Briggs 8HP mods (Customized Racing Tractors)

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  #1  
Old 01-27-04, 05:32 PM
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Thumbs up Briggs 8HP mods (Customized Racing Tractors)

Ok, here is a question for you 4 stroke gurus, as I am a 2 stroke kinda guy (get your mind outta the gutter!) We have 4 lawn tractors that we use for mower racing, and are getting about 25mph out of the stock engines..what can I do to increase the ponies in these engines?? The only thing we have done, is get rid of the govenors, and straight pipe the mufflers. What else is there to do? Increase compression? Advance timing? Help me out here please!!

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  #2  
Old 01-27-04, 06:44 PM
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There are a lot of things you can do, but the big question is what will the rule allow? It's a little known fact that Briggs & Stratton is the largest manufacturer of racing engines out there. A big market for them is the go-cart crowd. I've seen mods that will increase the HP by about 80%. I would think that there has to be some kind of rules to keep the playing field level. If the rules are few and loose and the pocketbook is deep you can whip the ponys hard. All it take is the bucks & time.

With the riders in the enclosed pictures I bet you don't spend much time mowing the grass.
 
  #3  
Old 01-27-04, 06:51 PM
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Jughead you are correct, not much mowing getting done...for the class we are racing in, there are no rules..we make them as we go..lol..we are the only ones racing...I am going to keep two mowers stock, just so we can go race in the city if we ever feel the need. But for the time being, I would like two of them to be modded...I am building a third, a rear engine Case, that will sport a 377 Rotax sled engine....should be good for fun!! We built a drag mower as well, with a 500cc AMF sled engine...it will do about 70mph...great for the drags!!

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  #4  
Old 01-27-04, 07:21 PM
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I don't think your last machine (in the picture) could be considered a lawn mower any more. I didn't know they drag raced'em. Kinda neat. The boys around here do tractor pulling with their lawn machines. Some of them put Harley motor cycle engines in their Cub Cadets, ect. When you start doing that you better have a good set belt. Check the Briggs & Stratton web site. They have a whole line of racing engines. Then check the go cart racing boys and they will give you a whole list of things you can do to those Briggs racing engines. I'm sure you can even run nitro and a blower if you wish to get real serious. How soon will it be before you need a sponsor to finance your lawn mower??
 
  #5  
Old 01-27-04, 08:57 PM
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Jughead, yeah I have checked out those Briggs sites and others...but the problem is they are dealing with Horizontal shaft engines, ,and I am dealing with a vertical....not that the same principles do not apply, just that the parts are not available...I am a builder, and I like to do these mods myself....I like porting and polishing, and I do lots of that to my chainsaws...but I need pointers on the 4 strokes...
 
  #6  
Old 01-27-04, 11:19 PM
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I've saw some of the fancy chain saws at the lumberjack olympics in Wisconsin years ago. Some of the same things apply to the 4 bangers. Using a tuned exhaust won't help you on a four banger, but I can think of a few things that would. The biggest way to increase HP is to increase compression. Lowering the deck height will give you a compression increase. Look as some of the go cart web sites to give you a good idea about how much milling you'll have to do. If I remember about 0.010 should give you a modest boost. Of course increasing compression will mean you'll have to go to aviation gas or alcohol. Try av gas first because going with alcohol will mean changing carbs. If you increase compression too much then your crank, rod, and piston start to take a beating. You might be able to find lighter, stronger pistons and rods that can be used from a racing engine. Pretty much all the issues that effect the air induction on a two cycle engine will apply to a four cycle as well. Increasing compression is standard on the two bangers as well. I saw someone who had a blower that would work on a small engine. That would be an obvious way to whip the ponys. I really haven't been confronted with the problems trying to find high performance parts for a vertical shaft engine. Most of the time I'm trying to find ways of keeping an engine glued together for 100 years, not 100 seconds.
 
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Old 01-28-04, 01:37 AM
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Hi Dennis!

