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Old 04-10-2012, 04:13 AM   #1
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Default HPI nitro engine modding

Ok, so I recently got an original HPI Nitro RS4 from a member here, and have a question about nitro engine modding.

I've modded a couple of mills already, with most noticeable gains coming from a Traxxas pro .15, and feel fairly comfortable with the processes involved.

It seems the lower end engines have the most room for improvement, which makes sense to me, but have noticed something about this HPI mill that I've only seen once before. This motor has a 3 port sleeve, but the case is only machined to match the two booster ports. My question is, will matching that port have any serious down sides? I know this is an underpowered and cheap engine, and that it would be much simpler to swap the engine out, but I get a much bigger thrill out of making power rather than just buying it, and this is just a 'basher' anyway.

I would assume that by increasing the fuel intake by making that blocked off port functional, I should also do something for air, which I think can be done by boreing out the air inlet from the rotating cylinder in the carb to the size of the carb body's venturi. Also, I would more than likely end up tinkering with the timing a bit as well.

Any ideas welcome, thanks.
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Umm, what does overkill mean again?
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Old 04-11-2012, 07:43 AM   #2
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Quote:
Originally Posted by meno1103 View Post
Ok, so I recently got an original HPI Nitro RS4 from a member here, and have a question about nitro engine modding.

I've modded a couple of mills already, with most noticeable gains coming from a Traxxas pro .15, and feel fairly comfortable with the processes involved.

It seems the lower end engines have the most room for improvement, which makes sense to me, but have noticed something about this HPI mill that I've only seen once before. This motor has a 3 port sleeve, but the case is only machined to match the two booster ports. My question is, will matching that port have any serious down sides? I know this is an underpowered and cheap engine, and that it would be much simpler to swap the engine out, but I get a much bigger thrill out of making power rather than just buying it, and this is just a 'basher' anyway.

I would assume that by increasing the fuel intake by making that blocked off port functional, I should also do something for air, which I think can be done by boreing out the air inlet from the rotating cylinder in the carb to the size of the carb body's venturi. Also, I would more than likely end up tinkering with the timing a bit as well.

Any ideas welcome, thanks.
The theory sounds like it should work .
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Old 04-12-2012, 12:05 AM   #3
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Default Well, metal dust flew...

I took the car out for a first fire-up after the tear down two days ago, and all went well. All except for the ever unquenchable thirst for power . Decided to dismantle the engine once again and grind away.

Note: I forgot to mention on my first post what this engine was exactly - it's a HPI Nitro Star .12E (not the best engine I've seen by far).

On these pictures, I took better ones, I think, of the stock parts than of the work I did on them. Taking pictures of the progress got a little difficult as I went along especially as I was getting tired (ended up seeing the sun rise).

First up is the sleeve. I did go over it once more with the buffing wheel before dropping it back in the case
This is one of the booster ports
Here it is after the first polish
The exhaust port
After widening the top corners
Other booster port
Another fang
Here is the intake port
After

Here is the main reason for taking a diamond bit to the engine: the case
This shows the exhaust port outlet and two booster inlets. As shown, the design of the engine places the exhaust and intake ports at an angle neither parallel nor perpendicular to the crankshaft.
Opposite of the exhaust outlet, there is a solid wall which blocks any air/fuel from entering the combustion chamber through the intake port on the sleeve.
This shows the blockage with the sleeve in
A pic of the solid wall with case laying down
After
Wish I had taken a picture of the sleeve in the case after work was done on both.

Crankshaft
Stock
Small change in timing on closing side and filled in with a little JB weld to help flow

Tiny 'turbo' swirl

Lastly, the carb
This shows how restricted the engine is with this carb. Of course, being a single needle carb, it isn't very potent anyway.
Not bored out all the way, but hopefully enough.

A lot of work, and keeping my fingers crossed that this engine is still useable. Will be reporting back after it either hauls @$$ or blows up
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Umm, what does overkill mean again?

Last edited by meno1103; 08-04-2012 at 05:58 AM. Reason: Added info
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Old 04-12-2012, 12:43 AM   #4
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Originally Posted by Connor B. View Post
The theory sounds like it should work .
I hope so!
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Umm, what does overkill mean again?
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Old 04-12-2012, 06:14 AM   #5
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Back when the RS4 came out, (I still have the original, been racing HPI since 96), the hot mod cheater motor, If I remember correctly, was putting the 15 FE piston and sleeve in the 12 FE case...I have all those parts..though I never got around to doing it..
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Old 04-12-2012, 07:26 PM   #6
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Originally Posted by fastharryDOTcom View Post
Back when the RS4 came out, (I still have the original, been racing HPI since 96), the hot mod cheater motor, If I remember correctly, was putting the 15 FE piston and sleeve in the 12 FE case...I have all those parts..though I never got around to doing it..
Hmm may have to look into that, sounds interesting. I have two HPI RS4 Rally's converted to on roads, one with all the graphite goodies and 2speed and the other with fiberglass stock chassis, Robinson racing pulleys and spur adapter. The frp one I used to race with against pro 4's and tc3/4s and did very well actually.
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Umm, what does overkill mean again?
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Old 04-12-2012, 08:33 PM   #7
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Well after some navigating in this website, I think I should have posted this in the 'Nitro Engine Zone' (if a mod has the time to move this, I would appreciate it).

I ran the car today. Was a little tricky getting the tune right after all the mods. It was running really rich with factory break-in settings of three turns out from fully closed, I ended up setting the carb 1 3/4 out from fully closed and the engine hit 283deg after a full tank. Gains were impressive, to me at least. The car still has factory gearing, but seemed just as fast as my modded Pro .15 in my 4tec in first gear - but then falls flat on its face. Next on the order list is a bigger clutch bell, or maybe even an overpriced two-speed
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Umm, what does overkill mean again?
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Old 04-20-2012, 12:59 AM   #8
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Well, I've ran the car some more after my last post. I found a 16t clutch bell laying around in one of my boxes (the bell came off of a SC8) and gave it a try. I didn't really want to skip from a 13t to a 16t but that's just what I had around. It lost a bit of acceleration from a dead stop of course, but it scooted pretty well through mid and top end. I'd say its running around 45-50 now, plenty of speed for this little car. Clutch grabs well and barely spins belted tires on street. I think I've found a happy medium here for playing around/'bashing'.
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Umm, what does overkill mean again?
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