Hi all! I should probably preface this post by saying its my first, and apologizing if its in the wrong spot or inappropriate for this forum. Set me straight if somethings not right! I've tried searching the forum and did not find any definitive answers. If i missed something I'll gladly delete this post. So as the ***le reads, I've been putting some thought into forced induction AND average-high compression. I'm trying to design (by design I mean determine what parts I will be using) an engine that's a semi-efficient but is force at the track. So the typical engine is designed to handle X lbs of total compression from combining natural compression and forced induction. Usually compression is lowered so the engine accepts more "boost". I'm wondering why the starting compression has to be lowered. Can one not just use less "boost" and high compression as long as the final compression matches the build? Since I'm long winded: If 2+2=4, and 3+1=4, what makes one better? In theory, wouldn't higher compression and lower boost hold better mpg and a smoother powerband (less lag, less "kick" when you hit boost)? After all, higher compression increases efficiency and forced induction adds extreme compression(and fuel to compensate).
I'm not an expert but in short.... 1: your compromising both ends. It won't be the best high compression motor or blower motor 2: if you start with 9.5:1 and add boost your running like a pound of boost before you can't run street gas. Seems like a lot of money wasted. 3: it would seem to me you would get more power from a heigher boosted cid than a low boost high compression motor of the same inches. But you take 10:1 add 13 pounds of boost and not only can you not find fuel but your going to break a lot of ****. 4: to answer the question if you want a reliable, drive able, motor with some zot as ****** says. I'd build a mild blower motor. You keep the over all compression to pump gas levels, you get "high" power and your not really straining any parts to make it in reliable or a gas hog. Would you rather a blown motor barely working to get 400 hp or an unblown motor working its *** off to get 400 hp
*disclaimier I really am not a professional but I'm sure one will Weigh in, though consider of its basically the opposite of what literally every one is doing there's probably a reason
Before answering, tell me what kind of "Forced Induction" are you talking ~ Roots Blower, Centrifugal Supercharger, Turbocharger ...... It makes a difference
A lot of questions need to be answered by you before accurate advice can be given. Like pump gas, e85, methanol, race fuel? What kind of forced induction as hotroddon asked? Street driven mostly and strip now and again or mostly strip? Going off an ***umption, for a true street/strip setup you want it to have a "dynamic compression" of no more than 13'ish for premium pump gas. I have gotten away with more but the tune up has to be dead nuts on. Cam overlap, total timing, cylinder head design and cooling all play into it besides the engines static compression ratio. http://www.blowerdriveservice.com/technical-charts.html Some say start with low static compression (like 8.5:1) and a bunch of boost to make crazy big power. I believe in starting with higher static then adding a bunch of boost and using an appropriate fuel and tune for a no hold barred set up. But for street you have to compromise as you need to fuel it from your local gas station. If your static is too low, the car is a turd that cannot get out of its own way until you stomp it and the boost comes on then all hell breaks loose. Typical cruising acceleration and leaves from stop lights will be sluggish. Even instantaneous boost set ups like roots superchargers don't build boost until the engine is under a load... i.e the loud pedal is matted. So for street I like a static of roughly 10:1 and 5psi of boost for premium gas. Easy on the total timing... like a couple degrees less than optimal for the na tune. On e85 you can get away with a couple more psi. This is all rough because I don't know the combo and not intended to be ragged edge at all. A trip to the drags, and you can fuel it up with race gas and turn up the wick with more aggressive pulleys if its a roots or if its turbo a manual boost controller. Hope this helps. Cannot get any closer with the info given.
