Venomantidote95

The only way a lower compression engine would make more power is adding cuin! Read more carefully!
Lowering compression by adding cuin is a stupid *** thing to say it infact has the opposite affect raising the cr, you are talking of using a camshaft to overcome the actual point I was trying to make! Everything cam heads intake, etc flatop wins! I do agree with the dish making a smooth surface that promotes the greatest flame propagation thus raising the efficiency of the burn rate in the cylinder. if there is somthing I am horribly ignorant on here please relay it in laimen terms where my redneck thinking can comprehend it!

simpkins_44

Hey I’m sorry I miss interpreted what was meant by your previous posts. And once again am unsure of your direction with “Everything cam heads intake, etc flatop wins!“ what do you mean by this???

I now also know that you do understand by what I mean about dynamic compression ratio. I only intend on using this to control actual cylinder pressures that are related in terms of volume. I previously have figured out that the most compression you can get safely with a 91 octane gas and air mixture of 14.7 is about 8.5 times its initial volume at atmospheric pressure. I only want to get to this point not exceed it. But to sum up my thinking, if my combustion chamber volume(including piston, quench, head gasket…etc) was 70.7cc and wanted to run a 91 octane high compression motor then I would only be able to take in up to 600.95cc(70.7cc times 8.5) of gas and air otherwise I would have to step up to a higher octane fuel. This also goes for the larger dished piston which brings combustion chamber volume(including gasket and stuff) to 80.9cc meaning I can only allow up to 687.65cc(80.9cc times 8.5) of air and gas before I would have to step up to a higher octane fuel. With this in mind I will only be using the cam to control the compression not the actual stroke itself on which static compression ratio is calculated on this is why I want to use dynamic compression ratio as my terms of approach to compression and compression ratio.

So if I had to choose a piston. I would choose the one piston that will allow for the highest amount of power without going to a higher octane gas. So wouldn’t the larger dished piston have the ability to create more power without going to a higher octane since it allows for a larger volume of air and gas then the flattop does??? And if so does this interfere with quench??? meaning two volume different pistons of the same height would have two different proper quench heights??? since quench is supposed to disturb the air and gas making it more stable(almost similar to a vortec)??? wouldn’t a larger volume interfere with that??? I understand the a dished piston promotes a good burn but does it do this just by shape or does it allow for quench to be improved some how without changing its distance???


p.s. (Venomantidote95) I wouldn’t consider anyone a laymen, we all have our fields that we prefer and that’s just how we’ve learnt. I know I am not an expert and I except that, I just want to try and do things in terms of thinking how I learnt. Thanks for your posts as well, and I hope they don’t stop just yet though.

67mustang302

It's actually quite common for lower compression engines to make just as much or sometimes more power than their higher compression counterparts. The reason is, the compression stroke exists primarily to provide heat from compression to get the fuel into a vaporized and burnable state and close to it's flash point. Once you've reached the necessary heat energy from compression to easily ignite and properly burn fuel, any compression beyond that is unneeded. But it still makes the air/fuel charge hotter and therefore more prone to detonation. Lower compression engines are often able to run more aggressive tunes because they're less prone to having combustion related failures.

Static compression is really there to work with camshaft timing to properly compress the charge in the cylinder where it needs it. In some instances with large race cams where the engine is very inefficient at low rpm, you need high static compression just to have enough cranking compression for the engine to even start.

Venomantidote95

Ok I see your point! Depends on what ur building! Yes I also agree on the cam/compression info! Im thinking maybe were on the same path just misunderstanding view points! Simpkins 44 no offense taken or given I hope! U sound like an engineer!?

simpkins_44

that’s some great information you go there 67mustang302. And I hope for my sake that even though I don’t plan on going too high for compression ratios that I can still pull off some serious power. I completely understand and agree with what your saying about compression and its promotions to vaporization and heat properties that increase combustion energy and how its benefits are limited. I am extremely interested in what you were saying about more aggressive tuning of lower compression engines because of its less likely combustion problems. I wish you would have touched more on this as I think it would greatly benefit others running lower compression set-ups.

