How far can I push/modify a 12hp on stock internals?

reddman

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I got a racing mower for cheap, and I'm trying to squeeze as much as I can out of her with minimal investment. I want to keep stock internals, as I'm on a very limited window of time. I'm doing the standard eyebrow shave, intake/exhaust porting, milling the head down, and advancing the timing, but I'm guessing on some of this. I need some input from people who know more about this stuff.

I am a machinist and off-road race car fabricator, so some of the stuff I'm making is kinda fancy. 1-1/2" titanium exhaust, 2" aluminum intake, but these are just materials I had sitting around, so really I'm still at zero dollars invested in upgrades. I am trying to stay close to that number.

The carb was mutilated by the previous owner, so it's gotta be replaced, and I think I should be shopping for a bigger unit to accomidate the higher flow. I don't know about interchangeability though, and am hoping someone can guide my in the right direction there. I also don't know what is a safe limit on compression. By my math, the stock compression is about 8:1, and if I shave .05" off that should put me about 9:1. I'd like to go even higher, and I have access to 115 octane leaded race gas, so I want to take it as high as I can without blowing the motor up.

Here is the beginning of the titanium exhaust, and the mower as it sits now while I beef up the front end. We are doing our next race on a rockcrawling course, and I think keeping the wheels on is going to be more important than power on this one. I still need that HP though!

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reddman

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I should add, this is a 28000 series motor. No idea on age. The cylinder is in good shape.
Model: 281707
Type: 0428-01
Code: 90082331

Oh, and any thoughts on improved cooling? I'm thinking of hanging a 10" electric fan above the motor to help shove air into the cooling blower. Think that's overthinking things or???
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bertsmobile1

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The rev killer is the counterweight.
Run that engine much over 4000 rpm with it one and it will exit through the crankcase.
You do not need it in a racing engine to damp out engine vibrations.
However once removed you will need to replace the the engine mounting self tapping bolt with through bolts.
Depending upon which chassis the mower has you might also need to put a stiffener plate under the engine.
Pay close attention to the ends of the steering links, they are normally buggered and putting extra load on them will cause them to part company and the mower to flip.
Pull the plastic bushes out of the front cross member and replace them with either full length nylon or gunmetal.
If you are going to leave the cross member loose then remove the support plates & check the flare that becomes the bush, generally they are rusted / worn through.
Glue some nylon/ teflon sheet on the cross member or support plates to take the twist out of the cross member when you turn.

Some bolt it solid, however that makes the mower more likely flip on corners.

On the subject of bushes , check the one in the steering shaft, some are plastic, some are brass and all of them are buggered.
With a bit of work you can also double the fan gear.
Flip the worm bear on the end of the steering shaft over so you are using full depth gears or the first hard corner will be the last for that day.

And don't forget to have fun.
 

reddman

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The rev killer is the counterweight.
Run that engine much over 4000 rpm with it one and it will exit through the crankcase.
You do not need it in a racing engine to damp out engine vibrations.
However once removed you will need to replace the the engine mounting self tapping bolt with through bolts.
Depending upon which chassis the mower has you might also need to put a stiffener plate under the engine.
Pay close attention to the ends of the steering links, they are normally buggered and putting extra load on them will cause them to part company and the mower to flip.
Pull the plastic bushes out of the front cross member and replace them with either full length nylon or gunmetal.
If you are going to leave the cross member loose then remove the support plates & check the flare that becomes the bush, generally they are rusted / worn through.
Glue some nylon/ teflon sheet on the cross member or support plates to take the twist out of the cross member when you turn.

Some bolt it solid, however that makes the mower more likely flip on corners.

On the subject of bushes , check the one in the steering shaft, some are plastic, some are brass and all of them are buggered.
With a bit of work you can also double the fan gear.
Flip the worm bear on the end of the steering shaft over so you are using full depth gears or the first hard corner will be the last for that day.

And don't forget to have fun.

Thanks for all the info!

I will have to look up what's involved in the counterweight removal. I'm hoping to avoid getting any deeper into the motor at this time. I pretty much only have this week to finish everything. Because I haven't touched the governor yet, am I safe to assume that the RPMs will stay in the safe range?

I've got some interesting plans for beefing up the front end, but I hadn't paid much attention to the steering joints on the push/pull link. I am making bronze bushings for everything up front tonight, and hoping to make some more progress on the exhaust.

What cross member are you referring to? The entire mowing deck and all related brackets are gone, and I can't think of a cross member anywhere on the chassis. I'm at work right now, so I can't go climb under it.

The steering gears seem tight. It's a rack and radial pinion setup, no worm gear here.

