Age : 25 Join date : 2013-03-16 Posts : 477 Forum Rep : 0 Location : Haddock,Georgia
Subject: BRIGGS OPPOSED TWIN PERFORMANCE PARTS&TIPS Sat Jul 27, 2013 12:53 am
Ok ive seen alot of people wanting to get more power out of there opposed twins well you can but it isn't cheap at all lol here is a list of websites that work on opposed engines but first here are some DO's and DONT's.
Q.why shouldn't i shave my opposed twin engine's heads wont it give me a boost in power?
A.sure it might seem like a good idea just because a normal flat head engine make more power when you shave the head doesn't mean a opposed twin will make more power in fact if you shave a opposed twin head your loosing power opposed twins gotta have room for the fuel to explode by shaving the heads your creating less space for the explosion to expand it could cause the engine to blow a head gasket so dont ever shave your engines heads.
Q.isnt a opposed twin the same as a normal flathead but instead it has a extra piston?
A.you cant tune/treat a opposed twin like a normal flathead because there not just a normal flathead with a extra piston there a whole different engine they are there own type of monsters just waiting to be modded if you treat a opposed twin like a normal flathead then you shouldn't own a opposed twin just remember to treat them like a different engine just like how you would treat a OHV different from a Flathead.
Q.Whats the cheapest mod i can do to a opposed twin and gain the most HP?
A.The cheapest mod would be to Port&Polish the intake and exhaust and another cheapest way is how you run your mufflers opposed twin's from factory don't like mufflers there meant to run with strait pipes just make sure your strait pipe is longer then 12" to get a opposed twin to run normal and not backfire.
Q.How could i supercharge my opposed twin?
A.Well i know of one way a Smogpump theres alot of people that say a smogpump can not and will not make boost on a engine well a few weeks ago i bought a smogpump to prove this fact i in fact made 15PSI at 4200RPM (with stock flywheel DON'T RECOMMEND) any higher i would of started to blow things apart my engine is no where near stock for a opposed twin i would recommend a steady 6psi would be fine for a smogpump there are 2 ways to run a smogpump the hard way is running it under the mower off the PTO shaft or running it off the flywheel im not gonna go into detail about how to hook it up when i get the chance to hook mine back up i will take plenty of photo's but i will tell you this the carb jets will need to be a bit bigger or the engine will run lean and it could destroy the engine.
Q.How could i turbocharge my opposed twin?
A.Its a bit harder then a supercharger due to oiling and what not best way to hook a turbocharger up is to get it as close to the carb as possible with out it touching the fuel pump on the front of the carb and get some radiator hose and attach it to the carb now for oiling this gets tricking if your engine has a oil filter then it has a oil pump most engines with these oil pumps have inspection ports right where the filer attaches to the engine you can tap into one of these ports and hook your turbo oil feed into that port and your drain into one of the engines oil inspection spots also known as oil plugs now if you don't have a oil filter on your engine then you might not have a oil pump so you would have to do the oil setup different you would hook everything else up the same but when it comes to the oiling you would need a small 12v pump a small bottle for the oil and you would run a line from the bottom of the bottle to the turbo oil inlet and the drain you would run to the top of the bottle now i would limit a turbo charger to a max of 7PSI unless you are running it with alot of mods now you will have to drill out the carb jets to run the engine rich at idle and at max throttle the engine would lean out to normal so you wont burn anything up and that's how you turbocharge a mower engine.
Q.Is it possible to turn one of these engines into a stroker what would i need and how would i do it
A.You will need a 42CI crank 40CI Rods you will have to have the top of the stock pistons milled for it to clear TDC also when you do this change the engine will suck in more air and fuel but it wont change much unless you get 46CI heads and port and polish the engine after doing all this your engine will be a stroker and it should make around 10-14 more ponies if done right.Also dont forget to buy a gasket kit when doing this ***Side note*** This can also be done with a 42CI/40CI/46CI block there is more info on this down below.
Q.How much does a stroker cost to setup and where do i get the parts
A.The pistons you can use the stock ones but you must shave off .060 on the top of the pistons for it to clear the head that's the only machining required and i cant really tell you how much that will cost because all machine shops charge different the 40CI Rods model numbers are Briggs and Stratton 393270 and are for a 16hp engine and cost $49.53 EA the 46CI heads cost are unknown but part number is Briggs and Stratton 691162 this is for one side of the engine and this one is for the other Briggs and Stratton 691163 and these are from a 20hp engine also dont forget the head gaskets and these parts you can order from briggs them self
Here is some more Stroker info i should of added a long time ago you can turn any opposed twin into a stroker my fav block to use is a 46ci block because it comes with bigger valves then the smaller 42ci and 40ci blocks
to turn a 46ci block into a stroker you will need 42CI crank 40CI rods any opposed twin piston 46CI heads and this is not a needed but recommend a adjustable carb.
to turn a 42ci block into a stroker your gonna need 40ci rods 46ci heads and any type of opposed twin piston
to turn a 40ci block into a stroker your gonna need 42ci crank 46ci heads and again any type of opposed twin piston
Note:IF and only if you have a 46ci block you can use a 42ci crank if you have a complete 46ci engine then you can use the crank thats already in it there is no need to use/buy a 42ci crank now if you have a 40ci then you will need to find a 46/42ci crank to use and just like the bigbore engines only the bigbore come with bigger valves but from what ive been told the 46ci heads have a bigger chamber in them ive never really noticed but thats what ive been told-Mowbandit ***PLEASE READ*** it is required that you have the top of the pistons milled for clearance or else they will hit the head and will cause the engine not to even turn over i do want to warn you over reving a opposed twin after doing the stroker conversion can cause a piston to cave in or crack.
These are the only websites i know of that deal with opposed twin parts.
***Just a side note i cannot and will not be held responsible if anything was to happen to the person modding the engine or the engine its self this is a DO IT AT YOUR OWN RISK always be careful when running a engine with out a governor and running a stock flywheel stock flywheels are known for breaking apart or cracking Ive never seen a flywheel explode from a small engine and i don't think i want to have a great day.***
EXTRA:There are some opposed twins out there with a 3.57" bore very rare and hard to find now the most common is 3.44" if you are unsure the size then measure the bore before you continue.
Last edited by MowBandit on Tue May 27, 2014 11:20 pm; edited 4 times in total
Age : 24 Join date : 2013-07-26 Posts : 221 Forum Rep : 0 Location : North Fort Myers, Florida
Subject: Re: BRIGGS OPPOSED TWIN PERFORMANCE PARTS&TIPS Sat Jul 27, 2013 10:03 am
I've heard that OHV rods are stonger than stock but is there any out there that will fit a model 42?
Join date : 2012-12-21 Posts : 2100 Forum Rep : 2 Location : Norway, Europe
Subject: Re: BRIGGS OPPOSED TWIN PERFORMANCE PARTS&TIPS Sat Jul 27, 2013 10:37 am
Nice write up Jay,
And would I never trust a stock rod. You need billet if your going to high revs.
Os it possible to put dual carbs either motorcycle or mower on an oppy?
Racers do this. Build a separate intake manifold for each cylinder, and hang your favourite carbs. Use a tandem throttle cable off a bike, ATV or sled, or rig your own linkage. The trick is to try and synch the throttles as closely as possible.
I could see a couple of Mikuni slide carbs waking one of these buggers up quite nicely.