Lots of things can be done for mo' power. Shaving the head can be done easily. A belt sander positioned in a vise upside down with the trigger locked down will shave a head well. Briggs uses thick enough head gaskets to compensate for minor imperfections, but the sander really does a good job considering.

The carb can be jetted, but you'll want to experiment to determine the best orfice sizes for the way your engine is built. As mentioned, you can port and polish, but the gain there is minimal. Getting rid of the pipe threads in the exhaust port will help the most here. Weld a flange on the end of your exhaust pipe and bolt it onto the block (there are bolt holes for this) instead of threading it in.

Installing larger valves can be done in many cases if the block allows.

Playing with the valve clearances...mainly the intake valve, can produce different effects. I like somewhere around .007 to .010 for intake valve clearance.

A poor mans blower can be made out of an A.I.R. pump used as a smog/emmisions device on most american mid '80's cars. This will require carb jetting.

Changing timing is as easy as removing the flywheel key and turning the flywheel a bit. Somewhere around half the width of the keyway would be a good place to start.

Do away with the points if any of your engines are equipped with them and use solid state ignition.

You can weld up and grind the camshaft to custom lift and duration specs. If you want to increase lift, but keep duration basically the same, you can grind off the backside of the cam lobes a little, but you will need to make sure your valves will seat far enough down to keep the desired vlave clearance. You may have to grind valve reliefs in the head if you have shaved the head and increase valve lift.

I've heard of using only one compression ring instead of two, but have never tried it. (to reduce friction).

Take the choke out of the carb for increased airflow if you can start it easily enough without one.

Removing the charging stator will also help some.

If I think of anything else, I'll post again. Hope that gives you some ideas!
 
  #8  
Old 01-28-04, 08:26 AM
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Thanks Cheese, I am still open to any more ideas if someone has them. I am also thinking of lightening the flywheel (keeping it balanced), was considering knife edging the crank lobes, I was also thinking of honing the cylinder out oversize and only putting in a new compression ring..using the old rings for less friction, thus less heat.

Does anyone have any ideas on a "tuned" pipe for the 4 strokes?
 
  #9  
Old 01-28-04, 04:00 PM
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Tuned pipes won't help a four stoke engine at all. In order to understand why you have to understand how a tuned pipe system works and why it WILL help a two cycle engine generate more power. Following the power stoke in a two banger the piston opens the exhaust port on the way down and there is a short blow down period when the exhaust gases are vented to the exhaust system. The combustion chamber/exhaust system should be close to atmospheric pressure at end of this time period. When the intake port is uncovered the decending piston acts like a pump and the fresh fuel/air/oil mix is pushed into the combustion chamber at more than atmospheric pressure behind the exhaust gases. The objective of this whole operation is for the induced fresh fuel/air charge to force out the spent exhaust gases by action of the decending piston and complete the scavenge process. Theory is one thing, the practical side is another. A small portion of the new fuel/air mixture always gets carried out of the exhaust port with the spent exhaust gases during the scavenge process because you can't have the timing perfect at more than one exact speed. If you could force that small portion of fuel/air mixture back into the combustion chamber while the exhaust port is still open you can increase power because you will have more fuel in the combustion chamber to burn. That's the function of the tuned pipes on the exhaust system. The idea is to have a 'discontinuity' in the exhaust pipe that will cause a reflected pressure wave to push that lost fuel/air charge back into the combustion chamber. That's why tuned pipes aren't straight. The point where the pipes go from tapering out to tapering in is where the 'discontinuity' is located. Of course, that tuned system will only work best at a single speed. The best option would be to match the tuned exhaust speed at a optimal speed on your power curve to get the performance you want.

Now consider the four stoke engine. After the power stoke completes and while the piston is still coming down your exhaust valve opens and you start the blow down/exhaust process. This process continues the whole time the piston is coming up and the combustion chamber is open to the atmosphere. Sometime before the piston reaches the top your intake valve opens and for a short overlap time both the exhaust valve and the intake valve are open. This is the only instant during the cycle when a tuned exhaust reverse pressure wave could communicate with the incoming fuel/air charge but you wouldn't want it now because it would tend to blow the fuel back into the intake manifold. That is the exact opposite of what you are trying to do at that instant. What you need for a four cycle is a blower on the intake manifold to force more air into the combustion chamber. The more air you can get in means the more fuel you can burn which means a bigger whip on the ponys.
 
