I have a 355 that is currently using trick flow alum heads with 64cc chambers, flat top pistons with 3.5cc valve reliefs, .025 deck clearance, and .015 heads gaskets. This combination works out to 10.6 to 1 compression and it works well on pump 93 octane gas. I will be switching the heads to vortec iron heads that have 61cc chambers. I need to lower my compression so I was going to install a .051 thick head gasket to lower my compression to 9.6 to 1. The problem is that this will leave me with .075 squish clearance with I think is too much to resist detonation. I could install a .035 head gasket that would raise my compession to 10.0 to 1 and leave me with .060 squish. The .060 squish would resist detonation better then the .075 clearance but would it actually work better because now it would have the higher 10 to 1 compression to deal with? Any thoughts on which combination will work better with 93 pump gas? Maybe both will work well with pump gas, maybe neither will work!

 

Optimally, you really want your squish to be .040, or even .035 if you run tight enough clearances that piston rock won't be a clearance issue.

My feeling is that if you are putting together a motor that will have more clearance in the squish area and you have two viable choices, then go with the one that gets you closest to .040.

Just my .02 cents.

 

That's a good question.

Optimally .040-.035 is going to work awesome more than that is going to hurt you and going to make the motor more detonation prone.

Why are you going from the Holley heads too the Vortec heads? Seems like head porting will be a better option over going to Iron Heads, even though the Vortecs are excellent examples. I can imagine that the Holleys have more potential and will not cause you so much compression ratio problems.

On top of that cam choice has a lot to do with it. If you have the right camshaft you can easliy run almost 11:1 on 93 with good Iron heads (such as the vortecs)

Bret

 

I am switching to the Vortec heads because the trick flow heads are going on a 406 that I am building for my corvette. The Vortec heads I am currently using on my 305 and I want to install them on my 355 that is in the Corvette and then install that engine in my Firebird. The vortec heads are already ported and they flow very well, 250's on the intake and 200's on the exhaust. If I can run the vortec heads at 11.0 to 1 compression on 93 octane gas with .040 squish clearance I will do that but I did not belive that was a viable option. I am using the Superram intake with the 219 cam on the 355.

 

Just an uneducated curious question. This "squish" that you are talking about, does it have anything to do with quench, and is the distance you are talking about the distance from the top of the piston at TDC to the bottom of the head?

Like they say, the only dumb question is the question not asked.

 

Yeah. Visualize a reverse dome piston and at the same time visualize an LT-1 head.

The "shore" area of the piston that is next to the "ocean" area (the reverse dome) comes up to that area on the head that is "inside" the gasket area, but not in the combustion chamber itself.
(you know, the area where the valves are?)

When the piston approaches the head, the fuel/air mix is "squished" between those two flat surfaces and it tries to escape by "running" to the more open area. This creates tremendous turbulence and shoves the whole mess over toward the spark plug.
The result is that you tend to dramatically reduce the chance of detonation. Why?
I THINK it's a combination of the turbulence and reduced surface area, but I'm not certain, since I'm not an engineer.

In a perfect world the "squish" area on the piston would just ~barely~ "kiss" the flat area on the head.

Of course, in that perfect world metals don't expand, pistons don't rock and rods don't stretch.

So in the real world you want to bring them as close as possible, without coming so close that a collision occurs, because that would be a bad thing. Very bad.

The real world ideal is about .035, but in most cases you actually want .040 for a .005 safety margin.

Anything beyond .040 rapidly reduces the effectiveness of the quenching process.

If I'm not mistaken it's an exponential relationship, although I don't know if it's a cube or square function.


Hopefully I haven't screwed up this explanation.

 

indeed, but I also don't know about using .015 gasket with aluminum heads, I would zero deck the block & use .039 head gaskets, or get a taller piston, I know for LT1's stock you can zero deck by getting taller pistons from GM, lt1 had +.030 pistons part number 10159438, so I dunno thats what I would do bro.

 

Ok so consider my setup and correct me if Im wrong then, it sounds ideal to me.

3.75" stroke, 6" rods, 1.125 comp.height srp pistons

(3.75/2) +6"+1.125" = 9"

Stock deck assuming 9.025 , zero deck down to 9" so deck and piston at TDC are flush.

Then run an LT4 gasket putting me at .039" from piston TDC to bottom of head. Am I forgetting anything, or should I not have the block zero decked since it is a little close to the .035"??

Two other things.
1. sorry to intrude on the post but it could possibly help also.
2. all parts are forged 4340

 

Enlighten me on those 'taller' gm pistons for an LT1. I'm not familiar. Usually, a +.030 piston refers to an overbore size available.