alright my 408 is going together finally and im wondering what i want my compression ratio at??? im going to be spraying a very healthy amount... the pistons are an 8 cc dish, zero deck, 64 CC heads, 4.030 bore, 4" stroke..

this puts me at 11.4:1 with a .040 cometic gasket and at 11.11 : 1 with a .051 gasket..


another thing... the advertised gasket thickness.. is that a squished thickness or before torquing thickness??? i wanna keep a good quench so what would be the better decision for me??

help ASAP would be appreciated... im ordering them up today!

 

I THINK it is measured before it is torqued.. maybe someone here can say for sure. BUT a very effective way of measuring gasket thickness after it has been torqued is.. place a small peice of lead between the head and the block deck where theres no gasket, and install the head and toque it down to spec. this will compress the lead to same thickness of the gasket, but the lead will not spring back. Therefore the thickness accurattly represents the installed thickness of the head gasket.

 

Most (if not all) gaskets are advertised with the compressed thickness. A non-compressed measurement would be pretty meaningless IMO.

 

Yeah it is the thickness of gasket after your heads are torqued down.

 

What type of gas are you planning on running?

 

Are we also forgetting another important component?

Camshaft and valve timing...DCR.

 

not to insult- but isn't the primary question here


do you want NA max performance or N20 max perforamance

personally, id pick NA, but i'm crazy, drunk and easy.

edit: i was drunk, not "dunk".

 

subscribing...

 

I agree. Most of the things you've named are already set. 0 Deck is kind of surprising, but oh well. That being said, I would go with the .04 gaskets to keep your quench down. By running .052 quench, you may have a harder time keeping detonation at bay (not cool on the giggle gas). That will put your static compression ratio at 11.4:1.

Now that you know what your static compression ratio will be, you need to decide what DCR you want to run. If you are running pump gas, I would run about 8.7:1 DCR. That being said, you'll want an intake valve closing of 70 degrees ABDC. (this isn't 50 degrees @ .050)
This is why I believe that Static Compression ratio should be chosen last, not first. If you already know what cam you want to run based on your goals, you can build the SCR to be what it needs to be to support that cam with the fuel you want to run.
Not that you did this wrong, I just would have preferred to build SCR based on the needs of my cam. Not choose the Cam based on the SCR that I have. Just give one of the sponsors a call. Let them know what your static comp ratio is, the fact that you want to spray it, and your goals as far a fuel choice, and have a cam ground.

 

I'm sorry but I have to disagree. Normally id say pick the pistons last- but too little too late.

at this point- sacrifice timing, not valvetrain timing. You are going to have a engine that wont right if you try to adjust your valvetrain events to get the desired compression ratio.

at this point, you HAVE to be thinking "retard timing heavily when spraying" not "close intake way earlier than I normally would"

 

Trey - I agree with you that normally the pistons/head cc should be chosen last based on the engines needs. That's what I was saying late in my last post. Looking at his setup, I don't think he's that bad off though. If he runs the exact cam that I just ordered - 243/250 619/631 114, (intake closed @.050 is 51.5 degrees + 20 deg to get to fully closed = 71.5) he'll be at 8.7:1DCR. Of course that's not the extreme largest cam he could run, but I think it'll work fine - especially on the giggle gas. Or he could go with a larger duration and bleed off a little more. I wouldn't go any smaller than a cam that size though. I also agree that he'll be needing to retard timing when spraying too.

 

I only disagreed in principal, I'm not going to suggest a cam.

 

Good tuning can make almost anything viable with N2O. On the shop car where I used to work (Fastech), the motor was about 14:1 358cid with a 200-300 shot. Only problem was pushing water from the water passages, but never blew any head gaskets. BTW, it ran 8.7s at 156 in the San Antonio race.

IMO, make the motor setup what you want and then tune it properly.

 

I agree with Reckless. get the motor to run as best as you can then tune in the NOS on top. I am a firm believer in compression to make motor power then add the power adder.

 

Your motor is what it is. You have to keep your quench, so going with thicker gaskets isn't an option unless you want the motor to eat itself. You might losse a fraction of a point, and induce 10X more detonation which ona nitorus motor is a recipe for disaster.

As stated, optimize it on motor, then look at dope. I'm a pretty firm believer that you can go with about a 200-250 shot before you REALLY, REALLY need to optimize specifically for nitrous. Thats not to say you can't make a car go faster by setting up a camshaft specifically for nitrous, etc... But, I think you can afford to be more "sloppy" below the 200-250 range.

The key thing to remember is that if you run at a higher altitude you can put more nitorus ina motr than at sea level. You might be able to run a 250hp fogger in Denver on and run fine with no problems and much less in the way of engine mods than a comaprably equipped car at sea level.

If you plan on going to much more than a 200+hp shot at sea level, you need to get some forged pistons with the top ring down the hole a bit and some Hell Fire rings plus a less compression. This is all pretty basic nitorus stuff though.

Above 200HP shot you need the piston, rings and a cam with more duration on the exhaust side to get rid of the heat and lower the compression even more. If you don't get rid of the heat ( blow down the cylinder earlier with more duration) you burn pistons. But, many of the grinds out there already have too much exahust bias on them, which is why I don't worry about them as much below 200HP.

If you think about "the old days" everyone and their brother threw a 250HP plate on whatever they had and had a good old time. We had a 68 Camaro that we street raced It was a 406 (stock rods with good bolts, heavy TRW forged pistons), 461 castings with a quicky port job, and a 292H Compcam, and 4.10's behind a 400 with a 300 stall. The car ran 11.80's on motor and 10.40's on spray. That combo was togther for many, many years....

Anyhow, enough ancient history.

First off. If you use a big fat nasty cam, you'll need that compression to boost DCR. Secondly, what type of fuel are you going to use. Thirdly which is something most folks don't ever pay attention to is multi-stage kits. I don't have the figures in front of me, but as an example. A single 400 HP kit creates something like 4 times the ammount of stress (cylinder pressure, etc...) as two 200 HP kits, especially when they are staged. So, if you are really concerned with lifting heads, etc... two or 3 small kits will be much easier on your motor and probably make the same power.

 

I'm no nitrous expert, but it seems to me that as you start squirting significant amounts of juice, despite the cooling effects of the nitrous vaporization and dissociation, the situation in the combustion chamber must start to look a lot like a bigger cylinder (e.g. perhaps 30% more fuel and oxygen) compressing into the same combustion volume, aka much higher effective CR and pressure.
Surely a point is reached at which you can make more power by further increasing the nitrous and keeping the engine alive with less CR?

 

What about a progressive controller? Lifting heads is my only real fear with the aluminum block. It has arp studs and cometic gaskets but still worried about it.

 

Ditto, I'm running 13.5-1 and will be using a 300-400 shot.

 

Bueller........

 

Sorry, been off for Christmas. Progressive controllers are nice to help keep from killing tires. I've never looked at one to controll cylinder pressure, but it might work fine for one. I'd have to do some more research to say one way or another.