I was just playing with a compression calculator, and realized that I could increase my compression quite a bit just be changing the head gaskets. Right now, I have a stock bottom end, and stock size gaskets (.051"). I see that Cometic has several gasket sizes smaller (.020-.041).

Based on the calculator I found, these are my options for head gasket thickness and compression ratio:
-.051" (current) = 10.26:1 compression
-.041" (Cometic) = 10.5:1 compression
-.036" (Cometic) = 10.63:1 compression
-.020" (Cometic) = 11.05:1 compression

I have a few questions about doing this:
1) Will these thinner gaskets provide a good seal?
2) How much hp will the additional compression give me?
3) Is there a chance that the pistons will hit the valves? My heads are milled down .030" (66.5cc).
4) Which gasket thickness would you go with?

 

Its all going to depend on the cam really and youll still want to check ptv with .030 off the heads. Just my opinion

 

Personally I'm running the stock LS9 gaskets because they are cheap and reliable. I have 12:1 compression on a stroked 427 LS3 on my current build. PTV is the major question, if you are running stock bottom flat top pistons.

Easy enough to check though, and cheap. Head on down to lowes and pick up a cheap generic spring to check that. Swap out the springs on one of your cylinders, both sides, and rotate the engine to TDC. I always just use #1 because it's easy to get to and index. From there just push the rocker down and see how much you got. If little, start measuring it, less than 0.100" is where things get interesting.

That's like the universal acceptable limit. It can be less, but you'd better have some badass springs in there to prevent valve float.

I would do that BEFORE buying anything, just to see where you are at. Cam will determine your real limits. Advancing the cam will decrease the PTV whereas retarding it will increase it.

Oh and you will need a valve spring compressor to do this, and another $5 for the checker springs. Any generic spring that's strong enough to hold the valve closed will do. Like I said, Lowes.

 

By the way, static compression is basically meaningless at the end of the day. So to your question, is it worth it? Perhaps, more the merrier, but that is really decided by the camshaft. You can't compress anything until the valve closes, so, there's that. 11:1 static doesn't mean **** unless the cam closes to valve soon enough to build pressure, and opens enough and long enough to draw in more air, which is actually the bigger factor. Longer you hold the valve open the better, because airmass translates to power. Compression has it's role, but airmass is everything.

 

Well, I can't find my cam card right now. The specs on TSP's website don't say what advance it comes with. I think it has either +3 or +4 advance. Either way, it doesn't change the valve events by much. I'll take a guess and say that it has +3 advance. With that, my Cam-Motion Timer says that my valve events are as follows:
IVO= 1, IVC= 43
EVO= 51, EVC= -3

Overlap= -2

 

I have the BTR LS .660" lift dual springs, so valve float shouldn't be an issue.

 

What I would do is plug in your cam specs and info into the calculators and see what your "effective" compression is. It may also be listed as "dynamic" though dynamic is technically theoretical. Either way, cam is going to affect these numbers. A 224/228 as you posted (see it in the sig now lol) isn't particularly huge.

I would venture to guess your PTV is pretty good, but there's really only one way to be sure, and that's checking it. If you plan to pull the heads anyway to do the gaskets, then this task is minor in comparison and cheap to accomplish, provided you have a compressor. Those can be rented too though, from autozone or other stores. I went ahead and just bought one on Summit. No complaints.

If PTV is good and your effective (dynamic) compression will be under 8.63:1 (limit for 93) with the new gaskets, then yeah you should get a good bump in power for the money. Good bang for the buck as I'm sure you are thinking as well. I have mine at 8.2:1 and I run it on pump all the time.

Don't forget to factor in any advance from your cam as well. If you put it in "straight up" then just go by the cam card. If not, you have to account for that in the calculator, especially if it's advanced. Retarding it will lower ECR/DCR.

 

Never use the head gasket to adjust static compression. The small amount of power you might gain isn't worth the effort.

However, changing the gasket to adjust quench is important to build an efficient engine. Get your quench down to about .040" or so and you will make better, more efficient power, with less chance of detonation. Some people say you can go down to about .035" on an LS.

