Assuming you get the dynamic compression ratio into the ideal range (i.e., depending on iron or alloy head) for pump gas...does it even matter what your static compression is??

Well it is a little tough getting 15:1 slugs to run on pump gas with a cam and aluminum heads.

Well it is a little tough getting 15:1 slugs to run on pump gas with a cam and aluminum heads.

I guess to be more specific if I calculate an 11.2 SCR and the DCR nets out to 8.1 should I be okay on pump gas or should I still be comcerned about the 11.2 SCR?? Does that make sense?

DCR might give you some insight on what kinda cam to pic. of u have a high compresion ration, and wanna run pump gas your gonna want a cam with a fair amount of overlap to blead of some cylinder presure, also and later opening intake will also help out a bit. but bottem line, i would say NO SCR realy determins if your motor is incheck for pump gas

Maybe a way to look at it is the SCR is how the engine is built. The DCR is how the engine operates. I would say that SCR is somewhat irrelevant, but this "old school" term has deep roots. Probably was not too cool to say "I have a 8.5:1 compression 327", unless there was a blower sitting on top. Years ago DCR was not really considered maybe because there was 100 octane fuel. I never heard the term until visiting this site. But the again I am not motorhead. Pat Kelley's DCR instructions just says 8.5 max period.

Maybe a way to look at it is the SCR is how the engine is built. The DCR is how the engine operates. I would say that SCR is somewhat irrelevant, but this "old school" term has deep roots. Probably was not too cool to say "I have a 8.5:1 compression 327", unless there was a blower sitting on top. Years ago DCR was not really considered maybe because there was 100 octane fuel. I never heard the term until visiting this site. But the again I am not motorhead. Pat Kelley's DCR instructions just says 8.5 max period.


8.5, with what heads, im sure u can squeeze 9 maybe a tad higher with aluminum heads, and what about a SOLID ignition system. or does it asume that spark is perfect

you might consider the rcr DCR calculator as it gives cylinder presssure as well. I was told of people running over 200 psi on pump gas.

Mr. Lt Whatsis, static compression REALLY has no bearing on anything.
It's simply a measure of volume.

It is completely irrelevant to a running engine. A running engine will never compress the mixture the full stroke, ever.

It's kinda like looking at some piston catalogs.... They call it an 11:1 piston, and most don't even give you any fine print.... 11:1 in what displacement engine, with what cc heads and gasket thickness? It's just advertising mumbo jumbo, like a 3/4 race cam...

Dynamic compression directly relates to the pressures produced in a running engine. It is the only thing that's important.

Mr. Lt Whatsis, static compression REALLY has no bearing on anything.
It's simply a measure of volume.

It is completely irrelevant to a running engine. A running engine will never compress the mixture the full stroke, ever.

It's kinda like looking at some piston catalogs.... They call it an 11:1 piston, and most don't even give you any fine print.... 11:1 in what displacement engine, with what cc heads and gasket thickness? It's just advertising mumbo jumbo, like a 3/4 race cam...

Dynamic compression directly relates to the pressures produced in a running engine. It is the only thing that's important.

im not talkin about "this pistons will give ya 11:1" i mean you have spent the time to actly figure it ALL out. yes im well aware that compresion is a mesure of cylinder volume at BDC VS voulume at TDC. i wouldnt say the SCR is totaly usless.

like a 3/4 race cam...


Hey now. We have been down the road with this one. There really is a 3/4 race cam and Isky publishes the specs. Just a name is all it is.:beers:


P.S. Lt.Whatsis. LOL.

read it again... the piston thing is just an example of another meaningless number.

Performance depends on cylinder pressure. Too little, no performance. Too much, uncontrolled ignition, pinging, bad bad bad.

Cylinder pressure depends on cam timing AND Static compression, both together. For every static compression (or most anyway) there will be a cam that will run best, and vice versa.

Hey now. We have been down the road with this one. There really is a 3/4 race cam and Isky publishes the specs. Just a name is all it is.:beers:


P.S. Lt.Whatsis. LOL.

this i would enjoy seeing.

this i would enjoy seeing.

Isky Max1 to be exact.

http://www.streetrodderweb.com/tech/0302_ford_flathead_reintroduced/photo_17.html

Getting back to the topic, DCR is a way taking SCR and making it fit the conditions in which the engine will run. I am sure that it can be tweaked a little higher with different components. Very few things in this world are absolute. Read the following and educate yourself, Lt. Longarm.
http://members.uia.net/pkelley2/DynamicCR.html

Isky Max1 to be exact.

http://www.streetrodderweb.com/tech/0302_ford_flathead_reintroduced/photo_17.html

Getting back to the topic, DCR is a way taking SCR and making it fit the conditions the in which the engine will run. I am sure that it can be tweaked a little higher with different components. Very few things in this world are absolute.

thanks

Look at it like this... Static compression is part of the equation used to determin dynamic compression. Simplfied, there comes a point where you just can't adjust the intake closing where ever you want to, to give you the dynamic you want with any static ratio. There are other side effects if you do, that could impare performance.

