Thread: Techinspector need your help with compression ratio!

Tweet

I have a 402 BBC. I'm having trouble finding my compression ratio. Thanks for any help. Bore: 4.125/ Stroke:3.76/ HeadChamberCC: 96.4cc's/ Piston Dome: 19cc dome/ Headgasket: Felpro .038" compressed, do I need to know gasket bore size? The block looked like it had been decked before, but I didn't measure the deck height when I had the heads off. I'm not sure how much difference it would make. How much deck height would there be if it wasn't decked at all? Anyways, I'm coming up with about 10-1 compression, but I thought some expertise might help me narrow it down. Thanks.

the fel-pro head gasket are 9.7 if you have the #f1027 that is a 4.370 bore you could be down in the hole as much as .025 get on KB pistons they have a tech list that will help you work it out the cr

I believe the part # was 8081PT2. Permatorque head gasket.

you need to get the size of the fire ring or run piston up to tdc grease up the piston and the head gasket and put on head tilt the engine on engine stand and fill thru spark plug hole and measure the cc this will tell want you really have. make sure clean off grease. them old 396 with the tin head gaskets were hi 9s to 10 with the small head chambers

with a 10cc heads gasket and your pistons and i the hole a bit i get 87cc?? could be not much better then a good shot at it but 91cc is 10-1 and 87cc is about 10.5 hard to say were the piston is in the hole you need to find this out and the size of the headgasket to get the cc

PART NUMBER: 396245 GRIND NUMBER: z-45
ENGINE INFO: CHEVY BIG BLOCK V8 396-402-427-454 CU.IN. APPLICATION:
SPECIAL REMARKS: SOLID PRODUCT USE: STREET

RPM RANGE: 2500-6500
Valve lift is calculated with zero lash and rocker ratio of 1.75
INTAKE TIMING DURATION: 278 deg.
OPEN: 31 BEFORE TDC
CLOSE: 67 AFTER BDC
CAM LIFT: .303"
VALVE LIFT: .530"
VALVE LASH: .018 HOT
.02 COLD
EXHAUST TIMING DURATION: 278 deg.
OPEN: 67 BEFORE BDC
CLOSE: 31 AFTER TDC
CAM LIFT: .303"
VALVE LIFT: .530"
VALVE LASH: .018 HOT
.02 COLD


LOBE CENTER: 108 deg. OVERLAP: 62 deg. CAM ADVANCE: 0 deg.
THE ABOVE TIMING IS CHECKED AT .020 (OPEN) LIFTER RISE
.020 (CLOSE)

The following timing is taken at .050" lifter rise and gives a more accurate determination of camshaft position.
INTAKE DURATION: 240 deg.
OPEN: 12 BEFORE TDC
CLOSE: 48 AFTER BDC
EXHAUST DURATION: 240 deg.
OPEN: 48 BEFORE BDC
CLOSE: 12 AFTER TDC


RECOMMENDED ISKY CAM KIT PARTS AND SETUP INFORMATION
LIFTERS: 202-H LIFTER TYPE: SOLID
SPRING: 8005-A TYPE: dual w/damper DIA.: 1.530" RATE: 400 lbs per inch
INSTALLED HEIGHT: 1.875" SEAT PRESSURE: 135 lbs OPEN PRESSURE: 395 lbs
COIL BIND: 1.160"
SPRING SHIMS: # 2
STEEL RETAINERS: 347-ST
TITANIUM RETAINERS: 91-TI
PUSHRODS: 203-96 TYPE: heat treated non-adj LENGTH: int - 8.281" / Exh - 9.250" DIA.: 3/8"
ROCKER ARMS: 204-96 TYPE: roller RATIO: 1.72
VALVE LOCKS: VL-3/8
VALVE LOCKS: VL-3/8
REV KIT:

This is the camshaft I have in there right now. And I found the piston I have on the KB website. It is actually a 17cc dome piston, .180 dome. Here is what I came up with, does it look right? This was calculated on the KB website with 48 degrees After BDC @ .050 lift. I used a deck height of .020 above the piston.
Static Compression: 9.847
Dynamic Compression: 8.762

If I use a zero deck...

Static compression: 10.283
Dynamic Compression: 9.145

Is this a good Dynamic ratio? For pump gas?

Originally Posted by pat mccarthy

with a 10cc heads gasket and your pistons and i the hole a bit i get 87cc?? could be not much better then a good shot at it but 91cc is 10-1 and 87cc is about 10.5 hard to say were the piston is in the hole you need to find this out and the size of the headgasket to get the cc

Well, when I assembled the engine I remember the piston being very close to the deck, almost dead even, if that matters. I'm sure you couldn't really tell a difference between .040" and .020" just by sight.

Originally Posted by 69elko

Well, when I assembled the engine I remember the piston being very close to the deck, almost dead even, if that matters. I'm sure you couldn't really tell a difference between .040" and .020" just by sight.

well i can but no matter . yes it will,, is it all together? this can be check very easly with a set of feeler s and a steel rule

Well I'm going to guess I have .010"-.020" of deck above the piston. The piston was almost even with the deck surface. It couldn't have been any more than that. How do you check it with the cylinder heads on?

