chcgobearsfan3

I'm wondering if I can get away with new pistons, cam, heads, and intake manifold while maintaining stock crank, etc. Please avoid the detonation comments, I'm aware.

Air_Adam

The bottom end doesn't care what the compression ratio is... but the REASON you need such a high compression (as in, a GIANT cam, if its going to be that high)... it won't like the RPMs that are required with a cam big enough to need 13-14:1 compression.

chcgobearsfan3

Well, the cam I plan on running isn't too big.. .530" 210/250 @050. TQ peaks at 5k, but it's a pretty flat curve from 2000-5000 which is the intended range. I'm looking at closer to 12.5 CR at the moment, but may have head work done to hit 13. Assuming I throw on some JE forged flat-tops, TFS 175/56ccc heads, and cam; the stock assembly can stay? I hope so, would save a few $.

TraviZ

iTrader: (2)

throw some rods in their for godsake, they are the weak link!

Twin_Turbo

iTrader: (25)

revs kill bottom ends....and with that kind of CR I assume you need it to battle bleed off @ low rpm from an insanely large cam, ie wanting to rev the snot out of it? If so..it'll go kaput!!

chcgobearsfan3

Ah hell, if I'm going to do rods I might as well go all of the way . Thanks

Air_Adam

I gotta ask... why so much compression with those heads and cam? They are fairly tame, and don't need anywhere near that much compression to run well. They would be fine with 9.5-10:1....?

chcgobearsfan3

There are quite a few reasons for increasing the compression ratio. Performance wise, there is a gain in torque which equates to more horsepower at a given RPM. Also, due to the higher compression of the A/F mixture, atomization, fuel economy, and thermal efficiency are improved. Why not run a tame cam that produces the same torque in more useful, lower RPMs? To me, it beats running a radical cam with 10:1 SCR, a choppy idle and low end along with crappy fuel economy. Just making the most out of what I have to work with.

Sonix

iTrader: (1)

Sigh, the reason I never bother on this forum anymore is because someone comes on with the "I know it all...but" attitude where they think they can run 14:1 CR with a small cam (210/250 sounds like a typo as well), and they think that they are gaining power with that astronomical CR. Not to mention the fact they think they can even get that high.. You'd need heads with 30cc chambers to get that on a STOCK stroke, 305 engine. Running that CR is a whole 'nother matter. One of those, "I know it all, but I still have this really BASIC question to ask" - which really lets everyone else know that you really don't know as much as you think eh?
Why not hop off your high horse, and just ask the basic question "how much HP does stock rotating assembly take"? Or better yet, how can I best spend $xyz in order to get the most bang for the buck?

chcgobearsfan3

Right bud, 13:1 is quite achievable with the TFS heads (and to prevent further argument from you, the RHS 200 cc runner heads with 50cc chambers would be fine, just drop the lift a bit because of the small bore and proportionally large volume runners for the engine, and I'd be set) and a bit of milling along with forged flat-tops or slightly domed pistons, and since you're the apparent know-it-all; I assume you know the amount of horsepower a stock assembly can take. Please, enlighten me. It's actually more an issue of piston speed, for your information. By the way, it'd take 50cc chambers to run 13:1. Did I not mention that I was just around 12.5:1 at the moment? You aren't aware of my intentions with the engine, but I'll fill you in before you continue running your mouth off. My intentions were achieving better efficiency while gaining high torque throughout street usable RPMs, rather than throwing in a 383 and ruining my fuel economy. Maybe I should throw in a RamJet 502? Or maybe could stick with the 305 and ditch the parts that make it crappy? Also, I don't recall mentioning cost or horsepower anywhere in the thread. Hm, spend $3500 on heads, rods, pistons, and intake manifold that achieve what I'm going for, OR **** away 5 grand into an engine that's not what I'm looking for. You tell me, Sonix. For what it's worth, I give you props, you're a better man than I, with all the research you've done on the topic; I mean really, you must have strained yourself... sticking your arm so far up to pull those numbers out of your ***; I couldn't have done it better myself. My BSFC, VE, TE (which just about offsets your advantage in cubes) are all far better than yours. I'm sure my performance is as well, anything else, sir?

PS: Your whole paragraph looked like a typo.



Attachment 187566



To everyone else, thank you for the input.

el_muerte

I love these threads. "Can X be done, and I don't care how bad of an idea it is?" Kudos for trying something different but no need to get snarky. My training tells me that the performance gains from bumping the CR up that much aren't worth the effort, unless you're doing something real special and have the money to back it - in which case you wouldn't be asking if you can do it on a stock bottom end.