Static compression ratio is a useful number for theoritcal calculations. DCR and SCR are "linked" to eachother.

Put it this way.

Static compression ratio * % of Engine bleed off = Dynamic compression ratio.



You could design an engine with a SCR of 20 : 1 and have it run on 87 octane if you designed the cam correctly. This engine probably wouldn't run very well, but hey its possible :)

For example, My motorcycle has a SCR of 12.5, and I've run it on 87 octane before.

A practical Example

Im working on a vintage English engine of very basic technoly to convert to pump gas..Cam inlet valve timing was the 1st and easest choice to make these changes
It was designed to use 'Pool' English petrol during WW2 and after during the rationing period upto 1952. This varied between 62 and 72 octane measured on modern method
The engine is advertised @ 5.4:1 compression...CC ing it it is actually 4.7:1

The inlet spec closes 18 deg after BDC @ 12/1000 tappet...when the tappet is set to factory spec of 6/1000 that makes the valve open at 0 Deg BDC
In this case Static= dynamic
What I am attempting to do is increase cylinder pressure (dynamic compression ratio) for modern 91 fuel.
A cam with a greater overlap would create the inlet valve to close ABDC on the compression stroke therefore reducing the effective comp stroke and therefore cylinder pressure therefore static ratio. (not what is needed for this application) This is how to reduce a high DCR...Advancing or retarding a cam will have the same effect...as will steeper or slower rams thu to a smaller degree.
In this practical example due to the inlet closing @ BTDC thewre is no room to alter SCR...I do note if the engine tappets are opened to around 10/1000 there is a very marked drop in performance as the SCR is reduced with the inlet then closing approx 6 deg later.
So as mentioned above..in this case the cam inlet timing being the easest choice to look at 1st...is not a choice.
I Digress here:
Nore is domed pistons or increasing compression height of the pistion.
Being a Side valve engine, and the top of the piston is actually in the compression chamber @TDC this reduces the effective cylinder height/volume, unlike a OHV engine. Then there is further complications as to planing deck/head as part of the compression chamber is also part of the cylinder and valve lift to head clearance.
Target SCR is around 7.5:1 to 8:1 run 91 octane.
This canbe obtained by filling in the head compression chamber...plus planing the heads 3mm

The max compression attanable is 5.7:1

I'm in here on the Isky stuff about the old 3/4 cam...

As for SCR and DCR!!

There are ways of fudging the results so that you CAN run a high SCR and DRC, but that it involves matching several things!!

And as I see it AND I have to say that I only know what I read, AND they are.

Use as long a rod as you can afford that fits!!

Use as eff. a small chamber head as you can!!

And finally, True flat-top pistons!!

I figure that from what I read, a 280/230 cam at 11 to 1+ CR. should run on 87 octane gas AND onna these days I'm gonna build the sucker to find out!!

pdq67

PS., AND I am bugging Dave F. about reviving this since he is now over PHR Mag b/c years ago this is where I got my info on this as well as from Mr. Ray T. Bochaz's two articles about using the late Mr. Fueling's BB aluminum torque truck heads on a 12.1 to 1 CR. 502 and burning 87 octane!!!!!!!!

And they did it!!

pdq67

Rather than configuring a camshaft to allow high compression engines to run on regular gas, I am interested in a cam that will raise the compression of a 502/450 hp engine that has a factory 8.75:1 CR. Does anyone know whether it would have to be a custom grind or does a vendor have one in their catalog?

coach,

A cam can't do it b/c like said earlier by Steptoe, you have to have the needed SCR before you can mess around w/ using the cam to adjust the DCR so that pump gas can be ran.. (If that's what you are after??)....

That engine only has 8.75 to 1 CR and won't ever have anymore unless you do one or both of two things!!

Blow it OR increase it's SCR mechanically by changing heads, quench, pistons etc., etc.. OR BOTH!!!

SD,

I highly recommend you read Pat Kelly's GREAT DCR site at least THREE time's and then come back and chat w/ us!!!

I did several years ago after reading about SCR/DCR in Ed Staffel's great BB book..!!!

pdq67

Gotta love this topic. It's a hot-button issue that has been discussed many times.

