Super Jet compression numbers and proper dome size

[Pablo]

What compression numbers should I be getting with 760 BB, flat tops, 38 domes? I got 150 for both holes. Figured higher. Will flat tops drop compression numbers vs. dome top pistons?

 

[GIL]

Yes the #s will drop big time! You have to run flat top domes. I have a 838cc 5 mil stroker w/85.5 flattops and I run 26cc domes to get 200psi. Keep in mind it all depends on how your motor is set up.

 

[SUPERTUNE]

Start with some ADA # YAK8628L15, 86mm bore with 28cc, and a .015 squish step in the dome.
Then you can see where your at.

 

[Pablo]

My current head is an ADA that was setup for TCurtis with these same 85mm flat top pistons. I thought they were 35 domes, but was corrected and told they should be 38's.
I had the head off last weekend looking at pistons/cylinder, but forgot to look at the domes to investigate size. I was having a hell of a tuning issue, so I checked compression and found it at 150 both sides. Any idea what type of compression numbers I should be getting with 38 or 35 domes. It's a cast, DASA cylinder, freestyle ported by DASA. I'm also thinking some of the culprit may be the lack of an actual base gasket. It appears that the whoever installed the cylinder used a liquid base gasket material. Almost like what is used to seal cases together. Can I assume that this decreased cylinder height (from no solid base gasket) will change my expected compression numbers.

 

[Waternut]

Hold on now... I was just guessing on the dome size based on what most people run on a 760 pump gas engine. I don't have any proof one way or another.

If the previous guy used a form of crankcase sealant between the cylinder and case, it would actually lower the ports and raise the compression numbers as it removes volume.

 

[GIL]

I am guessing that if u ran 35 or 38s that you would not have enuf comp to even run.

 

[Pablo]

Hold on now... I was just guessing on the dome size based on what most people run on a 760 pump gas engine. I don't have any proof one way or another.

If the previous guy used a form of crankcase sealant between the cylinder and case, it would actually lower the ports and raise the compression numbers as it removes volume.

I didn't label you as the "informer." Quit being sensitive...must be that time of the month. LOL! I have no idea what size the domes are. I'll prolly pull them this week and get the real answer.

 

[Pablo]

I am guessing that if u ran 35 or 38s that you would not have enuf comp to even run.

Huh? 35's with that bore would not give enough compression, really? 35's should give 180's compression on stock bore, correct? IDK either way. I'm just surprised by that info. I'll find out what size they are and report. Does anyone have ballpark numbers for where the compression should be with different dome sizes on my setup? ported cylinder/cases 760BB w/ flat tops and whatever domes (30?, 33?, 35?...).

 

[GIL]

I am NOT slamming you, just asking a serious question-do you know how to check squish? If so, hit me up and I will send u some domes to try along with a oem base gasket. I am thinking a 020 base gasket and some YAK XS 25L35 domes might get you in the ballpark. The L35 domes gave me waay too much squish because I run a thicker base gasket to raise my port timing a fuzz. Let me know what you find after you get your domes out and read the #s.

 

[Pablo]

no offense here Gil....just doing some "sheet" talking. I don't know how to check squish, but would like to learn. I'll hit you up in a minute. I'm always looking for info on tuning. Thanks brother.

I am NOT slamming you, just asking a serious question-do you know how to check squish? If so, hit me up and I will send u some domes to try along with a oem base gasket. I am thinking a 020 base gasket and some YAK XS 25L35 domes might get you in the ballpark. The L35 domes gave me waay too much squish because I run a thicker base gasket to raise my port timing a fuzz. Let me know what you find after you get your domes out and read the #s.

 

[SUPERTUNE]

Huh? 35's with that bore would not give enough compression, really? 35's should give 180's compression on stock bore, correct? IDK either way. I'm just surprised by that info. I'll find out what size they are and report. Does anyone have ballpark numbers for where the compression should be with different dome sizes on my setup? ported cylinder/cases 760BB w/ flat tops and whatever domes (30?, 33?, 35?...).

No, not correct. Your brain is thinking like your using Yamaha top pistons!
They have a WAY more positive displacement cc's than flattops do!