RED_DRAGON_85

regardless of weather or not the bottom end can take it, how will you keep detonation in control?
if you retard the timing to reduce detonation, you have basically negated the bump in CR.
the PROPER CR will give the most power.
you really want to have your timing as advanced as possible and set your CR around that to some degree.
if your cylinder is 12* around its rotation before it ignites, you have essentially wasted that displaced volume, which cannot be afforded on a 305 if you are looking for power.

now, if you plan on running race gas, then yes, 13:1+ will work just fine with aluminum heads and a mild cam. it will also run like a raped ape, but the question is still why...
you will spend more on gas than you would have on a bigger motor that had properly put together parts...

im not trying to be condescending or critical, just hoping to point out something that may save you a lot of frustration and $$$ in the end.

there is a reason 14:1 compression sounds so radical.... because it is!

chcgobearsfan3

****, I had written up a couple paragraphs and got the, "token has expired." The gist of it was that I was going to run methanol/water injection, which is roughly $2 a gallon (wiper fluid). That should last you through a tank of gas and some, but if you're feeling like a big spender, you can always add more methanol into the mix. That that'll bring the cost up a bit, but regardless, it's still cheaper than driving around with racing fuel. I'll skip the detailed reasoning on why I'm working with the 305, but essentially it's because I'm curious and have done countless hours of math, physics, and geometry behind optimizing it. Then I realized I had only been working with pistons and the top end; I figured someone on here would have some info to save a bit of time. The main response I was looking for was the one TraviZ gave me, thanks by the way. For what it's worth, I'm not doing this for the "I run 13:1 compression on my bad *** 305!!!!!" I'm doing it because I enjoy the challenge of trying to perfect what I was given, engine wise. Regardless of what size block, from 305 to 454, there's a lot that goes into configuring an optimal setup. If I wanted straight up power, I'd just buy a crate 350, 383, 400, 454, 572, etc.

chcgobearsfan3

For anyone interested--
"This is from a study NACA (the previous name for NASA) did on water injection a long time ago.

The following graph goes like this:

x axis = Fuel/Air ratio (invert it to convert to Air/Fuel)
y axis = Break Mean Effective Pressure

They injected several mixtures. They injected 60% as much of the mixtures as they did fuel. The ignition timing was fixed at 30 degrees BTDC, and the engine is non-intercooled.

Red = No Anti-detonation injection
Blue = Water Only
Green = 70/30 Methanol/Water

Not that with water alone you can lean out the mixture as much as you want and still make more torque (and thus more power) than you could with any air-fuel ratio without Water Injection.

With Meth/Water at this mixture torque could be increased as much as 65%. For comparison, a 65% increase in torque on the STi would yield 495 lb-ft of torque instead of 300."

Courtesy of NAISOC and NASA, just putting this out there.


Increasing CR is in many ways the same as boosting, by the way. The same principles apply to N/A motors as far as combustion temperatures, BMEP are concerned. Keep in mind, in many states they run 10% ethanol gas, which makes the mixture ~60/40.

Lastly, in my situation it comes out cheaper to run high CR than to bolt on a $2000 roots blower, which is inefficient as hell, or a screw charger which costs a ton. In the end, I'll put out torque equal to that of a roots blower with a 13:1 CR, without the loss of efficiency, but instead a gain! I feel like Glenn Beck speaking at UCLA, posting this stuff.. Is there anyone who is actually into the theory behind the power on this board? It seems like everyone is dead set on "350, stroke it to 383, or f-off." I mean no offense, really, I'm just curious.

Sonix

iTrader: (1)

Groan... I'm glad there are other members here that are grounded on earth (get it? Earth - NASA? teehee...), saves me the time typing up reasons why it's a bad idea, or trying to prove my snarky-ness and bitterness.

I'm a little busy, but there are many holes in your plan. Unfortunately. Good theory, I can't knock that, but the reality just isn't there - or else someone would have beaten you to it.
If you love the idea, go for it, let us know how it works - but i'm fairly sure some of us have a pretty good idea of how it will work already...

Batass

Sounds like you're into experiments and free thinking, so just try it.

I figure if a stock bottom end can handle the actual compression from forced induction, then it should handle it naturally.

dimented24x7

iTrader: (2)

If this was a honda civic going for max NA 4 cyl. HP, you know, like running a huge single cam with an RPM band up to 12,000 RPM, then I would say go for it.

But, its not. Keep in mind that increasing compression is one of those things that results in decreasing marginal returns. IOW, a linear increase in compression does not result in the same linear increase in efficiency. Yeah, it might give you a 10-15% boost in thermal efficiency, equating to a similar increase in HP. But it also makes the engine more difficult to handle as it will be more prone to detonation, especially with a small cam, and require more exotic fuels. And, on top of that, the chambers also need to be very closely matched to eachother when running a CR that high.

Why not increase potential HP by 14% and upgrade to a 350? Good heads and cam along with something more reasonable like 10:1 compression will yield a motor that can run on more conventional fuels and still make good horsepower.

