Yamaha 701 With High Compression And Tight Squish

[METALMULISHA559]

I recently finished breaking in my SBT Yamaha 61x/62T 701 and decided to put my ADA girdled head on with 35cc domes. After installing the head I did a compression test which read about 195 PSI in both holes. With the stock head, the PSI is about 175. I also tested compression with a Snap-On guage and an Autozone loaner and the results were about the same (190 to 195 PSI). This made me go an check squish with the ADA head installed and the readings were about 0.034 in both holes. I am limited on time as I plan to ride this weekend so I am tempted to just run it with 91 octane pump gas and see what happens but I want some more opinions first. Will is be relatively safe to run compression that high with pump gas? I ride freestyle and do mostly old school tricks like subs, fountains, and slides. I do like to go WOT on occasion but usually only for 10 seconds or less.

Full Specs:
Yamaha 61x/62T SBT (stock bore)
Limited B Pipe
TDR waterbox
Lightened Flywheel
MSD enhancer
Yamasaki coil
ADA girdled head with 35cc domes
Dual Mikuni SBN 38
VForce 2 Reeds

I am also considering putting the stock head (64u) back on until I can get custom cut domes to resolve the squish issue. Please let me know as I am torn on which way to go at this point. I want to just send it! lol

 

[te4250]

I wouldn't run it on 91, I've seen deto on 180psi cranking with 93. Can you just go somewhere to get some av gas to mix 50/50?

 

[long beach local]

Another option could be adding a thicker base gasket I use a .025 to add squish and also lowers compression too. WOT extended runs causing detonation have been a problem for me. It seems like my stuff runs a tad better with adding a gallon of race gas to the mix versus 50/50 av gas .

 

[aboarder]

Play it safe. That's tight squish. Either use the stock head or thicker base gasket. If you can't get a gasket by the weekend, buy some gasket paper and cut your own. That would probably be the easiest way to get the thicker base gasket. It's very likely that motor will blow if you run as is.

 

[DylanS]

I’ve run tons of motors at .035 squish clearance including big bore stroker motors and never once had a piston hit my head.
.032 is pushing it.
I had a buddy that ran at .030 that did have a piston hit the head when he left the water and never let off the throttle.
.04 is the general safe amount.
Run the stock head if you’re worried for the weekend, then get cha self a nice thin base gasket from @SUPERJET-113 and have your domes cut for .04.
I think you could get away with 93 without wide open blasts across the lake but it wouldn’t hurt to mix some Av gas for your setup.

 

[METALMULISHA559]

Play it safe. That's tight squish. Either use the stock head or thicker base gasket. If you can't get a gasket by the weekend, buy some gasket paper and cut your own. That would probably be the easiest way to get the thicker base gasket. It's very likely that motor will blow if you run as is.

Well put. This motor only has like 15 gallons of gas through it so the last thing I want to do is blow it up. (The Wife would kill me also if I need to spend more money and time on it also lol).
I guess a thicker base gasket is the cheapest and best option if I want to run this head.

 

[Storbeck]

It would not surprise me if the cylinder was decked slightly which is what is causing the tight squish and high compression. It would also not surprise me if the shapes of the ports in the sleeves are all wrong (seems to be a thing on sbt motors) so you might have very low port timing which would increase cranking compression.

In either of those cases the thicker base gasket would be a step in the right direction.

 

[mikidymac]

Pipe, high compression head, enhancer, lightened flywheel is a great start to a performance engine but then you do an SBT reman?

Our fingers and toes are crossed for you my friend.

SBT bores everything to 1.0 over no matter what, decks the cylinders then cuts the heads to lower compression, not the best base for a performance engine.

 

[fx1mark]

It would not surprise me if the cylinder was decked slightly which is what is causing the tight squish and high compression. It would also not surprise me if the shapes of the ports in the sleeves are all wrong (seems to be a thing on sbt motors) so you might have very low port timing which would increase cranking compression.

In either of those cases the thicker base gasket would be a step in the right direction.

SBT cylinders do have small ports. I was running an sbt cylinder. I found a post online talking about the small ports. (I will see if I can find it) I got out my endoscope and looked at the ports on my SBT cylinder compared to another motor I have that has a stock cylinder (both 61x) and the ports on the SBT cylinder were smaller. I ended up buying a good used oem cylinder and the ports were quit a bit larger on the oem cylinder. There was quite a bit of performance gain swapping to the oem cylinder. I sold my still good SBT cylinder on ebay. I didn't do compression comparisons between the two.

 

[mikidymac]

SBT cylinders do have small ports. I was running an sbt cylinder. I found a post online talking about the small ports. (I will see if I can find it) I got out my endoscope and looked at the ports on my SBT cylinder compared to another motor I have that has a stock cylinder (both 61x) and the ports on the SBT cylinder were smaller. I ended up buying a good used oem cylinder and the ports were quit a bit larger on the oem cylinder. There was quite a bit of performance gain swapping to the oem cylinder. I sold my still good SBT cylinder on ebay. I didn't do compression comparisons between the two.

But what about your SBT warranty?

 

[METALMULISHA559]

Pipe, high compression head, enhancer, lightened flywheel is a great start to a performance engine but then you do an SBT reman?

Our fingers and toes are crossed for you my friend.

SBT bores everything to 1.0 over no matter what, decks the cylinders then cuts the heads to lower compression, not the best base for a performance engine.

My last motor was an SBT so when I blew it (water ingestion), The cheapest and fastest option was an SBT swap. I could not afford a proper rebuild at that point and even more so now.

 

[mikidymac]

My last motor was an SBT so when I blew it (water ingestion), The cheapest and fastest option was an SBT swap. I could not afford a proper rebuild at that point and even more so now.

I can understand and for a bone stock replacement, SBT typically works fine but since you are adding performance parts I can only say good luck.

 

[METALMULISHA559]

But what about your SBT warranty?

View attachment 425808

Warranty is only good for one year and expires in a few months. I already burnt that bridge by pulling the stock head off to install this ADA head. I just wanted to get this one fully broken in before I installed the ADA head. So since I broke it in officially, I opted to install my ADA head again to have the final bolt on upgrade installed so I don't have to mess with it again for a long time.

 

[mikidymac]

Yes, I was joking about the warranty.

 

[fx1mark]

Here is the post I was referring to. My port sizes were identical. https://www.x-h2o.com/threads/what’s-up-with-these-small-ports-on-61x-cylinder.189857/

 

[METALMULISHA559]

Okay guys, after much debate with several awesome people ^^^, I've decided to go with a thicker base gasket (0.040) so I can still use the ADA head with 35cc domes. Gasket is set to arrive this Thursday so I'll have the motor put back together Thursday night so I can re-test squish and compression. Squish should be around 0.55 ish and PSI should be about 185. Fingers crossed! otherwise my weekend is shot lol.