Thread: DCR's??.

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Today's gas require a lower compression ratio.That statement is without question true. The thing that gets me is why would anyone build a engine that the SCR is too high and then use the DCR to bleed of the compression via cam timing or for that matter any other devise??. You would think a more straight forward approach would to build based off the SCR. Then try to keep as much combustion pressure as possible. It just seems like a patch work solution to the SCR using the DCR.

EPA says exhaust pipe will smell so-so-------------SO car manufacturers and petro companies have to scent the engine to smell that way----------whats so difficult to accomplish that??????????

Originally Posted by 1gary

Today's gas require a lower compression ratio.That statement is without question true. The thing that gets me is why would anyone build a engine that the SCR is too high and then use the DCR to bleed of the compression via cam timing or for that matter any other devise??. You would think a more straight forward approach would to build based off the SCR. Then try to keep as much combustion pressure as possible. It just seems like a patch work solution to the SCR using the DCR.

most big dur cams will do that job . there more to this.it.s not so cut and dry i will not get in to it all on here . but i done it a time or two with real engine you put oil and fuel not paper engines back before any soft ware was out or home computers .this was when i did not have the machines and tooling to rework stuff.i will not say if i still do it

Does a higher SCR than gas can allow and then relieve some of that with a DCR imply there is some mechanical advantage to a higher SCR even when you are losing some of it with a DCR??.

Any engine, put together with proper parts and care, will operate well enough to take grandma to bingo and back. It is not necessary that you know the static compression ratio or the dynamic compression ratio if you are not interested in blueprinting the motor and taking maximum performance from it.

Static compression ratio (SCR) is just what it says, static. Static means still, at rest. So, SCR has little meaning other than an initial indication of the type of fuel that you may have to use in the motor to prevent detonation. The missing ingredients are rod length, stroke and the intake valve closing point. You must understand at this point that compression does not begin in the motor until the intake valve is on its seat with the piston coming up the bore on its compression stroke. As long as the intake valve is open, even if the piston is coming up the bore, there will be no compression. The piston will simply push the mixture that was just drawn into the cylinder, back out of the cylinder through the open intake valve. So this is where you either build a motor or you don't. If you use a high SCR with a short cam, the intake valve will close too early and trap more mixture than the fuel can stand, so the motor detonates. If you use a high SCR with a long cam, the intake valve will close later and allow the motor to push a little of the charge back out the valve so that cylinder pressure is within the limits of the fuel used. The idea is to close the valve at exactly the right moment in relation to the position of the piston, so that you capture just the right amount of mixture that will make max power without detonating on the fuel used. If you don't know how to figure the DCR, then the camshaft you choose and screw into the motor is just a guess. Now, after a fellow has built a number of motors, he gets an idea of what will work and what will not work, but newbies don't have that luxury.

Dynamic compression ratio (DCR) takes into account the static compression ratio, the rod length, the stroke of the motor and the intake valve closing point, so it gives us a more complete picture of how the motor will perform and will also allow us to choose exactly the right cam to accomplish what we wish to accomplish with this motor. For instance, if we use the same DCR calculator all the time, then we will get a handle on what we can get by with and what we can't. Don't try to use first one calculator and then another. They're not the same. Settle in on one calculator and use it exclusively and you will know exactly what cam timing figures will work with the SCR you are using and you'll know that the motor won't rattle on the fuel that you buy down at the corner station. I've used the one on Keith Black's site for decades and know that I can recommend a camshaft that will generate 8.0:1 up to 8.5:1 DCR and operate well on just about any brand of pump gas. There are fellows I know who will push the KB calculator to nearly 9.0:1 DCR and get away with it on pump gas, but I'm content to cap the number at 8.5:1. I'd rather err on the side of caution in the event I get a bad load of fuel.

If none of this makes sense to you, PM me and I'll give you my cell number so we can talk it through.

gary i would take teck up on that phone call if you have anymore question. as for 9to1 dcr like i said i done it many times. back in the 80s i was doing it with out knowing it very well .no soft ware in my hands to see it at that time. but i have the list of my old cams and did run them threw to revisit the DCR engines i built then but...so i knew something was going on... i just knew my old TRW big dome piston did not have holes in them when many of buddys did using the same gas there more to it like i said many over look how well the engine is sealed i all ways used open breathers

Pat I knew what Richard posted.No not ego,but honestly knew that.I am going to PM him.I have been wanting to talk to him anyways.Humm-open breathers huh. Interesting.

Originally Posted by 1gary

Pat I knew what Richard posted.No not ego,but honestly knew that.I am going to PM him.I have been wanting to talk to him anyways.Humm-open breathers huh. Interesting.

yes that was one of the 4 or five things i did that my buddy with holes in his pistons did not do at that time did not do the other you may know