  #10  
Old 01-28-04, 06:37 PM
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I understand the theory of 2 stroke pipes 100%...I build them all the time.

There is also a reason for certain lengths of pipes on 4 strokes, I am just unsure of it. It will depend upon the cam, the size of the valve, disp. of the engine (I think), rpm, weather, and type of fuel as well...

Scavenging of exhaust gasses is a well known theory, the right shape and length can help pull exhaust gasses from the engine...hence the theory of headers I do believe...I am just wondering if there is a formula for 4 strokes like there is for 2 strokes.

I am just looking to see if anyone here has experimented with this stuff...if not...guess i will be the guinea pig...lol
 
  #11  
Old 01-28-04, 07:59 PM
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There is some tuned pipes installed on four stoke engines, but they STILL won't do you any good. Again you have to understand how those pipes improve performance. On an 8 cylinder Chevy engine, for instance, the exhaust for the right bank of cylinders is collected in a common cast iron manifold. Each cylinder's exhaust is in communication with the other three. When you do this each cylinder is subjected to back pressure and the piston has to work harder pumping the exhaust out of the engine against a pressure that comes partly from the other cylinder's exhaust. This takes power, this power could be used to drive the wheels, instead it's used to pump the exhaust from the engine. Now, to eliminate this loss of HP you install a header. Each header has a separate pipe coming from it and these pipes combine in a single pipe that goes to a muffler and or a catalytic convertor. You tune these individual pipes so that the individual exhaust pulses from each of the cylinders won't interfere with each other, in fact, they aid each other by lining themselves up when they are again combined in the common exhaust pipe and tend to pull themselves out of the exhaust pipe. Each individual pipe coming from the header, going to the combiner, is a tuned length. At a particular engine speed the exhaust pulses will combine in such a way as to enhance the exhaust flow from the cylinder. In that way back pressure is reduced in a band of speeds. Since we are talking single cylinder engines here this whole theory of headers and tuned pipes goes out the window since there is only a single stream of exhaust pulses to work with.

Take a look at a drag racing engine. You always see short individual pipes coming for each cylinder. Some of those boys have million dollar budgets to work with. I'll bet that their rules say that the pipes have to be a certain minimum length to protect the driver from the exhaust and keep from setting the engine on fire, but I've never heard of them doing any fancy tuning of those pipes. If anyone would be engaged in fancy and/or expensive exhaust technology it would the nitro methane drag racing crowd.

Exhaust gases will only move due to a pressure differential. Furthermore the flow will be from high pressure to low. I know that I'm stating the obvious, but I sometimes see proposed designs that seem to ignore those simple facts. Sometimes you see some screwball who thinks he can get water to flow uphill by itself with out any outside help. Trust me, when I was going to school I made some assumptions like that on some tests and was told that I better go back and reread the first law of thermodynamics.

Having said all that there are only a few things that I can think of that will help. I'm sure you've heard them before. First keep any bends in the exhaust line very gentle. Sharp bends cause a pressure drop that depends directly on the speed of the gas. Secondly keep the pipe as short as is reasonable. Pressure drop is also a function of length. Thirdly insulate the pipe if you can. Hot gas is less dense (and easier to pump) than cold gas.

If a specific length of exhaust pipe is an advantage in a four cycle single cylinder engine I'm unaware of it, but I am always willing to be educated. Racers always seem to persue the last ounce of power from their engines and sometimes even manage to advance the art & science of engine design. If you want to try out some new exhaust system designs go for it. LOL
 

Last edited by jughead; 01-28-04 at 10:20 PM.
  #12  
Old 01-28-04, 11:56 PM
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Hey Jughead...I thank you for explanation on headers...it makes perfect sense...I did not understand that part before.

