I just got my new motor together and was messing around on some compression ratio calculators and keep getting a higher number then what my engine builder says it is.

Engine Specs:
383
4.0315 bore
3.75 stroke
64 cc heads(actually measure 64.1 cc's0
5 cc flat top SRP 4 valve relief pistons
deck height is 9.011
Head gasket bore is 4.175 and .041 thickness (FelPro 1003)
Top of piston is 3.990
Ring land is .203

When I enter it in a calculator it doesnt ask me for the cc of the head gasket (9.1), and that is 81.27 cc vlave relief so I keep coming up with 10.7 to 1 and my engine swears it is 9.657 to 1 compression ratio. What am I doing wrong?

Also If somebody could dyno this motor that would be great, Specs on rest of motor:

Hyd roller cam 234/242 @ 050 535/555 lift 112ls
dart pro 1 heads 200cc, 64cc aluminum heads
Performer RPM, 1 5/8" headers, 750 DP

I think this is it. Any more let me know.

Thanks, Erik

I need deck height (how far down in the hole). But by assuming that figure it's coming out at 10.72:1

Jody

Erik,

I think the difference in the compression between what your builder is telling you and what you are getting is that he has the "piston set down in the hole". If I run your numbers assuming a 0.011 deck I am geeting just under 10.7 static CR. If this is right, then setting the piston "down in the hole" by .045 for a total of 0.056 top of piston to deck height I get a CR of 9.65 to 1.

I am no expert on this by any means, but I have been doing a lot of playing with these kinds of calcs lately as I am trying to designa 383 for my 67.

Hope this helps,

Victor

Camcojb,

You beat me to it. Every time I start to write a reply on one of these things the phone rings. graemlins/clonk.gif

Victor

With flat tops you will be closer to 11:1. I run 5cc SRP (2 relief flat top pistons) 64cc heads, a .008" deck height and a .026" thick by 4.080" diameter head gasket. My compression comes out to 11.3:1.

My engine guy is the smartest smallblock guy I know. He gave me a sheet of paper with all these measurements on it, that include even the space between cylinder bore and the piston in cc's. I talked to him twice on the phone and he did the math and comes up everytime with 9.657 compression ratio. Maybe he is factoring in something more then the norm. He measured everything out. I keep coming up with 10.7 to 1 as well. If I am reading this right, my total valve relief is 81.27. I guess this includes the negative space of the gasket opening as well. I dont know. I cant help but trust him. When you get down and do all the math with a mic is there like a more advanced way to measure it all?


What about the horsepower numbers, any ideas??

Thanks, Erik

The numbers you are giving us don't add up to 10.7:1 compression.

What is the deck height? You said in your 1st post "Top of piston is 3.990" -- that doesn't make any sense. Do you mean that the top of the piston is 8.990"? If it is that would leave your piston about .035" below the deck and your compression ratio very well may in fact be 10.7:1.

The problem with that scenario is that with a .041" thick head gasket and a piston .035 in the hole your quench height is aweful at .076". You really want to have a quench height right about .040" total. The cure is to have your machinist zero deck your block.

I think the 3.99" figure is the piston diameter; that minus the bore gives just over .004" piston to bore. Those figures come up to 10.72:1 on my calculator, except I do not know how far in the hole the piston is. Where did you see it's got a .035" deck height?

If it's "0" deck it's 11:1 compression. Even at .040" down it's 10:1 so if your figures are correct your builder is wrong. Here's a good calculator I use, might have to sign up for the site to use it though.

http://www.smokemup.com/auto_math/compression_ratio.php

Jody

Jody, I figured 8.990" piston height minus standard 9.025" deck = -.035" deck height. But that's just a guess at what the 3.990 was.

. . . and wouldn't a 3.990" diameter piston and a 4.0135" bore make for .0115" piston to wall clearance? That would be pretty loose . . .

I dunno . . . graemlins/clonk.gif

Yeah, I think he mis-typed 3.99 for 8.99. But then 9.011" minus 8.99" = .021" in the hole, not .035". That still makes it over 10:1 compression.

I should quit doing my math in my head; used to be pretty good at it!

Jody

I believe the 3.990 number is the diameter of the top ring land, as the bore is listed as 4.0315, so a piston skirt diameter of 3.990 would equal piston /wall clearance of .0415. Using Pat Kelley's software, and using a deck clearance of .011(spec deck measurement of 9.011, standard of 0 deck being 9.000. stock industry standard spec. of 9.025), I come up with 10.73:1, using all other specifications as listed. I don't know how the builder could have arrived at 9.65:1, unless the pistons are way down in the hole, and I doubt that SRP would make a piston that would give that much deck clearance, unless they were made for a 3.850 stroke engine. Not a likely situation.

