Chev 350 gurus (help required)

[Johnny]

dzorde wrote:

> ...
> Cyl 1, 3, 5, 7 between 237psi and 240psi
> Cyl 2, 4, 6, 8 between 223psi and 227psi
> 
> The cam specs are:
> 
> inlet opens 23btdc, closes 61 abdc, duration 264deg, cam lift 0.269"
> exhaust opens 68bbdc, closes 26atdc, duration 274deg, cam lift 0.285"
> ...
> 1.  Is this cam an economy cam ?  The guy that ground it said it would be
> perfectly suited for a TPI speed density system (I think so too, got lots of
> power up to 5300rpm, then it runs out of power quite noticeably).

The thing only has .427 inches of lift at the valve. I don't think you
could call it anything but economy. That duration has to be "advertised"
for having the small lift, especially if you are getting those kinds of
compressions at cranking speed. You should use "duration @ .050 lift"
specs to help comparisons between cams and manufacturers. 

If you want it to die off at 5300, you are fine with the cam you have,
and if you aren't over advanced on the ignition, the only way out is to
either run better fuel, or reduce the compression.

> 2.  Can you calculate the compression ration from the psi reading ?  If so
> what is the equation ?

There are just too many other variables to do it acurately. However, you
can do it by the fluid measurement method without diassembly, but it can
be a pain to get it right, especially with the spark plug at the bottom
(as opposed to a hemi). 

Its really easy to calculate it with flat top pistons as long as you
know the deck height and the chamber volume. Deck height is easy to
measure, chamber volume requires a little more expertise, but not much
more. If you don't know what they are, you will have to pull one head to
see. I also noticed that one side of the engine has noticiably and
consistently less compression than the other. I would be suspicious of
the deck heights not being the same or one head being more milled than
the other. You have to pull both heads to get to the bottom of that
though.

> 3.  If I need a bigger cam, any suggestions ?  Don't forget a standard TPI
> is only good for around 5500rpm.

You have to decide what you want your max rpm to be first. If the TPI
cuts off at 5500 and you don't want to change it, find a cam profile
that starts to fall off at that rpm. The cam you have now is pretty
close to that.  I think what you will find is that even with flat top
pistons, if you bore or stroke the engine much, your compression will
start to get up into the 10:1 and higher range pretty quickly. This will
not cut it on pump gas, at least not unless you retard the ignition
beyond where the engine runs right. Here's what I would do:
Nail down exactly what your timing curve looks like first. This is the
easiest and cheapest solution and you need to know where you are at
there anyway. You shouldn't need more that about 38 degrees total
advance. If you have that or less, and it doesn't ping at say, WOT and
4000 and up rpm, then you need to change the curve and nothing else.
Might need to get a few sets of springs to play with, and also an
adjustable vacuum advance unit can be very helpful here.

If you have less than 36-38 degrees total advance, and it pings at WOT
high rpm, then the only thing you can do is either increase the cam
overlap, which will also move your torque curve up in rpm and not do too
much to lower your cylinder pressure at the high end anyway unless you
change it more than you would want to, or run better fuel, or reduce the
compression. Easiest ways to reduce the compression are; use a thicker
head gasket, increase the chamber volume (good excuse to CC the heads),
or run a dished piston. I have also found that you can get away with
more compression running aluminum heads than you can iron heads.

Things you can pretty much bank on:
You should be able to run 92 octane unleaded with 9:1 compression and
iron heads.

What the guy said the cam is and what it actually measures to, can be
two completely different things.

Above also applies to head volumes. Until you actually measure it, you
are just guessing.

Your face will be sore after spending a few hours mapping out your
advance curve on a running engine. Try and find someone with a
distributor machine.

Whoever made them lower the octane should be shot.

The above advice it worth what you paid for it.  ;)

-j-