Alright now... You need to get back to basics here as you don't seem to have a clue to what your doing or getting bad info and/or advice.
You said you have flat top pistons. Now I have to ask WHO's pistons? What kind of flat top pistons do you have?
Kawasaki small pin pistons and the older Dominator pistons made by R&D are the same 2* degree flat top style piston.
The Riva pistons are different, they are a true 0* degree flat top piston and have no positive cc displacement unlike the Kaw and R&D piston which do have some positive displacement.
So the first thing you need to do BEFORE taking off the head, is to go to Radio Shack get some 60/40 solder in .063.
With the plugs out bend a piece 90* degrees about "1 1/2 from the end.
Now with the piston down the bore, thread in the solder in the cylinder and hold solder against the cylinder wall pointing to the front so your over the wrist pin of the piston, with coupler cover removed, grab by hand and roll engine over to past TDC to hopefully squish the solder. Pull out solder and now measure in 2 places where it's crushed. At the very end and inside next to where the solder is not crushed. This will tell you if you have the correct squish angle.
With o-ringed head I like to see .002-.005 difference with the tighter squish at the outside.
.045-.048 is the normal average squish.
Squish needs to be with in this range first before deciding what cc domes you need.
Now take off the head and check your piston deck height at TDC and look for any numbering on the pistons to ID them as to what you have.
The next thing would to measure the top deck to top of exhaust port in mm to use the online degree calculator, http://john.rushworth.com/Pics/Piston/ or use a degree wheel to measure when the exhaust port opens. Once to get this far and have some very specific answers to these questions then some good advice can be recommended.

 

[Pablo]

Many thanks Chuck. Riva pistons, dominator sleeves with porting by DASA. It may be a few days before I can report back with numbers.

No, not correct. Your brain is thinking like your using Yamaha top pistons!
They have a WAY more positive displacement cc's than flattops do!

Alright now... You need to get back to basics here as you don't seem to have a clue to what your doing or getting bad info and/or advice.
You said you have flat top pistons. Now I have to ask WHO's pistons? What kind of flat top pistons do you have?
Kawasaki small pin pistons and the older Dominator pistons made by R&D are the same 2* degree flat top style piston.
The Riva pistons are different, they are a true 0* degree flat top piston and have no positive cc displacement unlike the Kaw and R&D piston which do have some positive displacement.
So the first thing you need to do BEFORE taking off the head, is to go to Radio Shack get some 60/40 solder in .063.
With the plugs out bend a piece 90* degrees about "1 1/2 from the end.
Now with the piston down the bore, thread in the solder in the cylinder and hold solder against the cylinder wall pointing to the front so your over the wrist pin of the piston, with coupler cover removed, grab by hand and roll engine over to past TDC to hopefully squish the solder. Pull out solder and now measure in 2 places where it's crushed. At the very end and inside next to where the solder is not crushed. This will tell you if you have the correct squish angle.
With o-ringed head I like to see .002-.005 difference with the tighter squish at the outside.
.045-.048 is the normal average squish.
Squish needs to be with in this range first before deciding what cc domes you need.
Now take off the head and check your piston deck height at TDC and look for any numbering on the pistons to ID them as to what you have.
The next thing would to measure the top deck to top of exhaust port in mm to use the online degree calculator, http://john.rushworth.com/Pics/Piston/ or use a degree wheel to measure when the exhaust port opens. Once to get this far and have some very specific answers to these questions then some good advice can be recommended.

 

[Waternut]

I didn't label you as the "informer." Quit being sensitive...must be that time of the month. LOL! I have no idea what size the domes are. I'll prolly pull them this week and get the real answer.

Just trying to keeps the facts straight is all...

 

[#ZERO]

The Riva flat tops pistons & ADA flat top domes are not really compatible because the Riva pistons are completely flat and the ADA domes have a 4-degree squish angle that is more suited for the R&D Dominator or Kawasaki style pistons which have a 2-degree angle. You may need a set of custom machined flat top ADA domes with the correct squish angle to obtain a good (MSV) maximum squish velocity depending on your deck height and other factors with smaller CC domes.

