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Topic Review (Newest First)
01-20-2013 11:51 AM
cobalt327 Then there's: Spec'ing Cams by Vizard. But the article stops short (by three pages) of telling the whole story. Supposedly an abridged version is in a couple of his Chevy "how to" books. Or you can rent the complete version. Only $100/hr.
01-20-2013 11:11 AM
F-BIRD'88 Very good article. The fine print explains much of what I am saying.
Simplifed, it tells you , its just not that simple.
01-20-2013 11:00 AM
cobalt327 At the risk of beating a dead horse, I'll just say this:

DCR is one of many tools in the engine builder's chest. And like any tool, it works best when used for what it was intended. Most guys know better than to pound a nail using a torque wrench (although sometimes I wonder); using DCR is no different.

For those interested, here is a different approach to the same concept as DCR. I've not used it yet but I'd be interested in hearing how it compares to DCR calculations made w/the "standard" DCR calculators (three are below), should anyone use it. This might be where some of the somewhat incomplete concepts posted in this thread came from, but either way this is a clearly presented case w/the math included. BTW, he uses "supra" a lot. It means basically "there's more info above".

Wallace Racing DCR calculator
Kelly DCR calculator
KB/Silvolite DCR calculator
01-20-2013 09:56 AM
thinwhiteduke
Quote:
Originally Posted by F-BIRD'88 View Post
Dynamic Compression is rooted in the fact that the intake valve closes after bottom dead center. Since your cylinder isn't sealed right away as the volume above the piston begins to decrease, you do not keep all of the air that is in the cylinder at BDC (Bottom Dead Center). What is compressed is what is left in the cylinder after the intake valve closes. This is why Dynamic Compression is always lower than Static Compression.

this statement is false and misleading.... When the engine is running at speed in its natureal best power rpm range the cylinder pressure is very high. Back pumping is little or non. On a tuned induction system engine.
( a engine with headers, cammed and developed induction ( a powerfull intake) the engine takes in more air , resulting in higher than 100% volumetric efficiently at WOT. cylinder pressure is high.

Back pumping before the inake valve closes only occurs at low rpm, when the engine is running out of its best power band. Not "on the cam".

Real running engine dynamic compression ( when its running "on the cam" @WOT) is not fixed and often and should be higher than 100% volumetrically efficient if you are any good at building and tuning engines.
induction ram effect-exhaust scavedging. A high performance tuned induction-exhaust system engine is not a simple air pump. Cylinder pressure is highest BMEP right around peak torque output rpm.
Air always flows in the direction high pressure toward low pressure relative. and it don't care which way the piston is moving in a cylinder.

In the running engine, running at speed you have a open intake valve, a rising piston and air still entering the engine.
inertia Ram effect.
Thats not possible without some compression. The air keeps moving into the cylinder from the intake port until one of two things happen. 1. the valve closes. 2. the air pressure in the cylinder is equal or higher than the air pressure just under/at the exit of the still open intake valve.
Good point, makes sense..

Duke
01-20-2013 08:25 AM
F-BIRD'88 Her name is Shirley....
01-20-2013 08:07 AM
1Gary Some of the experience I bring to this forum is a numinous yrs on forums like this one.Without question every thread reaches a point where it is doomed to repeat it's self.I have I think made my position on this topic clear.
01-19-2013 04:31 PM
GunnerTHB
Quote:
Originally Posted by F-BIRD'88 View Post
Dynamic Compression is rooted in the fact that the intake valve closes after bottom dead center. Since your cylinder isn't sealed right away as the volume above the piston begins to decrease, you do not keep all of the air that is in the cylinder at BDC (Bottom Dead Center). What is compressed is what is left in the cylinder after the intake valve closes. This is why Dynamic Compression is always lower than Static Compression.

this statement is false and misleading.... When the engine is running at speed in its natureal best power rpm range the cylinder pressure is very high. Back pumping is little or non. On a tuned induction system engine.
( a engine with headers, cammed and developed induction ( a powerfull intake) the engine takes in more air , resulting in higher than 100% volumetric efficiently at WOT. cylinder pressure is high.

Back pumping before the inake valve closes only occurs at low rpm, when the engine is running out of its best power band. Not "on the cam".

