Compression ratio sounds right?

[Wizard]

Hybrid 16V DOHC 2.5L common block, "all motor" n/a and hope to have torque to get more lower rpm using longer intake runners, needed for gen 1 short caravan.

http://www.rosspistons.com/calculator.php gave this result 9.57:1 ratio:

head chamber 50cc
piston 10cc
bore 3.46 (bored .020 over)
stroke 4.094
deck .020
HG crush thickness: .068

HG crush thickness assumed for 16V DOHC HG.
10cc dish was based on picking a number between TI 2.2L 8cc and TII 2.2L 14.5cc dish volumes.

I know dish volume will increase some with milling valve reliefs into stock piston, however, What I don't have is T1 2.5L dish volume also TBI 2.5L dish volume. Care to come up with this info?

If this numbers bit off, please do to revise.

Cheers, Wizard

[89acclaim]

For the T1 it is 24cc though that is with a deck of -.084. For The 2.5 TBI I have been told 16.4cc, with a deck of -.008. I say mill the block to make the pistons have a tight quench with the head, and fly cut the 2.5 pistons. I like this program for calculating the CR. http://members.uia.net/pkelley2/DynamicCR.html and this is a usefull thread 782 chamber volume???


Daniel Merrill




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[Wizard]

Pat Kelly's article on dynamic compression ratio. Yow. Good one that I ever saw.

Thanks, Daniel! Google didn't pick this one up, even I couldn't find the info I wanted on TD forum too.

Cheers, Wizard

[Wizard]

Boy that brings up more Q's!

Now the revised numbers with the suggestions:

Using TBI 2.5 pistons (1.64 cc dish) for common block (89-95), fly cut them for 16V valve reliefs means I roughly gain .5 cc, so say 17 cc dish.

TBI 2.5 piston (modified) 17 cc
Chamber vol (could be less) 50 cc
Crushed gasket (assumed) .05
deck on tight side: .002
bore (.020 over) 3.46
stroke 4.094

Gave 9.41:1 (static comp ratio)

What is the average crush thickness for 16V DOHC gasket (MLS or like)? I thought .068 is bit thick?

On modifying turbo 2.2 pistons, I did figuring on the comp height and dish depth of 2.2 turbo piston's dish of .19 means completely cut below original dish depth by .031 to get 2.5 comp height 1.35 with flat top. This means any fly cuts for valve reliefs will make the crown too thin in places and too high comp ratio. So going with TBI 2.5 piston is safer I think.

Now, I don't know the cam shaft specs for 2.0 DOHC and 2.4 DOHC so I can calculate dynamic compression ratio?

Thanks & Cheers, Wizard

[89acclaim]

Here are some cam specks for the 2.4 and 2.0 http://www.fwdmopar.com/sites/dennis...umorslies.html
A little different ones from http://forums.neons.org/viewtopic.ph...hlight=buildup
SRT4 and NA 2.4 are same from what I have seen.


Daniel Merrill




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[Wizard]

Thanks, I have since found more info from crane cam website. It lists the stock specs for SOHC/DOHC 2.0, 2.4, 2.2 and 2.5.

Have to get around to collect these info and bang numbers together to get dynamic compression ratio.

Cheers, Wizard

[turbometh]

Just a quick question. I notice in the spec's you have 17cc for the piston. Are you entering 17 or -17 (dish or dome) or does the program ask the differance. My program you have to enter the negitive if the piston is a dish.

[89acclaim]

Negative for dish positive for domed.

[turbometh]

Dont listen to me. I'm fat. I checked again it it does it the way you show. I should have kept my trap shut.

[Wizard]

It is good to talk. We learn this way!

Cheers, Wizard

[moparzrule]

If you're going N/A I'd go for atleast 10:1 compression if not more.

[89acclaim]

Too bad you aren’t going to run a 87-88 block. You could use the early t1 pistons and mill the block get good quench and have around 10 to 1.

[Wizard]

87-88 block is not exactly common is it? And this is over 17-18 years ago now.
I thought using common block is easier way around to obtaining a block.

Cheers, Wizard

[89acclaim]

The 87-88 block isn't hard to find. The only reason that the 89+ blocks is called the common block is that it is the same block between the 2.2 and the 2.5. If you us a non-common block you will have to find a different belt that will fit. The non-common block is 12mm taller than the common block.

Daniel Merrill

[Wizard]

Are we talking about those '84-'85 T1 2.2 flat top with small dish? (9cc) And have to mill the reliefs in those pistons for the 16 valve DOHC or magnum SOHC head.

Ah, I noticed the 86-88 2.5L tall deck uses 157mm rods, weight 669 grams. Good enough to hold up to stress of N/A 150+ HP & 167+ ft-lb? Remember 2.5L vs 2.4L and custom intake (longer) and higher compression ratio 10:1 may push up the torque bit more.

BTW, I have not yet started on dynamic compression ratio. Gulps.
Have much to understand and figure out the when intake valve closing at what so I can come up with this. Planning to go on 87-91 octane gas.

Cheers, Wizard