https://honda-tech.com/zerothread?id=1659352
I'm considering machining down the dome a bit on these PCT CTR pistons as well as rounding out the edges to prevent hot spots. Plus, by removing some material, the compression ratio of nearly 12:1 will lower, which is great because I'm wanting to run 93 octane. But by doing this, would it compromise the structural integrity of these casted pistons?

Also, I've seen many people on here run high CRs 11.5+:1 with high lift/relatively low duration cams on pump gas <<<taking dynamic compression in consideration... and I don't understand how their motors haven't blown up from detonation, more-or-less lasted tens of thousands of miles. Are their ignition timing retarded? Does anyone care to explain??

 

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

Also, I've seen many people on here run high CRs 11.5+:1 with high lift/relatively low duration cams on pump gas &lt;&lt;&lt;taking dynamic compression in consideration... and I don't understand how their motors haven't blown up from detonation, more-or-less lasted tens of thousands of miles. Are their ignition timing retarded? Does anyone care to explain??</TD></TR></TABLE>

Give me their compression ratio's and cam spec's and I can explain it.

 

Alright, for example Evil Nick M here on H-T is running:
b20b, stock sleeves, b16b head
cp 12.5:1 pistons
93 octane, no problems
tuned on AEM EMS
and shifting at 8500rpms
not sure which cams

How many miles can this motor hold up to? One can only wonder given all the variables such as how it's been built, tuned, maintained and driven.
But, apparently he is not facing detonation?!

Anyhow, I'm really just concerned with the first question.

 

The cam specs are what is important. The static compression ratio means nothing (as far as detonation is concerned) without them. The engine doesn't start building compression until the intake valve closes. The compression ratio that is measured from the point that the intake valve closes is called the effective compression ratio. I think you refered to it as the dynamic compression ratio but that is incorrect. The dynamic compression ratio is the effective compression ratio times the volumetric efficiency. All engines that are built correctly will have an effective compression ratio of about 8 - 8.5:1. It doesn't matter if it's a Honda, a BMW, or a Ferrari they will all be in that range (in this case with iron block/aluminum head running 93 Octane fuel - iron block and heads will be lower).

As you raise the rpm that peak torque is produced you have to raise the static comression ratio or you won't be able to make any power. This is because when you have a higher revving engine you will have a cam with more duration and the intake valve will close later which lowers the effective compression ratio. To compensate for this you raise the static compression ratio.

As far as the 12.5:1 piston goes, that is only an approximationand the only true way to know what the static compression ratio is, is to measure is with a burette. This is especially true since he did a head swap.

If the engine is built and tuned correctly and is not detonating there is no reason the engine should not last a long time.

 

So a longer duration cam will ebb off gases during overlap, therefore decreasing the dynamic and/or effective compression ratio, correct?

So how does a higher lift cam influence the compression ratio?
Like I've seen certain cams capable of producing power at higher RPMs. Like the JUN IIIs for instance I've seen time and again generate power up to 9000rpms... specs are:
12.0 mm 265.3 @ 1mm 11.5 mm 265.3 @ 1mm
Perhaps, the setup including the effective compression ratio might of allowed one motor to rev higher to produce more power over another.

Also, the Jun III cams, I believe all of Rocket's cams as well as the Skunk2 stage 3's and Toda spec C's have the same amount of duration on both the intake and exhaust cams... what aspects of the motor will change when using an intake cam with more duration than the exhaust cam?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SleepEMike &raquo;</TD></TR><TR><TD CLASS="quote">So a longer duration cam will ebb off gases during overlap, therefore decreasing the dynamic and/or effective compression ratio, correct?
</TD></TR></TABLE>

I'm not exactly sure what you mean by ebb off gasss during overlap so if you could elaborate that would be great. The reason the longer duration cam lowers the effective compression ratio is because the intake valve closes later (lets say it closes 70 degrees ABDC rather than 50 degrees ABDC). You know how when you measure the static compression ratio you measure the swept volume (the volume the piston covers from TDC to BDC) plus the clearance volume divided by the clearance volume? When you measure the effective ratio the swept volume is smaller because the piston has already moved up whatever amount corresponds to 70 degrees ABDC (in our example). To determine how much the piston has moved up you need to know the stroke, rod length, point the int. valve closes in degrees ABDC and possibly piston pin offset if it has any. Then it takes a little trigonometry to figure it out.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">
So how does a higher lift cam influence the compression ratio?
Like I've seen certain cams capable of producing power at higher RPMs. Like the JUN IIIs for instance I've seen time and again generate power up to 9000rpms... specs are:
12.0 mm 265.3 @ 1mm 11.5 mm 265.3 @ 1mm
Perhaps, the setup including the effective compression ratio might of allowed one motor to rev higher to produce more power over another.
</TD></TR></TABLE>

Higher lift has nothing to do with compression ratio. EXCEPT when you take into account this extremely misleading way of marketing cams @ 1mm or @ .050". These numbers are not that helpful unless you also have the seat-to-seat timing. It is pretty rare for cam companies to give seat-to-seat timing anymore. I just went to the Crane Cams site and while they do give seat-to-seat for American V8's the Honda's are only listed @ .050". A lot of people think that the engine isn't flowing much air under .050" but that just isn't true - especially on a 4-valve head. Low lifts are really where a 4-valve head shines. Although they do flow more air at max lift than a 2 valve head it is not that huge of a difference but at low lifts they flow a hell of a lot more compared to a 2-valve head.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">
Also, the Jun III cams, I believe all of Rocket's cams as well as the Skunk2 stage 3's and Toda spec C's have the same amount of duration on both the intake and exhaust cams... what aspects of the motor will change when using an intake cam with more duration than the exhaust cam?</TD></TR></TABLE>

The amount of intake and exhaust duration you need is based on a number of factors the main one being how much the intake and exhaust ports flow (in CFM). On one engine increasing the int. duration may make more power and on another it may make less. The fact that these companies only make 3 or 4 profiles for one engine is by far not optimal. If it were a perfect world every combination of parts would get a different cam profile.