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The Wife and the Car
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Discussion Starter #1
Ive searched and searched but couldnt find anything.

I know that the cam design / elevation etc all play a critical part in this.
Yet we should be able to get some idea if people who have built their engines chip in.

What is your cranking compression on a comp gauge.
What is the highest you have ever seen or heard of anybody running.

Just need two bits of info
Example:
Calculated CR 10:1
Cranking com = 200 PSI

Just trying to get a feel for what the limits are in terms of PSI.
 

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88 ED6
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Highest I've seen...was on my old D15B2 with just a milled head.
~260-270psi with the engine warm. Cold, it was around 170-190.
 

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I know this isn't what you want to hear, but what are you trying to accomplish from this? Cranking compression numbers really are not that important unless they're suspiciously low. Cylinder-to-cylinder variation is what really matters for a compression test.

For what it's worth...

b7/z6 mini-me
2 deg retarded stock z6 camshaft
75 m above sea level
215 psi
 

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1988 Honda CRX
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6,676 Posts
^^^^ +1

But I think you know that already.

I have seen 250 psi at 11:1, but I also saw 180 at 11:1
 

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Premium Member
The Wife and the Car
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Discussion Starter #7
Thanks guys appricate the inputs. Doing this for two reasons.
a. Want to see if the simulator numbers are close.
b. Need to know what we can expect to see at 13:1 and 14:1

We may not even be able to test it cause the stock Starter motor will stop working at around 13:1 so I have been told.
 

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McLOVIN
2002 Civic Si
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800 Posts
Wont it depend on the cam? When reffering to the starter not turning? Because the Camshaft will dictate your dynamic CR depending on lift/duration/lsa....

So you could have 2 motors built exactly the same, but with 2 different cams and get different PSI

This is just my understanding
 

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90 CRX Si
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Wont it depend on the cam? When reffering to the starter not turning? Because the Camshaft will dictate your dynamic CR depending on lift/duration/lsa....

So you could have 2 motors built exactly the same, but with 2 different cams and get different PSI

This is just my understanding
This is how I understand it as well. Like someone mentioned a high PSI with a stock cam, well there's not much overlap to bleed off compression. If you have a larger cam with a different profile, it will be different on the same engine.
 

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Classic Man
Civic
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it depends how the test is done.

you can sit there and crank a 9:1 engine all day and it will make over 200psi.

the way i was taught to do a proper compression test was to allow only 4 compression strokes. with the TB wide open. and the injectors disconnected, and all spark plugs removed. on a engine that is at or close to operating temps.

my 11:1 D17 only makes 175-180psi like that. my d16z6 8.5:1 cr makes 150psi like this.
 

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1988 Honda CRX
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It is not cam overlap that influences it. It is intake cam closing point. The intake cam always closes after bottom dead centre on the compression stroke. If the piston has moved say 20mm up the compression stroke before the valve closes, then on a D16 you might have only 70mm of the stroke volume being compressed.

If it loses 10mm up the compression stroke then you have 80mm of the compression stroke compressed. This is 8% more air squeezed into the same chamber volume.

Without changing the grind and therefore total overlap, but simply advancing or retarding the cam, you change the intake closing point.

Cranking speed also makes a difference as there is always some leak down and the slower you crank, the longer the air has to escape through the small leaks that must exist. This means that as you get to the last test, if the starter is slowing from the battery draining, the reading might be a bit lower than otherwise
 

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I always run the starter until there is a negligible change in the cranking pressure. The engine isn't going to defy physics, so once the pressure has reached its "normal" value, it will stop rising. I'm not trying to single you out, danz, but if you had a magical starter/battery that did last all day, the cranking compression wouldn't be any different at 8pm as it was at 4pm when you started. However, it will often require more than 4 cycles.

The number of cranks it takes before the engine reaches this pressure is sort of a loose indication of the "quality of the seal". I can't think of how to describe it physically right now.
 

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I agree with Dan on this.

Oh your both Dan, guess I picked a winner then either way ;-)

Seriously I rarely disagree with Danz, but this time I'm with ddd4114 who coincidentally I also rarely disagree with.
 

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The Wife and the Car
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Discussion Starter #14
Guys Im painfully aware of how the cam effects comp. Its a lot like the exhaust port on a two stroke. i.e. Lower Ex port duration gives you more CR and Higher Ex Port duration give lower numbers. So it all depends on your Valve Events.
How I do it. Is remove plugs. Crank engine till the needle stops climbing with the TB Wide Open.
My sim numbers say I should see over 320 PSI. Lets see.
We managed to find Av-Gas. So now I have to do two maps one for pump and one for Av-Gas.
 

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Ron

I know you know about IVC point, but someone else commented about it being related to valve overlap rather than IVC. I was jut correcting the error in a reply.
 

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92 Civic Hatch
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What benefit does this have for you being able to type in variables and simulate it?
Just cause you can simulate the cranking PSI of the engine, does that really give you the ability to do anything beneficial?

If it does, cool beans! But, its just seems kind of needless IMO

Maybe you could dial in your cold start corrections better or something initially on a fresh tune......

7.8:1 CR
Vitaras and stock cam- 155psi
 

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Premium Member
Honda City
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What benefit does this have for you being able to type in variables and simulate it?
Just cause you can simulate the cranking PSI of the engine, does that really give you the ability to do anything beneficial?

If it does, cool beans! But, its just seems kind of needless IMO

Maybe you could dial in your cold start corrections better or something initially on a fresh tune......

7.8:1 CR
Vitaras and stock cam- 155psi
Engine sim.

You can try different exhaust lengths, intake lengths, cam profiles, compression, etc before you buy a single part. Saves a shit load of money and time.
 

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1988 Honda CRX
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It helps you more accurately predict at what static CR you will get detonation with a particular fuel.
 

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The thing is, simulations aren't there for getting an EXACT match. They're there for seeing relative changes and how they might affect your engine. There are so many variables that affect power and knocking that are damn hard to accurately simulate, that unless you're an OEM or something, matching things like cranking compression seems futile to me. To predict knocking - or even changes in power for that matter - you need to predict the change in the burn profile, and to do that, you need intense 3D CFD software and probably a single-cylinder engine with a quartz wall and fancy equipment to validate your simulation. It just doesn't seem worth it to me. If you're doing simulation work, measure everything as accurately as you can, make sure the torque curves are pretty close, then iterate dimensions, valve timing, etc. to see what happens. The effects of a compression ratio change are very hard to simulate accurately. You're much better off using basic Otto cycle equations to get a rough idea.
 
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