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What pistons do I need to get a given compression ratio?



What pistons do I need to get a given compression ratio?

 

Well, let's start off with a target compression ratio so we have something to work with. Say, 10:1. First off, there's no such thing as a "10:1 piston" anymore than there's such thing as an 8:1 piston. It's all relevant to SEVERAL factors, A) The bore & stroke of the engine, or basically the "displacement" of the engine, B) The TYPE of piston being used, whether it's a dish, an inverted dome, a flat top, a dome, etc., AND more importantly the compression height and positive or negative displacement of the piston top. For instance; a domed piston is obviously going to have a positive displacement which compresses the air into a smaller area thus increasing the compression, and a dish or inverted dome piston has a negative displacement causing a larger area between the piston top and the combustion chamber to compress less air thus lowering the compression ratio.

You also have to factor-in the valve relief area. A piston with large, deep valve reliefs is obviously going to have more of a negative displacement than a piston that has very small valve reliefs, or no valve reliefs at all. C) The combustion chamber size of the heads. Bigger chambers mean less compression, and smaller chambers mean higher compression. D) The head gasket thickness. A thicker gasket ads "area" to the whole ball of wax, which lowers your compression, and a thinner gasket decreases the "area" which will increase compression. It is VERY important to know how thick of a head gasket you will be running because it ends-up being about .213cc per every .001" of gasket thickness, so going from a typical .040" head gasket down to a .020" you'll lose over 4ccs, which in loose terms is about a half of a point in compression. In other words, if you have 9.5:1 compression with a typical .040" thick gasket and you drop down to a .020" thickness gasket, you will end-up with upwards of, or over 10:1. This gets real touchy when building forced induction engines where the static compression can be critical to the amount of boost and type of fuel you plan on running or serious detonation problems WILL happen. And last but not least, E) How far "in the hole" the pistons sit. The manufacturer gives you specs with a set of pistons telling you what the yield in compression will be with a certain deck height. They pretty much all use .040" head gasket thicknesses for their calculations as a standard rule of thumb, for whatever that's worth to you. Just know this; if you buy a set of pistons, let's say for a small block Chevy which have a 9.025" deck height, and the piston manufacturer says their pistons make 10:1, but at a 9.000" deck, you certainly won't have 10:1 compression with the pistons sitting .025" down in the holes if you didn't cut the decks down .025"! In fact, using typical generic math for a typical 4.030" bore, you'll be down about a full half point in compression than where you think you are. That's an easy 5 - 10 HP. A general rule of thumb, for any "typical" 400- 425 HP street engine is about 15 HP per point in compression difference, or about 3%. Obviously there are all kinds of factors that will sway that one way or the other, plus it's a sliding scale in that the lower the amount of compression you have, the more HP gain you'll see, and in high compression engines (say 15:1 or more), the less gain you'll see, but typically, two identical 400 HP engines... one having 9:1 compression and the other 10:1, there will be "about" a 15 HP difference between the two. What's even more critical isn;t how much power you;'ll gain or lose, it's what's controllable by the type of fuel you're using. In other words, it's MUCH more important to lose 10 HP by stepping down the compression on an engine that has too much, like say 11:1 for pump gas, than the gains you'll actually get by not having detonation and being able to run full timing. A laid back engine won't make as much power as one that is timed and tuned where it is supposed to be. So what you think you lost in power by stepping down in compression, you'll gain back by 5 fold or better by not having detonation and by not having to lay it back to avoid detonation.

You will never know what type of pistons you’ll be using, or what size chambers in the heads you need, until you establish what compression ratio you want to end-up with first. Of course there are a zillion variables on this, and 10 zillion guys that will say that they run something different, but as a GENERAL rule of thumb, on a stock engine with cast iron heads that you want to run regular gas in (87 octane), you want to stay around the 8.5:1 or so area, unless you have a computer system with a knock sensor to control timing to prevent detonation. You can run a little more compression in that engine if you want to run higher octane gas, such as 9:1 safely.

If it is a performance engine with cast iron heads and running premium fuel (91 – 93 octane), you don’t really want to get above 10:1. You’d be better off running between 9:1 and 9.5:1 which will allow you to run full timing rather than having to back it off to prevent detonation (pinging) and making the engine “lazy” by not utilizing full timing.

If it has aluminum heads and you are running premium fuel, you can bump it up to the 10:1 to the 10.5:1 area, but you don’t really want to exceed that area by much or you’ll have detonation problems. A "general" rule of thumb is that with aluminum heads you can bump your compression up by about 1 full point over what you should run with cast iron heads. This is simply because aluminum heads cool much faster between combustion cycles which helps keep detonation a bit more under control. Yes, some people run more compression, and so do we sometimes on some engines on pump gas, but it depends on things like the cam profile, and the type of car it is. You really want to stay below about 10:1 though on any typical street performance engine running typical premium pump gas.