I can give a general answer to the question it is old world technology so it may not apply to a modern LS with hair driers and an intercooler but it is an answer. Instead of thinking about boost as a compression adder thing of it as a fuel adder. The added zot comes from packing more fuel into the combustion chamber. Added compression is just a by product. Lower static compression is a by product as well. you dished the piston that would have normally been domed or flat topped in a naturally aspirated engine, when you did this you made more room to stuff more fuel mix in, the fuel mix the bigger the explosion. Now take this a step further, if you were talking to a powder man he would tell you that the longer you contain an explosion the more powerful it becomes. Higher compression will detonate quicker, so by the by product of dishing the piston you not only have more room to pack more fuel in there but you also have made it so that you can contain the explosion longer, bigger bang. Anyway that is how it was explained to me by an induction guy an entire lifetime ago. it may not apply to modern theory but it works for me.
its all just cylinder pressure in the end.....just run less or more boost or nitrous, whatever it takes to reach your target of "x" cyl pressure. I turbo'd a 10.5:1 motor and ran 15# boost, it was built for it though and was quite happy.
Any room, or plans for an intercooler, or "water injection." ? High Inlet air temperature has a pretty profound effect on limiting much advance the engine will stand without detonation. Boost without an intercooler results in HOT air even with "efficient" superchargers.
Compression ratio affects the efficiency of the engine. Boost (and displacement and RPM) affect how much work the engine can do. The limit for both of these, is detonation, which is determined by combustion pressure and temperature, vs fuel quality. If you want to make the most power, you set up the engine to move the most air. If you want peak efficiency, you increase compression ratio as high as you can.
Think of it as cylinder pressure. Your static compression ratio is based on normal atmospheric pressure of about 14 pounds per square inch. Add 7 pounds of boost and you are over filling the cylinder by 50%. This means your cylinder pressure will go up by 50%. In other words a 10:1 compression ratio turns into 15:1. This is way too high for gas, and is getting up into diesel territory. Might be ok on pure alcohol. But, if you start with 6:1 then 7 psi boost turns it into 9:1 which is more like it. This is just a general idea, there are other factors that go into it. For example when you compress air it gets hot, and hot air generates higher pressure than cold air. This is why intercoolers are used. But, if you take your supercharger pressure, as a percentage of atmospheric pressure, and add it to your compression ratio you will get the idea.
Here is a chart showing static compression and what effective compression ration can be attained any giver blower boost. Here is a link to calculating your boost by blower type and by tooth count. http://dyersblowers.com/pulley_boost_charts.html Remember, 12:1 effective compression is about all you can run on 92 gas. Warren
Just run straight alcohol and the engine will take as much physical compression and boost as your head gaskets can handle. Yeah I know this isn't the answer but it works on the track
It's funny how technology changes things. That rule has been true forever, yet I was looking at the specs for the 2016 Mazda Miata yesterday and it has a compression ration of 13:1 and runs on 91 octane. My how things change with electronics. Many modern motorcycles are in the 12-13:1 range too. I remember not that long ago when those were pure race motor numbers
There are a lot of factors that enter into it. Years ago Bruce Crower and others were running 13:1 compression on unleaded pump gas, on the street, in Chev V8 powered cars and pickup trucks. The secret was a trick camshaft with a radical intake profile and a milder exhaust. This allowed air to blow back out of the cylinder at low speed, lowering compression pressure, while the mild exhaust tamed the rumpty idle. At higher speeds the cylinders filled completely but at higher speeds you don't get the spark knock because breathing is restricted by the carb and intake. The result was a 20% increase in gas mileage, a slight increase in power, and no knock or drivability problems. I don't know why they never followed through on this. I believe a lot of people fooled around with the idea over the years but it never came to anything. Another trick to tame knock is water injection. Usually a mix of water and alcohol is injected into the intake. This allows high compression without knocking. There are a lot of angles.
My coupe has a 355 ci small block 9.5 compression with 8/71 weiand roots blower run with 5 percent underdrive pulleys it rocks with Canadian 94 octane pump gas and msd boost timing controller..