I'm thinking as Eric pointed out ahbove that .041" is way too much piston clearance. I think he typoed his 3.99 number.

Jody

3.990" is probably the diameter of the top ring land of a 4.030 bore piston. Maybe the builder was trying to calculate the crevice volume around the top ring land to add into his c/r calculations, as the specs posted also gave a .203" ring land thickness figure. Most c/r calculations don't use this volume as it changes with piston top temp. That is one of the reasons the ring lands are smaller diameter, so they won't scuff when engine is under power.
Maybe a mistake in this calculation is where he arrived at the 9.65:1 number, that or as I mentioned before, he has 3.850 stroke pistons in a 3.75 stroke motor, giving him an extra .050" deck clearance.

I think Blue 69 figured it out . . . that makes sense.

Yea, youre "around" 11:1
My 383 has 67cc chambers
SRP flat tops
5 in the hole
39 gasket
and its around 10.8:1

My engine builder is right on his calcualtions, I talked to him 3 times now and he is dead on everytime. I am just not sure how he gets it compared to all the internet calculators out there. He is taking it a step further then the rest of the calculators do. He is calculating all the area in the head chamber as well as the head gasket and the area betwwen the piston and the walls. This equals 81.27.

My engine calculations are as follows:

383
9.015 deck height
.011 piston in the hole
.041 head gasket (felpr 1003)
cylinder bore is 4.0315
piston diamater is 4.026"
top ring land is 3.999"
bwteen piston and cylinder wall is .203
64 cc heads
-5cc 4 vlave relief SRP pistons

Like I said before, this guy is amazing when it comes to small blocks and trust him 100%. there is just something I cant figure out when it comes to doing these calculations. I know he is taking it a step further then the calculators do which in my mind make it more accurate.

Thanks, Erik

Sorry Erik, but if your figures are accurate he's wrong. There is not enough "little areas" not figured in these calculations to make over one full point of compression difference. They do figure the gap to the top ring, the piston to bore gap, etc. I mean you drop the piston .020" further in the hole which is a big amount and the compression ratio only changes a couple of tenths of a point. One full point difference is huge.

At any rate, does it really matter? No blower on this engine right?


Bore
(Inches)
Stroke
(Inches)
Connecting Rod Length
(Inches)
Cylinder Head Volume
(cc)
Deck Height
(Inches)
Head Gasket Bore
(Inches)
Head Gasket Thickness
(Inches)
Piston to Cylinder Wall Clearance
(Inches)
Top Ring Land Height
(Inches)
Piston Dome Volume
(cc)
Negative number is dished piston. Positive number is a domed piston.



You Entered:
Bore 4.0315 (inches)
Stroke 3.75 (inches)
Rod Length 5.7 (inches)
Cyl. Head Vol. 64.1 (cc)
Deck Height 0.011 (inches)
Head Gasket Bore 4.175 (inches)
Head Gasket Thickness 0.041 (inches)
Piston to Cyl. Wall Clearance 0.0055 (inches)
Top Ring Land Height 0.203 (inches)
Piston Dome Vol. -5.0 (cc)


Compression Ratio:
10.722 : 1

Bore / Stroke Ratio:
1.075 : 1

Rod / Stroke Ratio:
1.52 : 1

Total volume:
865.275 cc's

Compressed volume:
80.702 cc's


Jody

Even when taking the crevice volume into consideration (which I calc out to be only .9 cc)I still get 10.64:1.

Maybe your machinist is mis-speaking and really means 10.6:1 instead of 9.6:1 ?????

When I told you your around 11:1, I mean any one thats built a 383 and is familiar with them can tell you of the top of their head that:
64cc chamber
flat top piston
39+/- gasket
will put you right around 11:1
Thats without doing any calculations.
Like everyone said your #s look ok, its your machinist calcs that are wrong, not to discredit him - because everyone makes mistakes. But on the other hand he should know, like most of us here that have done 383s, that with the 3 items I listed above the comp will be around 11:1+/-

To add some real world experience, my 383 is 10.8:1 and will detonate with 37deg of timing on 93 octane, thats with TFS alum heads, polished chambers. Anotherwords - I'm at 10.8:1 for sure, and thats with 67cc chambers, go to 64 and I'd be right around 11:1
Hope that helps

Camcojb: My motor is a 6" rod motor, not a 5.7" rod motor and also doesn't 865.275cc's makes a 427 (7 liters) smallblock not a 383. My 383 is 784.8cc total. This is all based off what my engine guy measured. My compressed volume is 81.27cc.

