Thread: compression/HP change calculation

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Does anyone know of a calculation or formula that determines HP change per compression ratio change? The reason I ask is that I just purchased a set of .060 pistons,(motor is a .030 at present) and the new piston has a compression height of .026 'less' than the original.Maybe the bigger bore will make up for this but since I am using the same heads and gaskets I have become curious about how much difference this actually makes.They are the pistons I wanted and I hope this doesn't sound like a dum question.

Thanks Denny.

Thanks .I have found that being .026 farther down in the hole may not take but a few HP away and the .030 increase in bore should equal or better that.Especially if it adds around a quarter point comp. increase.Was just one of those things that had me bugged with curiosity.

There's something else you should take into consideration and that is that you are making the squish worse by placing the piston further from the bottom of the head surface at TDC. Increased squish = increased tendency to detonate. You may want to include a shave and a haircut on the block deck.

Thanks guys. I am considering decking but hate to pay the cost for less than 10hp.What I have is a silvolite piston with 1.779 compression height+1.50 which is half the stroke,(3.00" stroke 283 motor),+5.7 rod length= 8.979. Subtract that from sbc 9.020 deck height and I come up with .041 in the hole. Using the compression calculator I get about 8.821:1 static,and 8.086:1 dynamic.Thats using a .016 gasket that Denny suggested with 58cc heads
This is my first attempt at figuring CR per engine spec's so I may be missing something.
Other spec's are piston head relief 6cc;gasket bore 4";cyl.bore 3.935";deck clearance .041;stroke 3.00";rod length 5.7";cam intake valve closing point 41 deg.ABDC.
So .041 deck clearance+.016 gasket=.057 yuck!

I don't know how you figured the DCR, but I come up with 7.586:1 by using your 8.979 SCR and figuring your cam intake closing point at 41 ABDC @0.050" lift. If that is the case, you're gonna be a little light. A good, crisp street motor should be between 8.00 and 8.25 on the DCR.

OK,TECH.the shave and a hair cut it is! Can't figure where I goofed. I used the calculator on the website of the piston mfr. and entered all the above info.,unless I was wrong by entering 6cc for piston head volume,all the other numbers sholud be correct. Do you think I could return the pistons and tell them my wife bought them and their the wrong color?

>Does anyone know of a calculation or formula that determines HP change per compression ratio change?

Although I can't cite a specific source to document it, I've read that generally, a one point change in compression is worth about 4% plus or minus. Again, that's a generalization and subject to quite a few variables.

Bob

bobscogin, the google search that Denny posted above may be what your looking for.

shevy, don't send anything back yet. Let's go back and re-figure. I'll have to do it later, have an appointment right now (Thurs 7:30 AM now). What is the combustion chamber volume in your heads? Have you mocked up a piston less rings on each corner of the block and verified the deck height? Or are you flying blind with published figures?

>bobscogin, the google search that Denny posted above may be what your looking for.

Yup, that may well have been it! I entered 100 hp into the equation, bumped the compression ratio from 8.0:1 to 9.0:1 and presto! -- 104 hp.

Bob

Blind figures right now Techinspector1,the pistons are in the mail to me so I,like other hotrodders like to set around and 2nd guess myself.When guys are building hotrods we are like women window shopping or catalog shopping you ever notice that?
bobscogin,,,that was my point earlier,alot of money for a few horsepower,just to increase one point.
Denny,,,if I could find a file big enough and straight enough I would.Seriously I probably should have the block decked .020 or more and go with a 'safer' head gasket.
Less than 9:1 compression with 58cc combustion chambers and a .016 gasket seems should have got a caddy motor or a hotrod lincoln.

Thanks Denny and I agree because all I was hoping for was around 280+HP.
But I am contemplating doing more to the top-end of the motor next year,different cam and heads and may wish I had got more compression while I had it tore down.But you may have a point there because it's going in a little s10 truck and with the right converter, gears and traction may be as quick as I'd like.280+ HP shouldn't be asking to much from a .060 over 283?

shevy, 280 hp from a 283 is, in my opinion, very ambitious. At the pinnacle of their development, 283's made 270 hp with 2-4 bbls and 283 hp with Rochester fuel injection. This was done with 11.0:1 SCR and a very large cam.

Don't worry about hp right now. Just get the pistons in your hands, take the block and pistons to the machine shop, have the block bored and honed, rods resized and piston hung on rods. Then put rod/piston assemblies (without rings) into cylinders 1,2,7 and 8. Using a dial indicator to find TDC and a depth gauge to check deck, find out what the deck height is on each corner. Gauge each piston at the 3 O'clock position, not at noon or 6 PM as you're standing at the side of the block. Write down each measurement, then publish all the info on here and let all us gearheads get our heads together as to the best way to proceed at that point.

IF YOU HAVE NOT PURCHASED THE CAM YET, DO NOT. IT SHOULD BE THE LAST THING YOU BUY FOR THE MOTOR AND WILL SET YOUR DCR.

Thanks Richard. I'll look this post up and post the measurements I get,but It will probably be sometime in July.