Author Topic: Super charged 42CI opposed. Valve springs, head gaskets, ect...  (Read 1781 times)

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Offline mr.modified

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Super charged 42CI opposed. Valve springs, head gaskets, ect...
« on: November 05, 2019, 08:09:34 PM »
I'm building a 42CI horizontal shaft opposed for an off road tractor. The plan is to use an AMR300 supercharger at about 10psi boost. It seems like maybe not as many people are racing these engines anymore and parts supplies are cutting back a bit. Although ARC still has rods and flywheels. I have two main questions:

1) I found on here that EC carbs has some heavier valve springs #260. They are listed at 45lbs installed and 115lbs at .575". I was just wondering what the stock briggs valve springs are rated at? I plan on running a stock cam so that should be nice and easy on things, but still I don't know what's really aggressive and what's not. I don't want to wear everything out right away if I don't have to. Anyone know what kind of RPM you can get with the EC carbs springs with stock valves?

2) Does anyone know if there is anyone who still has copper head gaskets for the opposed? I'm a bit worried about using stock head gaskets with the supercharger, even though 10psi should be fairly mild from what I can tell. I would think copper ones would be more reliable and would prefer something better than the stock ones if I can get them.

Thanks for any info, -Don



Offline BIG AL 202

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Re: Super charged 42CI opposed. Valve springs, head gaskets, ect...
« Reply #1 on: November 08, 2019, 12:17:53 PM »
A supercharged Oppy? The rods won't take that kind of boost. Do you realize that's kind of a fragile engine? I'm not saying don't try it, more power to you but it is going to need lots of TLC. There are better engines to make power. With that said copper gaskets installed correctly should hold, should be available at EC carb.
RPM's good question, the stock cam will sign off between 3,000 and 3,400, maybe lower.
45 pound springs are a little heavy but will work.
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Offline Mower Maniac

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Re: Super charged 42CI opposed. Valve springs, head gaskets, ect...
« Reply #2 on: November 08, 2019, 11:22:43 PM »
Copper gaskets unlimited has gaskets for the opposed. You could take out the inner springs on the EC springs to make the spring pressure a little more mild.
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Offline BIG AL 202

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Re: Super charged 42CI opposed. Valve springs, head gaskets, ect...
« Reply #3 on: November 09, 2019, 11:49:08 AM »
That's a good idea.
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Offline mr.modified

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Re: Super charged 42CI opposed. Valve springs, head gaskets, ect...
« Reply #4 on: November 15, 2019, 06:40:11 PM »
Thanks for the info.

Yeah, the stock rods barely handle stock use I know. I'm gonna run ARC rods.

I checked out copper gaskets unlimited, don't see gaskets for the opposed listed in the lawn and mini bike section but I'll send them an email and ask. I also found a guy on ebay who said he would make them if I sent a pattern.

Are the lifter/tappet bosses likely to break at higher spring pressures? I know go kart racers back when they used 5hp flatheads would weld up the one. The exhaust side I think was pretty thin and would break. On the opposed it's a fairly uniform block, maybe they are alright.

I don't plan on running really high rpm if I don't have too, but probably a bit over stock. Mostly I'm worried about the pistons holding up, I would think they would be the first thing to go in my setup with the supercharger. We'll see I guess. I was going to make an oil sprayer to squirt the bottom of both pistons, with a cooler in the line to get rid of some of the extra heat. I've seen people break blocks on drag race setups but not sure if these bigger engines are prone to that or not. I just picked up an aluminum welder so I'm set to do some serious hacking if needed.

Here's what I'll be doing with it. I'm the guy with the orange simplicity. Building a new machine for next year. My current machine is a bit too tipsy and a bit too gutless.


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Re: Super charged 42CI opposed. Valve springs, head gaskets, ect...
« Reply #5 on: November 15, 2019, 11:44:34 PM »
After watching 1/4 of the video......well......I donít think a supercharger will be much benefit.
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Offline mr.modified

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Re: Super charged 42CI opposed. Valve springs, head gaskets, ect...
« Reply #6 on: November 19, 2019, 08:42:10 AM »
I figured a couple seconds between each obstacle. Plus quick power to rip up over things right when I want it instead of trying to anticipate it ahead of time. And in some of the courses there were really tight corners where you had to back up if you didn't take it exactly right. If I had something with more power, I could pretty much lock the brakes up coming into it and then just dump the clutch and burn around like a go kart. My new build is going to be much lower center of gravity and with smaller tires.

Also, I'm going to have full suspension. I don't think a tractor built on a stock platform will be able to touch it. Except maybe in the mud, it won't do as good as the guys running big aggressive rears.

And if all else fails, there's the intimidation factor too. Gotta make the other guys try a bit too hard and mess up in a few spots......

Here's the build thread if you want to check it out, I have a drawing on there showing the rough engine and transmission placement.
http://www.atltf.com/t6224-simplicity-broadmoor-r-s-project#top