Head or base gasket

[Mekanix]

Somebody recently asked me a question in terms of performance increases on a 951.
Would you change the base gasket for plane the head to raise compression.

I think both would have the same effect but I'm wondering about port timing and how that would affect things.

What do you guys think?

 

[mikidymac]

No, it will not only change your port timing but increase your squish which will cause detonation.
That is not the way to increase performance.
@etemplet is selling a high compression R&D 951 head if you want more punch.

 

[Mekanix]

Squish is at 2.6mm or over100 thou on this build with a 6 hole gasket.

 

[mikidymac]

Set the squish to spec. Even with a high compression head the squish should be within the book spec. Don't adjust the base gasket to increase compression, just to set the squish to spec.

That being said you might and this is a big maybe get a little more performance from setting it at the low end of the stock squish range.

When a stock head is cut from a good shop that knows what it is doing like Group K, recutting the squish area is always part of the job.

 

[Mekanix]

Set the squish to spec. Even with a high compression head the squish should be within the book spec. Don't adjust the base gasket to increase compression, just to set the squish to spec.

That being said you might and this is a big maybe get a little more performance from setting it at the low end of the stock squish range.

When a stock head is cut from a good ship that know's what it is doing like Group K, recutting the squish area is always part of the job.

951 s don't have a squish spec in all the books that I could find. The only thing that it mentions is doing combustion chamber volume with a burette and then adjusting the base gasket for the correct volume.


For the 787 yeah I completely agree and that's what I usually do is adjust the base gasket till I get the right squish. I just never use the six hole gasket in the kit.
Instead I do a squish measurement without a gasket and then adjust as necessary

But what's a 951 the squish is very wide and they control everything by cylinder volume I'm wondering if squish even matters.

Sorry to ask this question by a guy who is building an engine and using the WSM top end kits with the six hole gasket but he had to get the head planed and wanted to know what gasket to use since you couldn't find the squish spec.

 

[mikidymac]

Seadoo switched to the cc spec for squish on all engines. I did my 951 this way. I don't remember where but there is the solder specs for the 951 floating around somewhere.

 

[Mekanix]

Looking for it but cant find it. Maybe it was something groupk said.

 

[mikidymac]

I honestly don’t remember, been a very long time.

 

[Mekanix]

That's ok. I reread all of the group k stuff. Nothing there.
All the manuals from 97 on are volume based. I wonder what the ideal setup would be as most of what I found causes detonation. So too close squish causes det, too high compression with wide squish causes det, porting changes cause det.... geeze. Oh and max octain is 92 here. I think I'll suggest he goes with checking squish now, planning the head to clean it up (corrosion from water port to combustion chamber) and then adjusting the gasket to get the same squish as before.

 

[drjack]

Does this help any?
Rotax Squish Gap Check Procedure - 2 Stroke

 

[Mekanix]

Does this help any?
Rotax Squish Gap Check Procedure - 2 Stroke

Yup that does. It's the first ive seen of a number anywhere.

 

[mikidymac]

Told you I remember, just can’t remember the details, lol.

 

[Mekanix]

Told you I remember, just can’t remember the details, lol.

Lol you did