Quote: Measured deck height to 1.4mm for an effective 10:1 compression

I would recommend reducing your base gasket by .025mm , I target 1mm and go for it .
.055" is what you have , remove .010" (.025mm) and you have .045" (1.143mm) and this is a much better deck height. your engine will be happy for your efforts

The 1mm deck height is a very important number , the larger the deck height the more heat and pressure will go to the rings . This extra clearance can also add to the potential for knocking .

2.7 Hot Rod Engine Build (Advice Needed)
 

I tried multiple shims and targeted 1mm. The issue I had was I would be under or over. I currently have the supplied shims from the engine builder and that got me around 1.4 to 1.5. I purchased 1mm shims and also used the factory shims to try to get to the 1mm but got tired of the multiple attempts and stuck with the final originally supplied shims 1.5mm. The engine will have dual plugs to aid with the higher compression but I’m trying to make progress.

Will the 1.5mm height really have detonation? Do I need to tear it back down? Those shims aren’t cheap at $200 a set.

I have not read all but I must have missed something here.

If you have 1.50mm Piston deck height are we talking positive or negative deck height?

If the piston is below the top of the cylinder at the edges we are talking positive. If it sits above the deck its negative. Be careful here as different people have this reversed. Just make sure where the piston sits when the crank is at TDC.

If its below the surface if the cylinder top, and you have 1.50mm clearance, your static CR will be less than if the clearance was 1.0mm. A simple way of checking is to use some plug numbers into your calculation. Increase the Volume at TDC and see where the CR comes.

Find the 1.0mm shims if that is what you need. What is the bore size of the gaskets required? I may have some. PM me and I will help you.

Another way to del with this is the bottom of the cylinders can be cut to remove 0.50mm. Not the better way, the gaskets are.

Before you do anything. have you measured thew chamber and dome volumes?

I'll throw in my two bits: I like to keep the piston to head clearance even tighter, shooting for .75mm. This increases squish and combustion chamber mixing. I know it helps with single plug heads. I don't know if it gives you any advantage with dual plug heads. It might not.

As far as the cylinder base shims go, the stock ones are .25mm and you can stack several of them. It's not necessary to have custom shims.

Thanks everyone for their input it’s good to know there are several knowledgeable people here. I spoke with Ian and he gave me some valuable advice. I do have 1mm shims that I did purchase from LN engineering and the effective number was around .85-.90 left to right.

Given my pistons and rod combo that may be okay. And I will be measuring and trying that once more. If I get the same values I feel that may be a good combo.

I’ll keep you all updated on the progress.

Thanks

I noticed that you used a GT3 oil pump in an air-cooled, two valve 911.
Why? Isn't the enter section simply an additional pump designed to create crank case vacuum. How would that benefit your engine?
Wouldn't an air-cooled engine benefit from more scavenging instead?

2.7 Hot Rod Engine Build (Advice Needed)
 

The extra screen is another scavenge pump.

I have read that the addition scavenging on a Naturally aspirated engine would help in reducing windage in the crank case and reduce resistance freeing up horsepower. Also beneficial at the higher RPMs. Another plus was the price I got it for was comparable to a new turbo or 964 pump.

GT3's also have a check valve in the top to keep 10 in Hg of vacuum in the crankcase. Yes, there's power to be had by having a vacuum in the crankcase on any engine. In nascar we ran over 20 in Hg, but we also had supplemental pin oiling and extra piston squirters, otherwise the crankcase vacuum will cause those parts to run dry.

On boosted engines, the blowby under boost is always high enough to overcome the vacuum generated by the extra scavenge stages, so it's not even possible under high boost.

Even with the benefit of a discount price, the pump is not appropriate for your build. Don't feel bad, a lot of inexperienced builders think more is always better.

All true but no benefit on this particular build.

Thanks for your input, but it’s staying inside. Couldn’t hurt and probably won’t.

Of course. When building these cool little projects we should all use what we have.
My comments were offered so others wouldn't make the same error.

Care to elaborate on its inappropriateness. I may learn something.

I'll take a shot at this question.
The scavenge pump in the 911 was designed to scavenge as much crankcase oil as necessary. Moving parts like oil pumps create parasitic loss so limiting that loss is a design imperative. Porsche improved the oil pump around 1975 by reducing the scavenge side and increasing the pressure. This had little effect on internal loss. Obviously, Porsche design engineers deemed the scavenge reduction beneficial when balanced with increased pressurized volume.

In or around 1978, Porsche decided that a larger pump was necessary for the increase challenges generated by the turbo engine. They increased both the scavenge side and pressure side probably to compensate for the pressure loss do to larger oil squirters and the increase sump oil that followed. Now keep in mind, Porsche now had a larger pump but they continued to use the smaller (what I call the 908 pump) in normally aspirated engines. In fact they continued that practice for 11 years. Why? I would assume that they deemed the turbo pump "inappropriate".

