Deck height and blown motors.

[kjewer1]

This is a long ass post I made on the DSMlink forums, getting some good response. Thought some poeple here might find it helpful. For the noobs that arent as familar with engine specs/tolerances, a google search for "piston quench" turns up some bad ass links. Read all of them on the first page there.




Excessive deck height equals blown motors?

Guys, I need some help here. I need to figure out why I'm tossing motors left and right before I build another one. I also need to put this all down just to help organize my thoughts on this subject. I have just started to research quench/squish after someone on tooners tipped me off on this possible contributing factor (NDgsx, nick something?). The vast majority of the info out there on this subject is for NA V8s, so I also need to run it through the DSM filter and see what pans out. This will get long, please bear with me

So the problem is blown motors. With little or no knock, AFRs from low 10s to high 12s, etc, I'm blowing motors. More often than not the real damage is actually from the spark plug coming apart and beating the shit out of the piston/cylinder wall. I think most people know my setup, if not the mod list is on my site, click the WWW link at the bottom of this post. The first was a 2.0 running low timing and 10.3:1 AFR on race gas, spark plug number 2 came apart and douched cylinder 2. Compression 90 psi. Second was a stroker running 11:1 on pump, but timing turned out to actually be 28 degrees (base at 10). I can see why that one went. Third was the last, 21 degrees, race gas, 29 psi, 12.5-12.8:1, plugs 1 and 3 came apart, douching cylinder 1 and 3. Cylinder 2 also had low compression with no spark plug damage (50 vs. 0 for 1 and 3).

It has been said that a piston to head clearance (or quench height, and a dozen other names) over .06 inches will just about defeat the purpose of quench/squish, and put the motor in danger of detonation. By a huge margin the most often recommended value is .040 inches. Getting it down to .040 will create turbulence giving a more homogenous mixture, faster burn rate, less timing advance required, less negative work being done, lower tendency to knock, cooler pistons especially around the ring land, lower exhaust gas temperatures, and the list goes on and on. Every source agrees that too much clearance is bad and just asking for trouble. NDgsx says he has run 4g63s at .030, and suggests 0 deck and 030 gaskets. He also makes the disclaimer that you wouldn’t run this tight on aluminum rods for obvious reasons. But he is the only person I have been able to get DSM specific info from, and my account at NABR is still locked for a case of mistaken identity it seems, so I can't search there.

So here are some things I feel could have conspired to cause the ultimate loss of this motor. Any one or two on it's own might be fine, before you say "I ran 22:1 and the motor is fine." But all of them together?

Contributing factor #1, HG thickness. The mitsu metal HG ALONE is .066 inches. The factory gasket is reportedly .01 less than that uncompressed. But its hard to find info on this, the above is taken from FFWD. Even if we say .05 compressed its still over the 040 recommendation. Another question in my mind is what is the recommended value with an 8500 rpm stroked turbo 4 banger? But back on topic. You pretty much need 0 deck clearance to even get in range.

Contributing factor #2, deck height. When my last motor was assembled, the machinist was afraid to cut the deck for me (for metal gasket use) because of the deck height. But in the end, thanks to the bearing crush being set wrong by the previous builder, the rods had to be reconed. So that dropped them down a bit. The pistons ended up being pretty well below the deck surface. Now, I don’t have that figure yet, I won’t be able to get out and measure it until Thursday probably. But it’s certainly more than the thickness of a head gasket! He asked me if that would be a problem, I had no idea what all this BS was at the time, so I said F it, drive it on. The shootout was the next week after all. Remember that most sources say that over .060 you are risking the motor, and I'm at least double that.

Contributing factor #3, piston design. Something that seems odd to me is the lack of any type of quench pad on stroker pistons. There is just a 1/4" rim around the edge that constitutes your quench area, with a nice round dish in the middle. It occurred to me while dropping a deuce this morning that with the piston pin moved up so high (into the oil ring), you probably can’t push the dish down enough to get compression back in spec when raising areas to make quench pads (mimicking the head). I only see an obvious quench pad design on regular pistons. I ASSume there is more room to play with the dish to make up for the added compression from the quench pads.

Contributing factor #4, stroker. With a stroked motor (low rod ratio) you get a lot of piston movement in the first dozen or whatever degrees after TDC. Since the main combustion event is some time after TDC, the gap between piston and head is made larger at that time than it would be on a motor with a better rod ratio. So not only is the quench height way too big to start with (little to no squish effect at TDC), it gets opened up big right away.

Contributing factor #5, extra timing required. A direct correlation between squish and required timing advance has been found. Less suish equals more timing required. With extra timing you increase pumping loss/negative work, and detonation risk. Trying to make up for that lost power perhaps you lean out more, raise boost, etc. trying to get power to where it should be at the current set point. That of course just pushes the limits farther and makes things worse.

Contributing factor #6, fuel pump inadequacies. As we have discussed before, and can be found still at www.posracing.net/afr.html, the 255 LP was not enough to keep up with target AFR at 64 lbs/min. Obvious separation between target AFR and measured AFR could be seen in all gears but 4th. Running lean in what is potentially a detonation prone motor is obviously bad.

Contributing factor #7, cam timing. I have no friggin idea where my cams are. With all the machining done to the block and head, its all screwed up. This could be helping or hurting the situation depending on where they end up, and what effect my cam gear changes had. An earlier closing intake valve will raise actual CR, I forget which way I adjusted the gears now, been too long. I'll look thursday.