 

Yeah I think the .020" ones would be way too thin for quench. Forgot to mention that completely. Was going through other posts and got caught up by the cam and ECR/DCR stuff, which is probably where the real power is in any particular NA setup.

 

I didn't install the heads or cam myself. I had it done at a car club member's shop. The cam comes with some advance, so it should have been installed "straight up".

I can get the head gaskets for about $160 from Cometic. The other head seal kit from Summitt is about $180. I'll do the work myself, so I'm looking at about $340 total. That's about the same price as the LS6 intake upgrade I made. If I can get 20 hp or more from a head gasket swap, then it's worth the effort to me.

I started thinking about head gaskets when I started putting together a list of parts for rebuilding the bottom end. I'm not rebuilding the bottom end anytime soon. I was just thinking that if I added a supercharger, I would need forged internals. The initial cost of the supercharger kit has kept me from getting serious about it. At the same time, I don't want to install a supercharger if it's just going to destroy my stock internals. That's a whole other topic though. I just thought that I could get a little bump in power right now, if I went with thinner head gaskets.

 

I haven't heard much about quench. I saw a Youtube video where Brian Tooley was talking about it, but he was explaining detonation and compression. So, what is this quench measurement? Is that the distance from the quench pad to the piston?

 

Top of the piston to the top of the gasket, basically. With your SBE, you should have zero deck (0cc pistons) so whatever gasket you use is basically going to be the quench thickness, unless you changed the deck height, which I doubt.

https://help.summitracing.com/app/an.../engine-quench

 

Quench, also referred to as "squish" or "piston-to-head clearance", is the distance between the flat part of the piston at TDC and the bottom of the the cylinder head. Be sure to take into account where your piston is at TDC with respect to the deck surface; a piston out of the hole by .010 reduces quench by .010, and vice versa.

 

That's why I mentioned deck height. It the block was milled say like .005" to clean it up, it should be subtracted from the gasket, as you said.

OP, pretty much just go with the .040" and you should be fine, with respect to PTV since the cam is different. I don't think it would be too much of an issue, but I always check these things because that's the kind of thing that straight up ends a motor.

 

Does the seal kit include new bolts too? Don't forget those. Unless you have a stud kit, must replace those too.

 

I haven't touched the block, so it should still be 0" deck height. If .035-.040 is ideal quench for an LS motor, then I'd rather go with the .036" gaskets from Cometic. The .041" (Fel-Pro) won't change my compression as much.

I know my cam duration is mild, but it has .600" lift. That's why I was concerned with PTV clearance. I wanted a cam that would work ok for boost and N/A. Most people call it a granny cam. lol.

My engine still has the 0cc flat top pistons, but I'm considering the +3cc dome forged pistons from Summitt. They have valve reliefs and a dome. They look weird, but I suppose they work. I was worried about my valves hitting the dome, but these appear to have some big valve reliefs where they are needed. I'd really like to take a look at one of these pistons in person. I might have to drive to their store up the road this week and take a peek. I don't think these domed pistons will decrease PTV clearance, but I'd like to take a closer look and see.

With the .036" gaskets and +3cc domed pistons, I can get my compression up to 11:1. That should be a significant difference from the 10.2:1 I have now. I'm looking at these Summitt pistons, because they are the only ones that come in stock size and won't decrease my compression.

 

In that case, you should run thicker gaskets since the pistons will be out of the hole. That will reduce your quench. I think the best thing for you is to make a decision on what you want to do. Is it going to be NA or boosted? Going in between isn't going to give you the performance potential of either build, and going dome leaves you solidly on the NA side of things. If you really want to bump the compression, shave the heads or get different ones with smaller chambers. Lot's of other ways to do this, and yes domes will help too.

Another way to "bump" compression is to boost it. Just saying lol.

 

Yeah, I know. I'm just worried about my stock internals. No one makes a forged flat top piston. It's either dished, or domed. Summitt has the only forged pistons in a stock size. Again, all of theirs are either domed or dished.

