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.
> 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.
> And how is that used to figure out
> compression ratio?
You need the volume of the reliefs, not the diameter.
> 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.
> 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.
> 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.
Hm. Pretty strong opinion. Sounds like whoever is behind it may know what
they're talking about, though.
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?
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.
> With both valves
> open for an extended period you lose cylinder pressure. You're trading high
> static compression numbers for low dynamic compression. That stuff has to
> go somewhere, so it results in unburned fuel going out the exhaust, and
> also blowing back up the intake
Basically, "might run ok, and might sound ok, but it's going to be a gas
guzzler with a lot less power than it should have". Not exactly a glowing
recommendation of an optimal build.
> I have the short block built with this cam and pistons and before I go on,
> now is the time to swap out pistons and cam. Although expensive to replace
> parts now, it's cheaper then doing it later if it runs like crap.
Agreed. With it on an engine stand, it's relatively easy to swap parts now to
build what you want. It may cost a bit more, but you can re-sell the parts you
remove to help recoup some of the cost.
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