Real running engine dynamic compression ( when its running "on the cam" @WOT) is not fixed and often and should be higher than 100% volumetrically efficient if you are any good at building and tuning engines.
induction ram effect-exhaust scavedging. A high performance tuned induction-exhaust system engine is not a simple air pump. Cylinder pressure is highest BMEP right around peak torque output rpm.
Air always flows in the direction high pressure toward low pressure relative. and it don't care which way the piston is moving in a cylinder.

In the running engine, running at speed you have a open intake valve, a rising piston and air still entering the engine.
inertia Ram effect.
Thats not possible without some compression. The air keeps moving into the cylinder from the intake port until one of two things happen. 1. the valve closes. 2. the air pressure in the cylinder is equal or higher than the air pressure just under/at the exit of the still open intake valve.
I stand corrected about bleeding air back into the intake. Other than that, nobody is arguing against you with regards to the rest of your post.
01-19-2013 03:53 PM
F-BIRD'88 Dynamic Compression is rooted in the fact that the intake valve closes after bottom dead center. Since your cylinder isn't sealed right away as the volume above the piston begins to decrease, you do not keep all of the air that is in the cylinder at BDC (Bottom Dead Center). What is compressed is what is left in the cylinder after the intake valve closes. This is why Dynamic Compression is always lower than Static Compression.

this statement is false and misleading.... When the engine is running at speed in its natureal best power rpm range the cylinder pressure is very high. Back pumping is little or non. On a tuned induction system engine.
( a engine with headers, cammed and developed induction ( a powerfull intake) the engine takes in more air , resulting in higher than 100% volumetric efficiently at WOT. cylinder pressure is high.

Back pumping before the inake valve closes only occurs at low rpm, when the engine is running out of its best power band. Not "on the cam".

Real running engine dynamic compression ( when its running "on the cam" @WOT) is not fixed and often and should be higher than 100% volumetrically efficient if you are any good at building and tuning engines.
induction ram effect-exhaust scavedging. A high performance tuned induction-exhaust system engine is not a simple air pump. Cylinder pressure is highest BMEP right around peak torque output rpm.
Air always flows in the direction high pressure toward low pressure relative. and it don't care which way the piston is moving in a cylinder.

In the running engine, running at speed you have a open intake valve, a rising piston and air still entering the engine.
inertia Ram effect.
Thats not possible without some compression. The air keeps moving into the cylinder from the intake port until one of two things happen. 1. the valve closes. 2. the air pressure in the cylinder is equal or higher than the air pressure just under/at the exit of the still open intake valve.
01-19-2013 01:15 PM
F-BIRD'88 You cannot just increase the cam timing or the intake valve closing point to adjust a high compression ratio engine to run on pump gas. it is not linear-progressive. The in cylinder pressure is high at running speed.
When the engine is actually running at speed the air does start to compress before the intake valve closes when its running at speed. Why ? because the air is still moving into the cylinder as the piston is rising.

Air is only bleed off at low engine speed. The exact point of compression and therefor the net resulting in cylinder pressure and temperature is very variable in the actual running engine based on many factors besides the intake valve closing point.
While its important to give an engine enough mechanical compression ratio, it is not a linear equation.
On a real street engine that has to operate in less than idea conditions, and for longer than a quick dyno test or 1/4 mile blast after complete cool down, once you get much beyond about 10.5:1cr the chance if it running correctly on pump gas dimishes REGUARDLESS OF CAM TIMING.

Most people won't be able to use 11:1 reliably on North American pump gas.
If you buy into this DCR calc consider it EXPERIMENTAL--very limited use. And you'd best have some method of detecting or anticipating detonation
and be able to adjust the ignition timing (retard it) on the fly.

And non computer controlled (no method of knock detection and on the fly-automatic spark timing adjustment) 11:1 cr engine needs about a 98 to 100 R+M/2 octane fuel for reliable use on the street. That has not changed in real terms in 100 years of 4 stroke internal combustion engine history.
01-19-2013 01:01 PM
F-BIRD'88 This is a direct quote copy of the text from the pat Kelly site:

Caveats: Running an engine at the upper limit of the DCR range requires that the engine be well built, with the correct quench distance, and kept cool (170). Hot intake air and hot coolant are an inducement to detonation. The average driver has no control over this when driving in traffic. If you anticipate hot conditions, pulling some timing out might be needed. A good cooling system is wise. Staying below 8.25 DCR is probably best for trouble free motoring.

