> -----Original Message-----
> From: David Gersic
>
> On Tuesday 13 October 2009 17:47:00 Jamie Calder wrote:
> > I was able to find the information needed for Ross to figure out the
> > compression ratio for the pistons built by them. They said "Job number
> > 105995 has a compression of 10.5:1 with valve pocket diameters of 2.150
> > intake and 1.700 exhaust."
>
> As previously noted, though, the "compression ratio" of a piston, by
> itself,
> is a meaningless number. You need to take the entire engine in to account
> to
> get the actual compression ratio. But at least now you know what pistons
> you
> have. That's good.
Agreed!
> > What do they mean by valve pockets?
>
> I'm guessing, but the pistons probably have some reliefs cut in to the top
> surface to make room for the valves to be open at TDC without hitting. The
> diameter of these reliefs are 2.15" and 1.70". That, then, should dictate
> how
> big a valve you can run without hitting. If you were to put 2.20" intake
> valves in, for example, they'd hit the piston unless you restricted the
> valve
> lift pretty severely.
Agreed!
> > And how is that used to figure out
> > compression ratio?
>
> You need the volume of the reliefs, not the diameter.
Agreed, and to figure static compression ratio you need all of that plus
bore size, combustion chamber volume, head gasket thickness, diameter of the
bore hole in the gasket, shape of the piston top (flat, domed or dished),
and distance from the deck to the piston top.
> > I did tell them that R/T heads with 2.02"/1.625" valves
> > will be used. Maybe they pulled up all the R/T head specs to figure it
> > out?
>
> Could be, but probably not, unless they also have the specs for the rods,
> crank, and cylinder diameter. More likely what you got was the advertised
> compression ratio for the pistons.
Agreed!
> > Also, what are the pros/cons of a 10.5:1 ratio. Seems a bit high.
>
> Very roughly, the higher compression ratio, the more potential power the
> engine makes, but the higher the risk of detonation. You can reduce the
> risk
> of detonation with cam specs, timing, engine management computers, higher
> octane fuel, water / alcohol injection, or some combination of these. Your
> goal is to get the most usable power from the engine, in the RPM range you
> intend to use it in, for the least fuel burned, without breaking anything.
Agreed, and you can also use a thicker head gasket. Cometic will make them
to your specs if they don't have one already that fits your need. If using
Felpro I'd go with their thickest one.
Personally, I wouldn't be too concerned about an "advertised" 10.5:1 ratio
piston because what will really matter is the cylinder "pressure" you end up
with. That can be optimized for your use with your head gasket and
camshaft choices.
> > Here's one opinion on the build:
> >
> > "First, dump the cam that you got from ***. The lift is horrible. The
> > duration numbers are weak, and that LSA is going to be bleeding off all
> > your compression. Then again, looking at your pistons I recognize one
> of
> > ****** typical tactics. He likes to sell people high compression parts
> like
> > those 10.3:1 pistons, and then put in a real narrow LSA like that 108
> > degree spread to keep a lot of overlap to make up for it.
I agree 100%....and I know who we're talking about. :)
> Given that you already have the short block and heads, you probably ought
> to
> check with a couple of cam grinding companies to get their recommendation
> on
> selecting a cam to work with this package.
>
> Got intake, fuel, and engine management figured out yet? You'll need to
> know
> if it's going to be using a carburetor or injectors. What intake you plan
> to
> use. Stock engine management computer, or something aftermarket? Any
> forced
> induction (blower, turbo, NOS) plans?
>
> It'd also help to know what you plan to use this engine for once you get
> it
> built. Is this a dragstrip motor? Street motor? Towing or not? Just
> looking
> for a nice seat-of-the-pants kick when you step on the gas? Mostly low
> speed,
> or high speed driving?
Intended use will determine pretty much everything else needed to build an
efficient combination. Anything other than track only use will require
compromises. A race only combination would absolutely suck on the street
while a strong street build will also perform well at the track.
> What's the transmission going to be behind it? Manual or automatic? 2WD or
> 4x4? Stock suspension or modified? If modified, how much from stock are
> you
> going with it? Stock sized tyres, or something bigger (or smaller)
> diameter?
> Even the planned differential ratio would be good to know.
Likewise, if automatic, the stall speed (and efficiency) of your torque
converter is very important. However, the stall speed is usually
determined by the power band of the engine and the "effective" gear ratio
(determined by actual gear ratio and tire diameter).
<snip>
Ray
http://www.dragtruk.com/ENTRIES/20KM1FD2KWBP.html
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