6M6 head on a 61X to bump up compression?

[Dustin Mustangs]

Some random nice guy at the local ramp last night (6-8 footers on Lake Michigan, woot!) mentioned he was running a 650 head on his 701 superjet to get his compression up. I've never heard of this before, but it does seem logical. Less displacement means less chamber volume for a given compression ratio. Given the 6m6 and 61x run the same ratio in stock form, that would give about an 8% increase in compression for the 61x with a 6m6 head.

Is this old news? Any downfalls to running this?? What is the resulting compression and/or what is the stock 6m6 chamber volume???


:Banane35:

 

[Big Kahuna]

It may work if you were to have the dome section recut, the 650 bore is smaller than they 701, may not hurt, but I would not try it.

 

[sflsurfrider]

Some random nice guy at the local ramp last night (6-8 footers on Lake Michigan, woot!) mentioned he was running a 650 head on his 701 superjet to get his compression up. I've never heard of this before, but it does seem logical. Less displacement means less chamber volume for a given compression ratio. Given the 6m6 and 61x run the same ratio in stock form, that would give about an 8% increase in compression for the 61x with a 6m6 head.

Is this old news? Any downfalls to running this?? What is the resulting compression and/or what is the stock 6m6 chamber volume???


:Banane35:

the squish would be off due to the narrower diameter of the head vs the wider bore of the sleeve, possibly smashing the piston against the head at high RPMs, id think.

thatd be a downfall

 

[Matt_E]

Errr.....stock 6M6 bore is 77mm, stock 701 bore is 81mm.
The 701 and 760 heads allow for 3mm and 2mm overbore, respectively.

I don't think that the 6M6 head will allow for 4+ mm overbore.
Sounds like a piston smasher and/or detonator to me.

 

[sjetrider]

Put it on and check squish, the squish is huge from factory on a 61X so it may very well work to increase compression with about .045 squish, but not sure how tha shape of the dome would perform.

 

[TB]

dustin if you really wanna try it i have a 6m6 head here in great shape....i dont think i'd try it but if you want to i have one.

 

[sflsurfrider]

Put it on and check squish, the squish is huge from factory on a 61X so it may very well work to increase compression with about .045 squish, but not sure how tha shape of the dome would perform.

the squish is high, but were talking about the mating surface of the head getting beat up by the pistons.

 

[Dustin Mustangs]

This isn't common practice then, eh? The guy did say he was having some issues with his ski, lol.

So the general consensus is, given the squish checks out, this might work?? Anyone happen to know the stock chamber volume for the 6m6??? I'd like to figure out what this would get me compression wise before I spend any time or money looking into the squish issue.


thong_bandit: I may take you up on that depending on what I find out...

 

[Matt_E]

Again, the bore size of the head will be smaller than the bore on the cylinder.

Think about it, and you'll see why that is a problem.

 

[TB]

k dustin it's in the back of my van and i dont think its going anywhere anytime soon

 

[sjetrider]

the squish is high, but were talking about the mating surface of the head getting beat up by the pistons.

????????? I guess I am confused then, if there is over .035 squish how in the he!! will the piston hit the freak'N head?????????????? Please educate me here. I dont know that there will be enough squish, just saying if there were it would not be a problem (also not saying it would be good for performance gains, but it would increase compression OBVIOUSLY) .

 

[Matt_E]

A good head gasket might take care of it, as it could provide the needed squish.
Check it to be sure.

But, I would not do it. Get the stock 701 head milled, it ain't that expensive.

 

[sjetrider]

A good head gasket might take care of it, as it could provide the needed squish.
Check it to be sure.

But, I would not do it. Get the stock 701 head milled, it ain't that expensive.

I agree with this but am curious as well and would like to see him try it if he is gonna burn a head gasket anyways.

 

[Dustin Mustangs]

Again, the bore size of the head will be smaller than the bore on the cylinder.

Think about it, and you'll see why that is a problem.

Oh, I understand what you are saying matt, but I figure this is at least worth looking into given I talked to a guy last night that claimed to be running this and no one, so far, can say for sure it won't work. Not to mention, looks like I can get my hands on one pretty easy.

 

[sflsurfrider]

????????? I guess I am confused then, if there is over .035 squish how in the he!! will the piston hit the freak'N head?????????????? Please educate me here. I dont know that there will be enough squish, just saying if there were it would not be a problem (also not saying it would be good for performance gains, but it would increase compression OBVIOUSLY) .

the piston crown's diamter is 81mm. the dome's diameter is 77mm. the edge of the piston comes up to right around the deck height and bells upward from there.

if the inner edge of the dome (at the mating surface) is hovering over the crown of the piston, itd mean that part of the piston isnt even under the dome!

essentially, its the same reason you wouldnt use 83mm domes in a bigbore 760.

 

[sflsurfrider]

A good head gasket might take care of it, as it could provide the needed squish.
Check it to be sure.

by "good", do you mean "thick"?

a thick gasket will solve the pistons slapping the mating surface of the head dilemma, but if the goal is increased compression, a thicker gasket will negate putting the 650 head on in the first place.

 

[sjetrider]

by "good", do you mean "thick"?

a thick gasket will solve the pistons slapping the mating surface of the head dilemma, but if the goal is increased compression, a thicker gasket will negate putting the 650 head on in the first place.

I agree with this.

I understand the theory of the piston POSSIBLY hitting the head, but what I said was IF the squish was above .035 it would not hit OBVIOUSLY. This is why you test squish on any set up.

 

[Matt_E]

Yes, that's what I meant.
With a std thickness gasket, the piston probably will not touch the head.

I can only guess at what this does to the squish band and combustion therein.
Try it if you must, but there are better ways.

 

[sflsurfrider]

I agree with this.

I understand the theory of the piston POSSIBLY hitting the head, but what I said was IF the squish was above .035 it would not hit OBVIOUSLY. This is why you test squish on any set up.

this is true. i still wouldnt do it though.

 

[Dustin Mustangs]

I'll be sure to set up the video camera when I light the fuse, lol. Really though, at this point I'm just considering getting one of these heads and dropping a mic on it. Thanks for all the thoughts on this...