AWilson

Everything I read says CR less than 10:1, more like 9.5:1 is what you want for a nice street machine with iron heads.

Lately DCR is coming into play. So which is most important? If it is DCR why do I not read about it in the Vizard book, or others? They use CR as the important number.
I need to know because it greatly effects my piston choice as the difference between the CR and DCR is about a .9 ratio.

Or someone has said a DCR of 7.75:1 is ideal.

MotorHead

You and the books are correct for ballpark compression ratio for iron heads. 9.5 to 10:1 is about the limit. Now this is taking into account someone has selected the correct "mild" performance cam for this motor. The cams generally run with a mild performance motor will usually put the DCR in the correct range so there will be no detonation problems.

It is the DCR you are concerned about, it has always been in play, DCR is what determines your "actual" compression ratio taking into account the cam intake closing angle.

You can still run into problems if the cam is too small and and timing is too advanced with at 9.5:1cr motor. You could theoretically run a 13:1 motor on the street on pump gas with a big enough cam. Bottom line is, do your DCR calculations carefully and stay under 8.5DCR

SteveG75

DCR is based on the fact that the mixture in a cylinder does not start compressing until the intake vavle closes. There are quite a few folks here running 11:1 CR engines on pump gas because they have a big enough cam with a late intake closing event.

Good discussion of DCR here:
http://www.empirenet.com/pkelley2/DynamicCR.html

TheSkunkWorks

Remember, the worse your quench is (.040" is great, >.060" sucks), the less DCR your engine will tolerate. And, IMHO iron heads just won't allow as much compression as aluminum, so you might consider backing the DCR down to 8.0, if not a bit less, for those. Also, even with everything looking good on paper (DCR, cranking compression, etc...) cylinder pressures and temps can still reach a critical point at high revs on pump gas whenever the power band is shifted upwards.

63mako

Stay under 8 DCR, 7.5 prefered W/ Iron heads. Keep your water temp low (Good radiator and water pump 160 - 180 thermostat). Shoot for .040 quench. Follow timing advice in sticky at top of Tech page. Make sure you are not running lean. Do the above your fine.

AWilson

63mako, I have learned to trust your advice. I am having trouble computing the DCR. All the programs I find don't exactly ask for info that is on my timing card. So I am unsure how to do this.

heres everything I know about my build!

Bore: 4.030
Stroke: 3.75
rods 5.7
heads: 63CC
deck height: 9.00
Gasket: .039 or best
CR: trying for 9.5:1. (a -20cc Weisco forged piston will get me to 9.6)

Engle EP-22HYD CAM Card info:
Valve Lift 480:
Gross cam lift:320
adv. dur.:274
duration at .050: 230
lobe centers: 110
intake opens- 8, closes-42
exhaust opens - 48, closes-2

I don't think I can change much here besides the piston dish size.

Can you or someone compute my DCR for me and then show me the numbers they entered in. I want to learn it.

Belgian1979vette

There was a discussion about this on the engine mod forum.

Vizard did discuss DCR in his books and several other articles as you can see in the thread. In fact dcr is just a method to more or less estimate your final compression pressure. This should stay definatly below 200 psi and to be even better in between 165 and 180 with alu heads, below or max at 165 with iron heads.

cardo0

U won't find the complete method of calculating DCR because u will need to use geometery and the cam card data for each cam - also recalculated if the cam timing is changed (adv or retarded). I challenge any of the DCR "experts" to provide a complete formula for DCR.
And u can read in D. Vizards "How to build a Hi Perf Chevy sb on a Budget" where he has an entire section on fuel octane and DCR. No where in that book does he publish a method for DCR calcs but recommeneds using the c.r. from the cam mfr (i will post the page # later). U will need to read D. Vizards "Camshafts & Vlvtrain" where he only prints 3 basic examples of cam DCR but not the full calcs. I can only guess that those that are boasting DCR calcs are doing a lot of guessing themselfs.

For a reference on c.r. try D. Vizard "How to rebuild the sb Chevy".

