OK so I have sort of a technical question here. We all know that the higher an engine's compression ratio, the harder the charge pushes the piston down, and the more power it makes. We all also know that there is an upper limit to the static compression ratio that you can run without running into problems with detonation. Something I was told about recently though is that although a static compression ratio of up to 11:1 is possible on some of these big old iron headed engines, a DYNAMIC compression ratio of only right around 8:1 is what we're actually shooting for. My question is, if we're all shooting for the 8:1 dynamic number, then why do we build our engines waaaay up to the razor edge at 10.5:1 static, only to cam them down to 8:1 dynamic in the end anyway? To put it a little bit more clearly, why not build and 8.5:1 static motor for waaay cheaper, barely cam it down at all, and still end up with the same 8:1 dynamic CR as you would with an expensive 10.5:1 motor, a motor which is also alot more difficult to get right.

One thing that popped into my mind was that I don't know what RPM dynamic CR is calculated at, and if it's done at a high RPM, then an engine that can only handle an 8:1 DCR at 5000 RPM (where the upward speed of the piston will really slam that air and heat it up, approaching detonation point, and the chambers will be at their hottest from rapid fire combustion) might be able to handle a much higher ratio at a lower speed. But I don't have any idea if that's the case first of all, and second of all, volumetric efficiency is at it's lowest at the lower RPM's, vacuum is at it's strongest, so then I'm not sure what to think.

I could keep theorizing here all night, but who here can enlighten me and the forum, and answer some of these questions?

Very curious, AleX

 

I'm no end all expert but here goes
Dynamic figures in the cam profile and the number is lower because in theory compression cannot take place until the valve is closed. I'm pretty sure it doesnt change with rpm. It could be called actual compression ratio as it is a better measure of the true CR with a given cam.
I think you build for a SCR that when combined with the cam you're going to run will give you the appropriate DCR. Some cams bleed off more than others so you would need to have those cams combined with a higher SCR so that the DCR is still high enough not to be a turd. A cam with a shorter duration will result in less bleed off so they cannot have as high a SCR or the DCR will be out of pump gas range.
Essentially you build your SCR to match the cam you will run.

Feel free to correct me if I am mistaken in any way.

 

I think your asking this so here ya go.

Static is what does with out any intereference with the cam, and I wouldn't try more then 10-1 with typical closed chamber heads like the 302/351W or 350 SBC. Maybe you can get away with 11-1 with the wedge chamber heads like the FE's or the Mopar 426 max wedge. They have a tendency to like running pig rich, and that may help with or need the high static compression.

Dynamic compression is like when you ran a Boss 302 cam or a Duntov 30-30 cam. Both these cams were over 300* of duration so they had a fairly low dynamic compression, and thusly need high static compression to get going becouse of low speed bleed off. I saw a guy use a Duntov in a low compression motor, and it ran worse then you could imagine. Even at speed.

Typical "stock" cams are set-up for higher dynamic compression to ease fuel requirements. Plus they cams put TQ where you need it down low.

 

BTW what kind of motor are you building?

From what I am reading it seems to me you want to or are building a rpm screamer?

 

Well thanks for the responses guys, but that's actually not what I'm trying to ask.

Also though first of all I'm actually building a nice conservative low end motor for my truck at the moment, I just tend to get curious about everything.

Anyway though, let me try to clarify what I'm asking. I know what DCR and SCR are, and that your choice of cam bleeds off a certain amount of pressure from the SCR because of how late on the upstroke the intake closes and all of that, and depending on how free your exhaust is, and what your volumetric efficiency is, that all determines the DCR. I get what the 2 numbers are.

What I was hoping to get into with this post, is why build a motor with a high SCR, when a high SCR motor, and a lower one, both need to come in at below 8:1 DCR when all is said and done anyway?

I'll try to illustrate my question with an example:

You build 2 motors, one has a SCR of 10.5:1. To get the DCR down to where it needs to be, 8:1, you install a very long duration cam, to bleed off pressure. To build this motor though, you needed to buy flat top pistons at 350$, small chamber heads at 700$, and a very radical specialized cam, maybe even a custom grind, at 250$. Say the rest of the engine cost 1000$ to build. You end up with a 2300$ engine, with a DCR of 8:1. Now on to your second engine. You build it to have a SCR of 8.5:1. The only custom part you need to buy for that is a cheapo 100$ mild cam, to bleed off a little pressure. This engine, with a base cost the same as the first, 1000$, costs you only 1100$, and you still end up with the same DCR of 8:1.

