NewbVetteGuy

Can anyone help me figure out the dynamic compression of an L82 motor with a top-end swap given the heads and cam below?

(Please show your work/teach me how to do it.)


The starting point is an L82 engine from a 1979 C3. (I think that gives the majority of the statistics required to get started.)

The heads have 64cc combustion chambers and are modified Profilers from ATK (www.high-performance-engines.com) so that they have non-raised exhaust ports. 2.02 / 1.60 valves.

CAM: Let's model a CompCams 260XFI HR13 roller cam which has a seat-to-seat duration of 260/270, a duration @ 0.050" of 210/218, an LSA of 113 deg, ICL of 109 deg.
(I'm not sure it matters but 1.6 ratio roller rockers are planned for this build, too.)

I'm also interested in how a change of the LSA would change the dynamic CR- let's say I went with an otherwise identical CAM that dropped the LSA down to 110 degs that would increase the dynamic CR, I think, but by how much?




I'll then take the formula and I'll try my hand at applying it to the stock L48 and L82 1979 motor to see if I can successfully estimate the dynamic CR from the factory and hopefully someone can double-check my work then. (Please no one post the dynamic CR of the stock engine options because I want to try it myself and see if I can figure it out and get to the right answer.)



Adam

NewbVetteGuy

Also: Let's say that I'm slightly "pushing it" with my static and dynamic compression and I error on the high side to attempt to maximize efficiency and I start finding out that I AM getting pinging / detonation when the weather turns hot or I get partial ethanol gas- What's my best course of action to reduce that knock / detonation risk particularly without adversely impacting performance.


Would I be best off to just retard the camshaft timing a tiny bit? (That would help right?)


From a few other threads, I'm pretty sure that a stock L82 with 64cc heads and the super thin FelPro gasket (0.015") will put me at almost exactly 10.2:1 static CR and a 0.040 quench (yes, my decimal points might be off on these numbers), but I really want to focus on optimizing my cam for efficiency / power (torque) so my understanding is that I should go with a slightly smaller LSA. -I'm happy to crawl up right next to the detonation limit hoping that I can either retard the cam shaft slightly or set my ignition-control EFI to retard the timing a bit OR just ensure I buy really good gas or keep an octane booster or two on hand if I get into hot weather/ higher detonation risk weather.


Note: I live in the Seattle area and it doesn't really get very hot here very often and the humidity is generally pretty high so if I'm not mistaken that helps reduce my detonation risk.

I'm not sure if it matters but when I have to remove the radiator to do the cam swap I'm installing a 4 row aluminum champion radiator (no, I can't be convinced to spend any more on the radiator; I'll go higher end on the fans, though) and I'm hoping that reduced engine temps will also help with detonation risk, again because I'd like to run with a cam profile that's going to maximize compression, efficiency, and torque. (not sure how much engine temp helps with detonation but I thought I'd heard here before that reduced temp with aluminum heads is why aluminum heads can deal with higher CR, so I figured that increased cooling might help, too.)


Adam

cv67

Not saying this one has all you need but lots of online ones out there
A cam card of all the valve events is good

http://www.wallaceracing.com/dynamic-cr.php

Shooting from the hip you have a nicer combustion chamber shape which is more tolerant+ alum. + lower temps think youll be fine.

MotorHead

This is the one I use http://cochise.uia.net/pkelley2/DynamicCR.html

I also have Engine Analyzer which is much more complex to set up but it gives you much more accurate results.

cv67

MH never checked out EA do you order it online or is there somewhere you can run stuff for free? Cause Im a cheapass.

StraubTech

Adam,
With all due respect don't worry about DCR. It is like herding cats. It is someones idea of creating a calculation that means nothing. How can can you use Dynamic and Ratio in the same sentence. It is useless sir except for bench racing...

cv67

StraubTech

and I will add all the following cam guys have the same opinion on DCR...useless:

Rick Jones
Ed Curtis
Mike Jones

So its just not me.

Priya

Show your work.

REELAV8R

https://www.jeepstrokers.com/forum/calculator/

This is the one I use.

We've been down this road of DCR being useless before. I'm not clear if it's the term that is found to be objectionable , or the whole concept.

Certainly compression can only really begin after the intake valve closes. So the remainder of the stroke after the valve closes in the remaining effective stroke. This is what I believe DCR is trying to address.

How much the the 10:1 cr is left after the intake valve closes. If 20% is lost due to valve closing point then you are left with 8:1 cr or DCR. The term may be inaccurate, but the concept gives one an idea of the ratio of total cr to remaining cr in order to try to understand how much pressure may be seen in the cylinder and if detonation may be a risk due to those pressures and the fuel used.