However, I still beieve there is something to the length of single exhaust pipes...maybe it is something in my pipe...lol...but I don't think so....I will get back to you with some more research...our V8 hotsaw head wrench had the pipes (8 individuals) built specifically for the particular engine....I will be talking with him tomorrow...and possibly he can get on here and explain what I don't understand...

Hopefully we can both learn something from this...

I did find this link for exhaust "tuning" ( and maybe "tuning" is the wrong word)

http://www.nightrider.com/biketech/c...austlength.htm
 
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Old 01-29-04, 09:40 PM
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Great explanation Jughead! I now have a better detailed understanding of tuned exhaust. Thanks!

From what I understand, there are minimum lengths for exhaust pipes for optimum performance. The theory is that cold air can get back to the valve if the pipe is too short. Some engines have exhaust valves that close a degree or two after TDC on the exhaust stroke, which could allow a small amount of air to be drawn back toward the valve. There should be enough hot exhaust gas in the pipe to allow this. It makes sense to me, but I've never spent much time pondering the idea.
 
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Old 01-30-04, 07:19 PM
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Your cold air theory has some merit. Air density depends on temperature. Cold air would be harder to pump than hot exhaust. Engine exhaust is primarily carbon dioxide and water vapor at about 700 to 800 degrees (if my memory is good) on a small engine. Having the exhaust valve close a few degrees after TDC just allows a little better cylinder scavenge. Gas has inertia like all things with mass and won't stop and reverse direction immediately even when subjected to a small suction force of the decending piston. There may be some merit to a longer exhaust pipe that I'm unaware of though. I don't think that there is any way for cold air to reach the piston, but it could get into the exhaust pipe and act like a 'plug' that would have to be blown out during the next exhaust cycle. This could require a bit more pumping power that would be subtracted for the shaft power. I've looked at the NHRA Jr. rules and they specify a 27" max exhaust pipe for the dragster engines. All these 'cars' are running 5HP single cylinder engines. Maybe there is an advantage to running longer pipes and hence the rule. I have another book written by a guy at MIT that did extensive research on the internal combustion engine that I haven't looked at in a while. I'll see if there is something in there regarding exhaust systems. There is lots of 'folk lore' out there and racer aften have a 'theory' about such things as this. Often there is an element in validity in such 'theories' but I always like to get to the root of 'the evil' and really understand the surrounding engineering.
 
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Old 05-08-04, 11:58 PM
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Dennis I don't have any answers for ya on that as a matter of fact I have a few questions for you. I have a MTD (2000 or 2001 i think) lawn tractor that I am attempting to rev up for play. Was wondering what were some of the first things to do to get some speed out of it???
Nathan
 
  #16  
Old 05-09-04, 08:45 AM
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Wow, I remember this post. It was a long time ago. I doubt the guy is still here.
 
  #17  
Old 05-09-04, 09:54 AM
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Racin' mower hotrods

