HI all.

I have a b16A with followings.
Crower stg 2 cams
Cam gear
Single layer head gaskit (OEM)
I/H/E
Crome tuned.

My rings went bad so i replaced the hole block with a newer one. after installing this new block i compression test my car the results were.

150
150
156
153

I at that time didnt take any tension as i thought that this engine will require some running as it was sitting in from a long time. I also tested after putting in some oil in the cylinder and all the cylinder were above 220..

last night i tested the compresion and there results were.

162
158
153
160

They are allmost same as i did first time. nothing changed. people told me that my cams are bleeding compression, but the result are lower then they should have to be.

I dont know where i am stading and where is the problem....

Ill be thankfull for your inputs.

 

if your cam has a agressive primary like the skunk pro series, buddy clubs,(not sure about the crower spec's) etc.. it will have an effect on the static compression from my experience.

on my build my compression test #'s were in the 290psi range with the stock gsr cams, after i switched to the pro 2's my static comp dropped to around 250psi and has been at 250psi for the last year, i check it every few months.

Are you holding the throttle wide open when you do the test, is the motor cold or warm??

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mx621 &raquo;</TD></TR><TR><TD CLASS="quote">
Are you holding the throttle wide open when you do the test, is the motor cold or warm??</TD></TR></TABLE>

Yes! all the test are done at WOT and warmed up engine...
I can look for the specs, but they are below or equal to SK2S1.

 

if the block was sitting for a long time?

how can we know if the 150psi is already the best that block can give?

i dunno just a thought

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by d15Beta &raquo;</TD></TR><TR><TD CLASS="quote">if the block was sitting for a long time?

how can we know if the 150psi is already the best that block can give?

i dunno just a thought</TD></TR></TABLE>

 

Correction -

<FONT SIZE="3">OVERLAP DOES NOT BLEED COMPRESSION - IT BLEEDS VACCUUM</FONT>

Overlap cannot bleed compression because overlap occurs around TDC at the end of the exhaust stroke while compression occurs after the piston has swung down through the intake stroke, passes though BDC and is on its way up again on da compression stroke.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">Correction -

<FONT SIZE="3">OVERLAP DOES NOT BLEED COMPRESSION - IT BLEEDS VACCUUM</FONT>

Overlap cannot bleed compression because overlap occurs around TDC at the end of the exhaust stroke while compression occurs after the piston has swung down through the intake stroke, passes though BDC and is on its way up again on da compression stroke.</TD></TR></TABLE>

In other words.. your compression test numbers with that huge cam are fine..relax.if your worried do a leakdown.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">Correction -

<FONT SIZE="3">OVERLAP DOES NOT BLEED COMPRESSION - IT BLEEDS VACCUUM</FONT>

Overlap cannot bleed compression because overlap occurs around TDC at the end of the exhaust stroke while compression occurs after the piston has swung down through the intake stroke, passes though BDC and is on its way up again on da compression stroke.</TD></TR></TABLE>

so how does an engine decrease dynamic compression?

is it only if the engine is creating more than 100% VE?

 

Overlap can increase dynamic compression if you have a properly tuned header like RMF etc. That's how cam/valve timing interacts with the header system.

Late intake valve closing (as opposed to early valve opening as in the overlap phase) bleeds compression at low rpms because you are pushing out the charge that doesn't have enough momentum behind it.

 

<FONT SIZE="3">OVERLAP BLEEDS COMPRESSION IS THE GREATEST MISCONCEPTION OF ALL TIME - HANDS DOWN</FONT>

 

Try negative lash and see where your compression goes to.

Also try your compression test on a half dead battery.

Does anyone ever tell you the condition of their battery when reporting compression numbers?

There are mulitiple factors at play. If you don't have an understanding of them then you'll be scratching your head like a monkey in front of a mirror.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">Overlap can increase dynamic compression if you have a properly tuned header like RMF etc. That's how cam/valve timing interacts with the header system.