Thanks again!
 

bertsmobile1

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The one the front wheels fit into.
The gear ( pinion ) that turns the fan gear ( rack ) is cut very helical sorry if Aust has caused confusion.
Without the counterweight the engine will accelerate a lot quicker.
The governor will tend to over correct so if you are leaving it in stretch the long spring the throttle rod passes through and bend new hooks on each end.
Down here they usually toss the governor and screw in the high speed throttle stop to limit the amount of throttle opening but you will need to check the local rules.
 

reddman

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Gotcha. Yeah that thing is pretty chintzy. It flexes a huge amount with the push/pull of the steering, I can't believe they went into production with that design. I've got all kinds of beefiness that is going to address the front end.

I'll have to look at the pinion if I ever leave work, but I want to say mine is straight cut teeth. Sounds like you know a thing or two about these though, so I may well be mistaken.

The previous owner attached a cable to the gov, and with a little tug it can be overridden. I may just tweak the spring like you suggest though. I did something similar on the Honda mower I actually use for my yard, and while she screams like a banshee, she just keeps on going. The racing I'm doing is pretty unofficial. The rules are what we make them. :laughing: Maybe I will pull the counterweight as you suggest. I don't know if I'll have time for that kind of surgery before this next event though.

What are your thoughts on compression? I am hoping to mill the head tomorrow night, but I don't know how high I dare go. I will be milling out pockets for valve clearance as needed, and will be helicoiling the threads for the plug so I can add a copper washer to it to keep the valves from hitting it as well.
 

sgkent

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In general it is torque when down low, and the RPM at the top that moves things. Since you are giving up a higher RPM I don't know how much the work you are doing is going to make that much more torque. You really can't flow that much more air thru it at a lower RPM and the things you are doing cater to higher RPM's. I would have looked to bore it to the largest piston the class would allow too.

Maybe you might want to also post in a go cart forum to get ideas.
 

reddman

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In general it is torque when down low, and the RPM at the top that moves things. Since you are giving up a higher RPM I don't know how much the work you are doing is going to make that much more torque. You really can't flow that much more air thru it at a lower RPM and the things you are doing cater to higher RPM's. I would have looked to bore it to the largest piston the class would allow too.

Maybe you might want to also post in a go cart forum to get ideas.

Why do you say I'm giving up the top end of the RPM range? I am shooting for a more hp oriented motor over torque. I think it'll spend the majority of it's time near the upper limit of its RPM range, which is why I'm trying to get air through the motor with less restriction. I am trying to do this on a shoestring budget, so I know I won't be getting the maximum performance attainable. This is more for racing with buddies than sanctioned events, so the rules are very basic. All my cash goes to my skiing addiction and desert racing, this is more of an experiment to play with something that I can blow up and not lose a lot of sleep over. That said, I would like the motor to keep its guts on the inside. :laughing:

I looked at several karting forums, but they are so specific with their built-for-racing 206 and Animal motors, and those that I did find which had a forum for more general B&S threads see so little traffic that I may not get a response this year. I tried on the busiest appropriate karting forum I could find, and it has 5 views in 24 hours, zero responses. The kart guys just don't seem to do anything with this family of motors.
 
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bertsmobile1

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You sure you got those numbers right
280707-0428-01 is a side banger so pushing up the compression ratio is not going to make all that much difference
A long intake tract and resonant exhaust will make a big difference but SV's breathe so badly pushing up the CR by itself is of little value and oft leads to detonation of the incoming charge by the outgoing exhaust.
Building up the head as per the KR Harley can make them flow a lot better but the angles of the inlet & exhaust tracts are critical.
A solid copper head gasket is worthwhile
One of the programmable electronic ignitions is also very worthwhile as the modules fitted as standard are so close to fixed ignition it is not funny
 

reddman

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You sure you got those numbers right
280707-0428-01 is a side banger so pushing up the compression ratio is not going to make all that much difference
A long intake tract and resonant exhaust will make a big difference but SV's breathe so badly pushing up the CR by itself is of little value and oft leads to detonation of the incoming charge by the outgoing exhaust.
Building up the head as per the KR Harley can make them flow a lot better but the angles of the inlet & exhaust tracts are critical.
A solid copper head gasket is worthwhile
One of the programmable electronic ignitions is also very worthwhile as the modules fitted as standard are so close to fixed ignition it is not funny

Yeah those #s are correct. I will be blending a very high octane fuel. I'll start with premium and cut it with 115 leaded as I dial in the timing to keep preignition/detonation in check to figure out my target octane. I may be up in the night, but I think I can control the problem with that approach. The flow through the intake and exhaust tracts looks like a joke on this motor, so while it doesn't sound like she's the best candidate for high HP, I have to assume that with my experience in aerodynamics I can at least create more laminar flow, without making them too big, and squeeze what is possible out of this thing. Because I can't find any solid info on compression ratio, I am just going to shoot for 9-9.5:1, that seems reasonable in my mind. If anyone disagrees on any of this, I would love to hear from someone with more B&S experience than myself. I'm not too proud to accept input from anyone, rather I would much appreciate it.
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