Chev 350 gurus (help required)

[Scot Sealander]

dzorde wrote:

> Guys,
> 
> In an earlier post I wrote my TPI chev 530 pinged on pump leaded with 
> 4deg static advance.  Now I thought it was due to too high a 
> compression, and so I've finally done a compression test. 

Normal TPI timing is at 6 degrees.  So you are setting timing 2 degrees 
less than a normal TPI system. (for info only.)


> The engine is standard flat top pistons and 520 iron headswith 1.5 
> ratio roller rockers, 305 TPI using aftermarket speed density ECU.

Flat tops and I assume 3884520 heads?  Those are 60.5 cc heads, 1.72/1.50 
valves.  If this is the correct casting number, that is the source of 
your problem.  Take a look at a ballpark figure on compression ratio.

Just using cylinder swept volume and head volume: (Using cubic inches)

             swept vol + head vol
Comp ratio = --------------------
                  head vol

       43.75 + 3.692
 CR = --------------- = 12.85 to 1
         3.692 

The actual number will be less due to the head gasker thickness and other 
factors.

        
> The compression test  was as follows:
> 
> Cyl 1, 3, 5, 7 between 237psi and 240psi
> Cyl 2, 4, 6, 8 between 223psi and 227psi

No surprises there.  The numbers are high.


> 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"

Sounds like a good TPI cam.
 

> I was told by an expert to put in a bigger cam to bleed of some 
> pressure and get down to 210psi.

You really don't want to do this. I don't think that will solve the 
problem at all.  But that is just my opinion, and you get what you pay 
for.... :-).


> I didn't tell him the cam specs, but he recons I have an economy cam 
> since my compression is so high. 

On it's own, it's tough to assume too much about the cam.


> The same guy also says the pinging could be due to the advance springs 
> in the dizzy softening when they get hot and give just that little bit 
> too much advance (need to check full range when engine is hot).

How did you get those advance springs in an electronic distributer?  Neat 
trick! ;-)  The TPI calculates advance in the ECM.  No weights at all in 
this application.  May be that it is time for a new expert?


> Now here are my questions:
> 
> 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).

Hey, it sounds good to me.

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

Not at all accurately.  Divide by 14.7 psi.  There is also heating of the 
air during compression to alter the value, and other things....


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

Keep the cam, change the heads.

> Help appreciated
> 
> Dan     dzorde at geocities.com
> 
> P.S. next thing to try is a tank full of BP100 racing fuel.

Try a different set of heads next..... ;-)  Of course this assumes that I 
got the head casting correct....

Scot Sealander    Sealand at clarityconnect.com