Street89vette

cam designers must not see the need for this information to be upfront. Lots of details on all the other specs.
Any links for finding the "Ground in Degrees" and "Intake opening / closing points" . Very hard to find information on HOWARD'S SBC Small Base Circle Retro-Fit Hyd Roller 286/294 .530"/.545" 110° Cam. All the other information is available:

Duration: 286/294 advance
@ 0.050": 233/241
lift @ 1.5-1 .530/.545
intake center line: 106
LSA: 110

intake valve closes @ ??

attempting to calculate dcr. Any help is appreciated.

Kevova

Try Wallace Racing for calculator. But it you Google it there are several choices in regards to sites.

Street89vette

thanks, great calculator. Intake closing point is where i can not find information on just about any cam but is required to find dynamic.

rklessdriver

Simple math to find the closing point..


To find the timing events for the intake:

Divide the Advertised Duration in half and subtract the Intake Center Line from the remainder.

That will give you the opening event. Intake Opens Before Top Dead Center (BTDC)

For the closing event subtract 180 from the duration and then subtract the opening number from that sum.

That will give you the closing event. Intake Closes After Bottom dead center (ABDC)

Will

bjankuski

In your case the intake duration seat to seat is 286 degrees.

Take 286/2 = 143 degrees

add this number to the ICL which in this case is 106 degrees

143 + 106 = 249 degrees from TDC

To get ABDC subtract 180 degrees from this number

249 - 180 = 69 degrees ABDC is the closing point and this is what you use to figure dynamic compression.

MatthewMiller

Just to make this more complicated, this assumes the cam installed straight up, and not advanced or retarded.

Street89vette

thanks again, it worked! 69' for the calculator + 8.63 dcr

ddahlgren

Why would you assume the lobe is symmetrical?

bjankuski

Lack of any additional information makes that assumption (Most cams are symmetrical) but the only real way to know is look at the cam card which was not listed.

ddahlgren

There are so many holes in the DCR crap you could use it to strain pasta

Kevova

Dynamic compression ratio is a calculation but it has little to do with cylinder pressure. The running compression psi is effected by induction system to exhaust. The dynamic calculated ratio is a consideration when building an engine with over 11 -1 or adding forced induction. Cams with a lot of overlap can have a lower cylinder pressure and need higher static compression ratio to get desired performance.

cardo0

Congrats on nailing your compression ratio. I think that DCR of 8.6 is a little high though from what I've read. Maybe consider a larger duration cam? . Custom grinds are fairly cheap also. Or you can install the cam retarded though it gives up power/torque. Did you measure the piston and chamber volumes? Can you open the head chambers?

ddahlgren

Ok we can play a little game with real world data. 368 sbc with 4.125 bore 268/274 @0.050 cam lift with 108 lobe separation and installed at a 110 intake lobe center 14.2 CR if it matters 720 lift on intake w/1.7 int rocker 18 degree heads w/2.150 intake valve.

What is the DCR and what octane fuel?

bjankuski

You need the seat numbers not the .050 numbers to determine the dynamic compression.

cardo0

I can tell you it's gonna use race fuel and DCR wont matter much.

Not trying to bust anyone's bubble rather just trying to help the OP get the cam to match his build. Yes I know DCR calcs are only as good as the data you provide and the math skills of the user. But its just a ball park tool that helps more than it errors. I like to think of it like a rule of thumb where there are more accurate methods but not nearly as easy to use.

There are countless conditions that can alter the cause of detonation but DCR gives us something to reference to for choosing a cam. We as hobbyists don't have the budget or equipment to optimize a camshaft like a sponsored race team. But the next best thing is a DCR calc which is a mathematical model using documented parameters for a simulation. And I will guess that modest performance street builds are not all that different or difficult to model.

Thats my 2 cents so take it or leave it. And I'll shut up now. Good night.

GREGGPENN

Also...choosing chamber size, piston config, etc... As a hobbyist, it helped me choose "the right parts" to make sure I didn't end up with super-high compression.

Oddly, my build shows 8.4 DCR using Paul Kelley's calculator but my cranking compression is only 155psi (unless the gauge I borrowed is wrong). With a 383, I expected higher cranking compression especially since it's low-end performance is so much stronger than a stock 350. I revisited this lately (for an unrelated reason) and am still trying to figure out why cranking compression isn't higher -- with only about 10-deg overlap in my 270/270 .544/.544 cam 112LSA.

FWIW, I had already shaved my heads to 56cc (or 54cc?) heads before stroking it. Hadn't planned on that originally. I ended up buying 20cc dishes to keep compression from landing too high. 8.4DCR seemed like a nice target. (Plus, I've seen theory that says dished pistons -- with a good chamber -- nets the tightest/smallest quench for good flame-front. At least that helped me feel better about the chain of events that led to smaller chambers and dished pistons! LOL)

The cam card showed the builder to install 108ICL (as ground in) though I suppose it's possible bullet didn't grind the cam "advanced" as planned? Or...maybe he installed on the -4 setting for the multi-position double-roller set?




As for the hypothetical...DCR requires chamber size and piston reliefs too. Throwing in Ddalgren's numbers quickly reveals PKelley's DCR calculator can't handle a cam THAT big. (I'm assuming adv duration is well over 300/300) Plus, I'm wondering how 14.2 static is possible? Are you using dome pistons? Flat-tops with zero deck, a .010 shim, and 4.125 bore with 56cc heads isn't even 14.2 SCR. It's less.

Doesn't really matter when the DCR calculator can't estimate effective stroke for a cams over about 275/300 (adv). With a cam that big, I'm wondering if it will actually run on gasoline? Or even E85? Using 275/300 and a small chamber, DCR wasn't much over 10DCR. Could adding 30, 40, 50 degrees could bring it back into QTrip land? Nothing more than curiosity though....

bjankuski

Dynamic compression is just a math calculation that determines the compression after the intake valve closes. It calculates the compression by determining the amount of swept volume left in the cylinder compared to the total non-swept volume left above the piston. You do not need a calculator to figure it out you can use geometry to determine the numbers if the calculator does not work.

Tom400CFI

It is straight forward. I also agree with you that you need seat to seat numbers....not .050" numbers.

ddahlgren

18 degree heads can be had in the 40cc chamber size with much better flame spread and less prone to detonation. Easily runs on gasoline at least real race gas will work fine. Not talking street engines as for the street I would think big inches rather than high tech/expensive.

GREGGPENN

Smaller, tighter chamber fits into theory about quenching to smaller size (and improving flame-front). It's been over 7 yrs but I remember reading why larger chamber was better in the AFR Eliminator "architecture". Maybe it had to do with flow rates in/out of the cylinders?

All I remember is thinking I might have hurt my build by shaving from 65cc to 56cc chambers. Seeing their are 40cc heads makes me wish I remember where I read up on the chamber size.


The calculation on DCR makes sense....to an extent. If it uses a percentage of the "swept" volume of the cylinder to pare down the SCR accordingly, it doesn't necessarily account for the varying ability of the valves to supply air during the intake cycle. (especially below .050)