9.8 or 10.1?
 

I need to decide my deck height. I measured cylinder head and piston dome volumes last night. Motor is short stroke 2.5, JE pistons, 2.2E heads cut for 90mm cylinders, cylinders are reconditioned, and had top surface machined, std crank and rods. Head volume is 68.4 CC. With a deck height of 1.5mm I'd be at 10.1:1, and with 1.75mm I would get 9.8:1. It seems to me that 9.8:1 is the way to go for pump gas/single plug with E cams. Anyone agree or disagree?
Thanks,
-Scott

Shouldn't deck height be 1mm (ideally)? I thought that higher deck heights are just as bad for detonation as higher CR can be.

Increasing the deck height dimension is actually lowering the piston relative to the head, reducing the CR. I believe Wayne recommends a range of 1.5mm to 1.75mm. I don't have the book in front of me, so I can't verify. (edit 7/21: Wayne's book states that Walt Watson recommends 1.25 mm to 1.5mm for "stock" engines). Machining the head for the bigger cylinders results in a reduction of the combustion chamber volume, necessitating a little bigger deck height number (thicker shims). So, I think my numbers are right were they should be.
-Scott

Yessir, that is basically correct,....:)

Too much deck height promotes detonation as well as too much compression. 1.0mm is correct and 1.5 to 1.75 is too much.

Hello Steve.

Have you actually tried running much less than 40 thou?

I'm intrigued to see what others have found..I have not yet built a real hot engine with much less than this..but have made a road engine with 25 which lived happily..so I'm tempted to try on a race motor..probably with 30 thou..

What do you think?

Kind regards
David

So, my choices are 10.1:1 (perhaps a little high for CR) and a deck height of 1.75mm (large enough to perhaps promote detonation). Which is a better choice?
Thanks for all of your help!
-Scott

You may also have the option of machining the pistons.
A little off the top...
http://forums.pelicanparts.com/uploa...1153330186.jpg

Before and After
http://forums.pelicanparts.com/uploa...1153330207.jpg

-Chris

I have assumed that 9.8:1 is the practical max CR for pump gas/single plug. Is this assumption correct?

It your piston diameter, I'd already say you are riding the edge at 9.8:1.

Good job, Chris.

Lowering the dome heights also promotes improved flame travel which is something single-plug motors desperately need, especially at those CR's.


David:

Running much less than .040 results in the pistons striking the heads at the edges. It all depends on the piston crown design and what your piston-to-valve clearances are.

It looks like I need to remeasure my volumes in order to insure that my data is correct, and think about having some material machined off the top of the pistons. I appreciate all of your responses.

-Scott

Hello, Scott.

Defintley helps IMHO to take the peak off the piston between the valve pockets.

To keep the CR up I do this like a ditch on 2L's..looks horrid!
The effective and tolerable CR will vary markedly with the cam overlap as I'm sure you know...big cams enable much higher CR's..

IMHO a most important variable for tolerable CR is charge , chamber, and especially exh valve temperature..so the ambient really makes a huge difference.

One of the little secrets of the aircooled motor is the use of inlet charge to cool the exhaust valve. at WOT.hence plenty of charge loss in the exhaust..I've wondered about an afterburner..

Kind regards
David

David-
Given the E cams, it seems like I should hold the CR to 9.8 at most. I re-measured volumes last night, and found that I can not obtain repeatable results. I will refine my measurement techniques before I decide what to do.
-Scott

Update:
I refined my measuring techniques and found that matters were worse that I originally imagined. With 1mm deck height, the pistons supplied would result in a CR of 10.9:1! The pistons were purchased form EBS (prior to Pelican carrying the product). I returned the pistons to EBS so that they could have a look. Following consultation with JE, they found my calculations and measurements to be accurate. The box the pistons were shipped in had "10.5:1, 2.7" written on it with a sharpie. The spec sheet within had measurements that matched the pistons, but it is clear that they would result in about 11.5:1 in a 2.7, not 10.5:1. So, taking the time to measure saved my bacon! EBS is machining the pistons to the correct dome volume. They were very responsive and are doing this work at their expense. I should end up with 9.8:1 at a deck height of 1mm. I'll post before and after shots of the pistons when they return.
-Scott

I have heard this before and yet the Euro Carrera 3.2 has 10.3, single plug and was designed to run on what would be rated 92-93 octane gas.at the maximum. Can you raise the CR as the piston diameter gets larger? pardon what may be a dumb question, but i am new here......lol

The factory compression numbers are higher than what you'd get if you actually measure them so you can't use them as a comparison. The Euro 10.3:1 pistons probably measure closer to 9.8:1. (I haven't measured a Euro setup.)

Compression ratio is varied by changing the piston dome volume. Imagine if you made a piston dome that filled the combustion chamber so there was no room in the combustion chamber at all when the piston was at the top of it's stroke. Such a motor would have a compression ratio of infinity:1
(CR = volume at bottom of stroke divided by volume at top of stroke. In our imaginary engine it would be CR = vol at bottom / 0.0)

I wouldn't build a 9.8:1 motor for a street driver these days. The fuel qualities aren't likely to improve. The last motor I did a machined the pistons to 9.5:1 so if he gets a little bad gas and gets stuck in traffic (high temps) no worries.
-Chris

I guess my second question is where are you guys getting all of this bad gas these days? All of the majors here in Florida sell 93 octane Super and I have never had a problem. Even with the outrageous prices, should we Porsche guys really be looking for the "best deal" on gas?

The best you can get in New Mexico is 90 octane and Cali isn't much better at 91. This is due to emissions; even in Florida, the days of 93 octane are numbered.

Not unless they ban it nationally. We are a flat and breezy place. No smog or ozone issues to speak of. We used to have emmissions inspections in the major metro areas but they killed those at least 6 years ago. Why do you think we never have to hide our bypass pipes? lol

Not to doubt Steve's words... but why does too much deck hight promote detonation?

If I understand things correctly, having a compression ratio too high heats the intake charge to the point that the fuel spontaneously combusts.

I'm just curious as to what the dynamics are for too much clearance (and lower temps) causing the mixture to ignite...

--
Joe
1986 951
1977 911