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littlemarv

There's no replacement for displacement

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littlemarv

Last year, I was at the local engine machine shop, and we were looking up some specs for a 12 horse cast iron Briggs. I vaguely remember seeing something along the lines of the 10-12-14 horsepower models all shared the same block ( and maybe even piston). The HP determining factor was governed speed?

At any rate, I have a 13 horsepower Briggs that I am going to rebuild. My main question is: Can I up it to 16 horsepower, and how do I do it? It does have the raised cooling fins on the cylinder head, I also remember that having something to do with it.

And how do you tell how much power your engine is putting out? Guess I've never seen a small engine dyno, but haven't been looking for one either....

The engine numbers are: 302431 0155-01 7704111. Looking at the manual, that means its 30 cubic inches. Can anyone date the engine?

Thanks for any info, I may have to stop up to the engine shop and look at that dang book again.

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Chris727

The first two digits of the code indicate the year. It would be a 1977 engine. I believe @BLT can narrow the production down to the day. As for the rebuild it can be done. The 12-13hp (30 CI) shared the same pistons and the 14-16 shared pistons (32 CI). I believe you can just have it bored to the spec of the 32 CI engine. I think the main differences between the 14 and 16 were achieved by different adjustments of the carb and governor. There may be differences in the porting of the cylinder heads also as if I recall. 

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BLT

The date code goes by year 77, then month 04 and finally the date, 1.  There is no individual S/N. Engines were batch built. Stroke is the. samw and I never checked to see if synchro weights are the same.

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littlemarv

Pulled the motor out a 7013 Baron, so that would jive with years of production. Seems like that book said there were engine "families" that shared parts, but I kind of figured I would have to jump from one to the other. If anyone knows about carb of governor adjustments or settings, I'm all ears. Or eyes, I guess. Boring and putting in a new piston is the easy part.

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wwbragg
1 hour ago, Brettw said:

Ecoboost would take issue with that

VW performance builders would agree.  You get more bang for the buck by adding high performance heads and carburation than you do by increasing displacement.  With that behind us, @littlemarve will probably have to re-jet the carb since that extra 2 cubic inches will want more gas.

I repowered a Simplicity 725 with a 13 HP HF motor.  It would only run with the choke half closed.  So I bought a set of carb jet files and just filed out the jet step by step until it ran better.  

 

Edited by wwbragg
expand response
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ShaunE
13 hours ago, wwbragg said:

With that behind us

Oh it's not behind us by a long shot. xD  

Carefull will those words or you'll end up with 47 pages of more scientific facts then there even are scientific facts. Similar to the "What's the best oil" question.

Two topics everyone is a Genius in.

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ShaunE
16 hours ago, littlemarv said:

Can I up it to 16 horsepower, and how do I do it?

As @Chris727 said, you can bore it to the correct piston size.  But the 16 has a larger intake valve than the 12 horse so you may not be getting the required fuel to get your full 16 ponies. Will you notice it?  I doubt it.

I personally agree with the title of your post & have said that statement many times.

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littlemarv

Food for thought. Why bore it and put a bigger piston in it, if it's choked down with a smaller intake valve? Maybe I should just rebuild it as a 13 horse. The plan  is to put it in a dedicated loader tractor. Really all it's going to be doing is driving the hydro and a hydraulic pump for the loader.

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wwbragg

 

4 hours ago, ShaunE said:

Oh it's not behind us by a long shot

I hate it when people say, "That, having been said..."  So I put it "behind us" instead.  But that supports my hypothesis that the extra displacement will be hungry and need more fuel.  If you are going to bore it out to the 16 HP specs, why not enlarge the valve sizes while it is at the machine shop?  I say valve sizes because if you have more fuel coming in, you have to allow for more exhaust to get out.  Think of it as a miniature "little deuce coupe."  "She's ported and relieved and she's stroked and bored"

But we are scaring @littlemarv away.  

3 hours ago, littlemarv said:

Maybe I should just rebuild it as a 13

Rebuilding it as a 13 would be quite adequate if the cylinder is still within tolerances.  But they no longer make the oversized pistons for that motor so if it out of spec, you can either take it out to the 16 HP specs or scrap the thing. I have a 300421 (12 HP) that needs a refresh.  But I also have a 16HP piston on the parts shelf so we will see how that goes.

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kwt

Same exhaust valve. Intake MAY  be different depending on year. I'm willing to bet that a 13hp has the same valve as a 16hp.

 

Same camshaft.

