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Can the head be milled on a 98 XPL?
- Thread starter maniac1
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Sportster-2001-951C-Stock
Well-Known Member
Is it warped beyond factory spec?
Why do you want to mill the head? If it's a performance issue I would say no. You don't want to do anything to increase compression, originally 951's were 150psi compression, Rotax intentionally lowered the compression to increase engine life. So like sportster suggested if the head is warped replace the head.
Lou
Lou
I found this on group k site, but there not giving any secrets.
Compression - Increasing compression ratio is always a very popular first modification, and the 951's respond well to "subtle” increases in this area. The use of compression gauges (to establish a measurement in “psi”) has been a very popular way for owners to compare compression. As the “Compression” document on our website points out, measuring compression in this way is, at best, a “ball park” measurement with limited meaning. It the case of the 951 Sea Doo, indicated compression measurement is even more worthless than it has been on earlier pwc's. The primary reason for this is the many variables that affect electric starter cranking speed.
Accurate comparative indicated compression measurements must be taken at cranking rpm speeds that are as close together as possible. For pwc's with smaller displacement individual cylinders (which in this case is “all” pwc's), it’s not too difficult for the starter to maintain a relatively narrow range of starter cranking speeds over a very wide range of indicated compression numbers. However the starter in the 951s has a lot more work to do than any other pwc starter. If a very high compression head is installed, a compression gauge will not be capable of showing an accurate measurement of the increase because the extra load of compression significantly slows the starter cranking speed. On the 951s, we have seen cylinder heads of several cc’s difference show indicated numbers only a few psi apart, while the accurate difference would have been 20 - 30 psi. All this said, “relative” indicated compression measurements might be a useful diagnostic tool on an otherwise unchanged 951 motor. However discussing various 951 compression ratios in terms of indicated psi is (in our experience) a total waste of time.
All this said, the compression ratio increase of our 951 kits is “very” conservative. Our testing showed that very strong overall acceleration can easily be accomplished without big increases in compression ratios.
All of our 951 head modifications include altering the squish angle of the stock head. he Sleeper head mod is unique to those done by other shops in the sense that it has a “staggered” compression ratio. That is, the front and rear cylinder head domes are cut to slightly different compression ratios. This staggered arrangement helps to equalize the operating temperatures of the two cylinders, and helps eliminate detonation.
Included in our head modification is a cooling system upgrade. The stock 951 head casting can “land lock” air pockets in the water jacketing around the combustion chambers. If this air pocketing happens during high-speed operation (on a modified motor), it can cause localized over heating that can easily lead to detonation. Our cooling upgrade completely eliminates this air pocketing. (Please note that our technicians are not permitted to discuss the specification information of our cylinder head modifications or cooling upgrades).
About “O” Ring Heads - There are many aftermarket heads available for the 951 engines. Many are called “O” ring type heads because they utilize a large rubber “O” ring, instead of the stock steel head gasket, for sealing between the cylinder and the cylinder head (as is done on the smaller Sea Doo engines). These heads are typically crafted from high quality billet aluminum, and often have more water jacket volume than the stock part. While many owners have had good results with these heads, they can represent two difficulty areas that should be kept in mind.
First and foremost is that the elimination of the head gasket itself incurs a 2.3cc reduction in combustion chamber volume. This means a 47cc domed head, used with no head gasket, is actually the equivalent of a (higher compression) 44.7cc dome being used “with” a head gasket. It’s very important that all dome volume terminology’s be defined as “with gasket” or without” gasket. Having 2.3cc too little volume can easily lead to detonation and swift engine damage.
Besides the issues of volume, we have experienced some sealing difficulties with “O” ring heads on 951 engines. It should be understood that an “O” ring can easily seal water away from the cylinder bores, however no rubber “O” ring on earth is strong enough to seal back the pressures of compression. “O” ring setups depend on relatively broad, and perfectly flat, metal-to-metal mating surfaces to effectively seal compression. The absolute flatness of these two surfaces is fundamental to maintaining a lasting long-term seal. Herein lies the problem with some “O” ring setups.