Now lets move on to the GT3 pump. By the time the GT3 engine arrived on the scene, Porsche had modified the oil flow to the top end, reducing it some 25%?
The pump they used was a 9000 rpm racing development that would create crankcase vacuum and theoretically more horse power. They created so much vacuum that they had to redesign the flywheel seal. Because of the spacial limitations of the Mezger case, they had to reduce the scavenge side of the pump by about 30%.

The new GT3 pump, had less scavenging volume than the turbo pump but now, in conjunction with other design features, it could make negative crankcase pressure. Because of the higher location of the vacuum pump pickup, it offers very little oil scavenging effect but will add to the parasitic loss (by the shear nature of it's size) of an engine that doesn't benefit for the vacuum.
More parasitic loss, no scavenging benefit, no need for larger oil pressure side (unless larger squirters are installed), no horse power producing vacuum creation, might just deem the GT3 oil pump an inappropriate pump for a 7200RPM engine.

Why do I say 7200 rpm? The 2.4/2.7 crank has a design flaw that limits it's functioning RPM range. The rod journals are so wide that it created a thin flyweight that flexes during higher RPM. That flexing creates an imbalance (a rocking couple) that creates a destructive harmonic that limits engine longevity.

I know "but I rev my 2.7 all the time". Well if you really work it hard, the engine will do it's best to spit off the flywheel. 2.8 RSR could rev because it had very special crank. A crank with narrower rod journals, wider fly weights and reduce rotating mass. They also ran a really cool light weight flywheel.

Thank you Henry. That's interesting.

We did a lot of Oil flow testing on the dyno some years ago when Stefan did the big pump for us. We compared the stock pumps at the time and the GT3 to his prototype pump. I need to dig out all that data and re read what we found.

Is it inappropriate for this particular engine. No not at all. The wording inappropriate reads as if it would not work. Could it be said "over the top" for the application, yes for sure. But at the cost it certainly will not fail the engine.

I can say this from experience and a lot of testing. The GT3 pump has played apart in saving the middle rod journals.

But then, according to the post that suggested it is inappropriate, and inexperienced engine builders use it, I guess I just found my position in life.

Crap, I'm too old to start over. What do I do now. Anyone need their lawn mowed?

So the word "inappropriate" has your panties in a bunch. You didn't take my comments personally did you? Oh my, maybe you did.
If so I apologize.

Henry, I think Neal's sarcasm is directed at this post.....
"Even with the benefit of a discount price, the pump is not appropriate for your build. Don't feel bad, a lot of inexperienced builders think more is always better.

It's not really how he portrayed my words but I think that's what has "his panties in a bunch"

I saw no panties in a bunch. Neil's post was lighthearted and self-deprecating, and also substantive. Henry you seem to be spoiling for a fight of some kind. I don't get it.

Henry, no offence taken. You have no need to apologize.

In all the years I have never heard a part called inappropriate. It was something that caught my eye. Seemed an unusual way of telling someone he had chosen the wrong part.

I come on here knowing most are DIY home builders. Sometimes you can advise and be helpful. I know that I have been blessed to do this work professionally and at times at a very high level. Those times and now, professionally, we have to make the right decisions as all too often it comes around and bites us in the rear end. We push what works but more often, what does not. My "failed did not work" pile is so high you need a 10ft step ladder to see over. That's how we learn. I remember in my F1 days, Niki used to say, you learn more from failure than you do from success.

This is the world I live in. The DIY guy lives in the "one time" world that either works or does not. There is never any room for failure. So when I read the person was using a GT3 pump, I look at that this way. Will it work or will it fail. I know for sure it will work and not fail him and his engine. The wording inappropriate stood out as to say, neither but to add a whole lot of confusion.

For the home built DIY, the fact it will work, not fail and its a good price, what's gained by suggesting anything other.

BTW, its commando all the way.

I guess I opened a can of worms. Not my intention.
I was just asking why and offering a thought about why the GT3 pump might not be the best choice for this type of build. Others will read this thread and hopefully will have more information to evaluate their own choice.
This looks like a great build and a cool thread, it was not my intention to derail it.

Well I’m glad my choices created a great discussion. Either way there’s a gt3 pump in there and it’s staying.

I spoke with Ian and he advises I reevaluate my piston deck height. After many combinations of shims. I got to a .89 deck height. With my current Rod and piston selection I’ll be fine. And the static compression ratio is at 10.6:1

As I continued to assemble the engine I installed all the pistons and cylinders.

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Rebuilt rockers by Craig Garr and WPC treated. Turbokraft rocker locks

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All timed. The Stomski tools are amazing

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Carrera tensioners installed using Supertec idlers.

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Sealed and ready for the next step.

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More updates to come. Thanks for all your input