If u request i would like to post my references on c.r when i have more time Mr. Wilson - just too busy now.
cardo0

Belgian1979vette

You can make a small sketch of your crank/rod/piston pin to come to the position of the piston in the bore. From there you can calc the volume above it.
You can use math too but it is a little bit more complicated

The Money Pit

I'm beginning to believe nobody really knows what's the best practice for selecting the "right parts" building a performance engine.

I read a CarCraft build where they made 540 ish hp with a 388 on 87 octane.They ran 9.7 or 9.8 cr,AFR 210 heads,a solid roller around 254 degrees @.050,then threw on a RPM intake.So here we have low DCR,big low velocity ports,too much streetable duration and a dual plane,all bolted together......making over 540 hp on 87 octane.

Perhaps we all worry too much about fractions of inches?

AWilson

I used a program off the internet and came up with DCR of 8.77. This is why it concerns me. If I input the right info then I am too high. But I bet I did not input the right info.

I used the download at the bottom of the link provided above by SteveG75:

http://www.empirenet.com/pkelley2/DynamicCR.html

S489

well i'm no DCR "expert", but here is a calculation:
http://i73.photobucket.com/albums/i2.../Temp00079.jpg
http://i73.photobucket.com/albums/i2.../Temp00081.jpg

the example calc is for a different combination; you can use the formulas to plug in your data . . .

dgruenke

Man, it's been a long time since I worked with any formulas like that.

Steve439

If you're using Pat Kelley's calculator you need to use the "advertised" intake closing angle, not the .050 one. You need the close at .006 or .004 or whatever they supply.

Little Mouse

Your stressing out over nothing with 9.5 compression, friend of mine runs around with factory 11.1 domed pistons iron heads a little
more cam then you have and the timing retarded a little. I had
flat top pistons 64cc iron heads mild 270 degree crane cam in my
pickup 3.07 rear turbo 400 auto, ran it around for yrs on medium
grade gas.

AWilson

I called the cam mfg. they said to take half the difference between the adv. duration 274 and the .050 230: Answer is 44.
Divide this by 2 : 44/2 = 22.
Ad the 22 to all the open and close numbers on the cam.
That means the intake close of 42+ 22 =66.
That is also the computed number when I run the spreadsheet. So it matches. So I think my DCR is 8.77.
UPDATE: It helps to run the program in the right order. I read the help instructions. Now my DCR is 7.55:1!! Maybe I am ok after all.

Steve439

That's a fun little application. I've played with that for hours.
I used it to choose the cam for my 439 - shooting for low 8s DCR
with 9.5 CR and aluminum heads.

Scott Marzahl

According to the Pat Kelly DCR calculator and assuming 0 deck height from your previous posts, I came up with 7.57.

TheSkunkWorks

The math for calculating DCR is not as easy as Pat's comprehensive program makes it look, so you're unlikely to see a definitive formula published in lieu of brief explanations that gloss over how to arrive at an answer. And no, I'm not going to bother trying to post one either, but that doesn't mean it can't be done.

Since virtually no compression can take place before the intake valve closes, the only reliable method for calculating DCR is by seat timing rather than @ .050" info. Yes, the profile of the lobes matters, but that's all about flow characteristics once the valve has left its seat. Thus it is advisable to always consider the advertised, @ .050" and even @ .200" durations when looking at which bumps on a stick you need.

When you compare the recommended static CR's and cam combinations meant for pump gas in most any cam catalog, I believe if you crunch the numbers that you'll find a pretty good match with this DCR stuff. And, IMHO, I put a ton more faith in knowing DCR than in attempting to theorize over what the cranking compression of any particular build may be.

That said, while DCR is undeniably an important function which has been at work since the first internal combustion engine fired up, no one should get hung up on DCR as being the Holy Grail of performance engine design. Knowing it is a good thing, but it won't make you the next David Vizard.

Scott Marzahl

It simply gives you a basic idea of what is occuring and its way over rated by a lot of people.