 

[Pablo]

We're starting to get aweful technical here. I'm getting 150 per hole now. I was really trying to find out should I have more compression than 150? I still need to pull the head and inspect the domes for size and get the numbers Chucky was talking about. Where can a beginning tuner get his hands on info that explains more about squish, squish angles, and MSV? If you guys have time to type and inform me, I'm all ears, but I'll be happy to read up on it as well. Maybe have time to pull the head tonight or tomorrow.

The Riva flat tops pistons & ADA flat top domes are not really compatible because the Riva pistons are completely flat and the ADA domes have a 4-degree squish angle that is more suited for the R&D Dominator or Kawasaki style pistons which have a 2-degree angle. You may need a set of custom machined flat top ADA domes with the correct squish angle to obtain a good (MSV) maximum squish velocity depending on your deck height and other factors with smaller CC domes.

 

[egbrig]

There is some good info on this board. As soon as you realize that psi in a hole is of little use except to monitor the condition of cylinder then you will be on your way.
If someone starts talking psi then go and find another post with the info.

 

[#ZERO]

We're starting to get aweful technical here. I'm getting 150 per hole now. I was really trying to find out should I have more compression than 150? I still need to pull the head and inspect the domes for size and get the numbers Chucky was talking about. Where can a beginning tuner get his hands on info that explains more about squish, squish angles, and MSV? If you guys have time to type and inform me, I'm all ears, but I'll be happy to read up on it as well. Maybe have time to pull the head tonight or tomorrow.

I posted some of this information below a while ago in the port timing thread and you'll have to go with smaller CC domes if you want to increase the compression.




The tapered angle on the squish band is there to help the squish action; it forces the charge to where the spark plug is located and is generally one or two degrees more than the angle of the piston. A narrow squish band ratio allows for more of the total intake charge to be located in the combustion dome resulting in more power potential during spark ignition near TDC. The down side is that there is more piston/dome area that is exposed to combustion on a highly loaded engine and heat build-up can become an issue that could eventually lead to detonation. As RPM increases, there is less time for combustion and a shallow dome designs will take less time to scavenge and refill vs. a head with a deep dome. Most pump gas engines tend to like a wide squish band width and smaller diameter domes with more volume. Race gas engines like narrow squish band widths and larger diameter domes with less volume. The squish band area or ratio can vary with each engine and usually is between 30 - 60 percent of the bore size and the larger squish ratios require greater squish clearances to get equal velocities. There are many factors that come into play to get the correct maximum squish velocity or (MSV), such as; RPM, squish clearance, squish ratio, rod ratio, compression ratio, port timing, blending radius, diverging squish angles, fuel type, etc. and it can get quite complicated. You'll need a computer program such as the Bimotion advance head design software to calculate all the factors. This squish program enables you to calculate the MSV of turbulence in the combustion chamber. This is an important design tool for finely targeting the hit in the power band and to determine a link between power band and fuel. The squish ratio, dome shape, compression, MSV, etc., all work synergistically while at the same time are all a compromise. There's a fairly delicate balance that needs to be maintained and small changes are better than larger ones. Increased MSV will help speed along the flame front and complete the combustion process more efficiently, but fuel choice will limit MSV. There is a point where the friction from the squish action can be so great and happen so fast that it generates enough force/heat to ignite the charge. Some basic MSV numbers that have been around for some time range from 15 - 30 meters per second. This is divided into two groups, high MSV and low MSV. Lower MSV numbers, around 15 - 19 m/s, generally make power from peak torque to max RPM or mid-top power. Higher MSV numbers from 20 - 30 m/s generally make power up to peak torque and then start to go flat. Also MSV numbers that are too high will kill off over-rev and lead to detonation.

Here are a few things that can either raise or lower the MSV.

