No Power Under Load

356 Porsche-related discussions and questions.
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Dan Epperly
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Re: No Power Under Load

#31 Post by Dan Epperly »

Ron LaDow wrote:
Chuck Watson wrote:How did you determine the leaks were there?
A leak-down test and an ear will tell you.

I didn't do a leak down, but no one who checked this engine out heard the tale tell sound of pfft pfft pfft that you usually hear with leaky heads.

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David Jones
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Re: No Power Under Load

#32 Post by David Jones »

Chuck, Do you have a competent mechanic who understands the 356 engine? If so he should be able to remove heads and cylinders, get them R&R'd and replace them. New rings are not rocket science. I am sure Vic or CJ could advise you on what rings to use. A tear down and rebuild should not take that long. If while doing a leak down test you use the tube in the ear or stethoscope with just a tube with open end technique you can quickly diagnose a cylinder to head leak. Make sure to use at least 100 psi pressure for the leak down test.
If I had known I would live this long I would have pushed the envelope a little harder.
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Chuck Watson
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Re: No Power Under Load

#33 Post by Chuck Watson »

The sound was coming from the crankcase (dipstick and filler) so I think it has to be rings or cyl wall wear.

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Martin Benade
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Re: No Power Under Load

#34 Post by Martin Benade »

Once you get the compression all up to par then you can continue figuring out what is wrong that makes it feeble above 4500 rpm. The compression issues should make it equally poor at all rpms.
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Chuck Watson
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Re: No Power Under Load

#35 Post by Chuck Watson »

Today's agenda:

I am suspicious about the 009 dizzy. I do not know where the timing was set when it initially ran poor at 4500 rpm. But when I set the timing at 34 degrees at 3500 rpm it bogs under load at low rpm (almost undriveable). I understand the 009 does not have enough advance, so maybe at high rpm's it was not advancing enough previously (before I adjusted it). Today I will check on the range of advance between idle and 3500 rpm. My thought is, with an 009 you get good low rpm performance or good high rpm performance, but not both.

I am also going to redo the compression/leak-down test with the engine warm. It was cold when I did them yesterday.

Worst case: more data points

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Martin Benade
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Re: No Power Under Load

#36 Post by Martin Benade »

Two opinions- if you set the timing at 3500 rpm it will be too advanced at low rpm. That is not a healthy choice for the engine, and it will not sound very nice, but it should run acceptably until it hurts itself. Also with those leakdown numbers it should be able to run acceptably, not that great, but passable. I am not saying that is good enough, however. One of the real experts can weigh in on these thoughts.
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Chuck Watson
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Re: No Power Under Load

#37 Post by Chuck Watson »

So....the advance at 1200 rpm was high as anticipated. About 19 degrees at the crank. But it runs so I snuck it out on the road to see if it came into form above 3 or 4000 rpm. It did not. Performed badly across the rpm range. I've ordered a refurbed Bosch 031 dizzy with the thought that the old 009 had seen better days. Process of elimination continues.....

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Wes Bender
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Re: No Power Under Load

#38 Post by Wes Bender »

Put that old 009 distributor in your spares kit for a "get me home" repair on the road.

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Wes
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Jacques Lefriant
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Re: No Power Under Load

#39 Post by Jacques Lefriant »

the 009 has a better curve for racing than the 031
 

Chuck Watson
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Re: No Power Under Load

#40 Post by Chuck Watson »

Jacques and/or Vic,

While I'm waiting for the next opportunity to work on my issue, I'm curious about why you prefer the 009 advance curve. I understand it comes on earlier, but if you're almost always above 3000 rpm why would that be a major factor? Would you not prefer to have a wider range of advance for those times when you do need to run below that rpm level? Help me understand please.

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Jacques Lefriant
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Re: No Power Under Load

#41 Post by Jacques Lefriant »

the more effective compression the less total advance is advised.
 

John Ward
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Re: No Power Under Load

#42 Post by John Ward »

Nobody mentioned valve springs. When you pull the heads, you might want to check them for weakness, or breakage.
 

Chuck Watson
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Re: No Power Under Load

#43 Post by Chuck Watson »

I understand your comment Jacques but can you give me some parameters? Are you suggesting less than, say, 28-30 degrees total advance on a high compression engine? Those are the lowest numbers I've seen mentioned anywhere.....plenty of others are saying that 35 or 36 is needed on a race engine.

John, I'll be sure to take a look at the springs, thanks.

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Ron LaDow
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Re: No Power Under Load

#44 Post by Ron LaDow »

Jacques is correct as a general rule, but it also depends heavily on the chamber shape and quench clearance.
For instance, a big dome and sloppy quench clearance will give you a higher CR, but need more ignition lead to burn over the top of the dome and all the way out to the walls, compared to chamber with a lower compression with a smaller dome and tight quench clearance.
Generally, good single-plug chambers can do well at 33-34*, 9.5:1, and loose chambers/big domes go up from there. But regardless, the engine tells you what it need from the dyno sheets, not the other way around.
Yes, higher RPM needs more lead also.
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Max Handley
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Re: No Power Under Load

#45 Post by Max Handley »

You mentioned 35 mm Venturies, had a similar problem years ago, were they recently installed, double check to see that one was not slid in upside down.
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