Decompression holes

[Danbot]

Has anyone plugged the decomp holes on their own? Is there any reason why a guy couldn't install a set screw or something similar from the outside. Looks like the hole goes through the cyl wall, into the cavity where the ves guillotine lives, and continues again through to the exhaust manifold opening.

So just for the sake of discussion: what if I was to drill out the first length of the hole (from the exhaust) for access, and then drill and tap 1/2" or so into the second length (between ves cavity and cyl wall) so a set screw could be installed. I don't even know yet if this is possible with the angles, but I don't see why not.

Additionally, what kind of change is this for the engine? At 7000+ rpm, is this hole leaking enough compression to create a power loss? Are there any other results to consider, ie fuel mapping, detonation...

 

[guidoxpress]

Dan, i wouldnt see an issue with the set screw...locktite the hell outta it, and be done i would think..

of coarse SLP does this, but they charge $25/cylinder...you should call SLP and see if they weld them closed, or do something like you mentioned.

i have been wanting to do this as well, just hate to set the cylinders in for an easy job..i might have a local welder plug weld them if SLP welds them.

i would LOVE to hear feedback on this for sure....

here is the info from SLP, and the link...

Some Polaris models are equipped with a decompression hole in the cylinder to make it easier to start.
Over time, the exhaust valves wear on the cylinders allowing compression to bleed off around the exhaust valve. This reduction in running compression reduces peak RPM and horsepower. If this problem exists, no matter what you do with clutching, you cannot attain the recommended RPM. Plugging the decompression holes eliminates this compression bleed off and increases your peak RPM. Cylinders are needed for modification.

Service available for:
2003-2005 700 and 800 Big Block
2005-2006 900 CFI
2008-2010 800 CFI

http://startinglineproducts.com/catalog.cfm?pageID=detail&catalogID=3&catID=18&productID=172

 

[Danbot]

Yeah I don't know how slp does their plug job. Anyone?

 

[guidoxpress]

if i get some time in on Friday at work, i will call them and ask how they do it...plug or weld...ill report back for sure.

i will get mine plugged this summer as well, on the 975 and the turbo...im sure it will be a plus no matter what...

 

[Danbot]

I got caught up on engine building and decided to play around with this on a spare cylinder. Here's my work:
-I drilled out the hole from the exhaust manifold side to 3/32" purely for access
-Then I drilled 1/4" into the second section (from the ves cavity in towards the cyl wall) to 3/64" which worked for the size of my set screw.
-Installed a very small set screw in the hole

It went well, I can't see how anything could go wrong. I am going to go ahead and mod my current cylinders the same way.

In this pic you can see the path of the hole which I have inserted a small wire through

All the drilling is done in this pic

In this pic you can kinda see the set screw in position to be installed

 

[guidoxpress]

Dano..

Just called SLP. They said they use a set screw to plug the decompression holes...

Good work buddy..u should work for them...or me

Post back your results for us...I wanna do this for sure...

 

[Danbot]

RESULTS!!

I just test rode this mod (my engine was apart for other repairs) and I am impressed. Throttle response is improved at any rpm, but especially from idle to 5000rpm. Couldn't be happier with this almost free mod.

 

[guidoxpress]

sweetness....

glad to hear. ill do the same when i get time this summer...on the 975 and the turbo...!!!

 

[Danbot]

One more thing I forgot to mention...
You can REALLY feel the extra compression when starting the sled, very solid compression strokes. Might be a bear to pull over in the cold temps!!

 

[diamonddave]

9's are already a bear to pull over in those -temps.

 

[900Spartan]

Can you say Armor Cord and you better eat your WHEATIES!

 

[94fordguy]

9's are already a bear to pull over in those -temps.

You took the words right out of my mouth

 

[diamonddave]

Did Mark get his Y yet?

 

[94fordguy]

Did Mark get his Y yet?

Yes, he told me he got it installed yesterday evening

 

[diamonddave]

You guys should come ride Baker Sunday

 

[Danbot]

9's are already a bear to pull over in those -temps.

Yup haha, not looking forward to cold mornings with these holes plugged, but I shouldn't have to worry about it till next season at least

 

[diamonddave]

Yup haha, not looking forward to cold mornings with these holes plugged, but I shouldn't have to worry about it till next season at least

You never know, I've found those holes so full of carbon and oil-naise that they couldn't have been working, maybe you might notice any difference...if your holes looked like the ones I've worked on.

BTW, what happened to your motor?

 

[Danbot]

My holes were open and functioning, its way stiffer to pull over now even in the warmer temps.

I dont know for sure what caused it, but I squeaked a pto piston, cylinder, and big end con rod bearing in Revy at the end of the first day.

 

[sno_mo_racer]

I got caught up on engine building and decided to play around with this on a spare cylinder. Here's my work:
-I drilled out the hole from the exhaust manifold side to 3/32" purely for access
-Then I drilled 1/4" into the second section (from the ves cavity in towards the cyl wall) to 3/64" which worked for the size of my set screw.
-Installed a very small set screw in the hole

It went well, I can't see how anything could go wrong. I am going to go ahead and mod my current cylinders the same way.

In this pic you can see the path of the hole which I have inserted a small wire through

All the drilling is done in this pic

In this pic you can kinda see the set screw in position to be installed

In that last pic Dan, so with that set screw in the middle of the valve area there I am guessing you put a plate on to block off the valves? Sorry a bit confused here, or is it to go further down and screw in out of the way of the valves and to just block off the hole from the valve to the exhaust port?

 

[Danbot]

Nope, the set screw is installed in the hole between the exhaust valve cavity and the cylinder wall. The screw goes in just beyond flush so that it wont interfere with the exhaust valves operation.