To paraphrase, why take 4 steps forward (raising the SCR super high to 10.5:1), only to take 3 back (bleeding pressure with a big cam to achieve 8:1), when you could have just taken one step forward (done 8:1 in the first place, and not bled any off)?

There must be some reason.

Do you understand what I'm asking a little better now at least though?

 

Well some of those figures are speculative. It's the same price for my xe274h as it is for the oem replacement 252h cam. Stock heads. And flat tops came in FEs to begin with. Maybe thats just the FE family though.

The cam that will require a 8.5:1 SCR to have the proper DCR will be a low rpm torque cam. And the cam that needs 10.5:1 SCR will be a higher rpm performer.
You may end up with the same DCR but the performance characteristics will be much different.

 

Static compression ratio is the difference in volume with the piston at bottom dead center and top dead center.

Dynamic compression ratio is the difference in volume when BOTH valves are closed and top dead center.

Closing the intake valve later aids in top end power and cylinder filling at high rpm's at the expense of low end, good for race cars,not trucks . For a pump gas iron head engine 8:1 DCR is max.

 

Ever do a compression test on a motor with a wiped exhaust lobe? A cam with zero lift and duration essentially. It will blow your tester apart with dynamic compression, off the chart, but of course that cylinder will be making NO power.

You're thinking backwards. When you put in a long duration, high lift cam, you are NOT doing it to reduce the DCR because you have a high SCR, you're doing it to get more air/fuel into the motor.

I suppose there are circumstances where you MIGHT adjust the DCR with the cam, but I would think that's mainly because you boxed yourself into a corner with detonation problems. That would be a wobbly crutch indeed.

Where did you get that number in red from? You build your motor with as much compression as available gas will allow and that number can change a LOT depending on your octane rating.

Also, high compression ratios help at ALL rpm ranges.

Yep. That's why we go through the expense of putting all that SCR into an engine.

 

HOLY COW....MY FACE IS MELTING WITH THIS NEW KNOWLEDGE!!! (face melting is a reference to Jack Black and the Pick of Destiny). I never knew about DCR. Oh man, now I have to rethink my entire build process to incorporate DCR. So, if 8:1 is what we shoot for in our iron headed FE's. What would we shoot for in turbo or supercharged applications? I ask because I am currently rebuilding my Omni GLH motor as well. And want to make sure I get the right cam for the job.

 

Um, look up that mythical 8:1 number ANYWHERE in either an engine build site or with an experienced engine builder and they're gonna say HUH?!?

 

explain please. I've always ever heard that around 8:1 DCR is the limit for running on pump(93).

 

Because any way you look at compression ratio, it depends on the fuel that is used. From gasolines variation in modern standards that go from 87 to 112 octane, or propane that is IIRC 114 octane, and alcohol too, the motors compression ratio, either static or dynamic, is dictated by the fuels resistance to detonation. And that number can be anywhere from 7:1 to 15:1. Or even farther outside those parameters.

Any compression ratio number that you use depends on the fuel that you are using. (Although, there are other parameters that determine resistance to detonation, NONE are as important as the fuel used)

 

Oh ok. Well yeah if you have a motor that is going to run on 112 then it can be built to a higher CR. The 8:1 figure is the premium fuels (93) max CR without detonation issues. So we are trying for that maximum and have to combine the right SCR with the cam of choice to be there. If we went to higher CRs then the octane would have to go up as well and the motor would no longer be pump friendly. I think we are in agreeance?

 

I have never heard of that 8:1 number as a max for pump gas compresion ratio. It depends on a lot of things.

The new 5.0 Mustang's SCR is 11:1. It runs on pump gas.

 

Im pretty sure that is with variable valve timing. Does it also have aluminum heads?

I heard of the 8:1 first, then looked it up and found every forum I read on agreed on it. Then when I was building the 445ci, I talked to Barry R at Survival Motorsports about what I wanted to do with it and he gave me options on piston and compatible cams that would be pump friendly. He didnt mention anything about DCR in the conversation, but when I figured each combo out they all came out to 7.9-8.1:1 DCR. Coincidence??? I dont think so.

My SCR is 9.86:1. Cam IVC 63°. DCR 8.01:1.
I've seen motors with 10.5:1 run on the pump. And I've seen 9.5:1 ping away. Its all about cam combo to keep DCR at the right level.