knock at 5psi in new build

[bison]

Dial back and waste spark can make for inaccurate readings....

With a cheap timing light. With a good digital dial back its not a problem. I actually prefer to leave it in the non waste spark mode with double the pulses. Then I can split hairs on the final CRA to within half a degree


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[tttype]

The saga continues

Removed 10* timing , no change.
Added 25% more fuel , got up to 7psi before detonation .

Started looking for a mechanical issue.

Pulled plugs, they varied a lot, so did a compression test. # in pic are cyl # & compression.

Put a camera into every cyl. All pistons had oil pooled in the dish.
Engine has 50 miles on it , runs well, NO smoke out exhaust !

 

[Boost231]

Time for a leak down and then i would be in contact with your engine builder.

I see the motor coming out in the near future.

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[Pronto]

Houston, we got a problem.

 

[jskarateka]

I may be way off here, but if so at least I'll learn something:

I thought that it was better to run low compression pistons with a turbocharged or supercharged motor because you wind up with more volume of air-fuel in the combustion chamber at the desired compression after boost.

For example; if you have 8 to 1 pistons and you boost to 12 to 1 compression, you'll have more air-fuel in the combustion chamber than if you have 9 to 1 pistons and you boost to 12 to 1 compression.

So my point/question would be; isn't it expected then that it would take much less boost to get the same compression with the high compression pistons? Is that maybe why he's getting knock at over 5lbs, like he might be if he put a turbocharger kit on a new LS3 stock SS Camaro.

Like I said, just trying to learn something, looking forward to the response from the guru's here.

 

[tttype]

jskarateka this isn't the best thread to ask that question, use the search button & you can read for hours about CR choices in our engines.

Leakdown on #6 showed about 20% " good " on gauge

#1 about 60% can hear leaking through valvecover & throttlebody . Intake rocker removed.

 

[No disintegrations]

20% is not good on a new engine. 20% is good enough to run but not for performance.

 

[Reggie West]

The leak down tests are telling you what you need to know. If the pistons are not sealing against the head correctly then you have a problem. The only way to find out what is going in is it pull it apart now.

To answer your compression ratio questions this is how that works.

The stock CR of 8 to 1 gives you a wide tuning range meaning that you have to push the turbo to make more power. So it takes 25 to 30 psi sometimes to max out your combination, maybe even more. Example, a built motor with ported and polished heads and a 70-1 turbo will make good power but you have to run the turbo up to 30 psi plus to get MAX power out of it.

Now take the same motor but build it with a 9 to 1 CR. You make more power at lower rpms AND lower boost levels, Why? Because the CR allows you to stuff, literally, more air into the cylinder for each combustion cycle. More air needs more fuel and if that is right you make more power. Back to my example, that same turbo will make max power at a lower boost level because the motor can flow more air. Boost is resistance to FLOW. The higher the boost level, the more RESISTANCE there is to flow. So the turbo is pushing harder to get that air in the cylinders.

A 9 to 1 motor is more responsive down low as well. And it took me putting more fuel in the air fuel ratio tables across the board to keep it happy.

Your statements about increased CR under boost are correct. You can see CRs up to 15 to I under boost and the motor has to be able to handle that as well.

It is all about correct fueling and timing.

The LS motors run 10.25 to 1 CR and can use boost from a blower or turbo to make power too. It is in the tuning. And modern ECMs have thousands of pages of code to correct air and fuel under a number of different parameters.

I asked one of the most knowledgeable builders I know of what the difference is with modern motors making 700 hp on pump gas. I thought maybe it was head design or something like that. Nope it is in the tuning and ECMs the engineers use to control timing with multiple timing maps and fuel tables. I was told in the Ford Mustanng the ECM has over 16,000 pages of code written in it. That would equate to 6 FEET of papers stacked on the floor.

Hope that helps.

 

[bison]

I may be way off here, but if so at least I'll learn something:

I thought that it was better to run low compression pistons with a turbocharged or supercharged motor because you wind up with more volume of air-fuel in the combustion chamber at the desired compression after boost.

For example; if you have 8 to 1 pistons and you boost to 12 to 1 compression, you'll have more air-fuel in the combustion chamber than if you have 9 to 1 pistons and you boost to 12 to 1 compression.

So my point/question would be; isn't it expected then that it would take much less boost to get the same compression with the high compression pistons? Is that maybe why he's getting knock at over 5lbs, like he might be if he put a turbocharger kit on a new LS3 stock SS Camaro.

Like I said, just trying to learn something, looking forward to the response from the guru's here.

Don't confuse manifold pressure and increased compression ratio with increased cylinder pressure. It's very possible to have lower cylinder pressures with more manifold pressure even while running a higher CR. The fuel, timing, camshaft, rpm, head design as well as the rod length to stroke ratio to name some have quite an impact on the cylinder pressure vs piston position in the bore


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