compression for SE 257cam

[wewe1140]

Mike,

I am interested in a build for my 07 Ultra.I would like to go 103 with Gerolomy heads and a SE 257 cam. What I would like to know is what do you think would be an acceptable CR for this cam. I am assuming in between 10.25 to 10.5 would be acceptable.

[Mike]

10.5 gives a corrected pressure of 9.3 (with the 257 cams), which is just about right. I like 9.3-9.4 generally.

[COVSpringer08]

How do you come up with the 9.3?

[Mike]

It's base on the "mechanical" compression, cylinder volume and the cam intake closing. The cylinder does not start making pressure until the intake valve closes. For example, a 103ci motor with 10.2:1 mechanical compression will have a "corrected" pressure of 9.2 when the intake valve closes at 45 degrees, as in SE211 cams. The same motor/mech comp with SE264 cams (60 degree closing time) will have a corrected compression of 8.4 and run lazy ( loss of torque) due to lack of cylinder pressure. Moving the mechanical to 11.2 would increase corrected to 9.4, restoring much of the lost torque.

It's the "corrected" pressure that we shoot for when selecting cams and/or compression ratios. To get our desired corrected, we can either add/subtract mechanical compression to match a given cam or choose a cam with timing that best suits a given mechanical pressure. The higher the corrected, the better our torque curve will be, but much above 9.5 can get into detonation problems on street motors. This is why 9.4 is our most often targeted value.

[COVSpringer08]

My 08 SE 110 has 9.3:1 static compression and with SE 255 cams has 9.06:1 corrected compression and 187 cranking? Does this look correct.. Do you mainly look at cold compression or HOT?

Thanks!

[Mike]

We typically look at cold compression numbers. Cold is also what most advertised numbers are referrencing and most commonly used computer modeling does not take this into account (more sophisticated models do, but again, it turns into a lot of data input that really does not change the compared differences we are looking for). Since the numbers are used for comparison only, calculating hot serves no purpose other than confusing the math. The differences, when comparing, would be the same whether hot or cold. Hot can be difficult to measure and can introduce significant variables if not using exactly the same temperature for each measurement or calculation. If one were to use hot, you would want to use hot for ALL calculations, never comparing hot to cold.

Having said all that, we take cranking pressure checks with WARM motors, so that cold oil does not mask possible problems when doing so for diagnostic reasons. We also routinely check this number to get baselines for various motor combo's and future reference. Doing same on a hot motor can damage the tool and just makes life difficult for the tech doing the diagnostic. Oddly, these numbers will often be HIGHER, on a well sealed motor, when done hot vs cold even though the computer model says the opposite. This is because the motor cranks easier (faster) causing an increase in VE.

[COVSpringer08]

[QUOTE=COVSpringer08;50723]My 08 SE 110 has 9.3:1 static compression and with SE 255 cams has 9.06:1 corrected compression and 187 cranking? Does this look correct?

I found a online calculator and tried all types of compressions and cams.
The Jims 120 has 10:1 and with 264 has only 8.31:1 and 167 cranking (is this correct) Looks if you had a Jims 120 the first thing to do is up the compression

And a 2005 103 has 9:0:1 and 7.8:1 with SE 253s.. I now see why people change to the RS 575s it ups the corrected to 8.3:1 (I'm learning now thanks Mike!!)

Thanks

[Mike]

Jims likes the lower corrected to ease starting and reduce detonation. Basically overcamed. Bringing the compression to the cam really helps (10.5 is now an option in the 120's).

[COVSpringer08]

I seen that on the site... I thinking about getting a Jims 120?

Are my numbers above correct? or off

What are my ACR good for far as cranking compression and when do you need CRs?

[Mike]

Your numbers are correct for comparison purposes. Actual measured numbers will vary due to gauge and method differences.

Typically, cranking pressures above 200 (as measured on a Snap-On gauge, throttle wide open, good battery, motor warm) would benefit from CR's. This number may go up or down depending on motor size. Of course, we like to know this before the motor is built and since computer modeling typically shows lower numbers than those actually measured, the computer number would be around 170lbs. We kind of know it intuitively by the mehcanical compression, cam and motor size. No real downside to having them (other than added cost) so many engine builders recommend them on all builds, to be safe. They can help also if the battery gets low.

[COVSpringer08]

Thank you..

I don't know how far down the rings are on the pistons or the CC for sure. I was just getting close numbers.

I'm going with the Zippers rs575 in my 08 110 could you run the numbers on it to get 9.4:1?

Thanks again.

[Mike]

For the really technical guys, you want to figure area above rings and head gasket I.D.. Personnally, I like to just measure the assembly with fluid to get the actual number.

[COVSpringer08]

Mike

On my 08 110 i'm going to get a set of Axtell clys to fixs my oil leaks.. Would you go with 4.060 bore or bigger? At this time i'm going to have the timken bearing upgrade done & have the crank trued.. I have heard the case has to be bored for anything larger than 4.000?? is this correct.

[Mike]

Yes, the cases will require boring for either the 4.060 SE or Axtell 4.125. Since the cases will be apart for the timken conversion, this is the time do go to the larger bore. We offer both of these services, if needed.