HS50 tecumseh - change exhaust valve size?

tecumseh power

tecumseh power

Member
#21
#21
I know from racing OHH engines, that imcreasing the exhaust valve size did result in lower bottom end, especially on tighter tracks. I had a couple of heads that were ported, had good valves and springs, and even one with a welded up chamber for more compression, that I used on smaller tracks.

These engines are drastically under square with the big bore and short 1.938 stroke, which by itself hinders low end torque.

I agree with Delray that building a stock or even slightly warmed over engine, the small valves might even be a good thing, especially for a heavier bike/rider.

Here is one of the stock valve ported heads I used to run. That head still made great power even having the small valves. In fact, I patterned the chamber the OVRM head on my 356 build after it.




 
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tecumseh power

tecumseh power

Member
#22
#22
On the big port/small valve thing - the cfm of any port is generally dictated by the choke point (smallest point at the window where he port turns) and the bowl first, then by valve and runner size.

In other words, you can take a stock port and put bigger valves in it without opening up the window area and improving the short side radius and often hurt flow and velocity, and hurt performance.

In the flip side, you can port the runner and bowl efficiently with a stock valve and make way more power than the head mentioned above.

All a big exhaust runner with a small valve will do is help with heat reduction, which is I'm sure why they did that. This is what everybody always does to nitrous drag race engines to help lower EGTs.

The big ports on the those engines with the small valves isn't going to hurt or help performance much because the valve/bowl size, and choke point has already determined how much cfm that port is going flow by the time it gets to the large runner.
 
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delray

delray

Well-Known Member
#23
#23
same here too. I am running a small valve,head port,milled,billet rod,timing advance,header,mikuni carb 140jet,255 dyno cam on a ohh motor and it runs just fine. pulls up to 6000+ rpm's all day for the last 3 years. but it is time to freshening it up and see how things look inside. I think where you would notes the big difference is in the 6000 + and higher rpm's for big valves. for just riding around in the back yard and having fun. the small valves should still work ok. of course this is based on a ohv Tecumseh engine and not a flathead.
 
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markus

markus

Well-Known Member
#24
#24
I went ahead and ordered one to try, Been sitting here watching them listed for sale for a month or so thinking about it. I would actually like to see them run a little cooler, so If that's the case that would be great for the conversions to early HS flywheels and shrouds where the cooling seems to be getting reduced a bit, hasn't been enough to cause a problem internally that have seen but I know its running a little warmer with the early shrouds and smaller/lighter wheel on them.

Thank guys :thumbsup:
 
delray

delray

Well-Known Member
#26
#26
Tecumseh power
I haven't built one of those in about 20 years.

Where did the time go??
Click to expand...
called getting married and maybe having kids...…..:laugh:
don't feel bad james. I like to still build one more big block chevy. for most people they may think it's starting to be a dinosaur engine. I be happy with a mild built BIG BLOCK with fuel injection. only 400+ hp street engine...:laugh:
maybe 55 chevy 210 or 57 210 both sedan models.

oh well for now it's minibikes...….:doah:
 
tecumseh power

tecumseh power

Member
#27
#27
No doubt, but I wouldn't trade them for the world!

I love dinosaur engines!

I'm that way about Pontiac engines. Everybody is going LS these days, but I cringe seeing one in a classic muscle car. I'm in the process of building a pretty healthy 468 Pontiac for my '67 GTO.


 
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markus

markus

Well-Known Member
#28
#28
tecumseh power said:
Cool! Let us know how it turns out. I wouldn't mind building another HS engine myself. I haven't built one of those in about 20 years.

Where did the time go??
Click to expand...
I will, I'm really curious to look at it up close an personal. the newest HS50 I have is a Sept. 1994 its the old valve SBH replacement block so I have yet to see the smaller bore and valve block yet in the HS size. I need to search out some good carb numbers for the later HS50's and try and figure out if the bore size got reduced along with the the other stuff.
 
massacre

massacre

#29
#29
delray said:
Tecumseh power
called getting married and maybe having kids...…..:laugh:
don't feel bad james. I like to still build one more big block chevy. for most people they may think it's starting to be a dinosaur engine. I be happy with a mild built BIG BLOCK with fuel injection. only 400+ hp street engine...:laugh:
maybe 55 chevy 210 or 57 210 both sedan models.

oh well for now it's minibikes...….:doah:
Click to expand...
BBC is one of the best engine designs in history! And i’m A Ford guy lol!
 
