Many of you have asked about which Shredder head chamber size is best, and some have been ordering our 13cc version simply because it has the lowest volume of the three. In an effort to increase customer understanding of our products and allow you guys to better know what you are buying, I'm writing this blog to clear a few things up.
TLDR:
- You want lots of HP and compression makes that but can lead to serious detonation if it's too high or the fuel octane too low.
- You can only burn pump gas up to a certain amount of compression.
- VP octane booster is good, but don't think you're going to clean up 87 without a lot of it.
- The idea is to maximize dynamic compression for the fuel you are using but NOT get into detonation.
- You'll need to use online calculators to figure out your static and dynamic compression ratios. (links below this TLDR)
- For 93 octane pump gas don't go over about 9:1 dynamic compression.
- For methanol you can safely go up to 18:1, but you'll need lots of ignition advance to maximize power.
- If you are running nitro methane you'll either go through time or explode.
- If you end up with too much dynamic compression you can use the calculators to find a cam that lowers it enough to run the fuel you want. The more duration and overlap in the cam profile the lower your dynamic compression ratio will be. You can also use a thicker head gasket, but it's best to run a 0.035" thick head gasket with your piston at zero deck and then relieve the chamber to get the compression you want.
- Any increase in displacement like a larger cylinder bore or stroker crank will increase compression ratios.
- Larger combustion chamber volumes will lower both static and dynamic compression.
- Piston down in the bore at top dead center dramatically lowers both static and dynamic compression.
- Note that most high performance port fuel injected automotive engines end up around 10.5 or 11:1 static but drop to 9:1 dynamic because they are designed to run pump gas. Newer stuff varies cam timing and with a knock sensor, digital ignition, and port or direct injection, can break rules that we small engine builders can't, so take their numbers with that bit of understanding. If you try to follow them your wings may melt, Icarus.
- Always remember that a hot running air cooled engine needs to either have really high octane fuel, be self-cooling by using methanol, or slightly less compression than a comparable liquid cooled engine because the heat makes them more prone to detonation.
Calculators:
Easy static compression (don't worry about the number of cylinders):
https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator
Even better Static and dynamic compression ratio calculator:
https://www.uempistons.com/p-27-compression-ratio-calculator
Just the dynamic when you have already calculated your static:
https://www.uempistons.com/p-28-effective-compression-ratio-calculator
Displacement:
https://spicerparts.com/calculators/engine-displacement-calculator
***For calculating dynamic compression you'll need your intake closing point in degrees after bottom dead center and all of the calculators use the degree that the valve is at .050" from closed for some reason. You can find this in your cam documentation. Dyno has them posted under the documents tab and file is listed as "xxx tech sheet" For example on a 308 cam you'll want to use the 50.3 ABDC + 15 degrees number as your closing event in the linked calculators.***
The featured image at the top of this blog shows a Predator 212 Hemi block with flattop piston at zero deck and a 0.043" head gasket with a 13cc Shredder head on it. You won't get away with pump gas, so it's what NOT to do unless you are running 110 octane or methanol.
Links with more explanations:
Hot Rod magazine explains compression:
https://www.motortrend.com/news/0311em-power-squeeze/
EnginePower shows how to build a 13:1 compression pump gas engine (start at 12:47):
https://www.youtube.com/watch?v=okYyReBncC8
Totally awesome super famous NHRA Pro Stock winning daddy engine building gangster badass tells us all about compression:
https://rehermorrison.com/tech-talk-8-compression-session/
And now the rest of my ramblings:
There are two popular ways to measure compression ratio in an engine, they are the static and dynamic ratios, and you can also measure cranking compression pressure if you don't have a compression release. If you are using a compression release, you'll need to go with the calculators.
What is static compression ratio? It is the difference between the swept cylinder volume and the combustion chamber volume, and that's about it. We'll use a Predator 212 hemi with a flattop piston sitting at zero deck when the crank is at top dead center as an example:
Swept Volume = Displacement @ 212cc + Chamber Volume @ 22cc = 234cc
Combustion Chamber Volume = 22cc
So without taking into account other things like how far down in the bore the piston is at TDC or how thick the head gasket is, with this simple example we get this:
234/22 = 10.63:1
Another way of saying this is the ratio of the amount of air and fuel sucked in verses the amount of room left after the piston comes up on the compression stroke and smashes it all together into the combustion chamber in the head. Imaging taking a deep breath and filling your lungs with 10L of air, but instead of exhaling it, you have some sort of ram in your lungs that smashes that 10L of air into a 1L pocket in the bottom of your lungs. Now light it on fire and you've got internal combustion. Time to get some steel lungs...
What is dynamic compression and why do we use it? Dynamic compression takes static compression to another level by factoring in intake and exhaust valve overlap events and connecting rod length. We can get further into the details on dwell time, overlap events, inertial supercharging by pressure wave tuning of intake and exhaust events, wet flow dynamics, and all sorts of other stuff later but for now what you need to know is that if the intake and exhaust valves are open at the same time, which happens with larger cam profiles, some of the intake charge is blown straight through into the exhaust port at engine speeds where cam timing isn't perfect, and connecting rod length also factors into this because it affects how long the piston sits at the top and bottom of the cylinder which has an effect on the charge entering and exiting the chamber. This gets more complex when you realize that you can use the camshaft profile to make a 13:1 static compression engine simultaneously be a 9:1 dynamic engine. 13:1 on pump gas?
What does all of this have to do with chamber size and why am I writing this? Fuel octane, chamber size, ignition timing, and camshaft profile all influence detonation. I've been getting a lot of questions about Shredder head chamber volumes and what is best, and all of that really depends on what fuel you are wanting to run, and what dynamic compression your static compression, cam profile, and connecting rod length have set the engine up for.
What is detonation? In a reciprocating internal combustion piston engine the fuel and air charge doesn't explode, it burns, ideally in nice controlled rings extending in all directions away from the sparkplug until all of the fuel is burned up and its chemical energy is converted to heat energy which acts on the crown of the piston to create torque that spins the crank. Detonation is not fuel burning, it's pockets of unburned fuel randomly exploding in the chamber, and it's super damaging to an engine. The sudden shockwaves created by explosions in the combustion chamber impact the crown of the piston and can break rings and ring lands while they rattle the piston pin, rod, bearing, and rod journal. You really don't want this because it can cause a rod bearing to spin. In extreme cases it will cave the crown of the piston in, and can cause the piston to break into pieces and the rod to come outside of the crankcase to say hello.
Compression ratios are very important in internal combustion engines. When you get everything right the engine runs great on the fuel it is designed to run on, but when you go too far either high or low you can have serious issues that can be a safety hazard and expensive for your wallet.