Once you know what compression area you are shooting for, THEN you can begin looking at what heads & pistons you want to run. One brand or model of head may only be offered with a certain sized combustion chamber, such as 64cc. Well then, you’ll have to look for pistons that obtain the compression ratio you’re targeting with that combustion chamber size. Cubic inches, the bore diameter, and the stroke all play a role in it. Piston manufacturers do provide basic tables that show what kind of static compression you’ll end-up with for an engine of a given size, using a particular piston of theirs with a given combustion chamber size in the head. One piston will yield several compression ratios using several sized chambers. In other words; a flat top piston that makes 10:1 compression with a 74cc chamber might make 11.5:1 compression with a 64cc chamber. You REALLY have to be careful when choosing the right piston / head combo for any given sized engine.

what pistons should I run

When you run into this, by using the tables & graphs the piston manufacturers provide for quick reference you can look to see what other piston they offer to lower the compression to what you’re looking for using the combustion chamber size your heads have (or are planning on choosing for your build). They’ll most likely offer a flat top piston that will make 10:1 or so with the 64cc chambers. Again, you want to be REALLY careful when laying out what you want to run to get the desired compression you’re looking for BEFORE you go buying anything. Assuming you already know what size engine you're building, you MUST also know what chambers the heads you want to run have to then find a piston that best suits your target compression.

Click on the image to the right and it'll enlarge the snapshot from SRP's piston page. We use a lot of SRP pistons in our street engines because they're about the best you can buy and will handle pretty much anything you can throw at them. Notice that is says these typical pistons require a 9.000" deck. If you go sticking these into your engine and you didn't mill the decks to obtain that 9.000" deck height, then your compression will be substantially lower than what it says you'll get for the 3 given common chamber sizes. This again is stuff the average Joe overlooks or doesn't know about and why some engines run stronger than others. Also take note at what a difference chamber sizes make in compression for this same piston. Yes, for a 64cc chamber you could call these "10:1 pistons", but if you have 70cc chambers, they're 9.6:1, and worse yet, most common large chamber Chevy heads had 74 or 76 cc chambers, which lowers the compression even more. It would be down to like 9.0:1. That's a far cry different from the 10:1 you THOUGHT you had!  Worse yet, and to compound this problem even more, if you didn't mill the block from the standard 9.025" deck, and that piston is sitting .025" in the hole, that's about 5 more cc's less, which is about 6 tenths of a pint less. So now you're looking at about 8.5:1 of compression, and now you have a turd of an engine and can't figure out why when you THOUGHT you had "10:1 pistons" in it. Small block Fords run into this same issue with the 351W  blocks because a lot of pistons these days call for a 9.480" deck where all of the 351W blocks from 71 on-up had 9.503" decks. Well that's a .023" difference and about 5ccs.

Sometimes you’ll run into situations where you simply can’t get a piston to obtain the compression ratio you’re looking for with the combustion chamber size of your heads, so you’ll have to see if that head manufacturer offers a head with another chamber size. Sometimes they do, but sometimes they don't. We run into this all the time with strokers and blown engines. The more stroke you add, the more the compression goes up. You can only get a piston with a dish just so deep before you run out of area for a dish any deeper to get you a low enough compression for a blown application. Also, sometimes you can't get a head with a large enough combustion chamber, so you run into the same scenario. This means you need to run a shorter connecting rod and then find a piston with a different compression height that allows more area to lower the compression or to allow for a deeper dish. If you can't find anything to get you what you're looking for, then you have to start doing things like going to thicker or thinner head gaskets, milling the heads to make the chambers smaller, or sometimes milling the tops of the pistons, such as making domes a little shorter, etc. It's all part of laying out and building a NICE engine, and why "pro" engine builders produce engines that out run the average Joe's engine, because what the average Joe THINKS he may have, might not be what it is in reality. It's also why a professionally built engine costs more, because sometimes you can spend a LOT of time researching and looking for parts to get you what you're looking for, and when you can't, a bit of extra work is involved to obtain the net results you wanted, or needed.

This is all part of setting up an engine so it runs properly that a lot of guys just don’t do, or know how to do, and why a lot of guys run into troubles and wonder why they blow head gaskets, or they can't time it properly because it pings from having too much compression, or runs like a complete turd because it doesn't have enough compression.

You have to take EVERYTHING into consideration BEFORE you buy any parts, and do a LOT of research on what parts are available with given chamber sizes, pistons types (with domes, flat tops, dishes, various compression heights, etc.) for a given bore / stroke / cubic inch size engine, and for how it is going to be set-up (stock, high performance, race, blown, nitroused, etc.) so you can gather all of the correct parts to get you the desired compression you’re looking for BEFORE the build. It’s not about whether you WANT to run a flat top, a dished, or a domed pistons. It’s a matter of what you HAVE to run to obtain your target compression ratio. You need to know what you want to run first, (type of fuel, compression needed that runs best with that fuel, etc.) and then look at ALL of the parts available to see what kind of combo you can come-up with to get you what you want.

All of this is exactly why we have our trademarked slogan; Knowledge Is Horsepower! Because it really is! 

what pistons should I run

If you want to learn more about this kind of stuff, in much greater detail, be sure to check out our Auto Shop Videos series by clicking HERE. We have one titled "Rotating Assemblies" that covers all of this kind of stuff, and MUCH more, plus we have our epic "Performance Engines & Strokers" title that has a TON of info in it that might interest you. These make great Christmas, Father's Day and Birthday gifts!

what pistons should I run

 

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