Air_Adam

Its not the mechanical parts that are a problem - its the fuel. If this was race gas, methanol, or the like then he'd be just fine. Its only going to eat itself alive with the pump gas he's planning on putting into it.

Batass

I don't see where he said he was running pump gas, but he did say he was aware of the detonation problems and isn't interested in hearing any opinions about that.

I knew a guy that was running 12:1 in his truck on pump gas with iron heads. He ignored the common logic and the pistons broke after two years. I was surprised it took that long, maybe the rings went much sooner...

If I was running E85, I'd run a 12.5:1 cr.

Tightenloop

My friend built BMW 2.8L stroker (he used crank from diesel engine) he wanted to get CR not less then 13.0. assembled engine showed awesome increase in power and torq compared to stock 2.5 engine. He also used wider cam. And hand ported aluminium cylinder heads.
Later he changed radiator for almost twice bigger to keep engine being cooled appropriate.
BTW he use premium gas (in Latvia it is 98 octane) and there are no any sings of detonation.
After that i can not say 2.5 sleeper.. he is really scout on the street. so I suggest you to try to build.

in future I plan to swap in 350 instead of 305. but gas prices are hell high in Latvia. So I will use LPG because it is twice cheaper. LPG advantage is high octane rating and therefor I can go high CR with no detonation. I also think about high CR motor.

RED_DRAGON_85

on that note, if you can get E-85, then by all means, it sounds like a great idea!
one thing to consider is the availability of parts for a 350 vs a 305.
pistons and heads expecially are more common in a 4" bore configuration

ljnowell

Hate to tell you this, you are setting yourself up for a big time failure. You really, really, need to get away from books on theory and start looking at real life engine situations. What you say is great, in theory, but it will not work the way you want it too in the real world. You really need to work on your attitude, asking a question and then bashing members isnt a way to get help. If you do not bleed off the cylinder pressure at lower rpms with a bigger cam you will run into all kinds of other problems you havent even thought of.

Increasing compression ratio is by no means anywhere close to boosting anything. Leave the formulas behind and look at proven combinations.

I can tell you for fact that you would be miles ahead by putting a mild 350 in your car and running with that.

Air_Adam

I had an iron headed 350 with 11.5:1-ish compression ratio and a relatively big cam (230/236) and while it was pretty good in cool/cold weather, it was a nightmare to keep tuned to keep from all kinds of detonation problems when the weather warmed up. And thats two full points lower than what he wants to do, and with a larger cam to-boot. And that CR was made in the most detonation-resistant way you can - very small chambers and a slightly dished piston. No domed pistons.

Some small-bore engines can handle a big CR without really having detonation problems - alot of Japanese 4-cyls have very high compression ratios... some as high as 12:1 - but thats due to the small bores and a better chamber design. Big bores are always more detnation prone - BBC engines are a good example. Its fairly common to see 11:1 small blocks run fine on pump gas, but you rarely see an 11:1 BBC that runs well on pump gas, because the giant bores and a less-than-ideal chamber/piston design make those engines prone to detonation problems.

dimented24x7

iTrader: (2)

CBF,

Like said above, its all about how much time is required to burn the mixture. Larger engines with less optimal CC designs (pretty much all conventional single spark plug 2V/cyl V8's), need more spark advance due to the time needed for the flame to propagate across the chamber. With smaller bore engines with centrally located spark plugs, there is less SA required to make peak power, and less dwell time after the mixture has ignited.

Once the mixture ignites, the pressures begin rising rapidly, and with a high CR and less than optimal chamber, the remaining unignited fuel will reach a point where it no longer needs a spark to ignite. Hence, some or all of the remaining fuel can go off in one shot, you know, detonation.

Even with heads like vortecs that set up counter rotating vortices to help transport the kernal across and around the chamber, the SA required at WOT will be at least around 26-28 degrees to make good power.

You probably could make the engine run with 13:1 CR or more, but it wouldn't tolerate a lot of timing. So, whatever gains you could have gotten from the increased CR will be pissed away by having to grossly retard the timing. The end result would either be an engine thats a pig, or a motor that hammers itself to pieces when you try to get peak power out of it.