The popular 718cc and 782cc Sea Doo engines use only “O” rings for head sealing. However the cast components of those engines are not nearly as large as the 951's, and so subject to considerably less sealing surface “movement”, compared to the 951 top end. If the top sealing surface of the 951 cylinder and the sealing surface of the “O” ring head are both lapped to assure the perfect flatness of both surfaces, the “O” ring head can seal well. However as time goes on, and repeated temperature deflection of both surfaces takes place, there becomes an increasing risk of a breach at the “O” ring. For owners who intend to remove their cylinder head periodically to inspect and dress the sealing surfaces, the “O” ring head can be a very practical choice. However we consider the stock 951 head gasket able to offer a better long term seal than any arrangement using an “O” ring. For this reason, all our 951 modifications are setup to retain the stock head gasket. We will prepare kits for 951 owners wishing to utilize an “O” ring head, however we will take no responsibility for the lasting seal of the head surface joint.
Compression - Increasing compression ratio is always a very popular first modification, and the 951's respond well to "subtle” increases in this area. The use of compression gauges (to establish a measurement in “psi”) has been a very popular way for owners to compare compression. As the “Compression” document on our website points out, measuring compression in this way is, at best, a “ball park” measurement with limited meaning. It the case of the 951 Sea Doo, indicated compression measurement is even more worthless than it has been on earlier pwc's. The primary reason for this is the many variables that affect electric starter cranking speed.
Accurate comparative indicated compression measurements must be taken at cranking rpm speeds that are as close together as possible. For pwc's with smaller displacement individual cylinders (which in this case is “all” pwc's), it’s not too difficult for the starter to maintain a relatively narrow range of starter cranking speeds over a very wide range of indicated compression numbers. However the starter in the 951s has a lot more work to do than any other pwc starter. If a very high compression head is installed, a compression gauge will not be capable of showing an accurate measurement of the increase because the extra load of compression significantly slows the starter cranking speed. On the 951s, we have seen cylinder heads of several cc’s difference show indicated numbers only a few psi apart, while the accurate difference would have been 20 - 30 psi. All this said, “relative” indicated compression measurements might be a useful diagnostic tool on an otherwise unchanged 951 motor. However discussing various 951 compression ratios in terms of indicated psi is (in our experience) a total waste of time.
All this said, the compression ratio increase of our 951 kits is “very” conservative. Our testing showed that very strong overall acceleration can easily be accomplished without big increases in compression ratios.
All of our 951 head modifications include altering the squish angle of the stock head. he Sleeper head mod is unique to those done by other shops in the sense that it has a “staggered” compression ratio. That is, the front and rear cylinder head domes are cut to slightly different compression ratios. This staggered arrangement helps to equalize the operating temperatures of the two cylinders, and helps eliminate detonation.
Included in our head modification is a cooling system upgrade. The stock 951 head casting can “land lock” air pockets in the water jacketing around the combustion chambers. If this air pocketing happens during high-speed operation (on a modified motor), it can cause localized over heating that can easily lead to detonation. Our cooling upgrade completely eliminates this air pocketing. (Please note that our technicians are not permitted to discuss the specification information of our cylinder head modifications or cooling upgrades).
About “O” Ring Heads - There are many aftermarket heads available for the 951 engines. Many are called “O” ring type heads because they utilize a large rubber “O” ring, instead of the stock steel head gasket, for sealing between the cylinder and the cylinder head (as is done on the smaller Sea Doo engines). These heads are typically crafted from high quality billet aluminum, and often have more water jacket volume than the stock part. While many owners have had good results with these heads, they can represent two difficulty areas that should be kept in mind.
First and foremost is that the elimination of the head gasket itself incurs a 2.3cc reduction in combustion chamber volume. This means a 47cc domed head, used with no head gasket, is actually the equivalent of a (higher compression) 44.7cc dome being used “with” a head gasket. It’s very important that all dome volume terminology’s be defined as “with gasket” or without” gasket. Having 2.3cc too little volume can easily lead to detonation and swift engine damage.
Besides the issues of volume, we have experienced some sealing difficulties with “O” ring heads on 951 engines. It should be understood that an “O” ring can easily seal water away from the cylinder bores, however no rubber “O” ring on earth is strong enough to seal back the pressures of compression. “O” ring setups depend on relatively broad, and perfectly flat, metal-to-metal mating surfaces to effectively seal compression. The absolute flatness of these two surfaces is fundamental to maintaining a lasting long-term seal. Herein lies the problem with some “O” ring setups.
