Configuring Compression Ratio...

[Addicted_2_II]

Ok,
I just wanted to make sure what my Compression Ratio was so I did some inestigation and I am confused with the results?

My 302 V8 .040 over:
Bore 4.04"
Stroke 3.0"
Deck .045
Gasket .038
Heads 54cc
Piston Flatop

When I run it through this calculator I get: http://www.texastransams.com/articles/compression_calc_4.htm
9.72:1 compression - This sounded a bit low so I ran the stock setting for my MII and I got: 7.7:1 when it shows stock to be 8.4! Am I not doing something right?

Also when Curb Weight is listed, 2660lb is that dry weight?
Chad

 

[Blue Thunder]

The dyno program that I wrote calculates your 308 as having 9.2:1 CR, which sounds about right, considering that motor's really high .045" deck clearance. As calculated by the (partially completed, still in progress, but I'm pretty sure this part of it is functioning correctly) program, a deck height of .035" would give 9.5:1 CR, a deck height of .025" would give 9.8:1 CR, a deck height of .015" would give 10.1:1 CR, and a zero deck height would give you 10.7:1 CR.

As a reference, my motor has a 3.25" stroke, a 4.030" bore, yielding 331.6(332) cubic inches, has a zero deck height, milled TFS heads with 57CC chambers, .038" gaskets, and it calculates the compression ratio as 10.96

You need to cut down that deck height, (to zero, preferably), but only do so with aluminum heads, because 10.7:1 is more than iron heads and pump gas will tolerate. If you're running iron heads, keep 9.7:1 as your goal. (.025 deck height, in your case) Just be sure to check piston to valve clearances with the very low deck height.

 

[Addicted_2_II]

The dyno program that I wrote calculates your 308 as having 9.2:1 CR, which sounds about right, considering that motor's really high .045" deck clearance. As calculated by the (partially completed, still in progress, but I'm pretty sure this part of it is functioning correctly) program, a deck height of .035" would give 9.5:1 CR, a deck height of .025" would give 9.8:1 CR, a deck height of .015" would give 10.1:1 CR, and a zero deck height would give you 10.7:1 CR.

As a reference, my motor has a 3.25" stroke, a 4.030" bore, yielding 331.6(332) cubic inches, has a zero deck height, milled TFS heads with 57CC chambers, .038" gaskets, and it calculates the compression ratio as 10.96

You need to cut down that deck height, (to zero, preferably), but only do so with aluminum heads, because 10.7:1 is more than iron heads and pump gas will tolerate. If you're running iron heads, keep 9.7:1 as your goal. (.025 deck height, in your case) Just be sure to check piston to valve clearances with the very low deck height.

That is great info... I currently have Iron heads and not sure yet if I want to go with Aluminum or not! I should be able to reach my goal for this next coming year with these iron heads! But as you say, I might take them in to get them milled down a bit! The strange thing is that I get around 178 PSI when doing compression test on this engine and those numbers are closer to 10.5:1 right? I think when I did the original specs for this engine I forgot to add in Deck height and head gasket width! Dohhhhh

You know the 9.2:1 does explain a lot! I wondered why I was able to run regular gas with really no pinging I thought my exhaust was just too loud to hear it LOL

I am going to start taking pictures of my Winter rebuild and I will also list out each part used and where I got it from so others will know what works and what doesn't!

Thanks again BT,
Chad

 

[Turbo II]

Also, don't forget to factor in valve relief dish volume in your calculations. Some flat tops (especially the ones with four reliefs) have quite a bit of volume (eg. Speed-Pro's Hypereutectic Flat Top Pistons list a volume of 12 cc's)

I was shopping for pistons last night for my rebuild

Francis "Turbo II"

 

[IIxs]

To decrease deck height you will have to shave the block, not the heads. Also, there's a difference between static compression and dynamic compression. Dynamic compression takes the cam profile into account, which is probably the reason for your good compression numbers.

BTW, nice car.

 

[Addicted_2_II]

To decrease deck height you will have to shave the block, not the heads. Also, there's a difference between static compression and dynamic compression. Dynamic compression takes the cam profile into account, which is probably the reason for your good compression numbers.

BTW, nice car.

Ahh good point... I knew that but just mis-spoke!

Here is a illustration I found that others may find useful:

IIxs, thanks for your feedback and your comment on my car! I hope it drives better after I get all the suspension work done on it!!!