 

I would roll with rings if the bore is good enough. If not, bore to 320000 specs, as those pistons are still available.

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RAC
21 hours ago, littlemarv said:

And how do you tell how much power your engine is putting out? Guess I've never seen a small engine dyno, but haven't been looking for one either....

Yep dyno (dynamometer), they make them for the small engines too. As for the no replacement for displacement thing... All things being equal, same cylinder filling efficiency, same valve timing, same ignition timing,  same compression ratio etc. bigger will be better. The thing is that all things are rarely equal.

For example, built a .060 over 14hp K321 Kohler once that made just a shade over 25 horse power on the dyno, with a stock lift & duration cam and stock size valves (it was a small dia. exhaust block to boot). There were K341's running the same class that were not making that according to the fella who was running the dyno. So how did the 2 cu. in. smaller engine make equal power with the same carb, cam and smaller exhaust valve? The swept volume filling efficiency was better for the smaller engine because they both were limited to the 1 inch carb bore, same intake valve diameter and lift.  So not only was the mathematical compression ratio slightly higher  because we used the small valve block significantly reducing the combustion chamber size (popping the piston up out of the block .200 added a little more compression ratio), but the real running compression ratio was significantly higher because with the same size carb and same size intake valve the smaller engine was getting the cylinder filled that much better. Re-degreeing the stock cam, a way longer connecting rod, stiffer valve springs, and a bunch more ignition timing all added up to the 25 hp. I think it peaked out in the high 4,000rpm range. Torque was not adversely effected because the K321 and K341 share the same stroke.

Want to get more power out of the smaller motor the easiest way? Slightly bigger carb (bore), multi angle valve job & mill the head down to reduce the combustion chamber clearance over the valves to increase the compression.

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wwbragg
44 minutes ago, RAC said:

Want to get more power out of the smaller motor the easiest way? Slightly bigger carb (bore), multi angle valve job & mill the head down to reduce the combustion chamber clearance over the valves to increase the compression.

https://www.bing.com/videos/search?q=i+told+you+so+dance&view=detail&mid=AF45FB540C15B815BA5CAF45FB540C15B815BA5C&FORM=VIRE

 

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Bill725

If you want to rebuild the existing engine and the bore is bad, then I would bore cylinder from 3-7/16 / 30 CI to 3-9/16 / 32 CI and use a standard 32 CI piston P/N 390364 / 792365. I would also have the engine machine shop change the intake valve to a 32 CI, P/N 261070 / 394435. I would have new valve guides installed and obviously a valve job. The camshafts are the same. I would check the 32 CI head gasket, P/N 270662 / 692231, profile against the cylinder head. It sounds like you have a 32 CI cylinder head, otherwise you will need the 32 CI cylinder head, P/N 691712. The existing crankshaft rod journal will either be polished if in good condition, or ground 0.020" undersize. I replace the crankshaft bearings. I would replace the piston pin, P/N 299691, connecting rod either way, standard rod is P/N 299632, 0.020" under rod is P/N 390306 and is NLA, But, there is one on eBay for sale for $70, and they are becoming harder to find. I would take the IPLs and compare the carb jets.

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TimJr

Has anyone actually overbored one that far?  Is the cylinder wall really that thick?

All the horsepower tricks are great, but you start messing with compression much on an inefficient old flathead and you end up needing some expensive fuel to keep that engine happy/alive.

When I built my pulling tractor with the iron 16hp Briggs, I CC'd the head and everything to figure out the compression ratio.  With milling and being overbored, I raised the compression ratio about .75.  Didn't seem like much, but with the inneficient combustion chamber of a flathead, it would pre-ignite/detonate on 87 octane if it was hot outside and the engine hot.  Granted, I had also made a modification to advance the timing ( I  had the flywheel repolarized to run a Magnetron), so that surely aggravated things too.

That being said, we have a local gas station that sells 90 octane Rec fuel - no ethanol in it.  It isn't cheap, but it is all I will run now in my stock tractors.  Kind of throws my whole " don't modify it because the fuel will be expensive" comment out the window.  With no timing mods, running 90 - 93 octane would probably keep an engine happy.  110 Racing gas ended up keeping mine from sputtering and hammering due to pre-ignition.......sure smelled good!

As a side note about horsepower ratings.  I can't remember the exact model numbers anymore, but back in the later 90's Briggs had 12hp and 14hp Vanguard twins.  They were identical engines except for one part number - the sticker for the shroud that said either 12hp or 14hp.  Learned it from a garden tractor puller.  He ran in a max 12hp class.  Well, per what all the factory literature said about the particular engine model/type/code for the engine on his tractor, he had a 12hp engine.  He used to carry around a bunch of Briggs literature to keep the rules guys at bay....

