adding a base gasket?

redsnake3

redsnake3

New member
Joined
Feb 3, 2005
Messages
1,171
Age
40
Location
labrador home of snow
i know that if i move the cylinders up with a extra gasket or two this will change the port timing therefore giving me back the compression i lost when i done the porting. my ? is would it be safe to use extra gaskets or should i try making my own thicker?
 

Putting in a thicker base gasket lowers your compression more. If you have porting, you should be removing layers from the head gasket to slightly increase compression and turbulence created by the narrrower squish band. pull a layer from the head gasket(use a opticool from bender), and run a two degree offset key in backwards to reduce the viper timming. I know you have the CPR head, remember i sold it to you!
 
Actually, by moving the cylinders up you will not gain any compression but probably loose some. I do beleive some places will sell the thicker base gaskets or you can buy some extras from Yamaha. There are quite a few good posts in here on that topic.
 
hey vipertripplexxx,
i totally forgot who it was that sold me the head, thanks it was a great deal.
i have the opti-cool from bender but i thought i read on here somewhere not to do it with this gasket. oh well im sure someone will let me know. about the key, where can i get one with the 2 degree offset? right now my compression is only like 110-115 with new rings, so i was lookin to get some more compression since i do alot of racin im lookin for the most power i can get.
 
base gaskets

I think the modification you are talking about is called "decking". An additional base gasket is added and the top of the cylinder is milled (decked) an equal amount to move the ports up relative to piston travel. The process is designed to change the port timing. I know enough about it to not try to do it on my own. If the engine mod gurus say it works on a particular engine then I might consider having it done by pros at the same time as porting and head modification.
 
i have done the porting to my sled myself, the head is already modified for the pipes. i dont want to have to shave down the cylinders mainly because there is noone around here qualified enough.
 
redsnake, do yourself a big favour and check your squish before you do this, if it's not the porting that caused the drop (which I very much doubt it is, porting doesn't 'cause drops like that) you will basicly be killing your motor by doing moer machine work, fix what's wrong rather than trying to go overboard on mods to fix it.
 
First off, you dont gain compression by raising the cylinder, you lose it, and you lose alot because your gaining the entire 69mm of bore area to added volume.
Second, if you raised the exh. port you will have lower compression then when you started, the port now opens sooner, and for a longer duration because of the raised roof.

Lastly, if you have a cpr head, you have a head that has lowered compression on the mag and center cylinder, the pto cylinder is already lower on a stock viper, he simply opens up the other 2 to match the lower one. So to gain back compression it all depends on how high the exh. port is and what the squish clearance is, the bigger the squish, the lower the compression. You dont ever want to raise a viper cylinder because the transfer ports are way hotter in port timing then they need to be anyways, if you raise the cylinder more you had better be spinning the motor to 10,000-12,000 plus rpm. Make it easy on yourself and have a educated path to follow by simply checking the squish(there is a post on how to do this in the tech section by myself), once you have numbers to work with you can easily see whats right and whats wrong with your current combo you have there.
 
Snake,
If you have premium gas avalible, then try what i told you. I also had my shelf pulled from my airbox. It actually seemed to be less tempramental with it out. If you have only 91 octane avalible then maybe try a 3 degree key, with one layer pulled. Even with the layer out i only got about 120 psi on the gauge. jet to wash.
 
Oh yea i always wanted to ask you whats the coldest temp you have ever seen up there?
I wanted to tell you i also adjusted my tps sensor, advanced it. I'll take a pic of it. I farted around with a timming light, and my friend with a screw driver in the sensor. we found that the highest timming readings were around a 1/4 throttle. After 1/2 it starts to go down. we both kinda thought that if you can get it to start going down sooner, the better off you are. I haven't burned down yet, knock on wood. I'm now running a megapower head with .045" squish. You can't go that low with a viper head cause the squish is alot wider. Without changing the jetting, and the 2 degree key, w/tps advanced, the wash got bigger, telling me the piston was running cooler. I have a tempa flow so temperature was not a factor. I'm going to test a four degree key this winter and see what happens. Exhaust temps are out the window with these motors seeing 1400+ sometimes, but the wash is good. my polaris runs 1250 with the probes the same distance from the skirt?
 
the coldest temprature her was probably about -60 celsius even then you would find me out riding around, not too many peple to ride wiht then tho'. im gonna check the squish tonight and figure out whats goin on. what do ya think i should do? i would love to get a set of megapower heads but cant find any used for sale. i definately want to see the pics of your tps.
 
Holy chit! -60 celsius is.....-76 fahrenheit ... thats way tooooo coooold! Noo way you would catch me outside. lot'sa frostbit victims!!
Yea I'll get ya a couple of pics, heads, tps, and airbox top mod. I really didn't want to mod the airbox but i came to the realization that with pipes it messes with the jetting if not ventilated.
 
You must log in or register to reply here.


Back
Top