This is up my alley. I have been out of lawnmower racing for a couple years but let me give some insight here. I have experimented with several single cylinder engines trying to come up with the best setup. What I have determined is that a Tecumseh 10 horse vertical (TVM220 model series) block works best. Reason being is that there are more and better stock carburetor options for Tecumseh's than for Briggs. This is an L-head configuration, rather than an overhead valve. I have had absolutely no luck with overheads, due in part to the lightweightness of the valve-train. The valves tend to float due to the fact I have been unable to come up with a satisfactory valve spring rate for these engines. Mind you, our organized racing league has rules to follow. One being that we must utilize lawnmower engines. What I have done to increase power is as follows: (Beware that although you may have a similar block, differences in specs between blocks may exist, even from engines straight from the factory AND you will need to make whatever measurements necessary to determine how far you can go with your block) 1-Weld and re-spec the cam to give .025" more lift and only slightly more duration (I don't have the background that Jughead does to accurately measure this) on the intake AND .020" more lift and no change in duration to the exhaust. 2- Machine the head down .018". 3- Machine reliefs opposite the valves in the head to allow for a minimum of .012" clearance between each valve and the combustion chamber in the head. NOTE: These measurements must take into account the use of Tecumseh head gasket part #36451, on average .047-.049" thick before compressed. 4- Deck the block .024" making sure the top piston comes no closer than .010" from the top of the bore. 5- Use carburetor part #632774, this is a large venturi, adjustable jet carb. (This only works with pump fuel (I use Cam2)). 6- Accurately cut off the flywheel fins, coming as close to the magnets as possible without actually removing any material from their posts. (I tried once removing material from the back side as well, but this didn't work since the balancing holes are machined here at the factory and it created a dangerously imbalanced flywheel.) 7- Machine yourself a bracket in order to advance your ignition coil. (Bearing in mind that advancing ignition has very little effect at high RPM's, only rather at lower RPM's (I compromised at advancing 6 degrees)). Make sure your advancing block is not so thick that the coil and magnets don't run past one another well. 8- Set your coil clearance at .008". 9- Use heavier valve springs. ( I have had good luck using Briggs part #691597 (these also have enough travel to allow for the extra cam lift)).
Well, my wife is on my ass to get off the computer. It is Mothers Day and I guess that would be best. By the way, my mod oval racer will do 50-55 mph on a track that is about 800' long (around). I have a dragster with a Briggs V-twin that does 70 mph. I recommend using a Peerless 600-series tranny (not the 601-series). These ar very solid tranny's. Also, we run solid (non-pivoting) front axles as well as a solid rear axle through the tranny with no other mods (other than welding the axle (offset, mind you) to the differential bullgear). We use 4.5-5" B-groove pulley's on the engine and 3.5-3.75" B-groove pulley's on the tranny, using "Gates" brand A belts. Gotta run, check back later.
PS I hope I didn't make any mistakes here but my wife is really on my ass.
 
  #18  
Old 05-10-04, 08:30 AM
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Puey, this is the kind of info that I was looking for..thanks!! I have done alot of research with this over the last few months, and found alot of info...I am currently working on an 8hp Tecumpseh..as our first race of the year is in two weeks!

I have found some interesting info on exhaust lengths as well for the straight pipes.

Nathan, the first things we did were play with the drive pulleys, changing their size to allow for a higher top speed, then we removed the governing system, and locked up the trannys for traction. We have been playing with these tractors for about a year now.
 
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Old 05-10-04, 09:59 AM
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Hi DennisG, did you ever come across anything on the "theory" of straight pipes for 4 cycle engines? According to everything I've learned, ANY exhaust pipe would be a "theoretical" restriction in the exhaust system and would cut down (if only slightly) on engine horsepower. I believe that drag racers are required to use a minimum straight pipe length for safety reasons only. A straight pipe would deflect the exhaust away from the driver and would move the hot exhaust gases away from the engine to help prevent fires. As previously posted on this thread, there are good solid reasons to use certain length pipes when combining the exhausts from MULTIPLE cylinders, and tuned exhausts will help a two-banger, but I've read precious little theory regarding exhaust pipe lengths when venting a SINGLE cylinder of a four-banger to the atmosphere.

PS:
I've aquired an old Monkey Wards lawn tractor with a hydrostatic drive and a 12HP Tecumseh engine that would be perfect for some lawn tractor competition. I don't think that doing anything this season is "in the cards" as I've got to build a new garage this season. My old facility just is inadequate for the kind of work I've been doing and I'm long overdue for an upgrade.
 

Last edited by jughead; 05-10-04 at 01:12 PM.
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Old 05-10-04, 10:07 AM
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My guess is that straight pipes would increase your HP. Thats what I have been told anyway.

I wish you luck Jughead!
 