One thing I'd like to point out. Many of those like Jim M. who say static compression ratio doesn't mean anything turn and talk about dynamic compression ratio as if it is a static number. The dynamic compression ratio changes with engine speed. Yes you can get close to dynamic CR reaching the static CR. That's a big part of what volumetric efficiency is all about. Cam timing takes advantage of the inertia in a moving volume of air to, in effect, super charge the cylinder. When the volume of air charging the cylinder is equal to the full cylinder volume, 100% VE has been achieved. What's more, it is possible to reach near 100% VE and even exceed it. However, the motor will likely have a very peaked response and ill manners below the peak. When VE is at it's max, static compression ratio does mean something.

I think some folks base their opinions of DCR vs SCR on whether or not the motor pings on pump gas. It is true that you can exceed reasonable limits on CR and still run pump gass if the DCR is reasonable. But when you reach the peak VE point, its a different story and all the assumptions change. The math used to describe what happens at ALL rpms is complex (in the non-real sense) :D -- it becomes a second order (or higher) system.

When I hear someone say something like "the dynamic CR with this cam and heads is 8:1" I think they're missing a big point. DCR at 3000 RPM is not the same as the DCR at 5000 RPM and it's peak will be at a particular RPM based on the mechanical factors of the motor. That fact of the matter is DCR vs. SCR is not so black and white.

If your trying to decide what to do, you first have to decide where you want to be. Do you want a broad torque curve with good low end torque? Do you want max horse power no matter what the RPM is? Must the engine run on pump gas or race fuel? Those decisions must come first. I must confess, I am a street performance guy. I want decent fuel milage. I don't really care what the peak HP is. I could spend the money just like anyone else to reach the max, but I personally would be disappointed with the result. Building a high SCR motor and using DCR to make a street motor that won't ping on pump gas is a waste of time IMHO.

You're very right, Dave, no doubt, and I do tend to oversimplify things sometimes, it's easier to understand that way, lol.

I also assume that anyone approaching 100% VE without a blower is A: not running on the street, B: not running mufflers of any type, C: not coming in here aksing us questions (he all ready knows more than us!)

And while I preach DCR, I also don't preach pushing into the 8.5 or 9:1 range, that can be very dangerous at high rpm, for exactly the reasons you stated.

As for Coaches question... the whole combo has to match to work right, AND it all has to match it's intended use. Your 8 3/4:1 502 will want to run a fairly short cam, which will in fact up the DCR. You could also close up the lobe centers (will give it a bit more radical idle, and slightly higher DCR cause the intake will close earlier.) You could also advance the cam a degree or 2, again, intake will close earlier, raising the DCR.

Really tho, compression and dcr isn't everything. Even with only 8.75:1, you could run a bit of cam for a decent top end and have 502 cubes for all the low end torque anyone can actually put to the pavement anyway, and probably get away with 89 octane gas and save a dime a gallon to boot!

Dave,

That's some great tech! Keep bringing it on. I agree with you about the static compression ratio being significant. Actually the static compression ratio is equal to the expansion ratio which is significant.

One more can of worms I will open here is the expansion ratio. This is what happens after TDC when the burning gases are expanding pushing the piston down. The expansion ratio and the static compression ratio are equal. With a high expansion ratio the work of the expanding gases are done sooner, and you can open the exhaust valve earlier to reduce pumping losses. The expansion ratio is not meaningless. Therefore the static compression ratio is not meaningless.


As far as the dynamic compression ratio goes I have heard of another term "Effective Compression Ratio" which is the "non variable" number that most are calling DCR. Effective compression is what is measured during a compression test without the inertial supercharging effect taking place.

Fun stuff!!!


Regards

CurtiSS 69

Thanks for all the useful information.

Jim - I was thinking along the lines of what you described - a way to perhaps squeak out a bit more performance without changing pistons, heads, etc. There's so much that can be done with cam technology that it seemed to be a logical route. Now if I can also do this while retaining the factory valvetrain - wow, that would be a home run!

ECR is what Pat K. really wants to change DCR to b/c it's more indicative to what is going on.

pdq67

Jim knows whats going on. He was making a point and it was well made. I just worry that some of the young'ns will spend a lot of money on building a turd if they don't fully understand the compromises they have to make to reach their goal. While I understand the theory, I don't have the experience that some of these guys here have. TGIF :beers:

I learned real quick about over-camming an engine when I installed a 292/230, 109/107, .480" lift hy-cam in my about 9.5 to 1 CR, 406 SB.

pdq67

I learned real quick about over-camming an engine when I installed
Im another who learnt back in our early days about over camming (and over Carbing)
As mentioned above, yes it is not as simple because DCR does change with rpm... Cams choice is more of an 'art' than "this one does that" there are so many profile parameters that can effect what happens...many are not in the advertised specs...like ramp rates.
If you have installed a cam with too much overlap...using adjustable timing chain gears, advamcing a couple degrees will drop the rpm range.