Fordnatics List Archive

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Mail From: (email redacted) (Dale Maurice)

Maybe not exactly a Ford topic, but since we've been discussing this
recently...

What is the consensus on exactly what happens to timing when increasing
or decreasing the rod ratio? It seems to me that when decreasing the
rod ratio either through stroking or changing the rod length that since
the geometry of the stroke has changed, the piston will now spend less
time at or TDC. And of course the opposite will happen if you increase
the rod ratio.

With spending less time at or near TDC you'd want to increase the
timing. When increasing the rod ratio you'd spend more time at
or near TDC and would need to decrease the timing. (??)

So exactly what are the cumulative effects of increasing or decreasing
rod ratio? And how should the compression ratio be adjusted in
conjuntion with whatever change you are making? What about advance?
More total advance quicker for small rod ratios and less advance
slower (?) for larger rod ratios??

Does any of this make sense?

dale

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Mail From: Eugene Y C Chu <(email redacted)>

Dale,

What you said about rod length to stroke ratios make perfect sense.
So if you can increase the duration in which the piston stays at TDC,
you would need less ignition timing advance. Remember the ignition
timing should ideally be set so that the resulting combustion provides
peak pressure just as the piston starts on its downward stroke. This
is the way to extract the maximum mechanical energy out of that
reaction.

The other benefit of increased rod length to stroke ratio is at the
other end; the piston stays at BDC longer as well, so the cylinder has
more time to fill on the intake stroke. This means you can use less
radical cam profiles to achieve similar flow efficiency.

Again, these effects allow you to change timing or cam profiles without
loss of perfromance, but you don't have to do it unless these parameters
were out on the fringes to begin with.

eyc

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Mail From: (email redacted) (Dave Williams)


-> What is the consensus on exactly what happens to timing when
-> increasing or decreasing the rod ratio?

As the rods get shorter, timing sensitivity decreases. Tendency to
detonate also decreases. These both relate to the reduced dwell time at
TDC.

I used to run a 12.5:1 302 with a mild cam on plain old 89 octane gas
with no trouble, but I did spend a lot of time tailoring the advance
curve. Lopping off the top of the distributor and welding on a GM HEI
made things much simpler; Delco put the advance mechanism on top, no
distributor disassembly required.

This read-only message was archived from a public mail list.
Mail From: (email redacted) (Brian Kelley)


Dale Maurice writes:

>What is the consensus on exactly what happens to timing when increasing
>or decreasing the rod ratio? It seems to me that when decreasing the
>rod ratio either through stroking or changing the rod length that since
>the geometry of the stroke has changed, the piston will now spend less
>time at or TDC.

Combustion chamber efficency seems to have the largest effect on how
much total timing advance is required. The current Winston Cup
engines don't run much advance. Last I heard, around 29-30 degrees.
Their combustion chambers are very small (so a large piston dome isn't
required to get the desired compression. Domes interfere with
efficient combustion and flame propagation) and extremely efficient.

The rod ratio may shift the optimal amount of total advance a little,
just like compression, cam, fuel, ignition and head material. With
sane rod ratios, a rod ratio change should not significantly alter the
amount of total advance. I'd say you should be much more concerned
with getting the right cam.

You're going to *tune* the amount of advance in any high output
engine. It isn't something you calculate on paper, set and then go
race. If it is at all serious, you're going to tune the optimal
advance on a dyno. If you can't do that, you tune it at the track or
on the street. In any case, you're going to give the engine what it
wants, not what your estimates on paper say it wants on paper.


>So exactly what are the cumulative effects of increasing or decreasing
>rod ratio? And how should the compression ratio be adjusted in
>conjuntion with whatever change you are making?

You don't adjust compression ratio because of a rod ratio change
unless you're running right on the ragged edge (and if you were, you
wouldn't be asking these questions). If you're running a sane
compression ratio, a rod ratio change shouldn't be enough to push
you into detonation with proper re-tuning.

If you're really interested in running on the ragged edge, you're
going to need the money to scrap some parts. If you're running
against engines that are on the edge, you'll probably need quite a bit
of dyno development time to be competitive. You'd also really want to
get advice from someone familiar with the specifics of the combination
in question (or very similar combinations). For that type of project
around here, I'd go to Diamond Racing (or possibly Roush, but I'd go
to Diamond).

Brian


- ---
(email redacted)

This read-only message was archived from a public mail list.
Mail From: <(email redacted)>

On Wed, 19 Oct 1994, Dale Maurice wrote:

> Maybe not exactly a Ford topic, but since we've been discussing this
> recently...
>
> What is the consensus on exactly what happens to timing when increasing
> or decreasing the rod ratio? It seems to me that when decreasing the
> rod ratio either through stroking or changing the rod length that since
> the geometry of the stroke has changed, the piston will now spend less
> time at or TDC. And of course the opposite will happen if you increase
> the rod ratio.

Actually, you've got that a little confused. The piston will spend the
same "time" at TDC, which can be measured in terms of radians per second.
What does change, however, is how fast the piston moves from BDC to TDC.

> With spending less time at or near TDC you'd want to increase the
> timing. When increasing the rod ratio you'd spend more time at
> or near TDC and would need to decrease the timing. (??)

Again, it's the travel time between BDC and TDC, and not the time spent
at TDC. However, in those terms, I would think that it might not be so
much an issue of timing, as it is one of duration. Possibly both.

Think of it this way: since for any given RPM, the rate of piston travel
is a little faster for the stroked motor; therefore, the cylinder has
less time for air to flow in. This can be overcome by enlarging the
ports and valves so that more air can flow per unit time; or, we can keep
the valve open longer (ie, increased duration). the exhaust side would
exhibit similar symptoms, but would be somewhat dependant on the intake
side (ie, you can only exhaust what you take in, plus a little extra due
to thermal expansion of the gas). What might occur is a little extra back
pressure, etc, which could also be compensated for with larger
ports/valves or longer exhaust duration.

> So exactly what are the cumulative effects of increasing or decreasing
> rod ratio? And how should the compression ratio be adjusted in
> conjuntion with whatever change you are making? What about advance?
> More total advance quicker for small rod ratios and less advance
> slower (?) for larger rod ratios??

Well, these are some interesting questions... Lessee...

1. Cumulative effects? Well, that's hard to say - it will change for
each RPM range, and for other factors...

2. As far as compression ratio, if you did nothing else to the motor,
you might be able to squeeze a touch more compression out of the motor,
due to the thinning out of the cylinder air/fuel charge. But that
thinning out might not really be significant until high RPMS...

3. In terms of advance, all else being equal, leave it be. It won't be
that significant of a change until massively high RPMS...

> Does any of this make sense?
>
> dale

YEah, actually it does. Hadn't thought about stuff like this in a long
times...

MArk