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Old 05-10-04, 01:01 PM
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I didn't realize this was an old thread but your welcome for the info. I was in a hurry when typing and missed a few items.
1- Remove the governor system. (Be sure to plug up the gov shaft hole) 2- Port and polish, as far as you dare, both ports. (Careful, don't poke through the back side) 3- Three angle valve job on each valve seat. 4- Flow the head. You can do this yourself using a die grinder and Scotch pads
With these mods, your engine should run about 6800-7000 rpm...be sure to run full synthetic oil and furthermore because of the higher rpm's, your engine will only handle about 3/4 capacity of oil, give or take. You will be lucky to get 6 races on the engine and you'll need to re-ring. I strongly suggest tearing down the engine every other race to inspect & measure the rod and crankshaft bearing areas. Also, it's a good idea to blueprint all your mods. so you know where you were at when inspecting. I don't know what you have for a machine shop on-site, but I used a local, reputable hot rod shop to do all my machining (providing all my own specs though) since I don't have access to such. If this is your case, expect to shell out a good chunk of change for such machining.
Again, remember to spec out all your figures yourself regarding clearances. The key being valve to head clearance (using a gasket with .047" thickness). Good luck.
 
  #22  
Old 05-10-04, 07:00 PM
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Jughead, glad to see you are still here, I missed our friendly sparring...lol..I haven't come up with much on the "theory" end, but have talked to alot of racers that do what we are doing, and they have a simple, yet effective means, it seems. They are taking the engine when it is done (mods) and putting a straight pipe, painted white, on it...then they run it, and wherever the paint peels first (hottest point) they are bucking it off. They (over a dozen different racers from different areas) seem to think their machines perform optimumly (is that a word?) there. I am going to try it in the next few weeks..and will let you know what I think of it.

Puey, I have everything you said written in my notes now...lol..and will be building this engine at some point this week. I don't have much for a machine shop, but I do alot of two stroke mods and know how to get around things and blueprint. Our engines are still running about 4000rpm..and if i can get one to do 6k plus...that would be a bonus.

Would you mind if I emailed you for more info?
 
  #23  
Old 05-10-04, 09:53 PM
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Neat trick, with the white paint. You could speed that operation up quite a bit by using one of those infrared thermometers. The one I use at work has a laser pointer that tells you exactly what point on the surface you are reading. I've used it on the exhaust manifolds of diesel engines before and it works fine. I suspect that the point you mention would be the point where the exhaust gas is at it's slowest and may very well depend upon the engine speed. You could confirm that theory with an infrared thermo gun. Good luck.
 
  #24  
Old 05-11-04, 11:46 AM
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Along the same lines as what you are talking about Jughead, we considered taking the pyrometer off my sled, and drilling several holes in a pipe and reading them at working temperature (of the engine). I do also agree with you that the hottest point would be the slowest exhaust flow, and I think that is the key...as soon as it starts slowing, let it escape, as it is only restricting at that point. It will definitely be a learning experience.
 
  #25  
Old 05-11-04, 07:33 PM
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I think you could just use common thermocouples and attach them to the OUTSIDE of the exhaust pipe and get a useful reading. You are really after just a RELATIVE reading anyway. Knowing the true exhaust gas temperature isn't by itself much use. Besides, if you inject something into the gas flow it will act as a restriction, and that's something you are trying to avoid in this case. Back when I was flying airplanes on a regular basis we used to have an exhaust gas temperature gauge in the ****pit. The idea was when you arrived at your cruising altitude you would monitor the exhaust gas temperature and lean the mixture. At most efficient cruise the exhaust gas temp would also be at max. We would usually then enrichen the mixture for an increase of about 25 degrees to give the exhaust valves a little break. They were usually sodium stem types anyway, but every little bit helps.
 
  #26  
Old 04-13-06, 02:38 PM
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well if they go over 20, i assume that u have changed pullies, if not, what i did is put the big one in the back, and the small one in front (engine driven) and switch them, then for older engines, compression test it, then if its not what u like it to be, either get a new engine, or replace some stuff with better, higher performance stuff.


oh yeah, and how to remove governor without breaking stuff? last time i tried iunno wat happened.
 
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Old 04-18-06, 05:10 AM
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A neighbor of mine built and raced go-carts.. Using a B&S 5 horse, the carters modify them to burn alcohol.. They get 8 HP out of the 5 HP block.. You might want to look into that..
 
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