Factors that raise the MSV:
Lower Compression Ratio
Less Squish Clearance
Larger Squish Area Ratio
Higher RPM
Smaller Bore
Longer Stroke
Shorter Rod


Factors that lower the MSV:
Higher Compression Ratio
Greater Squish Clearance
Smaller Squish Area Ratio
Lower RPM
Larger Bore
Shorter Stroke
Longer Rod

Here's the Bimotion head software: http://www.bimotion.se/

Free squish velocity program: http://www.torqsoft.net/squish-velocity.html

 

[Pablo]

Holy cheet, #zero! I was actually able to follow most of what you explained. My tuning knowledge base is low, but I'm learning every day. Since my compression #'s seem low, I'm assuming it's cylinder/piston/ring wear. We'll find out soon enough. Should squish be effected by cylinder/piston/ring wear? I'm gonna hit the links up you gave me and see what I can cull out of it. What are the effects of squish values that are too high or too low? Detonation is one guess and the other??? I couldn't pull the head tonight due to family stuff but will most likely get it tomorrow afternoon and get squish values and dome sizes.

I posted some of this information below a while ago in the port timing thread and you'll have to go with smaller CC domes if you want to increase the compression.



The tapered angle on the squish band is there to help the squish action; it forces the charge to where the spark plug is located and is generally one or two degrees more than the angle of the piston. A narrow squish band ratio allows for more of the total intake charge to be located in the combustion dome resulting in more power potential during spark ignition near TDC. The down side is that there is more piston/dome area that is exposed to combustion on a highly loaded engine and heat build-up can become an issue that could eventually lead to detonation. As RPM increases, there is less time for combustion and a shallow dome designs will take less time to scavenge and refill vs. a head with a deep dome. Most pump gas engines tend to like a wide squish band width and smaller diameter domes with more volume. Race gas engines like narrow squish band widths and larger diameter domes with less volume. The squish band area or ratio can vary with each engine and usually is between 30 - 60 percent of the bore size and the larger squish ratios require greater squish clearances to get equal velocities. There are many factors that come into play to get the correct maximum squish velocity or (MSV), such as; RPM, squish clearance, squish ratio, rod ratio, compression ratio, port timing, blending radius, diverging squish angles, fuel type, etc. and it can get quite complicated. You'll need a computer program such as the Bimotion advance head design software to calculate all the factors. This squish program enables you to calculate the MSV of turbulence in the combustion chamber. This is an important design tool for finely targeting the hit in the power band and to determine a link between power band and fuel. The squish ratio, dome shape, compression, MSV, etc., all work synergistically while at the same time are all a compromise. There's a fairly delicate balance that needs to be maintained and small changes are better than larger ones. Increased MSV will help speed along the flame front and complete the combustion process more efficiently, but fuel choice will limit MSV. There is a point where the friction from the squish action can be so great and happen so fast that it generates enough force/heat to ignite the charge. Some basic MSV numbers that have been around for some time range from 15 - 30 meters per second. This is divided into two groups, high MSV and low MSV. Lower MSV numbers, around 15 - 19 m/s, generally make power from peak torque to max RPM or mid-top power. Higher MSV numbers from 20 - 30 m/s generally make power up to peak torque and then start to go flat. Also MSV numbers that are too high will kill off over-rev and lead to detonation.

Here are a few things that can either raise or lower the MSV.

Factors that raise the MSV:
Lower Compression Ratio
Less Squish Clearance
Larger Squish Area Ratio
Higher RPM
Smaller Bore
Longer Stroke
Shorter Rod


Factors that lower the MSV:
Higher Compression Ratio
Greater Squish Clearance
Smaller Squish Area Ratio
Lower RPM
Larger Bore
Shorter Stroke
Longer Rod

Here's the Bimotion head software: http://www.bimotion.se/

Free squish velocity program: http://www.torqsoft.net/squish-velocity.html

 

[GIL]

Hey Bro, Chuckie and #0 are ALOT smarter than I will ever be. If you end up needing my back up domes you are more than welcome to them. If it were me, and at 1 time it WAS...... I would pick one smart person and follow their every detail !!! When I bought my Lamey I got what I asked for and then a week later changed my mind as to what it was I wanted. I talked to alot of people and got good advice from all them which was to pick a person and go all the way with that 1 person/builder. I know exactly what to do for my motor but that don't mean it works on any other motor. Lets face it, CHuckie can probaly get big Hp #s out of a 300SX motor!!!

 

[Pablo]

Got squish numbers last night. Didn't have time to pull the head though. average squish was 0.040, with a low of 0.039 and high of 0.041. I failed to get squish angle as well. Compression checked out again at 150.