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> Compression Ratio, Too High...Too Low ???
2-OH!
post Sep 6 2007, 05:44 PM
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Ok folks,

We just finished rebuilding the motor...94mm pistons and cylinders to match...71mm stroke...but old 1.7 case...CC'd the heads and come up with 44 cc's per cylinder...

Do the calculations and we come out with 1971 cc's...Great !!!

But, The problem is the compression ratio is now 10.64 to 1...

The question of the day:
Should I take the cylinders and heads back off and shim them to get a little relief...Probably not going to get much...Might knock it down to 10 or 9.75 (if I'm lucky)...

DO NOT want this thing to clatter and diesel on me when I get it back together...

OK, let me have it...What are your thoughts ???

2-OH!
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yeahmag
post Sep 6 2007, 05:57 PM
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Way, way to high. You willl need some big honking shims under the cylinders. CR is dependent on the cam. What cam are you running? What does the cam manufacturer recommend?

-Aaron
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mike373
post Sep 6 2007, 05:58 PM
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Only if you plan on running jet fuel.... But seriously.... 10.64 is on the high side... and the cam that you are using will either benefit from higher or lower compression. I just built my 2056 with a Raby cam... and set the compression to 9.3 to 1. And some people I talked with said THAT was high.
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ConeDodger
post Sep 6 2007, 05:58 PM
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QUOTE(2-OH! @ Sep 6 2007, 04:44 PM) *

Ok folks,

We just finished rebuilding the motor...94mm pistons and cylinders to match...71mm stroke...but old 1.7 case...CC'd the heads and come up with 44 cc's per cylinder...

Do the calculations and we come out with 1971 cc's...Great !!!

But, The problem is the compression ratio is now 10.64 to 1...

The question of the day:
Should I take the cylinders and heads back off and shim them to get a little relief...Probably not going to get much...Might knock it down to 10 or 9.75 (if I'm lucky)...

DO NOT want this thing to clatter and diesel on me when I get it back together...

OK, let me have it...What are your thoughts ???

2-OH!


I personally would not be comfortable past 9.5:1. Jake might be, but he is building to a much higher precision than I did. I say shim it...
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swl
post Sep 6 2007, 06:03 PM
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slight highjack:
Educate me - how does the cam get involved in the compression ratio? Not being argumentative - just want to understand.
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swl
post Sep 6 2007, 06:05 PM
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should have thought about it more - has to do with where the piston is when the intake valve closes - right?
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yeahmag
post Sep 6 2007, 06:12 PM
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Long story short...

A high duration cam bleeds off compression due to overlap between the intake and exhaust valves so the CR is effectively lower. The higher the duration the cam is the higer the static CR can be. There are lots of "buts" with this...

Again, the cam manufacturer has a CR in mind when they build it...

-Aaron
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2-OH!
post Sep 6 2007, 06:15 PM
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Using a Web Cam 73 Grind...Still running D-Jet...

Good Idea, I will contact WEB and see what they think...See what the 73 was built for...

2-OH!
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yeahmag
post Sep 6 2007, 06:21 PM
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My guess would be 8:1 on that cam. What did you heads cc at? What's your deck height? Pistons have a dish/notch in them? If so what does it cc at?

Just trying to get you to double check your math.

-Aaron
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jd74914
post Sep 6 2007, 07:15 PM
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Ok, that compression is definitely too high for D-jet. I think Aaron is right; if that cam is Djet friendly it probably will like near stock static compression.

The cam bleeding off compression spoken of above is referred to as dynamic compression. In your case webcam will probably give you the desired compression in static terms. This website has a good dynamic compression calculator.

http://www.kb-silvolite.com/calc.php?action=comp

For TIV's I have read that the dynamic compression should be around 7 to 7.5:1, but that spec is for rather high performance builds I believe, so not totally applicable to yours with stock FI.

Anyways, good luck (IMG:style_emoticons/default/smile.gif)
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2-OH!
post Sep 6 2007, 09:43 PM
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I agree with Aaron above...

Somebody check my math...

Can someone give me a formula for determining the compression ratio...Here is what I used without my formula so I can get an unbiased opinion...

Heads cc = 44cc
Stroke = 71 mm = Height
Cyl. Dia = 94 mm = divide by two for Radius
Formula ???

mm to cm conversion... 1mm = .1cm

Pistons do hve a very slight dish, but are really almost flat...

2-OH!
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Twystd1
post Sep 6 2007, 10:03 PM
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Whats your deck height?

I.E. From top of piston to top of cylinder.
(Piston and cylinder installed, then torqued down a bit and measure for volume)

Theres a bit of volume there that you need to include into your calculations.

Or have you already included that volume into your calcs?

Clayton
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2-OH!
post Sep 6 2007, 10:14 PM
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Thanks Clayton...

The way I measured it was with the cylinder hand tight using a spacer and nut to hold it in place...

Calipers, using the depth gauge end...71.77mm, BDC... .55 TDC...Leaving a balance of 71.22mm...

Actually, Looking at my notes, I'm not sure about that...That could be the problem in the math...

In short, is there anyway to check the CR without disassembling at least one head ???
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Borderline
post Sep 6 2007, 10:59 PM
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My calcs:

Cyl vol: 492.7 cc
head vol: 44 cc
deck ht (guess 1mm min.) vol: 7cc

CR= (492.7 + 44 + 7)/ (44+7) = 10.66!!!

Add 1 mm shim....deck ht vol doubles to 14cc

CR = (492.7 + 44 + 14) / (44+14) = 9.49 !!


Are you sure the head vol is 44cc? Seems pretty low.

I would think you want to get inn the low 9's at least.
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Cap'n Krusty
post Sep 6 2007, 11:23 PM
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QUOTE(2-OH! @ Sep 6 2007, 05:15 PM) *

Using a Web Cam 73 Grind...Still running D-Jet...

Good Idea, I will contact WEB and see what they think...See what the 73 was built for...

2-OH!


WEB thinks that cam works with D-jet, that's what they think. It has certain, shall we say, drivability issues? Monster HCs (that's unburned fuel) at idle, poor off throttle response, requires serious rethinking of how the MPS is set up. their theory and reality don't agree all that much. There are worse cams, but not by much. The Cap'n
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Brett W
post Sep 7 2007, 12:24 AM
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Toss the Djet, buy a nice cam Web 163/86b, or similar variant. Put some carbs on there with a nice exhaust and you will have a very good street engine. Yes you will have to run premium, but watch your timing and you will be fine.

Do not throw a shim under the heads. Decreasing the compression by increasing the deck height is an awful way to accomplish the goal. Your 9.5 engine will ping worse than your 10.5 engine will. Just put more cam in and lower the dynamic compression ratio. Or open the chambers up and decrease the compressed volume.

If you would go with a programmable fuel injection system you could probably go a little higher and still run it on pump gas.
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Twystd1
post Sep 7 2007, 12:28 AM
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What was the deck height measurement?

CCC
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Bleyseng
post Sep 7 2007, 09:34 AM
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QUOTE(Twystd1 @ Sep 6 2007, 11:28 PM) *

What was the deck height measurement?

CCC


yeah, what is it again?
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2-OH!
post Sep 7 2007, 09:40 AM
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Thanks for the help guys...

Looking at my notes, I'm not sure of the deck height...Guess I'll be removing at least one head this weekend instead of building it up...

2-OH!
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rhodyguy
post Sep 7 2007, 09:49 AM
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rick, was your case 0 decked during the machining process?

k
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