Contributing factor #8, plug heat range. Running BPR7ES plugs, while people have reported problem with them at the 126 mph level. Some people run them just fine though.

Contributing factor #9, compression ratio. WTF is my compression ratio? Static compression is 200 PSI. Thats fairly high. Just for grins I divided 160 by 7.8 and 175 by 8.5, and I get 20.5 in both cases. For a rough estimate I divided 200 by 20.5 and got 9.75:1. Seems reasonable to me At any rate, 10:1 seems a bit too high for what I am trying to do. And why the hell is it so high? With a deck height that is far more than it should be CR should be lower than predicted by wiseco. Even with some material taken from the block and head, it may cancel some of it out, but I would still expect a decent CR. And what’s going to happen if I tighten up the deck height? I'll be at 12:1 before I know it. And I keep punching the numbers into Ross's and other calculators, and CR is always around 12 to 15:1, WTF. Using wiseco’s info for dish (-21cc), 44 to 51 cc for head, 05 for gasket, etc. Something isn’t adding up. There aren't any other options for pistons to lower my CR, and a thicker HG is obviously out of the question. HTF am I going to get CR down to where I will be more comfortable with it?

Contributing factor #10, knock vs. preignition. I keep hearing that preignition won’t show up on a knock sensor. This would make sense because in most cases I don’t register any knock when the motor blows up. Small amounts in some cases, like 2 degrees retard. Not sure what to make of this.

So basically I ended up with a motor with a thick ass gasket, serious deck height, and no quench pads, giving me no quench effect making it detonation prone, coupled with a lean AFR, preignition or knock that is not showing up on the sensor, a CR of 10:1, and plugs that are potentially too hot. LMAO. Perhaps it’s no surprise it blew up. Maybe this is why other people can run 30 degrees of timing with the CAS dremeled out and 14:1 AFR and the motor lives through it, while I blow up at 21 degrees and 12.8:1 on 117 octane.

Other things I have questions about now are piston to wall clearance and ring end gap. As I understand it the more power you make the looser you need to run, generally speaking. But the looser you run the more rock you get and the looser you have to run your piston to head clearance. So what is the compromise? I know people out there have this all figured out already, and I would like to get some advice here. I can't afford to figure this out by trial and error


__________________
Kevin Jewer - 95 GSX
11.328 at 126
www.posracing.net



I'll add more to this later if there is some interest.

[kjewer1]

Well, I guess there is no interest, but here is some more anyway

Regular wiseco piston:

http://www.posracing.net/files/9_18_024.jpg


Magnus quench pad design custom piston:

http://www.utdsm.org/gallery/view_ph...um09&id=quench

[scottsee]

Well, the obvious reason I haven’t responded, is because I’m right out of the womb when it comes to engine design & fabrication. I’ll bring this up though. On the 1st contributing factor, I wonder what the head gasket does under the high heat its put through. The physics of any object under heat will create the molecules to expand. So if your running a thicker head gasket, unless the material is extremely resistant to thermal tempetures your gaskets going to expand in one direction, whether that direction is in, out, or expanding the deck head clearance, it beyond me. I would imagine a head gasket made out of high heat resistant metal, like aluminum would transfer heat quicker through the block, and retain less heat for expansion. Im guessing of course.

[kjewer1]

That may be part of the reason you are supposed to run thinner gaskets. Well, relatively thin. Some poeple go real thick to lower CR and the result is usually not that good.

[scottsee]

im kinda currious why your blowing spark plugs. are you runnging a stock ingnition system?

[kjewer1]

The igintion is stock. The consensus is that knock is taking the plugs apart. The debate is why I dont see it on the logger.

[JoeWagon]

I'm following the thread on DSMlink. I hate how everything is a lot more complex than it should be. You spend a good deal of change on a nice setup, and there is plenty chance that you will break something instead of setting a personal best. Good luck with that motor.

[helliviknow]

ok im really not sure i want to comment on this, mainly because i read that and felt like a first grader reading mcbeth or something lol.
Is it possible to dremmel out some material from the dish to lower your CR a little more? Are you planning any major changes to fuel delivery?

[kjewer1]

Its starting to look like my high compression might be from the cam settings. Others with the same motor are getting a much better CR even with a tighter deck height. I'm going to CC the head though, if I remember correctly this head is machined down to the service limit. And that certainly wouldnt help things. If it CCs real low I'm going to get a new head. I've got alot invested in this one though, sucks.

Looks like I'm going to use the MAgnus spec pistons, and I should get new rods. Because my rods are now shorter, I'll have to chop the block down to get the deck height correct (difference between the piston top and the deck). Rods arent cheap though, and if I am going to buy new rods I'd rather not get another set of eagles, but some crower or carillo. Double the price though. Still undecided there.

The magnus pistons and a deck height of 0 combined with an 036 gasket should make the motor less likely to knock than it is now. I am also going to spend some time learning how to degree cams on a 4g63. Should be easy enough with the motor on a stand. Just need to buy the tools.

Fuel system will be upgraded to a fuel cell and aeromotive A1000 pump. I could just barely get by on a supra pump, but I dont want to have to upgrade this ever again.

No major changes to the car this year, just trying to build a better motor

[scottsee]

the performance shops must just grin from ear to ear when they see you walking in the doors.