engine. I really want to hear that thing rev burning liquid horsepower. I've played with the stuff myself, mixing up my own 1/2A Texaco fuel for SAM antique model airplane contests. I run 35% in a decompressed Cox .049 reed valve engine and manage 4 to 5 minutes run time on 5 ml of fuel, in a very light 1/2A Playboy Senior. That thing is almost out of site in 5 minutes...oh wait, this it the minibike forum. Anyway, I did manage to watch all 50-some episodes of the Surfers. My wife wasn't pleased but I was thrilled. Those guys were great! As a reader of Hot Rod magazine since the early `80's I've heard bits and pieces of their story but not like that. Thank you for that link. Nice work on your engine. It's great to see folks thinking outside of the box and fabbing up their own parts. I will continue to follow this thread with much interest (and pretty much everything else you choose to post).



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Thanks for the kind words and I'm glad you like The Surfers series too. ;)

That wasn't on nitro was it? Or was it?
No no, I was just making sure it would run and used some heptane since it's not as stinky and persistent as gasoline. The engine needs a fuel system and has to be moved to a more secure mount before I can go snapping the throttle.


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Also on my mind is that running nitro will require great care with the way the ECU works. The ECU handles dwell and I found some instances where it would/could fire the coil and cause kickback when the engine is just stopping. It's not like breaker points where you can smoke the coil by leaving the key on if the points are closed. In controlling spark, the ECU has a failsafe to prevent that where it will cut coil power (which fires it) if spark is not needed within a given time. If the engine has a cylinder full of fuel and has just stopped when the coil fires off some confined nitro, that's bad. I could mitigate it somewhat with full EFI by not having fuel injected below some stall RPM, but the coil fires no matter what and for now a carburetor means no cutting fuel.

Things I wouldn't have thought about in any other instance.


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It's been a while a here's why:

It's a dual standoff/shower/etc injector setup that bolts onto the carb. The injectors are always in atmospheric pressure, so no manifold-referenced pressure reg is needed. The injectors have separate feeds so running two fuels is possible. They are also angled toward each other slightly. Obviously the center hole still needs to be bored to size and injector holders needs to be made.

I was going to wait until it was finished and on the engine, but I don't know when it will be finished. Hoping for the next few days, but other stuff might get in the way.


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Maybe after I get this thing straight. ;)

My laptop battery was on borrowed time and it quit just as I was trying to get a data log. What I did catch is that the engine is pulling really poor vacuum; only about 3-5"Hg. I'll have to check the MAP line, but suspect it's pulling air through the carb somewhere. It's a really junky HL-324 clone and never sealed around any of the gaskets.
Do you have a TPS on the carb? How does it know how much the carb is opened? It sounds like its running just fine. Looks like a fun project.


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There is no TPS, but it will likely need one in order to get acceleration enrichment to work. I found out the vacuum was weak because I was going to use the MAP signal to get some acceleration enrichment for better snap, but found out it was operating in just one or two rows across the table regardless of throttle or RPM.


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I'm going to play with the tune to see if I can improve the idle vacuum before resorting to a TPS.

It may not be an exciting development, but this has been 13 years in the making:

Back in 2006 I got some upgrade MAP sensors for use with a MegaSquirt-I like I'm using now. MS1 has factory support for four common sensors, but these are non-standard. Making them work involves tweaking the firmware, recompiling it, and tweaking large config files in the tuning software. There were no write-ups or support to speak of back then(and really still aren't). I revisited several times, but lack of knowledge and necessity kept shelving the project.

Since then I've learned a lot about sensors, electronics, and computer code. This week I buckled-down and got it all figured out.

The original sensor pegs at 250kPa absolute. Standard sea-level pressure is 101.3kPa, so it can read 150kPa above that or to about 21psi of boost.

The new one does a bit more:

I actually do have a combination of little turbos and a blower that could be combined into a compound turbo twincharged setup capable of pegging this sensor on this engine, but not so much hardware that could. Parts don't just break at that level, they land in the parking lot.