FloridaMiniBikes

FloridaMiniBikes

#30
#30
Back in the 80's I ran a BB 402 Chevelle as the family car. Love and miss the power and handling. Wife said if I kept it, the prices would have never skyrocketed. Now reverting back to my youth with the minibikes, and be danged if I could pull the head off a big block without throwing out my back for a month now. Those little HS50 heads are a lot easier to handle.
 
Orson_Yancey

Orson_Yancey

Member
#31
#31
Folks, Regarding trying to figure-out what year did the HS50 go to the slighter smaller bore and smaller exhaust valve,
my Tecumseh Technician's Handbook "3 to 11 HP 4-Cycle L-Head Engine", #740049,
page 98 has a chart "Solid State And External Ignition" which lists the specifications.
HS50 G & earlier, HSSK50 M & earlier at 197cc with 2.812" bore.
Whereas
HS50 H & later, HSSK50-55 N & later at 195cc with 2.795" bore.

I wonder if Markus or someone has information that can tell what year did the
HS50 H, HSSK50-55 N come out?

The chart seems to imply that the transition to slightly smaller bore, smaller exhaust valve
took place during the period of the electronic ignition.
And most of us are in agreement that electronic ignition started on the HS engines
in 1985. Hence, the transition to slighter smaller bore and smaller exhaust valve
must be sometime after 1985.

(The chart on page 95, "Engine Specifications Standard Point Ignition"
lists only one HS50 and it is 197cc.)
 
delray

delray

Well-Known Member
#32
#32
yes Orson_Yancey they still carried the large bore,big valve into the late 80's for sure with electronic ignition when the cut off year was?
 
tecumseh power

tecumseh power

Member
#33
#33
That may be the case. I haven't built an HS50 in a long time, but all the ones I fooled with back in the day were big valve engines, but they were points engines too, so that may be the case.

Does anybody know when the first OHH engines came out? I became aware of them in the early to mid 90's. (Maybe 93 or 94)

I bought one at the local lawnmower shop to see what was up with them and what I could do with one, and when I realized the internals of my HS engines fit them, that pretty much ended my HS involvement.
 
delray

delray

Well-Known Member
#34
#34
something else too consider when purchasing a newer block with the small bore/valve. for some reason Tecumseh decided to run little extensions for the coil bracket. I believe this was tied into the vertical engine coil bracket. looks like they use the same casting setup for the coil bracket.
on a horz engine they use the external mount magnets making the coil sit farther out and on the vertical engines the mag sat in the flywheel. I guess that's there way of fixing that problem.


on a ohh motor next to the flathead block you can see the webbing casted between the mounts and the flathead open so the coil could drop in and clear when mounted to a vertical engine.


also the tin work is different from the vintage tin.



like I said early. Tecumseh change alot little things when they started to build the small bore/valve engine...:doah:
 
delray

delray

Well-Known Member
#35
#35
here is a good example of a electronic ignition engine that still had large bore and valve. note also in the picture it has the little extensions for the coil. engine is david's hs-50. don't have a date code,but I do remember the factory recoil cover that came with this engine and it was the late 80's early 90's style cover. you can see it in ther back ground.

 
markus

markus

Well-Known Member
#37
#37
you dont want extension the lugs on there if your converting to early style shrouding and points flywheel anyway or they would not clear. thats also how they were able keep on offering replacements that fit the old engines as well.


 
delray

delray

Well-Known Member
#39
#39
here is one more crazy thing that Tecumseh did in there very last years of production of the flathead. they ran a aluminum flywheel on a horz flathead engine and the extensions had to be remove so the coil would aline with the flywheel.


looks like they use that for more then one type of application.
 
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