This is assuming 100% efficiency on each intake stroke. If less is achieved then you will have lower cylinder pressures. If more due to inertial ramming is achieved then you may exceed 100% efficiency and have higher cylinder pressures and a greater "DCR" due to taking in more air than the effective stroke of the engine. Volumetric efficiency or VE is the term used.

Priya

I'm no expert but seems clear enough to me its a sound and useful concept.

REELAV8R

I don't think he really can. His line of work is dependent on his unique set of skills, knowledge and technique. If everyone knew what he did by him telling us that would not be good for business. Each cam designer has his own proprietary knowledge. It's only makes good business sense to not give that away.

Priya

I can't tell whether or not you're being serious.

cv67

Betting he is
Straub is a well known cam designer whos good at his craft
How many times have we seen a guy who has the right cam only to have the forum tell him to put it in 8 deg retarded all sorts of goofy stuff due to a perfect DCR and the car dont run. Unless its way off look at static put it in go from there.

augiedoggy

Well my understanding of dynamic compression is its the reason im able to run 91 octane with no problems with 9.7:1 static compression and iron heads? if its not the fact that my cam bleeds off pressure to bring the actual or (Dynamic) compression down to a safe level then what is it? the smaller quench?

I know that helps along with chamber shape and lack of sharp edges..

StraubTech

Mam,
Just look up the definitions of Ratio and Dynamic. With every 1/4 of a degree of rotation you have a change in pressure for a split second. It's worthless.

Now what is worth something is opening and closing events. These are the critical numbers to making an engine perform.

augiedoggy

I may be wrong, but I think Priya is a woman..

those opening and closing events are what I thought was the main effect on dynamic compression in practical terms.

StraubTech

Page 38 of HotRod 2016 September

2016 Drag Weekend Overall Fastest
Pump gas 91 octane 10.8 to 1 BBC 565
8 Second Street Car
Over all winner for the weekend with an average of 9.1647.

StraubTech

Thank you...I "PC"d it.

427Hotrod

I'm in the camp of "I don't use dynamic compression calculators". The times I ran the numbers, they came up with recommendations that I knew from experience weren't going to work in real life. As discussed, things change constantly in a running engine, from idle to initial crack of the throttle to part throttle to WOT. Everything changes due to altitude, temp, baro, ignition timing curve etc. It's all so variable.


Let's say for example we were right on the edge of detonation with a given combo and 9.7 CR. Does that mean the cam is correct for that engine? Picking a cam based on whatever pressure it will create on a calculator just doesn't make sense. I can take a 7.0 CR engine and cam it so it will detonate. I can take a 14.0 CR engine and have it make full dyno pulls with a given cam (think back to the old Crower ( think it was Crower?) fuel mileage cams in the 70's). But neither instance means I've picked the right cam for the combination. The first one will be so short it will limit top end power most likely and the second will be so large it would be a stone just about everywhere.


The better the head overall, not only allows...but REQUIRES a typically smaller cam to perform the best. Weaker ones need more cam (duration) to get RPM and higher peak power... but they will suffer on the lower end. There's that tradeoff stuff again. That's why I tell people to concentrate on heads. Good heads help everywhere...bad heads NEED help everywhere and the compromises become more obvious.

I always think back to a bunch of dyno testing I was involved in with some old Busch engines. Those things only had 9.0 compression, but the heads flowed so well the cylinders got packed and you had better have the best race gas available. Even with a 390 cfm restricted carb choking it (but with that much airflow you can bet they flowed a LOT more than 390 cfm!). The cams in them were relatively small considering the RPM range they worked in. Good heads.

Look at the latest bad boy factory LS stuff. It's not necessarily that the exhaust is so bad...it's that the intake is so good. The COPO Camaro's with the 396 and 427 engines have a 40* split! The 427's have a much improved/larger head with 2.20" intakes vs 2.065" and exhausts that are 1.615" vs 1.590". Even though they are feeding 8% more cubes, that doesn't tell all the story. The 427 has a .175" longer stroke and a .060" larger bore. It also uses .100" shorter rods.

I can't swear the cam lobes shapes are identical,( I haven't seen Cam Doctor results) but I believe they are. What GM did was use 1.8 rockers on both intake/exhaust which changes the opening rate of the valve...and apparently that worked pretty darn well with the heads, stroke, rod length etc. Now of course these are race motors, but the 396 only has 10.4 DR vs. the 427 at 13.0 CR.

Also interestingly, these are 8000+ RPM engines with "only" a 233* intake lobe!

Anyway, I continue to learn about cams/heads constantly. There's a lot of math involved to get you in the right game and then there's "tribal knowledge" from folks who've tested the stuff in the real world. Chris/Scott and I have lots of fun conversations about cams/heads when we get together We haven't always agreed...but I will say I've been swayed a good bit by the thought/effort they've put into learning and the results of the work they've performed. Hard to argue with success. Not that other ways won't get you down the track/street just as well...but there are several ways to get things done.

JIM