Late intake valve closing (as opposed to early valve opening as in the overlap phase) bleeds compression at low rpms because you are pushing out the charge that doesn't have enough momentum behind it.</TD></TR></TABLE>

ah ok. I had it backwards. got it now

so the only time it can bleed compression is between the exhaust and intake strokes at TDC. But it only bleeds dynamic compression when the engine produces more than 100% VE, correct?

then it bleeds vacuum at BDC if the intake valve is still closing after the intake stroke while the piston is moving back up the bore.




Modified by 98vtec at 3:07 PM 4/29/2008

 

Almost. Re-read what you wrote and what I wrote.

 

MOTU FTW.

Seriously, that is a very good explaination.

 

The dynamic compression bleed (actually low vacume draw) on very large cams happens only at the cranking rate or low RPM's. Vtec motors are as subject to this as others unless you do your compression test with VTEC locked. These numbers, dynamic compression, will change with RPM's, if not, then a larger cam would be usless. ALL cams run better with more compression, if you have the fuel to support it.
If you want big compression test numbers open your EX. vale adjustment by .030, just don't run the motor.

 

hmm. thanks guys...

ONE more thing, If i shave head to gain compression will it be right ??


I am thinking,,
to go with OEm gaskit and 1mm shaved head...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">Almost. Re-read what you wrote and what I wrote.</TD></TR></TABLE>

http://auto.howstuffworks.com/engine1.htm

i am watching that trying to come up with all the possibilities (even if they arent physically possible) of what you are saying. I dunno why i am having a difficult time grasping this, lol.

If you have a cam that is still closing the intake valve while the piston is making its way back up the bore, how are you not losing compression? maybe compression isnt the right term. But you would lose X amount of combustible air because it is no longer a part of the combustion process until the intake valve opens again.

now next, during the period change of the exhaust stroke and the intake stroke. I get what you are saying about losing vacuum. If the cam makes the intake valve open late while the piston is traveling towards BDC, then you lose vacuum which reduces the engines ability to fill the combustion chamber which in turn reduces VE?

then you say it can increase dynamic compression if the header is tuned correctly. is this because the header allows for the chamber to be "emptied" of the exhaust gases and allow for the engine to intake more charge?




Modified by 98vtec at 3:08 PM 4/29/2008

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 98vtec &raquo;</TD></TR><TR><TD CLASS="quote">so the only time it can bleed compression is between the exhaust and intake strokes at TDC.</TD></TR></TABLE>

One phase at a time.

Ask yourself what is the piston/motor doing during this phase.

- The exhaust stroke pushes out gases,
- The intake stroke pulls in gasses,

Where is compression occuring here? especially when compression checking at 200 rpms.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">

One phase at a time.

Ask yourself what is the piston/motor doing during this phase.</TD></TR></TABLE>

piston coming to TDC
exhaust - clearing the chamber
intake - filling the chamber
piston moving to BDC


Modified by 98vtec at 3:09 PM 4/29/2008

 

Piston moving down to BDC?

 

yea, thats what i meant. BTDC as in bottom TDC not Before TDC, lol. bad vernacular.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">

One phase at a time.

Ask yourself what is the piston/motor doing during this phase.

- The exhaust stroke pushes out gases,
- The intake stroke pulls in gasses,

Where is compression occuring here? especially when compression checking at 200 rpms.
</TD></TR></TABLE>

there is no compression happening at that time.

 

There's couple things going on in the 4 stroke motor.

You can get more VE by getting stuff to flow into the cylinder earlier at the beginning of the intake stroke (positive effects of overlap) and keep flowing into the cylinder at the end of the intake stroke and very beginning of the compressin stroke (dynamic compression, **** ramming into the cylinder).

for critical thinking.

 

myt be OT, MOTU, does that help with stacking up flow on the intake side.. with regards to u mentioning "getting more VE"

?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by d15Beta &raquo;</TD></TR><TR><TD CLASS="quote">myt be OT, MOTU, does that help with stacking up flow on the intake side.. with regards to u mentioning "getting more VE"

?</TD></TR></TABLE> That is RPM and velocity matched to the correct IN cam closing number.