compression ratio discussion

[Shark]

I've managed to put together a spreadsheet that will figure all the static numbers. What I'm still struggling with is understanding how to take the advertised cam specs and get the IVC point.

ive been using the andreas stroker calc. i think its from madXJ. an added column in there that uses the IVC of several of the most popular stroker cams to calc the DCR would be very useful. it wouldnt be hard if i knew the IVCs.

[gradon]

So I was crunching #'s on the KB link and I had a SCR of 10.28:1(stock pistons, decked .040, 58cc--just an option). I will be using the Mopar 4529230AB cam. I have not found the Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees to put in the formula. The advertised Intake Closes (ABDC) deg is 56. Now this cam has 40* of overlap, so should help bleed off some cylinder pressure (and lower DCR, right?). I used Dino's cam spec page @http://www.angelfire.com/my/fan/Jeep4.0Camshafts.htm. Anyhow Flash, I haven't attempted to find the formula to determine what the ABDC @ .050, so maybe you can help me out.

[Heck]

I just finished a "simple stroker", in that all I changed was the crank, rods, and a Crane #753905 cam.
For those of you thinking you can lower your dynamic compression ratio using a bigger cam, realize that mine lost most of it's drivability below about 1100 rpm. Meaning at crawl speed idle, where the stock engine performed so well, it stalls out easily. Of course it runs like a flaming cat on the highway and gets the exact same mpg as before (measured over 5 tanks with a calculator). But I payed for the money I saved.

[Shark]

I just finished a "simple stroker", in that all I changed was the crank, rods, and a Crane #753905 cam.
For those of you thinking you can lower your dynamic compression ratio using a bigger cam, realize that mine lost most of it's drivability below about 1100 rpm. Meaning at crawl speed idle, where the stock engine performed so well, it stalls out easily. Of course it runs like a flaming cat on the highway and gets the exact same mpg as before (measured over 5 tanks with a calculator). But I payed for the money I saved.

what octane fuel do you run and whats you altitude?

[oletshot]

Since this is in the basics section, I'm going to see if I'm understanding what's going on with dynamic compression.

Lets start with 4.6 stroker with a static compression ratio of 10:1.
The ratio is this high because the builder used stock aftermarket 4.0 pistons with the standard cast dish.
In order to run on pump gas the builder chooses a cam that will lower his dynamic compression to a more reasonable ratio, allowing to run pump gas.
This seems like a great solution, but....

If I understand what is happening here correctly, the builder has choosen a cam that leaves the intake vavle open longer on the compression stroker, thus lower his compression ratio (can compress gases with a vavle open). Hasn't, in laymen's terms, he just shortened his stroke by whatever distance the piston has moved towards TDC. Seems counter-productive, go thru all the work of lengthening stroke with the crank, only to shorten it with the cam to get it to run.
Dishing piston's more if your using 258 rods, or custom pistons (with custom dish) with 4.0 rods seems like a much more effective way of lowering compression.
I wonder how close to stock stroke people using their cam to adjust DCR are? I'm sure they still have more stroke than a stock 4.0, but how much more?

Let me know if I'm understanding this correctly, as I am a cam MORON.

[brewmenn]

It only effectively shortens the compression stroke. The power stroke remains the same.

[oletshot]

Oh yeah, forgot about other side of the stroke.
Kind of all wet with my example also, ran some numbers on KB's compression ratio calc.

stock 4.0 spec's came up as 8.8:1 SCR, 7.6:1 DCR, adjusted stroke was 2.875"

4.6 stroker with 2229 silvolites, 68-231-4 cam, 258 rods, came up 9.7SCR, 8.7 DCR, adjusted stroke was 3.488"

So even though stroke was lengthened by .484" with the crank, throw in the cam and the adjusted stroke comes out .613" longer.

[1bolt]

if you look at it that way it starts to make sense why strokers so often have ping problems. Not only do we have bleeding edge factory timing curve, and nearly always poor quench even if it's better than the poor factory numbers it's usually worse than ideal because the closest you can get to ideal is head gasket thickness. but I bet most strokers aren't done with zero deck height and the thinest head gasket... I'd bet very few of them are actually. Add in the extra "adjusted stroke" and it all starts to make sense intuitively.

It also starts to paint a clear picture of just where all that low torque comes from....

[Flash]

I've managed to put together a spreadsheet that will figure all the static numbers. What I'm still struggling with is understanding how to take the advertised cam specs and get the IVC point.

ive been using the andreas stroker calc. i think its from madXJ. an added column in there that uses the IVC of several of the most popular stroker cams to calc the DCR would be very useful. it wouldnt be hard if i knew the IVCs.

Do you have link to this calc???? Do's it figure DCR with a "@ .050" spec????????? I'm not liking the KB sites calc.

I don't thing it's vary accurite.....But then again, figuring valve close at .050" open is tough to be accurate!!!

Flash

[seanyb505]

http://www.madxj.com/

It's the second row under "engine." It does not have fields to incorporate cams, but Im sure they can easily be added.

[oletshot]

I figured if I used KB's calc for both of my calculations, both numbers would be equally inaccurate.

Since I wasn't looking for numbers that were set in stone, and all I really wanted was a comparison between the two engines, I thought that calc would be fine.

Plus, (and this is the real reason for using KB's calc) it was the only one I knew of, that gave adjusted stroke.

[Flash]

http://www.madxj.com/

It's the second row under "engine." It does not have fields to incorporate cams, but Im sure they can easily be added.

Thanks but i don't have ms excel, so i can't read it...........

Any body have DCR calc. that you can us like KB's?

There was one i found a while back but didn't save it to my favorites and can't seam to find it now


Flash

[Flash]

So I was crunching #'s on the KB link and I had a SCR of 10.28:1(stock pistons, decked .040, 58cc--just an option). I will be using the Mopar 4529230AB cam. I have not found the Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees to put in the formula. The advertised Intake Closes (ABDC) deg is 56. Now this cam has 40* of overlap, so should help bleed off some cylinder pressure (and lower DCR, right?). I used Dino's cam spec page @http://www.angelfire.com/my/fan/Jeep4.0Camshafts.htm. Anyhow Flash, I haven't attempted to find the formula to determine what the ABDC @ .050, so maybe you can help me out.

OK sence my DCR calc. does seat to seat duration spec's, and the MPP 230 IVC spec is 56, i did some number mixing until i got SCR of 10.28:1
the MPP 230 cam would make a DCR of 8.78:1........looks like your SCR need to go down, or you need a bigger cam
If you advance that cam 4*, you DCR would be 8.55:1 still to high i thing and you would loose even more bottom end torque!!!!!!!

Flash

[Shark]

i could add a cams row on the top half of the stroker calc to output to a row for DCR on the bottom half but im not sure if my linux version of excel is backwards compatible to ms excel. i can read and edit excel files but im not sure i can save in excel format to share.

did i miss how to get the IVC value from advertised cam specs?

flask you really need to get excel or another spreadsheet program if only for the stroker calc alone. very useful tool. luckily for me, most of the software for linux is free and is pretty well made. i havent used windows at home in over 2 years and i have never looked back.

[brewmenn]

Thanks but i don't have ms excel, so i can't read it...........

I use this... http://www.openoffice.org/ to read it.