The Crower idea was similar to a modified Miller / Atkinson Cycle theory. What they do is delay the intake closing to a very late event, as late as 90° BTDC o nthe compression stroke. With the engine set up at a very high compression ratio, Bruce went as far as 17:1 which raises the expansion ration. BUT by leaving the intake open so long, it pumps a ton of the intake fuel Back into the manifold, thus lowering the effective compression ratio to say 8:1. The compression ratio is now lower than the expansion ratio. The heat from the burned fuel pushes the piston back down, expanding the air volume beyond when compression began. This allows the pressure in the combustion chamber to be close to atmosphere at the end of the power stroke meaning you get all the available energy from the combustion process, creating great efficiency. For any given portion of air, the greater expansion ratio allows more energy to be converted from heat to useful mechanical energy, meaning the engine is more efficient. The problem was the motor was really sluggish down low and the emissions were terrible. You can help it with a supercharger, making a true Miller cycle engine. Modified Atkinson's are common now, especially in Hybrid's (My Fusion has one) where the electric motor makes up for the lack of low RPM power, and EFI has cleaned up the emissions issues.
I've built, repaired and tuned a few blown engines (only small and big block Chevy). If you're going to run boost, that's great. Lots of fun and big power/grin potential. Must haves, that will make your engine fun AND let it survive, forged pistons, zero gap rings, Clevite H bearings, top quality rods, a good steel crank (internally balanced) and double keyed, head studs and the best valve springs are a bargain, chromoly pushrods, good roller rockers and stud girdles. 8-1 compression will allow you to run 5-7 pounds of boost on pump gas. With a simple pulley change, a 6-71 street blower on a 355 SBC should make up to about 24 psi lbs of boost, (racing gas and tuneup changes required). As you increase boost, you increase heat in the air charge, decreasing efficiency. If you're careful and make changes to your baseline tune-up in small steps you can avoid disaster LOL. I run Methanol in my blown SBC. It's 370 cu in, 10-1 compression. With 17 lbs of boost it's calculated to make roughly 930 hp. I'd like to have a blown street engine but I can't help myself. If more is good, then too much is better. Build it. You will like it.
I'm running just over 9 to 1 on pump gas with 6-7 PSI boost with many thousands of street miles and some track time too.
Thanks for all the info guys! I was initially looking for general principles, but now see that I should have given you folks a situation start with. As of now, it would be a 302/460 on pump gas used for street/ strip driving. Efficiency for city driving and cruising, power when I stomped on it. I would be using a turbo with an intercooler. Probably not methanol injection unless I could turn the boost down for the street and turn it back up along with methanol for the track? Ill be sure to get your opinions on a build plan before I shell out the money for parts. So drawing off of all of your responses thus far, my "Compression v. Boost For Dummies (of course this doesn't include any specific number provided). This is after a quick skim of all posts so far. I haven't had the time to fully read into them.: Forced induction adds more air and fuel at a quick rate, basically simulating a larger displacement with a very low restriction of airflow. It gets a lower amount of energy from each unit of fuel burned, but adds so much more fuel that the collective amount of energy is far greater than a n/a engine. Compression makes the most of the air and fuel being added, but is limited by the octane content of the gas. An engine that's more focused on boost is able to reach higher overall power because more fuel can be added due to the low compression, but its a dog before building boost because a horribly low amount of energy is able to be extracted from the fuel due to the low compression (excessive lag). The amount of fuel and air is limited before boost because, well, its running n/a. Its limited like any n/a engine. Compression is good for n/a engines. A number of factors limit the amount of air and fuel drawn in, so compression makes the most of what you can get. Compression gives you a smooth powerband starting at low rpm's, but you'll never make as much power as a turbo/blown engine because there's simply no way for an engine to draw in enough fuel and air by itself.
So to touch on meth/water injection... I know it helps cool everything down and in turn prevents detonation. Methanol is an octane booster, and water cools the best. (So the more methanol the better when factored into the additional fuel added ?) I'd like a truly wicked engine when the boost is turned up, so I would probably want to run straight methanol or a high concentration so I could run the maximum amount of boost (this is all under the ***umption that my internals are bulletproof), but i've been told a 50/50 mix is better than straight methanol? Wuga's table on the results of different boost/ static combos is really valuable to a novice like myself. Does anything like that exist for methanol added? How much of an effect does it have? Is it so effective that I can get away with race gas compression if i add methanol (id inject it when boost exceeded a certain point, taking the engine out of pump gas territory)? On another note: How much boost makes forced induction worth it? Does it differ between a turbo/ roots/ centrifugal setup? Ideally I would run my compression on the higher side, but I don't want to be running such low boost that the setup isn't worth the time, money, and h***le.
