760 Domes on 701 Question

[Pro-pulsion]

I have a pair of 760 31cc domes, they came with the ADA head when I bought it. I never ran them as I was running a dry pipe and recommended by the manufacturer to run 37cc domes to keep compression at about stock numbers. But I had to move on to a B pipe, and now that I can play with compression if I decide to, I was wondering about what kind of numbers the 760 31cc domes will generate on a 701. I have been told that I can run them, would there be any concerns with that if compression numbers are known to stay in the pump gas range? Wouldn't the squish band be out past the top of the cylinders instead of matching the bore?

 

[37]

Yes, you can run 85mm domes on an 82mm bore. You'll likely get a bit higher compression ratio equal to 30.5 or 30cc (~12.5:1 to 12.7:1) than you would with the correct size. Quite a few OEM chambers are larger than the bore. The 2003-2011 SX-R is a great example with an 80mm bore and 82mm domes. Edit: I had that backwards... it had an 82mm bore and 80mm dome.

31cc domes are outside of pump gas range unless you never see wide-open throttle. I'm a firm believer that compression ratio determines fuel requirements, not cranking compression. 37cc is the smallest you'd want for non-ethanol 91 octane.

 

[Pro-pulsion]

Thank you sir, that's quite interesting about the domes being larger from the OEM. I never knew that.

 

[cookn]

you just dont want to run smaller bore domes with a larger cylinder bore, it would just tighten the squish band up to the point of detonation

 

[Pro-pulsion]

Agreed, I have 35cc domes for the 701 application, the 37cc domes I have are also meant for my 701 but I often wondered how having the larger 760 domes at a smaller dome size by the cc numbers would affect compression numbers. It was just theory at the time but I wondered if the larger 760 domes at 31cc, would be the same as the smaller 701 35cc domes in compression.

 

[CRJ]

I had ATP 760 domes on my 650 for a few days to ride, not the best idea, and compression was lowered, i was just advised that carbon will build up on top of the skirt. As well, the skirt doesn't have proper top support, so the chance of the skirt shifting when it warms up is a bit higher depending on how tight it is in there. Just what I was told, it still ran fine though!

 

[37]

It was just theory at the time but I wondered if the larger 760 domes at 31cc, would be the same as the smaller 701 35cc domes in compression.

It depends entirely on the dome design and squish geometry. If there's a large gap created at the outer perimeter of the squish band then it will help prevent detonation up until the point when it packs up with carbon deposits. At that time it will act like a standard smaller CC dome (say, 30.5cc or 30cc). It's best to assume that whatever area is created beyond your cylinder bore diameter isn't really there. CC'ing the assembled dome volume will give you a false result if the gap is too big.

This is probably a "picture is worth 1000 words" moment but I don't have one handy.

It will definitely work and run fine. You just have to be careful with the results. Assume the 85mm dome on an 82mm bore is 0.5~1cc smaller than the marked size and you'll be on the right track.

@CRJ makes a great point about sealing. Make sure your o-ring matches the top of the sleeve correctly. It should but it doesn't hurt to check.

 

[Pro-pulsion]

Great info guys, thank you kindly for this. I always had that gut feeling to keeping those 760 domes on the shelf and now I am quite happy I did. One follow question which is semi-related...is there any real, noticeable improvement with increasing compression on the 701? I know this next part is comparing apples to oranges if I had the experience of a Yami prior, but I did have a Kawi 650sx. I went from stock compression at something like 140 to a Mariner high compression head at I believe it was 205 or maybe 215 compression, never noticed a thing for difference. I was running 98 octane on that setup too. Never blew it up, but popped a couple of head gaskets. I went back to the stock head and did same day comparative testing...never noticed a change in performance. Is it a whole different take on the 701 or is the "felt" difference negligible at best?

 

[CRJ]

Some will say it does, im on the other side. I didnt notice much improvement with my head and 150 to 180psi. I noticed more with pipe and carb size upgrade. I now keep a stock head so I can run any fuel 87+, and its reliable. (I like to venture off solo at times)

 

[Pro-pulsion]

Agreed CRJ, I've been running 37cc domes at about 155 compression and love it. Don't figure I need to change. The crank will be happy as well since higher compression does shorten bearing life. I'm with ya on going solo lol

 

[cookn]

You can still have a reliable engine with higher compression, I know people that run 200psi on pump gas and have never had an issue. fuel requirements are not just based off compression alone but also the squish clearance and timing advance. Sure you can run things on the safer side and get more hours out of your engine but what fun is that when you can dial in your setup for kill mode and get some big air. Setup properly your engine will still last for a long time but its all in what you are trying to get out of that setup.

 

[JetManiac]

You can still have a reliable engine with higher compression, I know people that run 200psi on pump gas and have never had an issue. fuel requirements are not just based off compression alone but also the squish clearance and timing advance. Sure you can run things on the safer side and get more hours out of your engine but what fun is that when you can dial in your setup for kill mode and get some big air. Setup properly your engine will still last for a long time but its all in what you are trying to get out of that setup.

Well said!

Great info guys, thank you kindly for this. I always had that gut feeling to keeping those 760 domes on the shelf and now I am quite happy I did. One follow question which is semi-related...is there any real, noticeable improvement with increasing compression on the 701? I know this next part is comparing apples to oranges if I had the experience of a Yami prior, but I did have a Kawi 650sx. I went from stock compression at something like 140 to a Mariner high compression head at I believe it was 205 or maybe 215 compression, never noticed a thing for difference. I was running 98 octane on that setup too. Never blew it up, but popped a couple of head gaskets. I went back to the stock head and did same day comparative testing...never noticed a change in performance. Is it a whole different take on the 701 or is the "felt" difference negligible at best?

The ADA head will raise compression alot or a little depending on which domes you choose, but power is also inceased with the much tighter squish than stock head and gasket and different dome shape. Power increase is quite noticable as many of our customers give us feedback.