Slant *        6        Forum
Home Home Home
The Place to Go for Slant Six Info!
Click here to help support the Slant Six Forum upgrade!
It is currently Fri Sep 25, 2020 2:17 am

All times are UTC-07:00




Post new topic  Reply to topic  [ 19 posts ]  Go to page 1 2 Next
Author Message
PostPosted: Sat May 04, 2019 4:27 pm 
Offline
Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 3124
Location: Indianapolis
Car Model*:
Some use IVC @ .050 like

Go Fast

http://www.gofastmath.com/Compression_R ... Calculator

UMC Pistons

https://uempistons.com/p-27-compression ... lator.html

Some Use Seat to Seat IVC like

Pat Kelley

http://cochise.uia.net/pkelley2/DynamicCR.html

Wallace Racing

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

asking, as for the engine that I am building, the various compression calculators agree on the static compression ratio
but are far apart on the calculated dynamic compression. One set calculates to the low side of ideal, the other set calculates to the high side of ideal.

The calculators that use IVC Seat to Seat calculate a dynamic compression of 6.81,, which I believe is on the low side of being practical.
The calculators that use IVC @ .050 ( with the 15 added when needed) calculate a dynamic compression of 8.5, which may be on the high end of being practical.

I have found this interesting as when I planned out the motor in the 83 D150, The same calculators did not give the exact same result, but they generally agreed with in a few points.

It appears that there is something about this combination of compression ratio and the intake closing ramp that is confusing one set of the calculators, but which one?


Last edited by DadTruck on Sun May 05, 2019 6:03 am, edited 1 time in total.

Top
   
PostPosted: Sat May 04, 2019 6:42 pm 
Offline
Turbo Slant 6

Joined: Thu Jun 07, 2012 4:29 pm
Posts: 604
Location: Houston
Car Model*: 68 Valiant
The real issue is that those calculators are not being confused by one particular aspect, but rather they are born confused and always will be.

I've talked to some very prominent engine guys and they've told me 'I don't know how to determine (not calculate) dynamic compression ratio'. I guess anyone can calculate anything as long as you understand going in that a calculation is only playing with numbers and can be made to say whatever you like, regardless of reality.

It's a very difficult thing to do determine dynamic comp ratio. I'll bet the best people at it - major car makers - have some very sophisticated programs that get them close. Most of all, those programs are developed and fine-tuned in conjunction with real-world measurements. There's no internet program that has only 7 or 8 variables that is gonna be on the money.

And....dynamic compression ratio changes with RPM. It has to. I looked at the Wallace calculator and it doesn't even list RPM as a variable.


Top
   
PostPosted: Sun May 05, 2019 10:57 am 
Offline
Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 3124
Location: Indianapolis
Car Model*:
Quote:
they are born confused and always will be


you are much more pragmatic then I am, working through the theoretical has never bothered me.
As George Box said "All models are wrong, however some are useful."

Said a simpler way, "For a model there is no need to ask the question "Is the model true?". If "truth" is to be the "whole truth" the answer must be "No".
The only question of interest is "Is the model illuminating and useful?".

And that was and is my question. Doc had already referenced the UEM Piston calculator,,
I was really just trying to find out what others had used and found useful in the area of dynamic compression calculators.



Thanks


Top
   
PostPosted: Sun May 05, 2019 12:14 pm 
Offline
Supercharged
User avatar

Joined: Thu Jan 27, 2005 8:32 pm
Posts: 7598
Location: SW Washington
Car Model*: 1965 Plymouth Barracuda, 1954 Dodge C1-B8
The main factors at play here are how fully did we fill the cylinder, when did compression truly start and how much more do we compress the mixture from that point. Most calculators appear to assume 100% filling, use the IVC (intake valve closing) point as when compression starts and use the piston position at IVC and connecting rod geometry to answer the last question. What we really want to know isn't entirely answered by the dynamic compression. The real question usually is what static compression ratio works well for the engine I have and the fuel I use?

_________________
Joshua


Top
   
PostPosted: Sun May 05, 2019 1:17 pm 
Offline
Turbo Slant 6

Joined: Thu Jun 07, 2012 4:29 pm
Posts: 604
Location: Houston
Car Model*: 68 Valiant
Yep. In the end, for most of us, all that matters is how much CR can you run for a given fuel. And even that will change based on cam selection and a bunch of other variables. For the near-stock Slant Six, the answer seems to be 9:1 on pump gas (and I don't consider myself to be a contributor to that consensus).

What's more...even if you did some magic and were able to run 9:5 to 1, the resulting HP increase is actually pretty small.


Top
   
PostPosted: Sun May 05, 2019 5:08 pm 
Offline
Supercharged
User avatar

Joined: Thu Jan 27, 2005 8:32 pm
Posts: 7598
Location: SW Washington
Car Model*: 1965 Plymouth Barracuda, 1954 Dodge C1-B8
A true 8.5:1 was borderline with a stock 1967 camshaft. It would have probably been okay with better oil control, but that would have meant a rebore to take out the cylinder taper. With a street performance camshaft 9:1 would have been fine in that car.

_________________
Joshua


Top
   
PostPosted: Mon May 06, 2019 9:35 am 
Offline
Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 3124
Location: Indianapolis
Car Model*:
Quote:
Most calculators appear to assume 100% filling,


Agree, and with the limited breathing ability of the slant head, it leads me to think that the dynamic compression calculators error to the high side for our motors.