Kohler did the about the same thing too.  I would have to look up the numbers again, but back in the late 90's, 18hp Command v-twins were almost identical to 20hp Command V-twins.  My 1997 Sovereign is an 18hp engine by number, but it is now missing the one part that made it 18hp.  The stopper screw that limited the throttle from opening all the way.  Basically it couldn't open up the throttle enough to hit 20hp with the screw in the linkage.  Bore, stroke, compression ratio, cam and carburetor were all the same part number.  Not every spec may be that way, but I compared parts lists one day many years ago to see for mine.

Sometimes it is just about marketing strategy or needing a different price point.  It's OK until they sell you an engine that doesn't make the power they advertised.  Then everyone gets sued like they did a number of years ago and we don't get horsepower ratings anymore.  Most of the OEMs got slapped for selling engines that they were rating by putting them on the dyno with no air filter, no exhaust and who knows what other little "tweaks" that a production engine/real world engine wouldn't have.

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wwbragg
1 hour ago, TimJr said:

Is the cylinder wall really that thick?

The diameter of the 300421 (12 HP) Briggs is 3.4375.  The diameter of the 32 CI engine (Briggs 326434 or similar) is 3.56.  That is only a difference of 1/8th inch or 1/16th on each side.  1/16th is .0625 - - a little over 60 thousandths on each side.   I have blades in my feeler gauge thicker than that.

 

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RAC

Have to admit, haven't done any over bore stuff on a Briggs except for a 7016 motor. But 1/8 overbore is a bunch. The aforementioned K321 that was .060 over ended up developing a crack between the fins about 3 down from the top.

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Bill725

The B&S CI 30 cu.in., 12 & 13HP and 32 cu.in., 14, 15 & 16HP engines have the same 3.25" stroke. The 30 cu.in. oversized pistons and intake valve are extremely hard to find. I am very lucky to have been able to find all three, 0.010", 0.020" and 0.030" over, along with two sets of intake and exhaust valves. MikeES has over bored a 30 cu.in. engine from 3.4375" to 3.5625", the same as a 32 cu.in. Hopefully he will provide comments. So, if you have the machine shop, install a 32 cu.in. intake valve, don't worry about exhaust valve size because the piston will push the exhaust gases out, the camshafts are the same P/N so they have the same lift and duration, use a 32 cu.in. cylinder head and use the same main jet in the carb, why would it not run?

Edited by Bill725
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wwbragg

Another option is to bore it out and have a sleeve pressed in.  That can take the cylinder back to the original specs.  You  can then rebuild it with the original sized piston and rings which are still available.

 

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SmilinSam

The 14hp in a 3314 I used to have was a 30 cu in engine.....Basically the same as the 12hp dual synchro engines used on the B-series tractors.

 

In all these years of doing these tractors all the 15 & 16 hp Briggs were 32's and all the 12, 13, & 14hp Briggs were 30's. At least all the examples I have come across...

 

Or did I miss something?

Edited by SmilinSam

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wwbragg
On 11/14/2019 at 7:14 PM, TimJr said:

Is the cylinder wall really that thick?

This began to worry me so I pulled the head on one of my 300421's.  It is a standard bore as indicated by the unstamped piston.  I measured from the cylinder wall to the outside if the cooling fin and found it to be 1 1/18 inch. (lets call that 18/16ths.).  Then I measured from outside the cooling fin to the OUTSIDE of the cylinder.  That cam to 15/16ths  - - - a difference of 3/16ths.  If I bore it out to the 32 CI spec I have to take out 1/16 on each side (as discussed above) leaving 1/8 on each side.  So the final question (ok - maybe not final) becomes, is a cylinder wall of 1/8 inch thick enough? 

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Edited by wwbragg
reorder pics

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RAC

Another question is do you have that thickness all the way around? Probably not. Pretty common to see a lot of core shift in those older castings. Sometimes you can tell by looking at the bottom of the cylinder to see if the bore is really in the middle of the cylinder castings.

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Chris727

This 30 to 32 cubic inch rebore has been achieved successfully several times by club members. @huffy has done two of these engines and in this post, @RayS points out that the valves superseded to the same numbers and should even be the same. 

https://simpletractors.com/forums/topic/50709-reboring-300421-12-hp-into-14-hp-engine/?tab=comments#comment-121676

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