compression ratio

[badsix]

what would be a good compression ratio for this engine? its a 1600N with a NPR big bore kit 87mm a ro200 cam duel 40 mm webers Bursch exh.and stock A heads. its just going to be a driver, but I would like to get as much as I can out of what I have but still on the reliable side.
thanks Jay D.

[S.J.Szabo]

Mr.D:

Well, tops of the NPRs are a copy of the SC and should produce a 9.3-.5 c.r. in a late head.

However, presuming that both the A heads and the NPRs are stock, there will be a well-known problem using them together in this state. The slope of the piston dome is 11deg. to match that of the late heads, while early heads have a slope of 15deg. This will produce an isolated quench area around the edge of the cyl., an unpredictable c.r., and potential interference issues.

The usual remedy is to machine the heads, tho some cut the pistons.

[neilbardsley]

Hasn't Jacques F produced a big bore kits that get around this issue?

http://forum.porsche356registry.org/viewtopic.php?f=19&t=38070&sid=f37ca4cb1e7c1c21d224bca742fe5ddd

[badsix]

SJ thanks for the reply. I have had the pistons cut to 22* and in the process of calculating the static compression. as near as I can tell at this point, I can be any place between 9.26 to 8.32 by varying the cyl. base gaskets from .25 to .75.
thanks Jay D.

[S.J.Szabo]

In that case, let us address your original question...

As your component set is pretty much the same as I have used, I would suggest that you should have no issues with the highest c.r. that you can achieve with valve/land clearance. The Webers will run better for it, and the A heads breathe well enough to get a good low-end torque as befits a driver. Just don't expect much in the upper rev range.

Curiosity prompts me to ask about your crank and rods...

[badsix]

SJ, crank is stock std., rods are late 912 style and I have the large oil pump ( new 3rd piece ). good question on land clearance what would be A safe valve to piston clearance? I have the spring off right now but I could throw them on and check valve to piston clearance.
thanks Jay D.

[Craig Richter]

No need to install those stiff valve springs you're planning to actually run for checking clearances. Go to the hardware store and find some longer, lighter springs that can be assembled and depressed by hand; makes checking things much easier.
Compression ratio is a mixed bag. Too much matched with a Normal cam will result in a motor very sensitive to on-off power, not what you're expecting and very uncomfortable in heavy traffic. More "spirited" camshaft? (I suggest 280* measured at .020" clearance MAX for street) Too much C/R (anything over 10:1) will overheat; too little gives up HP because the compression is going out the exhaust valve during the increased overlap.
It's tricky to get these things right, and why good engine builders get the big bucks!

[badsix]

No need to install those stiff valve springs you're planning to actually run for checking clearances. Go to the hardware store and find some longer, lighter springs that can be assembled and depressed by hand; makes checking things much easier.
Compression ratio is a mixed bag. Too much matched with a Normal cam will result in a motor very sensitive to on-off power, not what you're expecting and very uncomfortable in heavy traffic. More "spirited" camshaft? (I suggest 280* measured at .020" clearance MAX for street) Too much C/R (anything over 10:1) will overheat; too little gives up HP because the compression is going out the exhaust valve
during the increased overlap.
It's tricky to get these things right, and why good engine builders get the big bucks!

THANKS Craig, for the tip on the springs I remember reading this what a good idea
Jay D.

[S.J.Szabo]

Mr.D:

Good choice of rods and oil pump, but if that's a stock A crank, you might want to dial back the c.r. a tad. Especially with the big-bore kit. Now, while I'm certainly not the last word in engine builds, I would think 8.5:1 would be good, 9:1 might be pressing your luck some.

I might not fit your plan to entertain such suggestions, but I would note that the B crank might be better suited to your needs.

Just $.02 worth.

[badsix]

thanks SJ, the 8.5 is kind of what I was shooting for but its looking like 8.68 with a 1mm base gasket, that's going to be good. I need to do some more calculations to be sure.
Jay D.