Why wouldn't I be able to run boost with the domed pistons?

 

Utinator,

Here's my thoughts on your question.

Theory & Formula
show a decrease in percent of hp gain as compression rises above ~10 to 1. Going from 8 to 1 to 10 to 1 we'd expect a bigger net gain (delta) in hp than going from 10 to 1 to 12 to 1 compression.

Here's an internet chart showing the general relationships. I'm sure one of the experts can expound in detail. Under 10 to 1 compression sucks



Conventional Wisdom & Old Wives Tales
say about 3 to 4 percent gain in hp for each point of compression increase. Based on the chart it might be more on the lower side of the chart and less on the upper side of the chart. David Vizzard probably has a chart. I've read a few times in a dedicated hp engine the difference in 10 to 1 and 11 to 1 is more hp than the theory says.

Quench Details
Ideal Quench Height - Hotrod Magazine

Highlights
Qench height (piston to head clearance) may be defined as the distance between the flat-top portion of the piston and the cylinder head surface at top dead center (TDC), including the head gasket’s compressed thickness and any positive or negative piston deck height that may exist.

Good rule of thumb is that you can run as little as 0.038 to 0.043 inch with steel connecting rods. This helps reduce detonation and increases burn efficiency.

Estimated gain for 1 PT of Compression
Say the HCI LS1 makes 435 whp and we bump compression 1 pt going from say 10.2 to 11.2

A three percent gain would be ~ 13 whp
A four percent gain would be ~ 17 whp
A five percent gain would be ~ 21 whp

Looking at our chart above seems like gain won't be dramatically huge. So probably around 10whp to 15whp

In reality depending on the engine and combo it might be more or less.

Engine Building Philosophy
I've been told but don't have online reference that selection of head gasket should be based to setting the correct quench distance for an NA engine not for setting compression. In others getting the correct quench will be more beneficial than an extra .2 in compression

Is it worth $350 to change head gaskets for a 10whp to 20whp gain assuming free labor? That's definitely an opinion question and the answer will vary.

For me no way with the engine in a car. It's not worth the back pain involved to an old fart like me.

Known good condition engine out of the car. For me no way. It's not worth the risk of screwing something up. Plus mod creep aka "while I'm in there might as well" will set in and I like a lot of folks will be like I really need a new set of CNC heads to make all this work worthwhile. Instead if costing $350 for head gaskets it will end up as $3500 new top end package.

Rebuild project - sure it's worth it to me.

 

Because boost increases compression. If you have domed pistons and you are at a 11:1 or 12:1 SCR, you won't be able to make much, if any boost before encountering detonation. You will be spending 5-7k for the option of throwing like 3 or 4 psi at it and hoping it doesn't knock on pump. It probably would in this case, with domed pistons in there and decked heads. Just a guess.

Conversely, you could drop the SCR down to a 8:1 - 9:1 or so with dished pistons and push 10 - 15 PSI at it. I'm not going to really get into the math on why this is the way it is, I'm just throwing out examples that are possible. In this case you actually get your money's worth out of that supercharger, procharger, or turbo.

Now I know the numbers might not be exact there, due to tons of other factors, but as an example, that pretty much lays that out. Boost adds LOTS of compression, as I said, so the higher you start, the less boost you can run. The only way around that is to changeover to another fuel type, like E85, race gas, or alcohol. Those fuels can take much more compression than 93.

It's best to decide what you want to do before buying parts. Do this before you buy anything and never "build for both" since I believe many of us here have learned that the hard way at some point. Pick a combo that will maximize your efforts and money, instead of getting mediocre results out of "combo" engine, if that makes sense.

I highly suggest getting good at checking PTV if you decide to go with domes, reliefs or not. Unless there is a hole in the piston or something, a dome is going to move the piston closer to the valve than it was before. There are tons of designs out there, but unless the reliefs actually go under the deck, you'll want to get good at checking PTV otherwise a hole will likely form in the piston at some point, from internal automatic uncommanded self clearancing.