nine-44

Ok, Rock's 951 is done and out, it's back to Project 949 once again.

I've been working on the 16V idea, got a couple of heads and another on the way. I wish I had the 2.5L 16V head right now, but I don't, it's a week away. Right now I have a 928S 32V head with 100mm bore and a 944S2 head, 104mm bore. while I still don't know the design of the 944S head, I'm asking the questions anyway.

In a turbo or boosted motor, if I have the options, should I go with a head with more, less or no squish area?? As previously stated in other posts, I'm going 16V for a more efficient combustion chamber to stay further away from detonation. The 951 head and piston combo have considerable squish area. I'm thinking that in a higher boost motor, the sqish area and it's affect may make it prone to set off detonation, it may effect preformance if I drop it tho. I'm out of my strong point here so I'm asking for someone who knows.

If I go with the greater squish area for performance, will it benefit more even with having to pull timing to stay out of detonation?

Or, do I go with less to no squish area and bring up the timing to get the performance?

oooor, am I just not thinking right and the squish area affects detonation less than I am thinking?

I just feel that, in that last few degrees BTDC, getting into the squish area, it gives the mixture a quick crunch and may set off the charge before I want or get hard to control detonation.

Sorry if I'm hard to follow on this, I know what I mean, just can't exactly explain it. That, and I'm not an expert in head, combustion chamber and combustion physics.

Here's some pics of what I'm talking about....

nine-44

Also, I'm thinking that if the 944S head has the squish area like that of the 944S2 or similar, I may be able to cut valve releifs in the 951 pistons and run them. I have checked the cc of the chambers on 951, 928S and the S2 heads. The 928S and the 951 are almost the same at roughly 58-60cc while the S2 is higher at about 48-50cc. I really don't know why it's a concern at this point tho, I'll most likely be running custom pistons anyway. here's a pic of the 944S and 951 pistons and a few of the project as well. I picked up a set of 928S4 log manifolds...

cjeckert

just woundering but is that the driver side exhaust manifold from a 928?

Geo

Andy, if by squish you mean the quench, the area at the edges of the combustion chamber where there is little space between the piston and the combustion chamber, then you want more quench.

I am not an automotive engineer, but I have a very good friend who is who has talked about quench a fair amount. More quench is supposed to help resistance to detonation, I believe by moving the mixture closer to the spark plug. Now, just exactly how this works or why I'm not sure.

I have an engineering textbook on internal combustion engines. I'll try to remember to look up tonight what the science behind this is.

500

Hey Andy,

In addition to the help you're already getting here, you might want to also try posting this over at the 944 turbo/turbo S forum. Seems to be a bunch of pretty knowledgable folk there that don't post in this section much.

nine-44

Yeah, 928S4 driver's header flipped over.

I wish I could find good info on heads and combustion chambers, seems to be the best kept secrets tho.

Geo, see what you can come up with if you would, I'd apreciate it greatly. I just need to buy a couple of million dollar race motors to pull apart for homework. That's probably about the only way I'll be able to find any real info, lol. I beleive that we mean the same thing by your description, the books I've read called it squish area.

Anyone know of any search ideas that would help? I've tried stuff like "head design" "combustion chamber design" "combustion theory" and such, no real info to be found, that or I'm just not seaching very well.

Also, as far as head work, I had a buddy tell me that you want to polish the chamber the best you can, it will make it absorb less heat and retain less carbon. The later I would beleive, the first part I think is more theory. Any merit in this? Or just worth a shot? Would it hurt atomisation in the chamber?

Allen, I would but I beleive that most of what they will tell me is that no 16V will survive and that big valve 8V is the only way to go. I already hear it enough on this forum, lol. I guess I could try, we'll see where this goes first.

Geo

More quench or squish generally yields more detonatoin resistance.

I've found a good section on it, but it's too much to write for now since I need to get ready for work. So, for the moement.....

"The usual explanation offered for the fact that arragements involving squish reduce the detonation tendency is based on the assumption of a cooling effect. It is pointed out that near the end of flame travel the end gas is located in a thin space where it makes excellent contact with cylinder walls, which are at a lower temperature than the gas. Because of this theory, the thin space in combustion chambers of this type is often called the quench area. While the cooling effect of a quench area may have some influence, there is also the possible influence of increased turbulence. Perhaps even more important is the fact that in nearly every case the combustion chamber becomes effectively more compact as teh quench area is increased."

From: The Internal-Combustion Engine in Theory and Practice, Volume 2: Combustion, Fuels, Materials, Design, Revised Edition, by Charles Fayette Taylor.

So, quench/squish is good for reducing tendency to detonate as I mentioned before and you have speculated. I'll try to quote more of the book later.

Just purely as an aside, when I build the engine for my SE-R, I had the combustion chamber welded up and partially clover leafed to increase quench even further and to increase CR (something you won't want to do obviously with forced induction).

nine-44

Cool, thanks for the info there Geo. I don't know how boost would effect this tho. I'm still worried about the last split second pressure surge that the squish effect setting it into detonation. Dunno, still learning. I have a while to build the motor, I do need to figure it out so I can select the head I will run so I can get on with the header and such. Thanks again, Andy

Geo

I'm certainly no expert Andy. Much of what I'm writing I'm simply repeating what I've heard or read from reputable sources.

I think improved squish/quench is good regardless of NA of FI.

I know my little monkey brain would have thought that the squish area would make a head more vulnerable to detonation, but that is clearly not the case. The 16v heads are much better in this regard.

Geo

BTW, of the heads you posted photos of, the first one would seem to be the best choice. Of course, I suspect it would also yeild the highest CR.

Campeck

for real? cause according to all the books ive read with a turbo motor you want less quench/squish....ill egt the book and copy some pages for you later andy!

nine-44

The first one is the 944S2 head, it has 10cc less volume to the chamber.

Just read this, didn't go into much of anything but..."the BWM Formula 1 turbopiston has minimal squish to help stabalise combustion"

Campeck

yeah! thats from what im talking about..now that i think about it maybe it was just that....i need to re-read it. i might be getting the F1 stuff mixed up with street turbos...

nine-44

From what I gather, the squish area increases reactivity in combustion. It also helps light throttle power and accelleration. But, when pushing the limits as far as boost and such, it can promote detonation, or atleast bring it on earlier and easier. I just hope if I go with lesser squish, it won't totally kill cruise power and accelleration. Dunno, I'm still learning...

Geo

Where are you getting this information?