Tale of the Crossflow

[slomove]

I don't want to hijack this thread, But I am ignorant of these matters and would like to learn. I am very surprised the pictured piston tops are counter bored to clear the valves but there are hard 90* edges and no chamfer to the counterbores. As I say I don't know what the norm is, but this is counter-intuitive to me for flow, heat "risers" and the very thin outside wall. Maybe someone could direct me to info on this.

Thanks

I suspect the detail features of the pistons were not designed with much consideration or optimization of heat transfer properties and under normal conditions it is probably good enough.

[bball7754]

Engine builder called today, and it's back together and running.

 

He said he was looking at the head gasket, and it appeared that it was starting to burn through somewhere else. Confess I didn't really understand the impact of this, but basically he felt that the "hole" in the gasket was too close to the exact same size as the cylinder bore, so he wanted one that was "larger". He used different terms.

 

Also decided that the C/R may have been too high. Almost all his engines are race engines running av-gas. So, he got a thicker head gasket from Dave Bean that also fit his other requirement. C/R is now around 10.

 

Set the timing back a degree to 34 total advance. He'd had it on the dyno, and the impact in power was minimal.

 

So, may never know but it might have been a perfect storm of the C/R, slightly too much advance, and maybe a bad tank of gas.

 

I may run some av-gas at the first track day I do, just to be safe.

 

Picking up the engine on Saturday.

[TurboWood]

I agree, those are some strange looking pistons. It looks like they were trying to minimize the squish volume. Are these originally interference engines?

 

Sharp corners on a piston are quite bad. It will get hot and be an initiation site for knock. What's even worse is the sharp corners are near the OD of the piston. This is exactly where knock will typically start because it's where end gas sits (the last bit of air/fuel to get burned). This gas will be the hottest and most prone to knock.

 

Good luck with the repairs. I would be cautious with using the same piston design.

 

Regards,

Daniel

[seschm1234]

Nicemd's CSR blog has a link to piston damage description for many difference failures.

 

http://www.boosttown.com/engine/piston_damage.pdf

 

Lots of useful information at his site (http://csr200.blogspot.com/) I find it of great help.

[Hettric]

Nicemd's CSR blog has a link to piston damage description for many difference failures.

 

http://www.boosttown.com/engine/piston_damage.pdf

 

Lots of useful information at his site (http://csr200.blogspot.com/) I find it of great help.

 

Amazing amount of info, thanks for posting

[rx7locost]

It looks to me like the head gasket may have too much land between the cylinders. Perhaps the engine was overbored too much? This would put the gasket directly in the combustion chamber and cause the head gasket to burn thru. Perhaps the piston travels slightly higher than the deck (block skimmed?) causing damage to the gasket and the land of the piston? Certainly places to look.

[The Stig]

Interesting post. My theory (if you really need another) is overheating combined with/caused by too much advance.

 

Looking back at images of my previous Crossflow - all steel, dry sumped 9000rpm monster I had the heater bypass connected and the builder insisted this was necessary to retain the correct coolant flow. And I never once had an issue with this engine, however hard I drove it.

 

Sorry to hear of any woes on a Crossflow but I still believe they are the best engine - not an opening for debate, just my personal opinion!

[Kitcat]

A 9000 rpm anything is nice! Must have been quite the motor. I have had an X-flow, Duratec and a Zetec se7en. Each had its plusses and minuses. Probably liked the Duratec best, liked X-flow's sound best and, as far as I can tell, even w/more power in the newer cars, I was just as fast in the X-flow as I seemed to be able to use the power more easily.

[bball7754]

Stig - agree with Mike, must have been one heck of a Xflow.

 

Regarding the heater bypass connection, my Blatchat research (my primary go to resource for Xflow info) shows that Roger King supports removing it if there is no heater. I'm ok with it.

 

C/R has been dropped to 9.9 and advance dropped to 34. He sets the timing on the dyno from the flywheel. Engine is still on the dyno, so I'm going to bring my timing light and see what it indicates at the pulley.

 

He's as nervous about this as I am.

 

Steve

[twobone]

Hey Stig, talk about crossflow. Look at how horizontal his headers are. The flow on those must be increadible

[twobone]

rX7locost - so sorry to hear about your experience. It raises an interesting question. Maybe I spend too much time on the internet, but it seems like a statistically high number of rebuilt motors go "pop" shortly after the rebuild. Makes me wonder what are the top root cause? Mixing old and new parts? Builder error?

 

Engines are a complex symphony of moving parts under extreme stress.

[rx7locost]

Are you perhaps mistaking me with bbal7754? I'm not aware of my "experience". I was just offering a possible cause. Perhaps you are psychic and I should watch out in the near future?

[twobone]

my bad

[slomove]

.....but it seems like a statistically high number of rebuilt motors go "pop" shortly after the rebuild. Makes me wonder what are the top root cause? Mixing old and new parts? Builder error?......

 

I had the engine rebuilt in 2008 by a local recommended builder and got the head back like in the pic below (actually 3 valves were like that). Note the improperly seated valve spring retainer clip. If I had not seen this by chance the first start-up might have been quite interesting. So, if the builder is working with a hangover or otherwise not paying attention I am not surprised.