Chad

 

[Blue Thunder]

I'd put longer rods in that puppy, instead of decking the block. ha!
Actually, 289 rods are 5.155", whereas the 302 rods are 5.09", a difference of .065". Throw some 289 rods in there, order up some .058" thick gaskets, and you're good to go. A side benefit of running 289 rods besides the obviously superior rod:stroke ratio is also the heavier ribbed caps they come with.

 

[Blue Thunder]

To decrease deck height you will have to shave the block, not the heads. Also, there's a difference between static compression and dynamic compression. Dynamic compression takes the cam profile into account, which is probably the reason for your good compression numbers.

BTW, nice car.

Yes. I've found that the actual dynamic cranking compression pressure is fairly similar among engines of different ratios and cam sizes and often hides the true mechanical compression. If the stock camshaft is kept, I guess the cranking pressure will go up if there's an increase to mechanical compression ratio, but who tears down an engine, and JUST adds more compression to it? So it's hard to find such engines for testing purposes. My Mustang has a 10.95:1 CR, and cranks at 185 psi, but I'm sure the solid lifters and large cam are substantially dropping the pressure. My brother's factory original '69 Ford Custom ex-police car with a 429 P.I. had 11.25:1 CR, and cranked at about 205psi, if I recall. Due to the relatively small factory camshaft, no doubt.

 

[IIxs]

Back in the early 80's a guy I worked with built a Pontiac 455 and focused on fuel mileage (in a 455???) Anyway, he took alot of material off the heads and put in flat tops to bump up the compression. He bought a Crane Economizer cam (remember those) and put it in 4 degrees advanced. He brought it to me and said it wouldn't hardly crank when it was hot. I worked with it awhile and when we did a compression test it was over 300 lbs. cranking. We finally put in a bigger cam and retimed it, rather than tear the motor back down. We just fooled the motor into thinking it had less compression. It was a dog but at least it got him back and forth to work.

Does your brother still have his '69? I had a '69 XL with a 429 for a long time and it was a great car. Back then I was a smoker and I remember they picked a goofy spot for the ashtray in those cars but the hideaway headlites were cool.

 

[Blue Thunder]

Nah, he sold it after several tickets, and the car started getting a little rusty around the edges.
Some of the stunts he pulled with that car when he was young and stupid:
He installed flashing red lights behind the grill and left them on as he and his friend cruised to Florida and back at 110-120mph. (He went down mostly just to buy illegal fireworks and smuggle them back here to NY)
He was doing about 80mph on an unknown back road, suddenly came upon 4-way stop signs, he locks up all the tires and slides through the intersection. A cop happened to be sitting there, pulls him over, and what does my dumbass brother tell him? "But officer, all four wheels were technically stopped at the stop sign." Arrested.
He's flying down the wrong way on a one-way street. Cop pulls him over, and screams, "Do you not realize this is a one way street??" "Yes, officer, I was only going one way." Arrested.
He's doing one of his Southern fireworks runs, comes upon a long line of cars stopped on the highway, so he tries to pass them in the left lane, and there's a bunch of Georgia State Police standing around checking cars for an escaped felon. They pull him over, guns drawn and all, arrest him and his friend,(who was sleeping in the backseat at the time, and who woke up with a .357 magnum pointed in his face.) They almost got sentenced to a chain gang, but the small town judge asked for all the money they were carrying, and then let them go. He even got the car back.
The best one of all; He's cruising in the '69 Custom at 75 mph or so down the highway, when this dodge passenger car(diplomat I think?) comes up behind him, and rides on his bumper. So my brother gets annoyed and lays on the throttle, and goes up to 100. The dodge falls back, then catches up, again, right behind him. So he speeds up again to 120 or 130, the dodge drops back again, then slowly creeps up behind him. My brother finally stands right on the throttle, I think the special police calibrated speedometer went to 160, and he was getting up there, when suddenly, a red flashing light starts coming from the dodge.(which at this point, was a LONG ways back) It was an unmarked police car, huh. My brother knew he had his plate, so he pulls over and waits. The dodge comes rolling up, smoke pouring out of it, the motor knocking and rattling. The cop jumps out, slams the door, storms up to my brother, and starts screaming at him about how he just blew up his brand new patrol car. After awhile, the guy calmed down and was checking out my brother's ex-police car, looking under the hood and all. He turned out to be a really cool guy, he didnt even arrest my brother, just gave him a ticket for "120+" since that's as fast as the cop's car would go. The judge really wasnt amused, though.

Alright, back to the topic, sry. hah.