Even when i use 45cc for shaved heads (1cc missing) 2ccs for the valve cuts on a flat top piston and 98mm bore, i get 11.5:1 the only way to get 12.5 :1 is to do a deck the block so that the pistons are coming up as high as the head gasket. is that what you did? 1mm, or .039" decking is pretty extreme.

http://www.csgnetwork.com/compcalc.html

Glen, this thread below is from 2011 when you were building your 98mm piston engine.
looks like even then, you were expecting 11:1

https://rennlist.com/forums/928-foru...ml#post8744795

I gave up a lot of CCs to you for the shaved and decked block and heads. stock euro is 48cc im giving you 45cc. gasket 1mm, piston cuts are 2mm etc.

i think you even posted how much you shaved the heads and decked the block.. the number was pretty small.

i have a feeling that you are in the 11-11.5:1 range, not 12.5.



your post back in 2011:

Good thread.

I've been thinking about my re-built engine. It's a Euro S engine (M28.11) with the heads decked 0.4mm and the block decked 0.009". It's got ultra-rare 98mm Euro S pistons of 1984 vintage so the smaller valve reliefs.

Working all the numbers with some estimates, I'm getting a static compression ratio of 11.1:1.

Anyone care to verify that or take a shot at the dynamic compression ratio?

I've been running a mix of premium gas (91-93) with 110 race gas to get 95 or 96 octane. Any opinion on if that's enough

 

Mark, I'm talking 12:1. 12.5:1 is the air/fuel ratio.

Since writing that thread I've had the block decked. That's pushing it up the extra 1/2 point. The reduced chamber volume is the real question. What's the area? It's the shaved heads, several times, that are tough to figure the volume for. I posted 0.4mm down. With 28cm^2 area that's 11 cc less. 37cc in the head?

No credit without showing your math.

Such interest! I know I need to have it cc'd to really know. It's more than the stock 10:1 the car came with or the 10.4:1 for later Euro S engines. I need to cc the thing to really know.

The point being, it's higher than stock and, as I said, I'm figuring about 12:1 cr.

Once I've got it apart, I'll be posting the pics and will make some measurements. Then we can all be happy.

 

What about converting to bolts?

I know studs are considered "better", are they necessary with a 16V? If the various 32V race engines (and Todd's 1,000hp Turbo) can handle stock bolts, why not a 16V?

Cool, friend of mine was there supporting a C6 ZO6 he built. Blue with a black hood and front splitter. He did a personal best 2:19:xx, not sure what tires he's running.

As far as head gaskets go. Turbo Todd has built a few motors with Cometic gaskets (I believe Kiborts is one of them). Any 944/928 engine I put together going forward will have those too.

 

I'm not sure I'd spend too much time worrying about this....we know the compression is more than stock....somewhere between 11.0 to 12.0.

That's close enough to know that detonation is a possibility....and, after all, that is all that matters, in this conversation.

 

I've got two "strange bore" engines that are getting "closer to the top of the pile" for assembly.

Has Cometic solved their seeping between the layers problems, or are there still special "hoops" to jump through to make them work?

 

I spray the hylomar between the layers.

Why? Because someone on the internet said to.

 

This would be going backwards, no? Studs are better as they don't wear the threads in the block and the torque is more consistent with known-clean threads.

He was in my run group. Wicked quick. Never lapped me more than twice in the 30 minute sessions. A 991 GT3 was very impressive while being poorly driven. Then again, my car is piloted by a software engineer, too, just with no computer to control the car.

Thanks.

 

I had a LONG talk with Todd on this today.

I'm going to sit down with him and go over these details better and jot it down and post up, maybe start a new thread on it.

He worked many years ago with Cometic (many calls with the owner of the company) to work out some kinks he initially had with them.

I agree it's going backwards, but it may be overkill and going to bolts may simplify this process after reading what Greg has gone through to ensure equal clampoing with studs.

If bolts are good enough for Todd's 1,000hp engine, I don't see how they are inadequite for a sub 400hp 16V even in track conditions.

Just a thought really, not trying to start a debate on this.

The only driver aids turned on during the Corvette's best lap was ABS. Active handling and traction control were powered off.

Joel, the owner of Corvette Sports International know how to setup a chassis. Many NASA trophies on his shelf. His team "CSI Performance" is who I pit crew for when his weekend load is too much for one person.

The car owner is also a very good driver. Few years ago he picked up a 944 spec racer to hone in his driving skills.

 

Great. Looking forward to it.

Just to clarify, I think the 991 GT3 was driving well on software that produced great speed with poor lines. I've seen similar a lot where new, costly cars are driven down the middle of the track. It was unusual that he was 15+ seconds faster a lap with that.

Greg,

I'll pull the heads in a couple of weeks, over the 4th. Thanks!

 

If you do truly have 12:1 CR, and you're running 3-4 deg ign advance over stock, it is likely that you are seeing detonation.

 

Fortunately, you can see detonation on the stock head gaskets very easily. The usually smooth fire ring gets pounded on....little dents in the surface. These will pound so badly that the fire ring will actually split open (allowing coolant loss into cylinder), usually before any other engine damage occurs.

I used this phenomenon to my advantage, for years, with the high output 951 engines.

Blowing a head gasket, compared to burning a piston in an Alusil bore, is a very easy thing to repair.

The Cometic gasket isn't built like this....seeing "beginner" detonation isn't as easy, nor will the gasket fail from minor detonation.

That's why I prefer the stock style gaskets, whenever possible.....I personally think a blown gasket is better than a damaged piston.

 

I completely agree with you on that Greg.

 

So here's the problem. That didn't leak in during disassembly as the 6-8 holes are dry. Clearly shows an exhaust stain on the gasket.

 

Here are pictures of the block and cylinder head for the problem #5 cylinder. Both show exhaust stains at the same spot. All the other cylinders look fine.

 

Here's a close-up of the gasket on the bad cylinder. It doesn't look like trouble to me. Perhaps sharper eye will notice something.