rasing base gasket and shaving heads

jeffgilbert

jeffgilbert

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Nov 14, 2007
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Blaketown NL
I have a 99 700 and i was thinking about getting it ported but i read that you can add 2 base gaskets stacked on top of the orignal makeing a total of 3 and shaving the heads 0.025-0.035 and this is supose to be good for 6 -8 hp.
If this is done is porting any benifit and would the base gasket have to put back to stock inorder for it to work. I dont want any less relibility because most of my going is just agressive trails.

Thanks Jeff
 
porting won't hurt reliabilty, as long as ports are properly chamfered and that ports' corner radius' aren't too small. stacking up gaskets to increase port timing is a "poor mans port job". i prefer porting the cylinder, instead of stacking up gaskets.
 
I would have a hard time to get them ported and back to me quickly right in the middle of the season being that I dont know anyone around my area that is into yamaha porting. So if I was stacking the gaskets would that be the proper hight to get the heads shaved and if the heads are shave that much, and the summer i get the cylinders ported and remove the extra base gaskets will there be enough squish zone.
 
jeffgilbert said:
I would have a hard time to get them ported and back to me quickly right in the middle of the season being that I dont know anyone around my area that is into yamaha porting. So if I was stacking the gaskets would that be the proper hight to get the heads shaved and if the heads are shave that much, and the summer i get the cylinders ported and remove the extra base gaskets will there be enough squish zone.
Click to expand...

no, there won't be enough squish zone. you would then have to get the squish band recut to get squish clearance correct again, but then the squish band will become too wide and give you too large a squish band area, which would yeild too high of MSV. you would also need to get the "dome" of the head recut too, but its just far easier to get unmodded used heads at that point. oh yeah, your trapped comp ratio will end up higher too, and maybe even too high to be safe on todays crap pump gas.
 
That is what I tought but it wouldnt be too hard to get a set of used heads.
But, is it safe to have my heads cut 0.025-0.030 with a rased base gasket?
 
jeffgilbert said:
That is what I tought but it wouldnt be too hard to get a set of used heads.
But, is it safe to have my heads cut 0.025-0.030 with a rased base gasket?
Click to expand...

you can cut .020 off heads, when adding 2 additional base gaskets. anymore than that, i would double check trapped volume and comp ratio 1st, before running. you would probably be fine w/.025" or .030" off too, but double check like i said.
 
mopar1rules said:
you can cut .020 off heads, when adding 2 additional base gaskets. anymore than that, i would double check trapped volume and comp ratio 1st, before running. you would probably be fine w/.025" or .030" off too, but double check like i said.
Click to expand...
i got basicly the same done to my wifes 98'. race logic porting with 3 base gaskets and .030 off the heads,.020 for the base gaskets and another .010 to bump compression a little.squish is at .052" . this was done 8 years ago and haven't had a problem running 91 octane.if your unsure about the fuel your getting i would only take .025" off to be safe. it is the poor mans port job but it does bring a 98' up to par with a 2000 and up srx.
 
Run the rest of the season as is, Summer send your cyl. in for porting and do it right. It will be less expensive in the long run and it will be done right. All of what Mopar has said is right. Its a patch job at best.
 
9801srx said:
i got basicly the same done to my wifes 98'. race logic porting with 3 base gaskets and .030 off the heads,.020 for the base gaskets and another .010 to bump compression a little.squish is at .052" . this was done 8 years ago and haven't had a problem running 91 octane.if your unsure about the fuel your getting i would only take .025" off to be safe. it is the poor mans port job but it does bring a 98' up to par with a 2000 and up srx.
Click to expand...

neil, you must have had some pretty good exhaust duration values then.....199* to be exact then from my calculations.
 
Im not getting this.....by stacking base gaskets your retarding port timing. By stacking the gaskets and cutting the heads you are keeping the the same volume per say in the cumbustion chambers all that is happening is you are changing the port timing. What am I missing here ?
 
Theories behind raising the Cylinders would take a long time to completely analize.

Here's some points:

-raising the cylinders raises the exhaust ports which extends the exhaust port duration[measured in degrees of crankshaft revolution] Its like putting a high lift, long duration camshaft in a car or an Apex. Porting a cylinder can also do this, but sometimes the port is just widened, depending on the engine.
-more exhaust port duration potentially increases the pumping capability of the cylinder as well as raising the RPM capability of the cylinder to operate effectively...this all can enable the cylinder to pump more air...
more air = more horsepower
-raising the cylinder also raises the transfer ports of an engine...OK if they were conservatively low to begin with...but potentially power + torque robbing if the transfer ports were already near the highest power making + efficient height[like the Vipers transfers, + to some extent the 2000-2002 SRXs, already are]

The potential problems with just raising the cylinders:

- you also raise the ports floors... again not the optimum solution as this as well can cause power robbing + efficiency problems.
-cylinder heads will still have to be machined + sometimes cylinders will have to be cut to get the proper deck height, squish, + compression ratio
-raising the optimum power making RPM of an engine is futile if the exhaust and ignition system is not tuned to match the new RPM[I beleive this is the source of a lot of the issues with poor performing "ported SRXs"] as we all know our SRX exhausts and CDIs are designed to run at a max of approx 8500RPM... so why have an engine with porting designed for 9000RPM!?]
-just raising the cylinders also sets the piston down in the cylinder[deck height] when it reaches its highest upwards travel[TDC]...again not desireable

To make a long story short:

-everything in the engine must be matched to the common goal, RPM + horsepower, riding use,+ fuel...ect. A proper engine builder matches the porting to all these things and you get a very powerful, useable rocket... or if done incorrectly you get an unrideable, unreliable SLUG!

Its as I have always said...

"SPEED IS MATH....THE BEST MATHEMATICIAN WINS!!!"

JM.02c
 
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Thanks for the explanation. on a 2 stroke 250 persay we always used a thinner base gasket to drop port timing .015 to gain HP. thats why I was asking. I realize theres a difference in every engine made and different things work for different engines !! So as you put it The best mathematition wins !!!
 
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