Although 67mustang302 I’m not 100% sure I follow you with the static compression relation to camshafts. Could you post more on this for me to follow up on? To me it seem more suited that dynamic compression relates to the cams more because of the influences valve timing has on it. I will admit though that I do like the basic idea of static compression it reveals the maximum availability of compression and hints at volumes. However I feel that static doesn’t tell the whole truth which is why I do prefer working with dynamic compression although it my be more numbers to crunch. There are a few tricks here and there that I have picked up from using dynamic compression more so then static. one of the most beneficial is the use of CAD and Solid Works and although these help me out I know I still have more yet to learn.

Also venomantidote95 I agree. I think we have just been misunderstanding each other although we may have been on the right paths. Also I’m glad you didn’t take offense by what was said and I also have not taken any offense. Although I enjoy the complement of being in relation to sounding like a engineer, however I am not one. I am only 20 years old. However I am in university and engineering has been a mind set for me. I have even joined a engineering group that builds formula SAE race cars, but fail to get my opinions voiced because I don’t belong to the engineering college. But I enjoyed the complement though thanks you. You sound like you have a good background and knowledge of engines. You work at a performance shop or something?


Ohh!! I also figured out some numbers using a stock 302 HO cam and the rotating assembly I’ve been working out. With the intake valve closing being where it is on a stock cam and the extended (3.25”)stroke that I chose, there is still 2.96 inches left in the stroke this allows for a 8.543:1(dynamic) compression ratio for the dished pistons well within where I wanted it. However the flattops pulled of a 9.63:1(dynamic) compression ratio(I did the math twice and it check out).I did this math with 58cc heads, it could also be done with larger heads. I think this would make a great kit for people who wouldn’t want a full stroker or to bore their motor. It would make decent power(with a stoke cam) and for people who would like to take it higher in the RPMs then a stroker kit of the same price could handle can do so. but maybe that’s just me. Let me know what you think. Links are bellow. hit me back!

Crank http://www.summitracing.com/parts/ESP-4302325054AU/

Rods http://www.summitracing.com/parts/SCA-6540092721/ or http://www.summitracing.com/parts/SCA-6540092721A/

Pistons (flattops) http://www.summitracing.com/parts/PBP-12347-STD/ or
(dished) http://www.summitracing.com/parts/PBP-12352-STD/

67mustang302

Dynamic compression is what matters. Performance camshafts generally result in poor(er) dynamic compression at lower rpm, which is why performance engines idle with lower vacuum. Higher static compression helps to offset the reduced cylinder filling at lower rpm and create a combustible mixture. It also obviously works with cam timing at higher rpm to create more actual(dynamic) compression. Too much dynamic compression somewhere and you can damage the engine or lose power without the right fuel, too little and you lose power or have an engine that runs poorly or not at all.

Engines are generally less sensitive to compression related issues at higher rpm than lower rpm in terms of heat/detonation. You can run 36* of timing with a 13:1 AFR at 5,000rpm and WOT.....but not at 2,000rpm, for instance.

Personally, I prefer running more modest compression and a custom cam to best fill the cylinder(dynamic compression) and having plenty of safety room to run an aggressive tune. Engines make more power from timing and fuel than they do from compression alone.

The best way to think of dynamic compression is the volume of air/fuel charge that is sealed in the cylinder(it may be higher or lower than actual atmospheric pressure inside the cylinder when the valve closes due to the engine configuration) vs what the volume is when the piston is at TDC. Valve timing plays a critical role in that, but it's actually more about cylinder filling that valve timing alone. Valve timing works with intake ports, manifolds etc to determine how much charge actually gets into the cylinder. It's basically a relationship of density of air/fuel charge under standard atmospheric conditions vs maximum compressed state in the cylinder. NASCAR Cup engines for example have less dynamic compression when running a restrictor plate....they simply can't get as much charge into the cylinder on account of the restriction.