It is really easy to get carried away with supercharging or turbocharging when another 50 or 100HP is just a twist of a knob, or a pulley change away. But, the more boost the harder on the engine to the point where the big expense is not the blower or turbo, but building a motor that won't blow up like a grenade under the boost. If you want to use a Ford 302 the easiest and cheapest route would be a Paxton centrifugal blower. They made tons of them for 5 liter Mustangs and used ones turn up fairly often for a few hundred bucks. Centrifugal blowers and turbos are the lightest, and take the least HP to run in other words, are more efficient. But have the disadvantage of not making much boost at low speeds, then building boost and power as you go faster. Positive displacement blowers make more power thru the rpm range but are generally less efficient. There are some positive displacement blowers that are better than others like the Eaton that was OEM equipment on some cars in the 90s like Buick and T bird. Didn't Ford sell a T bird with a blown 302? Of course if you really want to fly ask the guys who are happily driving around with 10:1 compression and 15 pounds boost on pump gas. They know secrets that are beyond mere mortals.
Pressure is pressure and volume is volume [don't confuse the two] An engine with too much pressure will detonate. An engine that breathes too much volume will.....make power [there's no such thing as too much volume] An engine with a low static compression ratio have large volume in the combustion chamber so it will need to pump in a lot of air to get boost. Atmospheric pressure is 14.7lbs at sea level. If you had an engine with a static compression ratio of 7:1 and had boost of 14.7lbs it has doubled the volume and should have the same cylinder pressure as a 14:1 compression engine. Low compression and high boost will make good power , but probably as flat as a pancake when off boost.
Three good answers. As you can see, it's a hard concept to explain. Let me try - at the risk of confusing you all over again. What it's really about is m*** of air-fuel mixture. Whatever m*** of mixture you put into the cylinder you want it at the optimum pressure, which is the highest you can go without detonation. More pressure makes for more complete combustion and hence greater efficiency, but only up to a point. That point depends on a lot of things, e.g. fuel type, combustion chamber shape, even stuff like cam and ignition timing, etc. It isn't a case of more pressure making more power as such: you're aiming for that same optimal pressure whether you've got forced induction or natural aspiration. So, let's call that optimal pressure X. At any given temperature, a greater m*** of air-fuel mixture at pressure X will have a correspondingly greater volume than a smaller m*** of air-fuel mixture. Therefore to achieve pressure X with a greater m*** of air-fuel mixture, as supplied by forced induction, you would need a combustion chamber of greater volume relative to piston displacement, i.e. - by definition - a lower static compression ratio. The greater the proportion of air-fuel m*** supplied by supercharging, the greater the additional power output, for any given engine displacement. All things being equal, a high-boost/low-compression engine will produce more power than a low-boost/high-compression engine. The question of supercharger efficiencies becomes moot if you think of the piston itself on the compression stroke as a kind of supercharger. Compressing the charge on the compression stroke takes energy; it's most of the engine's pumping losses. With forced induction you are adding another supercharger to that: you'll find that the extra pumping losses due to the supercharger will be more than the saving due to the lower static CR, but the net proportional increase in pumping losses will tend to hover around the net proportional increase in the m*** of air-fuel mixture the engine is handling. Thermal effects do tend to mess with this a bit, which is something turbocharging exploits. So, lowering static CR with forced induction is about getting the optimal pressure right, not about "protecting the engine." Blown engines don't break because of superchargers pumping them up until they explode. They break firstly because greater m***es of air-fuel mixture produce greater levels of bmep and torque than the chosen components can handle, and secondly because not getting the optimal pressure right allows detonation to occur.
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