>>Unless you have actually measured the engine (CCed the chambers and pistons in the bores), these calculations are estimations, at best. Treat them as such. The published volumes for heads and pistons can, and do, vary (crankshafts and rods, too). It is best to err on the low side. When contemplating an engine of around 8.4 DCR or higher, measurments are essential, or you could be building another motor.


The underlined part is mine.
01-19-2013 12:12 PM
F-BIRD'88
Quote:
Originally Posted by 1Gary View Post
Now I am reading your words,so let me see if I got this right.You have blown up three engines because of poor parts choices ignoring DCR's in one of them and trying to use the DCR formula in two of them which is a band-aid repair.The concept of the use of DCR's formulas is to formulate a build in advance of purchasing those parts not afterwards.Then because of poor driver feel not knowing a engine at the top end is leaning over on the top end,you suggesting a devise"engine ears" to detect the mystic of detonation which if DCR's where properly applied in the first place would have been on a bucket list checked off so as not to be a problem.

I am going to suggest this "old school" thinking doesn't work and I came from that time period.When the gas changed it was a game changer.That that type of thinking needed to change with it and no less the use of DCR formulas is in no small was a huge part of moving forward.Not backwards.

I am for the record suggesting with a admitted history of three damaged engines because of poor execution of the DCR's formulas is enough proof that if applied correctly wouldn't have happened.

I am also saying for the record that a DCR of no more than 8.5 is disheartening to me as well.It does sound low,doesn't it??. But it is what is. And we all have to learn to accept it or end up paying for the consequences.
I was sure you would come up with a twisted lawyer version spin of what I stated to suit you own needs and agenda.

The two engines were my friends that blew up. Just as I said they would by (him/not me) buying into the DCR crap.
The 10.70:1 cr was too high for the pump gas and purpose even thou DCR theory suggested it would be fine.
I told him it was over simplifed BS that will not work and that exactly what it did. Just like I said it would.
He bought into it just like tech does.

The 12.65:1 engine was a dual fuel engine (Race gas and methanol) that was never intended to run on pump gas. I forget to reduce the timing for pump gas, one day. Get a grip. &^%% happens.

By the way I repaired it and had it back up and running in 3 days for less than the cost of a set of pistons.
1 piston new rings and a cylinder sleeve. Its still running somewhere.

This had nothing to do with driver feel.
01-18-2013 02:51 PM
1Gary I remember a quote from the movie Heart Like A wheel where after in the early days Sherry blew up one of her engines in her FED,Connie telling her she needed to feel the car nosing over long before the engine let go.
I know in real life how true that is.Test the limits of the parts??. We certainly do. But are real logical about how we go about that. Best lap in the 1/8 was 4.56@ 150mph. But we limited the driver to only .002 on the bottle and it was in the middle of the lap.

Oh,he doesn't have any "engine ears" on either,but most certainly a sensitive butt.
01-18-2013 02:23 PM
OneMoreTime These days serious engine builders work everything out on paper before they ever order a part. then they use a scientific rigor to evaluate the results. today we also use computer controlled injection systems and computer controlled ignition systems to get as close as we can to perfect for that build. Setting up design parameters is critical to the success of the build. One of the parameters is picking the design fuel for use in the engine as an example for a street engine we use pump gas as the design fuel and work around that parameter. Power band is another so where do we want the power band to be. Is this a Tow motor or sports motor just what is the intended use of the vehicle..Once we know all of this then we build a motor to suit the intended purpose..No longer do we just throw parts at the build.

Now if you do go to an experienced engine builder most likely he has various builds saved in his data base and can most likely have a parts list for that build. For the guys stepping over the edge the we accept that sacrifices to the gods of speed will be made and we will bust some parts on the dyno or race track at times and this is the price of progress..

Sam
01-18-2013 01:56 PM
1Gary
Quote:
Originally Posted by F-BIRD'88 View Post
WhiteDuke:

Your cr counds out the 11.48:1 but I have reservations on some of the stuff.
Like how you got valve to piston clearance with a tiny single valve relief of 1 cc.
Some of it seems od to me..

Any idea on the fuel specs beyond "98 octane" rated.

Here , our fuel is rated as a average of the two different bench tests.
R and M... Motor octane is always lower and emulates a engine more accorratly under load in the car.