Static compression ratios are just a starting point for determining cam, fuel, tune etc. You can build an engine at 11.0:1 compression that runs on pump gas like a raped ape(the new 2011 Mustang 5.0) or you could have an engine that's 9.5:1 and detonates on the same fuel with a molested tune and runs like crap as a result. Static compression is just a number, dynamic compression is what's actually going on.

simpkins_44

Once again great information 67mustang302! Its especially interesting, and makes total sense. The more and more I read this the more and more I become unsure about the piston choices and how its effects on valve timing is so important. Although I knew I wanted a engine to run peek power between shift points and not at lower RPMs like right now with the stock cam. I wasn’t really thinking much of rpm making to much of a difference on my compression. It makes sense though, only so much air can flow into a cylinder in a short period of time. This is why I wanted some small 58cc chamber high flowing heads. wouldn’t this work as well and then I would still have the ability to run a more aggressive cam for higher RPMs then the stock Cam???? Also wouldn’t this push me towards using the flattops because of its ability to have a higher static compression like Venomantidote95 was saying??? Or would it be more or if not just as beneficial to use the dished pistons with a somewhat less aggressive cam for lower RPMs??? Or could it still support a more aggressive cam???

You also stated that you prefer running a modest compression and an aggressive cam. What is your idea of modest compression??? And what kind of cams are we talking about here? I also understand how a engine with proper timing and fuel will run better then one designed with compression alone in mind. But is it to much to ask for proper timing and good compression while having semi-aggressive to aggressive cam???

I try to think of dynamic compression as you described thus is why I was looking at cam timing relations. however it has proven to be hard to get information on valve closure time on aftermarket cams and each setup has differing results. I get what your saying on the NASCAR cars when they are running a restricted engine as I have designed the throttle body and helped on intake design for a formula SAE race car running a 20mm restrictor. But my car will hopefully not be restricted in intake size and flow. And I would believe NASCAR engines have a larger stroke to generate more suction but I could be wrong. I’m curious on my static compression ratio but am also more interested in cam, and tuning as I feel this will help me determine the better setup. I understand that the new 5.0L are good powered motors who are good examples of timing in relation to power and compression however they also have 3 more cams and variable timing on their side not to mention lightweight internals. I have enjoyed your posts as they have helped me think a lot more and hope they don’t stop coming. Thank you.

67mustang302

It all depends on the engine. I'm running a 302, so flat tops are the order of the day since it's hard to get reasonable compression without them, at least not without using a ridiculously small chamber. I'm at 9.25:1 on a flat top with 5cc valve reliefs and a 60cc chamber. Cam is a custom grind from Ed at Flowtech. If I could run a dish and a smaller chamber to keep the compression up it be a better setup, but everything has it's limits. I could even run 10 or 10.5:1 without much issue. Strokers in that compression range with dished pistons turning 6-7k rpm make tons of power.

Cams need proper static compression to function....which is why engines back in the day with 8 or 8.5:1 didn't have much steam past 4-5k rpm, a cam that makes more power up high needs more static compression. Partly though you're going to be limited by parts availability....pistons, heads, gaskets etc. And compression doesn't have to be dead nuts on, just within a proper range for whatever you intend to do.

.boB

I'll throw in my 2 cents.

Cylinder pressure makes power. The more pressure you can build, the more power you can make. Dynamic compression is very important if you're going to run on pump gas.

Small things like quench and chamber design will change the formula. Modern aftermarket cylinder heads and pistons are quite good, and all the top brands will pretty close in those terms. Those small changes would only be noticed if you're racing and need every 0.001 sec; or particpating in a dyno challange. So don't sweat them too much. Either of your piston shoices will perform very close to each other in that respect.

As has been discussed, intake valve closing point will determine dynamic compression in your application. A later closing point will lower DCR.

BUT (and here's my actual point) the cam will have a later closing point because it has more duration. Cam lobes tend to be fairly symmetrical. If the valve closes later, that means it opens sooner. More duration generally creates more power, although usually at a higher rpm range.

The flat top piston with a matching cam will make more power; all else being equal. My street/race engine has 13.2:1 static compression. But, the intake valve doesn't close until 86* ABDC, and I live at high altitude. DCR is a little under 8.5 - perfect. It will run all day on 91 pump gas.

Venomantidote95

Well I think by now u have the brain power to make heads n tails of this, no one person is right, depends on what ur building n whats the use!

Hope we helped!