This implies that there is an optimal point for the CR to make peak power with a specific engine type and combustion chamber layout. On the low side, the decreasing efficiency causes power loss. On the high side, the increasing tendancy to go into detonation and having to retard the timing or take other measures causes power loss.

chcgobearsfan3

Okay, but running a supercharger at 8psi with 9:1 SCR is equal to running with just about 14:1 CR, 13.9 if you want to be precise... That's far from unheard of, no? That's when an intercooler comes into play. However, instead of running 14:1 TCR with a blower; I'm opting to run 14:1 with a 70 water/30 methanol. How is this any different? A high SCR is far more efficient than running with a typical roots/carb blower setup.. Hell, it's still more efficient than running a centrifugal. The engine will draw in the air it needs from vacuum alone rather than having a belt driven SC blow it. Like a supercharger, you'll still have high velocity exhaust, hence the much longer duration on the exhaust side while minimizing overlap, or creating negative valve overlap (EGR essentially). Both the 14:1 TCR SC engine and 14:1 N/A will put out the same amount of pressure,heat, and torque; the only difference is that N/A is more efficient. They can both run with low lift in street RPMs, which is why a lot of people just bolt on superchargers without changing cams, although this will choke the motor above 5k, but who drives that on the street? Well aside from being more efficient, the N/A will require the water/meth injection to prevent detonation, along with the knock sensor.

TCR formula is as follows by the way:

(Boost (psi) / 14.7) + 1) x SCR

dimented24x7

iTrader: (2)

I think your getting confused with how boost works. Boost can be thought of as a displacement adder. IOW, the blower, turbo, or what have you will take in more air than the engine could ingest on its own. The ammount of power that can be developed by an engine is essentially a function of its thermal efficiency and quantity of air/fuel taken into the cylinder. A 14:1 dynamic CR NA engine will NOT make the same ammount of torque (and hence, power) as an engine running boost and making an apparent dynamic CR of 14:1. Dont fall into that trap. The two systems are not equivelent. The reason being is that the NA engine has a lower mass of air in the cylinder. Under ideal conditions it may be efficient, but it will make less power than an engine under boost. CR is not a replacement for boost.

ljnowell

Seems like its been said, but I still dont think its been heard.

RED_DRAGON_85

wow
a blower at 8psi is putting an EXTRA 8 psi of air into the cylinder.
do some math and that gives a ratio of about 1.5 under OPTIMAL CONDITIONS
meaning that at 100% volumetric efficiency, there is 1.5 times the ammount of air in the cylinder than would be there if the engine ingested it on its own.

typically VE for a engine is around 72-76% on a mostly stock engine.
this means that only 72-76% of the air that SHOULD be in the engine actually is...
thats why superchargers were invented
the original idea for superchargers was to add air into airplane engines during combat.
at high altitudes, the supercharger would be switched on and voila, more power.
so you can see that a supercharger is really just a VE modifier, it makes the engine feel bigger than it is.

back to reality, your high CR engine will have nowhere near the power of a engine with an 8psi blower.
say you start with 9.5:1 CR and you increase it to 14:1
you increased your CR by 38%, but you will not see a 38% increase in power.
you will be lucky to see 5%

as everyone has said, there is an optimal point based on what engine characteristics you start with
14:1 is not optimal unless you are running race gas of some kind

chcgobearsfan3

It's dependent on the car as a whole, but seeing as we're sticking to the engine... Volumetric efficiency over 100%, on a NA engine, is easily achievable if the cam, heads, intake manifold, and exhaust manifold match up well. In my case, they do. If you specifically design the cam to run from 2-5k RPM, you can carry over 100% VE across the board. An engine will draw in air as needed. Also, the point of injecting methanol and water is to effectively increase the octane rating of pump gas while maintaining cooler air, cylinder, and stabilizing combustion. This allows for you to run higher timing, more safely. Make sense? Airplanes? As Sonix said, I thought we were keeping the discussion "grounded." For what it's worth, adjusting CR from 9.5:1 to 14:1 will yield an increase of ~13.4% at the point of peak torque. I may suggest the same to you, do some math. Although, it's been said that theory and math should be thrown out the window, so maybe math isn't the best way to go about it, eh? Silly engineers and physicists.

RED_DRAGON_85

FWIW, i am about to graduate and acutally be able to put PE (professional engineer) after my name, so i know a thing or two about math and theory.
the thing is, you have to realize that all the math and theory in the world are only valid under the conditions that the math was done in...
meaning ideal world, no friction, no heat loss or gain, ect.

at this point, all i can say is go for it.

dimented24x7

iTrader: (2)

Theory is good for exploring things. But, theory to a good extent is an idealization. There are many more variables in reality that just aren't accounted for in the theory. To a good extent, the OEMs are using more compression to boost efficiency. But, this requires more optimal combustion chamber designs to allow for more turbulence and a faster burn, as well as better injection technologies like direct injection. All of these you wont have with a standard small block. This is why you see people usually not exceed 11:1 compression on most street motors. Will it work the way you propose? Probably with the right witches brew of fuel. But, who wants to have a motor that cant use readily available fuels? I wouldn't. Its nice to be able to just throw regular in the tank when you pull up to the pump and still make 325 or more horsepower out of a more conventional engine. Not to mention the peace of mind in that you have a motor that will operate safely under all conditions, not just optimal ones.