Quote:
all that matters is how much CR can you run for a given fuel


So the recommendation is to use static compression as the main factor when determining octane level. I cannot say that I disagree. The advantage of using static compression
is that the formula is locked down and universally understood. The disadvantage to using only static compression is that it does not provide for input from the selection of the cam shaft.
And the cam is possibly the biggest single element that defines an engine's personality.


Last edited by DadTruck on Mon May 06, 2019 9:53 am, edited 1 time in total.

Top
   
PostPosted: Mon May 06, 2019 9:49 am 
Offline
Turbo Slant 6

Joined: Thu Jun 07, 2012 4:29 pm
Posts: 604
Location: Houston
Car Model*: 68 Valiant
Even engines that can achieve 100% VE (or better) only do it at their optimum RPM range.

That's why engines with 'hot' cams run crummy at low RPM's. And it's why fancy engines use variable cam timing.


Top
   
PostPosted: Mon May 06, 2019 10:22 am 
Offline
Board Sponsor & Moderator
User avatar

Joined: Mon Oct 21, 2002 11:08 am
Posts: 14539
Location: Blacksburg, VA
Car Model*:
I always compute static CR, then pick a cam from my experience, along with plenty of numbers and interviews of (and rides with) other racers.

Unless I am miscounting, I have run 10 different cams over the years, and with different cam timing (degreed centerline) for some, and different CRs for some, and different head flows for some. All on 225 engines.

Lou

_________________
"You mean you still have a Slant 6 in that thing?"


Top
   
PostPosted: Mon May 06, 2019 11:22 am 
Offline
Turbo Slant 6

Joined: Thu Jun 07, 2012 4:29 pm
Posts: 604
Location: Houston
Car Model*: 68 Valiant
I'm a fan of compression ratio. An engine with 'high' CR has a good, crisp throttle feel and you just know it's giving you all it's got.

Unfortunately, I'm an even bigger fan of engines that aren't detonating.....


Top
   
PostPosted: Mon May 06, 2019 11:37 am 
Offline
Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 3124
Location: Indianapolis
Car Model*:
Quote:
10 different cams over the years, and with different cam timing (degreed centerline) for some, and different CRs for some


and this is not being critical, but if over the span of experience described above, if you had collected dynamic compression data, wonder if it would have pointed to a
common index, for motors that performed well. The comparative date would need to be generated using the same calculator for all.


Top
   
PostPosted: Mon May 06, 2019 12:57 pm 
Offline
Board Sponsor & Moderator
User avatar

Joined: Mon Oct 21, 2002 11:08 am
Posts: 14539
Location: Blacksburg, VA
Car Model*:
I could probably come up with all the parameters if you want to feed the calculators, but it would take some effort. I'll put that project on my long-ish list... I had some notable fails early on, and those were extremely instructive.

Lou

_________________
"You mean you still have a Slant 6 in that thing?"


Top
   
PostPosted: Mon May 06, 2019 3:20 pm 
Offline
Board Sponsor & Moderator
User avatar

Joined: Thu Oct 17, 2002 7:27 pm
Posts: 12704
Location: Park Forest, Illinoisy
Car Model*: 68 Valiant
My experience has been that a moderately low compression Slant (9.5:1) ran better and better the more cam I stuffed in it and more RPM it spun. I also found out that when I broke down and added more compression (11.2:1) it ran more better. :mrgreen:

_________________
Official Cookie and Mater Tormentor.


Top
   
PostPosted: Mon May 06, 2019 4:41 pm 
Offline
Supercharged

Joined: Wed Sep 17, 2008 6:48 pm
Posts: 3124
Location: Indianapolis
Car Model*:
Quote:
my long-ish list.


inputting the data is the ez part, if you do pull it together it would be interesting to
see what corelates, especially since there are good ones and fails.


Top
   
PostPosted: Mon May 06, 2019 6:11 pm 
Offline
Guru
User avatar

Joined: Sat Oct 12, 2002 11:22 am
Posts: 3684
Location: Sonoma, Calif.
Car Model*: Many Darts and a Dacuda
Dynamic compression is actual running compression that varies with RPM. It is hard to truly measure DCR.
Effective compression is the calculated compression number most of these calculators give us.
As already stated... This calculated number is a good "point of reference" because it addresses the cam profile.

For me, I find (measure) the actual point where the valve hits the seat, at the lash (valve clearance) I will use. (UEM says to use @ .050 and add 15 degrees but I use the actual IVC I have measured)

My experience:
Up to 7.7 "result" engines run OK on any fuel you put in them and will take a lot of ignition timing but you left some performance behind.
7.8 to 8.0 'result' engines run well on pump gas. I look for this range for most pump gas engine builds.
8.0 to 8.3 'result' engine builds run butter... but needing better fuel... Pump premium or race gas "blends")
Any SL6 engine build over 8 to 1 on the UEM calculator are more pron to detonation on today's pump fuel.

I have also noticed slight output differences between difference calculators so it is important to stick with one that you have experience with.
DD


Top
   
Display posts from previous:  Sort by  
Post new topic  Reply to topic  [ 19 posts ]  Go to page 1 2 Next

All times are UTC-07:00


Who is online

Users browsing this forum: No registered users and 10 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Limited