Static vs Dynamic CR

[Chris]

Found a few numbers out there for the CR's. Looking at the stroker calculator up top here, it says the CR for a stock engine is 8.76:1. From my reading, that is the SCR, which translates to a roughly estimated 8.45:1 DCR. Seems that 8.7:1 DCR is the highest reccomended to be able to run 87 octane, and those numbers possible move into the mid grade range. 8.0:1 is figured as the ideal DCR for regular gas. I guess my question here, is either: 1 - How do I get the DCR numbers for my setup or 2 - What is the easiest way to convert the SCR numbers from the calculator to see what my DCR is gonna be. My math skill leave much to be desired, and if I attemp this it won't be pretty...

[Chris]

Head Piston Part No./ Gasket Deck Cam Part No./ Static CR Dyn. CR
CC Dish CC CC/Comp Ht. Intake Closed ABDC
Ht. (in) (in)
58 OE/13.5 10/0.052 0.031 OE-'96-up/14 9.66:1 9.29:1
58 OE/13.5 10/0.052 0.031 Crane 750501/21 9.66:1 9.01:1
58 OE/13.5 10/0.052 0.031 Crane 753905/26 9.66:1 8.91:1
58 SP-H802CP/18.8* 8.85/0.043 0.025 Crane 753905/26 9.44:1 8.71:1
58 KB-2228/11.5 8.85/0.043 0.025 Crane 753905/26 10.10:1 9.39:1
58 SP-677P/17.5 8.85/0.043 0.025 Crane 753905/26 9.57:1 8.83:1
58 SPH802CP/18.8* 8.85/0.043 0.000 Crane 753941/35 9.92:1 8.77:1
58 SPH802CP/18.8* 8.85/0.043 0.000 Comp 68-2394/36 9.92:1 8.72:1
62 Custom/25** 8.44/0.040 0.000 Comp 68-2394/36 9.02:1 7.95:1


*Note: This piston dish is commonly listed as 17.5cc. The number shown is our measured amount, including ring
land volume (about 1.3cc)
** The Golen Engine. Piston dish estimated from advertised CR.
SP= Sealed Power, KB =Keith Black Silvolite, OE= Original Equipment



According to what I'm reading, 8.7:1 DCR is the teetering point for regular vs mid grade. Over that usually requires supreme, and anything over 9.0:1 won't run on pump gas without a ping. Let me give credit to my sources. All of these numbers are coming from a fourwheeler.com article on the buildup of a 4.6. Using this information, can anyone come up with a math formula to get the DCR from the SCR? Has this already been answered somewhere and I'm trying to reinvent the wheel? It looks like the low 9.? SCR we are shooting for will work, but I wish I had some way of figuring out what the estimated DCR is from that to be able to read my numbers a little more clearly.

[Muad'Dib]

Try this post:

viewtopic.php?f=15&t=118

[Chris]

Wow, that helped! I'm gonna have to read through that a few times to try and grasp it all. I found a dynamic compression calculator that does the math for me...WooHoo... Have I already mentioned I suck at math.... According to it, with the last setup I posted in the project forum, my SCR will be 9.17209 with a quench of .0705 (our stroker calculator had it figured at 9.2:1 and a quench of .0705, so it looks dead on there and I don't think I got any figures wrong). Switch it over to DCR and it drops the ratio to 7.09683 DCR. anything below 7.1 DCR and you start giving up power. These numbers were done using the Crane 753905 cam. It looks I won't have any problems running 87 octane, and I can even start raising my compression some without any side effects. I'm gonna play around with more numbers and try out different cams. I wonder what the stock cam will give me?

[Flash]

SCR is the ratio of volume at BDC and TDC example: if you have a 9.0:1 SCR motor, at BDC(Bottom Dead Center) you would have 9 times a much space in the sylinder as you do at TDC(Top Dead Center)

D.C.R is the compression that the eng see's when it running!!!

Here is a much more detailed explination of it.

viewtopic.php?f=27&t=73

If your still confused after you have read all that..............Ask a way and i will try and do a better job of it!


Flash

[Chris]

I have a headache now, but I think I've got it...lol

[gradon]

When I was reading up on DCRs, it was good to shoot for a low 8 range and that once you got above 8.5, you would need race fuel, thus that's the critical point I've been using. I haven't tried spec'ing out the factory DCRs or common stroker build DCRs with the common Crane or Comp cams.

[darkcorp]

Hi, everyone!
I'm building stroker and want to use parts that are cheap in my country. That are 100mm pistons, stock rods and cam. I'm gonna shorten stock conrods to fit my pistons. As they have piston pin height of 43.5 mm (1.713 inch) I need to shorten conrods by 8.44 mm (0.332"). Also piston pin diameter is 25 mm instead of stock 23.65.
I run propane (LPG) and it's my DD. But sometimes I travel such locations where no LPG can be found, then I run petrol.
So I'm choosing right config. Which one will be better?
As I understand Renix cam is 1.5 degrees advanced and 91-95 cam is degrees retarded.

[darkcorp]

As I understand Renix cam is 1.5 degrees advanced and 91-95 cam is * degrees retarded.

*8 degrees retarded.

Anyone got thoughts?

[jeepxj3]

I'm gonna shorten stock conrods to fit my pistons. As they have piston pin height of 43.5 mm (1.713 inch) I need to shorten conrods by 8.44 mm (0.332").

How are you planning to shorten the conrods?

[darkcorp]

It's written in this post:
I have shortened rods for stroked motorcyles by making a fixture to hold the rod that aligns the wristpin hole to the crankpin hole and is adjustable for length with a threaded fastner , and your desired rod length center to center set by some form of stop. Insert rod, apply heat from O/A torch ,squeese rod to length , let cool, remove from fixture ,repeat as neccisary. stress relive in oven at 350-400 degrees for 1-2- hours. Rod should be restrained in fixture so the material displacement can be directed to the middle of the beam of the rod. It will sort of look like a snake that ate a large rodent. There are several companies that will make rods for anyting that you might want, so you might want to check around first.

My friend have made conductor and tried to resize a couple of conrods. That wasn't successful because he didn't use oven to relive at 350-400 degrees for 1-2- hours thus the conrods went screw. Now I've got extra 9 conrods for him to reseize using oven. I hope he could.

Actually, shortening the conrods was very popular practice in autosport a couple of decades ago. They work good in high rev motors so I have no doubt they will work in our six-line.

[Cheromaniac]

By all means use the stock connecting rods and cam but please DON'T use those pistons. You cannot safely shorten the connecting rods to the degree required to use those pistons so I'd strongly advise against it. You could instead do the following to run your engine on propane:

4.6L Modified "poor man's" stroker propane special

Jeep 4.2L 3.895" stroke crank
Jeep 4.0L 6.125" rods
Bulltear custom forged +0.020" bore pistons, compression height 1.380", dish volume 16cc
10.25:1 CR
Stock '91-'95 HO camshaft
DIY ported HO 1.91"/1.50" 57cc cylinder head
Victor Reinz 0.043" head gasket
0.043" quench height

Keep a bottle of octane booster handy in case you need to use petrol instead of LPG.

[darkcorp]

Dr. Dino Savva, thank you for advice!

[SilverXJ]

You cannot safely shorten the connecting rods to the degree required to use those pistons so I'd strongly advise against it.

x2. A .332" change isn't going to work to well

[jeepxj3]

I want to see a cut and shortened rod.