We do have a 98 octane R+M/2 pump gas here at some limited gas stations.
Sunoco GT-98. It generally is good for a 11.5 to 12.5:1cr engine if the tune and conditions are good. It is not readily available.
Here we also have a 100 R+M/2 octane unleaded sold at very limited locations some stations.
Delta Sonic---- it is Sunoco GT-100 sold as "CAM2" Its good stuff.

Our 94 octane premimum is quite good. I cannot tell you the absolute cr limit.
Some people get in trouble around 10.00:1 some have better luck with a bit more cr up to around 11:1. Beyond that it is real hit and miss. Depending on daily conditions and tune up.

many many stations in the US in particular sell very inconsistant fuel quality and octane. Many epople have touble even at les than a 10:1 ratio their local pump premimum gas. Especially in the Mid West states.

I do know that your fuel is rated differently than ours. And that generally the European fuel and oil quality standards are higher
than North America.

Having been there done that... I always warn people to take all this DCR stuff with a grain of salt when planning a engine build for pump gas for true street driven use.

Had a friend here that bought into the DCR stuff hook line and sinker and even after ignoring my warning and explaination of the hows and whys,,, destroyed two perfectly good 350's @ 10.70:1 cr. What happended was exactly what I predicted would happen. We rebuilt them with 9.8:1 and all was good..

I built on a t 12.65:1 with plenty of cam etc etc.. it went fine for a while on 94 with conservative (reduced) timing, but really liked and wanted 110 unleaded.
Came unglued on me one day when I forgot to reduce the timing for 94 pump gas use. Never heard a thing.
Broke a ring land. But it was fun. ( On this one I knew perfectly well that the 94 octane fuel was not going to be enough for this engine so no surprise) Like said it really liked and need the 110 octane unleaded. Went like a raped ape....

I suggest you tread lightly on it with pump gas. (watch the water and engine air inlet temperature)
stay off the throttle or reduce the timing a bit if when conditions are not optimum.
As the engine ages the octane requirement will go up. (carbon buildup- oil consumption, deposits in the combustion chamber)

Do you have any way to detect detonation @ WOT? "engine ears" or a knock detector?
I built my own amplified "engine ears" for under $50. Unfortulatly on that day I smoked that 350 I did not have my "ears" on.
Now I am reading your words,so let me see if I got this right.You have blown up three engines because of poor parts choices ignoring DCR's in one of them and trying to use the DCR formula in two of them which is a band-aid repair.The concept of the use of DCR's formulas is to formulate a build in advance of purchasing those parts not afterwards.Then because of poor driver feel not knowing a engine at the top end is leaning over on the top end,you suggesting a devise"engine ears" to detect the mystic of detonation which if DCR's where properly applied in the first place would have been on a bucket list checked off so as not to be a problem.

I am going to suggest this "old school" thinking doesn't work and I came from that time period.When the gas changed it was a game changer.That that type of thinking needed to change with it and no less the use of DCR formulas is in no small was a huge part of moving forward.Not backwards.

I am for the record suggesting with a admitted history of three damaged engines because of poor execution of the DCR's formulas is enough proof that if applied correctly wouldn't have happened.

I am also saying for the record that a DCR of no more than 8.5 is disheartening to me as well.It does sound low,doesn't it??. But it is what is. And we all have to learn to accept it or end up paying for the consequences.
01-18-2013 11:25 AM
GunnerTHB There are several calculators out there that can calculate DCR for you. I've always heard target values of 8.5:1 for pump gas and 9:1 for race gas, although I'm sure that is pretty generalized and you could probably exceed that a little bit with good engine design.

Dynamic Compression is rooted in the fact that the intake valve closes after bottom dead center. Since your cylinder isn't sealed right away as the volume above the piston begins to decrease, you do not keep all of the air that is in the cylinder at BDC (Bottom Dead Center). What is compressed is what is left in the cylinder after the intake valve closes. This is why Dynamic Compression is always lower than Static Compression.

So one of the important things you need to know to calculate DCR is how long (in degrees of rotation) the intake valve stays open after the piston hits bottom dead center*.

When you are looking at your fuel you will have critical values, such as maximum pressure and temperature the fuel will take before autoignition, which is another thing to take into consideration when you get this intricate into engine design.

*Thanks to techinspector for teaching me how to do this several years ago.
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