And why to avoid them.

it’s been a sexxy topic over the last few years to brag about your DCR knowledge online or talk about it like it’s been a secret recently discovered.

the fact is you’ll get five different DCR numbers using five different calculators. The biggest reason is the intake valve closing point used in them is mostly just a wild guess.

the only accurate ones to use are the ones that use valve on seat numbers… not an advertised#, not .050”, not .006” or an unknown, which most calculators use.

they’ll just say “intake closing ABDC”. Which means nothing unless the valve is seated. Most will add an arbitrary number to the .050” closing point , which also assumes every lobe ramp in the world has the exact same actual valve on seat closing from the .050” down

some arbitrarily add 15 degrees to the closing number within the program, some say add 20, some don’t say anything, but add it without you even knowing how much they’ve added in the program. That’s why you get all kinds of different DCR answers using different calculators

but let’s say you find a good one that actually uses valve closed timing and you get an accurate DCR number…what does it mean to you? The truth is, not much. The internet experts will say things like “oh I always shoot for a max 8.5 DCR for pump gas”. Or some other whacked out answer. what does 8.5 mean,, or 7.5, or 8.25478453?

they don’t take into account the fuel used, conditions, engine set up, combustion chamber design and its burn characteristics or resistance to detonate. Is it a bad wedge style chamber? Hemi or a very good compact small bore ?

you’ll never hear a pro engine builder talk about his DCR. If they do , it’s for a very specific combo that they have done many times and know what it can tolerate. That means nothing to any other engine build

 

This one arbitrarily adds 15. Why not 13.75, why not 18.3487?

it’s because it’s a wild guess that will give you some bs number that means nothing

 

You are not the first person to question these dynamic compression calculators. Same complaint on how they decide on the numbers and is it really accurate. I do like their non dynamic compression ratio calculator. Having the gasket bore in the equation does change the actual compression ratio somewhat.

 

most online static comp calculators don’t include a crevice volume input. That is usually anywhere from .6 cc to 2cc depending on the crevice to wall clearance with a .250” down dimension. Pistons with huge valve reliefs will usually have a much deeper top ring down distance which adds cc’s

stock cast pistons are about .025” smaller than the bore while forged 2618’s can be .045”

 

Use a calculator that has a crevice volume input

or figure it out yourself and add it to the chamber volume

 

So it is the distance between the piston and wall and from the piston top to the top ring? These Mahle have .025" piston to wall clearance.

 

yes that’s the space. How far down is the ring from piston top?

 

Good question. Mark would know that. On this iron head motor, a .1 to 1 drop should not change it enough to cause issues.

 

It all adds up. That 10.2 valve relief is more like 11.2 + when calculated right.

that’s why I got a lower number than you guys when I calculated it.

what Mark should do is talk to Mahle about making that piston available with no reliefs at all. It’s a machined relief, so it’s easy to eliminate that step. It’s a 4032 material piston,, 99% of the guys wanting that material don’t need any reliefs

the old TRW/Speedpro flat tops for the 350 didn’t have any valve reliefs…anyone using those pieces of **** never had a problem with valve to piston clearance.they were similar tight fitting like the 4032’s

 

They work for mid 9 to 1 iron head builds like mine.

 

Christian, you have the pistons there. Just measure how far down the ring land is from the top. Iirc it should be about .250.

 

Mark, I'm not too worried about it. The motor is together, basically need to throw on accessories, fluids, prime oil, fire and break in the cam. Slightly under 9.5, should be around 9.4 to 1. If it is only 9.3 to 1, won't mean enough to notice.

 

So who has a decent one to use

 

Best one I’ve found is from Diamond Pistons

https://diamondracing.net/p-10-compr...alculator.html

 

dp

 

Isn't that a small block chevy valve relief pattern ?