64 cc heads
piston .011 in the hole = 2.30 cc's
piston to ring land = .87cc's
5cc valve relief on piston
head gasket = 9.1cc cc's

I dont know but I have to take his word for it. He says he tried three different calculators online and everyone gave him anywhere from 8 to 1 to 10.4 to 1 compression. So he believes in doing things the mathmatical way. I wish i new what it was, but I am sure I probably wouldn't understand it. I am no engineer by any means. I know alot of these calculators only ask for the basics and leave out alot of info that you need. I do jknow alot of people think they automatically have a 10 to 1 or 11 to 1 motor and they really don't.

I think its time to put this baby to rest and talk about something else.

Later, Erik

Out of all the calculators online this one asks for the right info and it comes in under 9.66.

http://www.classicar.com/garage/shopmathcr1.asp

No body brought up horsepower numbers yet!!!

Thanks, Erik

Erik,

Rod length doesn't change the static compression ratio, that's why I didn't ask. By the way I just plugged your numbers into the calculator you linked to and came up with 10.7:1 compression. You are correct, having 10:1 pistons doesn't mean you'll have anywhere near that amount of compression in the real world. The calculator I'm using matches exactly with what Arias, CP pistons, and Ross calculate also. I'd assume they know since they build custom pistons. I can change your head cc by 1/2 a cc or so and come up with the exact same total volume he claims; the compression still comes in at 10.6:1 or so.

We'll all agree to disagree with your builder on this one. As far as HP I don't have access (and do not use) any of the calculators out there; I go by dyno numbers and track times. I'm sure someone out there will help you with an estimate.

Jody

Erik
I dont think that 5 of us (one being the moderator) are trying to bs you in any way. But its 5 to 1 on this, so take our word for it, you comp is right around 10.7-10.8. We're all here trying to help you out. If I were you talk with another engine builder/machinist to get a second opinion. And I dont know if I'd trust any "internet" calculators, as some of them say my 502 is makin 750hp, which is total bs, since it dynoed at 626, and my Moroso slide ruler says it takes 610hp to push 3300lbs to 133mph in the 1/4. So dont believe all that internet stuff, because it gets twisted up by peaple who dont know what they're doing or wanna make them selves look good with big #s.
Where is "CNC blocks" when you need him (a machine shop guy)
Get a 2nd opinion - is my only advise if you dont trust all 5 of us. graemlins/waving.gif

Are you sure you're figuring with 64 cc heads, and not 74? My 383 with 73 cc heads, same gasket, similar clearances, and 2 V.R. pistons, comes in just a little over 10:1 CR.

The 865.275 cc that Camcojb refers to is the total of swept and unswept volume, not engine displacement. 865.275cc(sv+usv)/80.702cc(usv)=10.72:1 cr.
Something in your engine builders math is messed up, the numbers just don't work out to what he is saying.
FWIW I am an automotive machinist with 20 years experience, ASE certfied Master Engine Machinist for over 10 years. (No that doesn't mean I know it all, but I do have a clue) ;)

Eric, have you calculated the DCR on the engine? With the aluminum heads and the cam selected 10.7 is probably a better static compression ratio make decent HP on pump gas.

Originally posted by Blue69:
FWIW I am an automotive machinist with 20 years experience, ASE certfied Master Engine Machinist for over 10 years. There's your 2nd opinion ;)
So dont worry about it, just stick it in the car and enjoy graemlins/thumbsup.gif

Thanks for everybodys input on this deal. I trust my engine guy completely and somehow, somewhere the math is just adding up. I plan to install this motor and hopefully make some low 12 second passes in my 68 Camaro. I just thought it would cool to have every spec on this motor written out somewhere. I would love to dyno it someday.

Thanks again for all your help and wish me luck on the install.

Erik

So, I just got off the phone with the local engine shop that builds all the race cars around here. This is the shop everybody in Ohio goes to, Fowlers Engines. They do all the machine work on my blocks as well as my engine guys blocks.

Anyways, it looks like we made a simple math mistake. We just divided swept volume by unswept volume and came up with the compression ratio. We didnt add the two together then divide the number.

784.7cc + 81.27cc / 81.27 = 10.65 Cr

I want to thank everybody again and sorry for math mistakes. I knew the guy s on this website would help me out.

Thanks again, Erik