I have a question for the engine guru's here. I understand how the cam/head relationship works, what I don't understand is when I see "that is too much cam for your compression ratio"....how does compression ratio fit into the mix? Thanks for the help.

Dave

 

Your static compression ration ie. 10:1 and your cam duration will determine your dynamic compression ratio which is very important for a street driven car that wants to use pump gas.

You need to be less than 8.5:1 DCR but above 7.5:1 DCR. Above you risk detonation and might need to run racing fuel and below you are going to have a lazy motor in which the line " too much cam for the compression" come in.

Do a search for "DCR"

 

Thanks MotorHead for the info. I will do a search on DCR.

Dave

 

Here's a good primer on "DCR" which includes a downloadable, comprehensive CR/"DCR" calculator.

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

Note that the theoretical "DCR" being referred to here is not the same as true dynamic compression ratio discussed by the real engine gurus such as David Vizard, Smokey Yunick, et al, which in reality is CR * VE (varies w/RPMs). IMHO, the "bigger" cam you run, the further you ought to consider backing away from 8.5:1 "DCR" to allow for high cylinder pressures at peak power levels. Nothing wrong with "only" having an ~8:1 "DCR" pump gas motor where a fairly healthy cam is involved.

 

Here's an online DCR calcualtor that I used. http://kb-silvolite.com/calc.php?action=comp

 

Thank you for the added info...you can never have to much of that.

Dave

 

I have been an Auto machines for 25 years.

The most important things are overlap and timing.

Hunt a program down called desktop dyno
It is a good program to help you see what works and what doesnt

 

This is a great calculator for static compression ratio. Not so good for DCR. Intake Closing Point (degrees) ABDC @ 0.050 lift plus 15 degrees is not accurate. It does not take into account varying ramp rate or VE. Pat Kelly's link The SkunkWorks refers to is much more accurate.

 

Thank you for the info 63mako. Still researching for the engine build.

Dave

 

Not meaning to invade the thread, but.....
That is a great write up, it helped me understand what forum members were telling me about DC/R, when I was trying to figure out what C/R to build my engine a few months ago. When I first started this I was just going to take the block to the machine shop and get them to build me an engine, but since I have been doing a lot of research, and gathering everybit of info i can find on engine building, cam selection, timing, C/R and now I am exploring DC/R. My knowledge of engine dynamics have increased by 500% in the last 2 years thanks to this forum's members and the questions that are never too dumb to ask. Thanks C/F

 

This DCR stuff spooks me.

My old cam I had in the 454 has the intake valve closing at 89 degrees.

Using the wallace racing calculator with 13.0 SCR the DCR would end up at 7.96, 12.5 SCR the DCR is 7.66. 12.0 SCR the DCR is 7.37.

So what do I do buy some aluminum heads slap in 13.0 SCR, 7.96 DCR
go to the station pump in some 93 fuel.

 

Playing devil's advocate, are we?

Actually, tho more extreme than what I had in mind, Little Mouse has given a prime example for why my earlier comments about a theoretical "DCR" figured in a calculator as opposed to true dynamic compression present in a working engine. Whenever considering "DCR" an important caveat to bear in mind is that the larger the difference between "DCR" and static CR (as is generally going to be the case with more radical engine combinations), the higher volumetric efficiency (more on VE below) may well likely be at peak power levels, and thus the higher the resultant cylinder pressures. Straightforward as that may or may not seem, understanding where along that scale, as well as which and by how much ingredients in the engine design formula (IVC, CR, octane...) need changes be made isn't so simple a matter in practice. While I still believe theoretical "DCR" info is well worth having as a reference, as was so aptly illustrated above it's a mistake to think that's all you need to know in order to spec out a sound build.

Now, more on VE...
Volumetric efficiency is the percentage of induction charge volume that actually gets trapped in the cylinder vs. total cylinder displacement volume. Although the point at which IVC occurs indeed plays a vital role in determining VE, that single event is as far from being the only relevant factor as is an intake valve from being the only part in an engine. Despite whatever theoretical "DCR" may be, whenever VE% is greater than the "DCR"/CR percentage, true DCR will be greater than the theoretical calculation. Also, it is possible for VE to exceed 100% in very hi-perf builds, which puts true DCR above even that of static CR. FWIW, in the quest to pack more induction charge in the cylinder, it's more the cam necessitating higher CR's rather than the other way around.

It makes my brain hurt too, but I hope that helps somewhat more than it is confusing.

 

It is a guide. Use it in combination with all other info to make an educated decision. It helps you from shooting in the dark. I am @ 100% VE from 4500 up on my build with the combo I have. My DCR figures @ 8.28 with aluminum heads. Tried to build in a margin of safety, .041 quench. high 12's A/F ratio, Dewitt's rad, 160 thermostat.

 

I agree. While DCR is a real thing and is actually what we are all shooting for.....as much as we can get on whatever fuel we use...there are WAY too many variables involved.

Those calculators are neat to play with and will give you some guidelines..but that's about it.

I'm in same boat......the calculators say my 540 would be fine on 93 octane with near 13.0 compression.....and that just isn't happening. Now if I put on really rotten heads and choke it down where the cylinders never get full at high RPM...I guess it could. But not when you're going over 100% VE. or even near it.

Everything can't be done with a calculator without having LOTS of input data.


JIM

 

Bigger cylinders more distance to travel for a complete burn domes in the way on a big block, all factors that make for more chance of detonation. Small block 383 build at best a nice flat top or dish piston small bore easy burn compaired to a big block.

No one recomended I pop in some 13.1 pistons drive down fuel up on that 91/93 octain fuel. But the DCR calculator says no problem go for it.

 

How are you determining this?

 

You know I ain't going to even look up that calc.

We built a few engines that customers have tried on pump gas...94

My buddy is one of them. I built him a 400 SB with 6 inch rods to correct the rod to piston ratio thus allowing higher revs.

This thing has a pretty wide overlap and duration. With timing set at full ...36 he runs 12.2 and can run the thing on nitrous and it will dip into the 10s

Timing and overlap are the key although you can run a lot more cam then the cr says in the specs

If you want a nice street strip thumper, stick with a cam with center line around 240 duration at .050

Now here is a trick. Get your heads cut for large ratio rockers ... 1.7s or better if you go to a ford rocker... dont laugh F O R D

Even a hydraulic tappet cam with the specs above with a CAM lift of 350 will have a valve lift of .525" with stock 1.5 to 1 rockers " aka 1.48" . The same cam with 1.6 rockers will have a lift of .560" and .595" with 1.7 to 1 rockers

Your may have to notch your pistons with the extra lift and you should use good springs.

If you want to get the most from a good cam, start by making sure you have a good set of machine heads or you really won't get the gains you want not to mention those 64 cc heads bump up a flat top CR to 10- 10.25 to 1 depending on the relief of the notch.

 

Comp Cams Desktop Dyno software.