If an engine has an 8:1 compression ratio and you want to increase stroke by .250, all else staying the same where will the compression ratio go to? My computations say not to much increase, like only to about 8.7:1. I don't think thats right?
I was thinking of a 383 va a 350. At near zero deck with flat top pistons and 72 cc heads, 350 (well .030 over) was around 9.5-1, 383 was around 10.7 as I recall. Only change was basically .252 stroke increase.
Need all the numbers (bore, stroke, piston dome or relief, head camber size, head gasket volume) other wise we’re all guessing.
"increase the stroke by .250.. all else staying the same.. " Ha ... there won't be any compression.. as the heads won't bolt on. Increasing stroke requires a reduction in rod length or piston pin height to compensate for the increased stroke. We're******** in the wind without some more information.
I threw some numbers together for a flat top piston 350, zero deck, .042 gasket and 68 cc heads and it came out to 10.3-1. With that 1/4” stroke increase and everything remaining the same it came out to 10.96. @squirrel is right on the money.
You can approximate the increase by making a couple of****umptions about the volumes required to calculate compression ratio. 1st:****ume the volume at tdc is constant, i.e. pistons have the same dome, the combustion chamber is unchanged and either the rod length or the compression height is changed to get the same volume when the pistons are at tdc. 2nd: let's use a 302 and a 327 Chevy as examples. The swept volume of the 302 is 37.7 in^3 and the swept volume of the 327 is 40.8 in^3 per cylinder. We're going to****ume that the piston is sealed right at the top of the piston so no crevice volumes to get in the way of ease of calculations. Finally compression ratio is the volume at tdc divided by volume at bdc. Volume at bdc is the swept volume of the cylinder and the volume at tdc. Or Vtdc/(Vbdc+Vtdc)=cr We can write in the cr as 1/8 giving Vtdc/(Vbdc+Vtdc)=1/8 rearranging gives Vtdc= 1/8(Vbdc+Vtdc)=1/8Vbdc+1/8Vtdc Collecting terms Vtdc-1/8Vtdc=1/8Vbdc And 7/8Vtdc=1/8Vbdc Or 7Vtdc=Vbdc We have Vbdc=37.7 for the swept volume of a 302. Therefore Vtdc=37.7/7=5.386 in^3 Now using that in the original equation and the swept volume of a 327 cylinder we get 5.386/(40.8+5.386)=0.117 Which is: cr=8.57:1 So just a little bit over a half point increase in the compression ratio. APPROXIMATELY And I really nerded out there. Sorry, the engineer in me escaped tonight... I promise not to do it again. Well, at least not for a while... Yup you guys got there a lot faster than me...
You can feel free to play with this. https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator I figure about .5-.6 on a basic 350.
Sure, you computer literate jocks make it look easy... I used pencil and paper. Old school. I did use a calculator and not a slide rule, but I still have one... Okay call me a troglodyte! I can take it!
welcome to the modern age. When I took geometry in high school in the mid 70s, our teacher showed us how to use a slide rule....and said don't worry, you'll never have to use one, because calculators exist now. I think the same thing applies to anything that has an equation now, you can find a bunch of online calculators that will replace all the required math
Dave do you still remember how to use your slide rule? I took mine out of the desk drawer a few month ago thought about playing with it, put it back in drawer and haven’t thought about until now. Ingenious devices! Dan
As an engineer I like to do math like this by hand on occasion, probably to prove to myself I still can. But the calculators make it super easy to compare.
As a matter of fact, yes, my brother and I had one as kids. We had an aunt who was a missionary nun/doctor who was stationed in Taiwan. She sent us one for Christmas when I was about 4. Never did get the hang of it.
1/2 stroke= .125. PI r squared times .125 and divided by the metric conversion to cc's. Add that to the current deck volume at tdc and you'll have a very close number. Current cubic inches divided by 8. One cylinder volume divided by 8 should give volume at tdc. That would be close.
As mentioned, if you increase the stroke .250, you will need to change crank, pistons, rods, or all three. No engine has the piston .250 down in the hole at TDC. So if the the only change is stroke, it will push the piston above the deck at TDC. Adding .250 to the crank stroke, requires figuring out the pin height on the pistons to be used, the rod length, and how far in the hole that will leave the piston at TDC. The size of the bore also affects how much compression gain you will get by adding the .250 to the stroke. In other words, it is very doubtful that the only thing that will change is the stroke. Once you have all the numbers there are a multitude of compression calculators online. Plug them in and see where you end up. If you want to do the math by hand you can do that as well. Or use a slide rule, calculator or somewhere in between they even made manual and electric adding machines (not to be confused with a calculator)
UEM Pistons has a good online CR calculator for both static and dynamic, but you'll need to know your numbers; guessing won't get the job done. If calculating DCR, and you have a solid cam, don't use intake closing @ .050 + 15 degrees. Use intake closing at .020 (advertised for solid cam).
Thanks to the guys that read my post and answered the simple and not gone of the edge. Depending on bore and stroke in the combinations I tried it came out less than 1.0 increase. I had expected more but it suits my initial combination thoughts. Thanks to those that ciphered.
Some of those 1970's 400 and 440 Mopar B and RB came darn close though. Bet you could do it with a slight trim to the dish top. LOL Old John Deere B 2-hole thumper could for sure.
The Jalopy Journal