The popular 718cc and 782cc Sea Doo engines use only “O” rings for head sealing. However the cast components of those engines are not nearly as large as the 951's, and so subject to considerably less sealing surface “movement”, compared to the 951 top end. If the top sealing surface of the 951 cylinder and the sealing surface of the “O” ring head are both lapped to assure the perfect flatness of both surfaces, the “O” ring head can seal well. However as time goes on, and repeated temperature deflection of both surfaces takes place, there becomes an increasing risk of a breach at the “O” ring. For owners who intend to remove their cylinder head periodically to inspect and dress the sealing surfaces, the “O” ring head can be a very practical choice. However we consider the stock 951 head gasket able to offer a better long term seal than any arrangement using an “O” ring. For this reason, all our 951 modifications are setup to retain the stock head gasket. We will prepare kits for 951 owners wishing to utilize an “O” ring head, however we will take no responsibility for the lasting seal of the head surface joint.
Sportster-2001-951C-Stock
Well-Known Member
I agree the 951 is already pushing design limits and this is why rotax detuned the later versions, performance modifications on these have a nasty habit of morphing into a wallectomy. If you want more power, then perhaps you need a 255hp supercharged 4-tec?
Sportster-2001-951C-Stock
Well-Known Member
Most things aren't marine engines. We used to build our bike motors and do stuff like that too, leave one piston ring out and shave the heads down. Those were bike motors though, they weren't running under a constant heavy load like a marine engine does, bike motors coast and cool off the pistons and rings. I'd hate to see you blow your motor up, really think it's more risky than it's worth to take more off than just a minimum, making sure it's flat to spec. The squish on the later 130psi motor is a fat 0.080+" usually, that's what I'd want to see on one, less than that and I'd be skeptical it won't detonate. I don't know what the squish for the 150psi motor should be, it's probably less and likely a big reason the earlier 951 versions were famously unreliable.
Also, I wouldn't recommend turning one of these past 7,000 RPM for more than 10~20 seconds, the rings will overheat and burn the oil film.
Ok good to know,
Like I said ya don't know if you don't ask.
I did the one on my Super jet and it is about 180 compression when I checked last summer, It was done on the stock head the day I bought it in 96. We run wide open for miles at a time on flat water.
The sea doo does the same and never a issue on both.
I don't ride sit downs anyway it is the wife's machine so I will just leave it stock as I appreciate and trust your input.
Thanks to all.
Now we just need the ice to melt up here.
Don
Like I said ya don't know if you don't ask.
I did the one on my Super jet and it is about 180 compression when I checked last summer, It was done on the stock head the day I bought it in 96. We run wide open for miles at a time on flat water.
The sea doo does the same and never a issue on both.
I don't ride sit downs anyway it is the wife's machine so I will just leave it stock as I appreciate and trust your input.
Thanks to all.
Now we just need the ice to melt up here.
Don
SabrToothSqrl
Well-Known Member
I 2nd LEAVE IT STOCK.... from experience. If you change up the head, you're also looking at jets, impeller, plugs, tuning, cooling, etc etc, and it just isn't worth it... for a few more HP your reliability and usability just tank fast...
seadoodoopartsofcf
Active Member
The 951 is pretty beastly imo...no need to mod it
Sportster-2001-951C-Stock
Well-Known Member
I guess if you don't care much about longevity you can mod stuff like that, be prepared for any consequences though. Maybe you're running 100octane and the best oil out there at an optimum ratio, it's hard to know. Maybe you only run a couple day per year, anything's possible, eventually every two stoke will seize if it's run, some highly tweeked bike motors won't go 50 hours without a tear down.
But if you've got a machine shop at your disposal and want to do that, I hope it works out.
Here's an article that describes some of the things you may run into with your mods and how to avoid them for as long as possible:
http://www.off-road.com/dirtbike/tech/dirt-bike-tech-twostroke-seizure-52428.html
IMO the 951 motor is so under engineered...the only way I would be talked into modifying one would be if it had been rebuilt top to bottom. And even then, I wouldnt expect it to last all that long.
Plus...I wouldnt expect any performance upgrade from that you could feel anyways.
Plus...I wouldnt expect any performance upgrade from that you could feel anyways.
SabrToothSqrl
Well-Known Member
under engineered... or over powered!
it was engineered to run safely on regular gas, even marina gas.
It can easily handle more compression provided you feed it quality fuel and know what your squish is and keep it above .065". Too many people simply slap an aftermarket o-ring head on and neglect to check squish afterwards. You can safely mill your OEM head to provide 150lbs compression provided the two aformentioned criteria are met. I'm not sure if you'll notice an appreciable increase in power. The 951 requires lots of small things/mods to get the most out of it if you aren't willing to buy a pipe.
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