I think I may have made a mistake on the KB calculator. It looks like they want you to take After BDC and add 15 degrees to it. So 48 After BDC + 15 is 63. This changes the dynamic compression completely... This is worst case scenario with .020" deck.

Static Compression: 9.847
Dynamic Compression: 7.981

Is this right? Could the dynamic compression be that low?

If this is correct, than I will take the heads off and check the deck height to make sure, and if it is .020" deck than I will go with a .015" head gasket to solve the quench issue and hopefully raise my dynamic compression up to a reasonable number.

With a deck of .020" and head gasket of .015" I get...
Static Compression: 10.419
Dynamic Compression: 8.432

Is this good enough?

well it could be with lsa at 108 i would have to go over all your parts with heads on it would be very hard if you could tilt the engine fill it up on TDC and cc it like i said

Won't the oil leak through the rings and give an innacurate reading? I have the ring gaps set big, and the heads are on so i can't put grease on the pistons.

Does anybody know about the whole Static and Dynamic compression ratios? Denny? Techinspector? I just need to know if my engine will run like crapola if the dynamic compression is at 7.981.

According to David Vizard:

Compression Pressure
About now there are going to be some of you who are wondering whether the engine you have just built and installed has enough compression for the cam you chose. Assuming your engine has good ring and valve seal, a simple way to determine whether or not this is the case is to check cylinder compression pressures. With the ring package and bore prep procedure I use, my own engines are almost always near zero leakage and we will look how to achieve that later. If the cylinders are sealing up well, I look for 190 psi as a lower limit with preferably 200 psi as a target when using 93-octane fuel. For every octane number less than 93, the compression pressure needs to be about 5 psi less to avoid detonation under normal circumstances.

http://www.popularhotrodding.com/tec...wer_gains.html

Wow, that Merc engine still skips by. My compression it 175 lbs. I completed that motor in 1974.

Denny - what does that mean?
I'm not real good at reading between the smilie faces.

I only meant to offer another method of testing a motors potential.
Granted its dependent on other systems, like valve and ring seal, but certainly a function of dynamic compression ratio on a properly built engine.

But then - I'm still on a steep learning curve.
Tell me what I'm missing here . . .

These guys have forgot more than I'LL ever know but don't try to figure CR without an actual measurement. A 'dial on a bridge' is the most acurate practical way to measure how far the piston is up but at least use the filler gage as suggested or buy a dial caliper if you can and make several checks around the piston should get you close.
If you have 8.4 DCR you are in good shape with hi octane and DCR is actually considered a measurement of detonation resistance. As suggested,do a cylinder pressure check.
Some guys are running well over 220 psi but if you follow the suggestion Denny gave you should be safe,,,,considering this is a street driven vehicle.
And as stated,cam intake closing point does have an effect on how the motor will tolerate compression ratios because it doesn't start untill the valve closes.

Originally Posted by DennyW

This is what Pat was talking about, at least that's what I thought he meant on the cam pick he mentioned.

Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine.

Two important points to remember:

The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine


Why it matters: A 355 engine with a 9:1 static CR using a 252 cam (110 LSA, 106 ICL) has an intake closing point of 52º ABDC and produces a running CR (DCR) of 7.93. The same 9:1 355 engine with a 292 cam (having an intake closing point of 72º ABDC) has a DCR of 6.87, over a full ratio lower. It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91 or better octane. The larger cam's DCR falls outside this range. It would have markedly less torque at lower RPM primarily due to low cylinder pressures, and a substantial amount of reversion back into the intake track. Higher RPM power would be down also since the engine would not be able to fully utilize the extra A/F mixture provided by the ramming effect of the late intake closing. To bring the 292 cam's DCR up to the 7.5 to 8.5:1 desirable for a street engine, the static CR needs to be raised to around 10:1 to 11.25:1. Race engines, using high octane race gas, can tolerate higher DCR's with 8.8:1 to 9:1 a good DCR to shoot for. The static CR needed to reach 9:1 DCR, for the 292 cam mentioned above, is around 12:1.
This lowering of the compression ratio, due to the late closing of the intake valve, is the primary reason cam manufactures specify a higher static compression ratio for their larger cams: to get the running or dynamic CR into the proper range.

ps: If you rotate the engine on a stand, if that's where it's at, you can rotate it enough, that the piston is on the upside of the engine tilt, and oil will not leak past the rings fast enough to matter.

Ok so basically I am good with 7.98 dynamic compression, since it is in the middle of the range of 7.5 to 8.5? That is what I was getting at, because I recall a post saying that you want to be as close to 8.5 to 1 as possible. But I wasn't sure if my motor would run like a dog with 7.98. That is what I don't want. So this means I will most likely be able to run on 91 pump gas? I might just take my heads off after break in and check the deck height anyways, because when I built the motor I did not know about squish clearances. If my engine has .020" deck and a .038" gasket= .058", that means I'm quite a bit out of the range of .033"-.045"? What kind of worried me was that I thought I was going to have around 10.5+ compression with this setup, and the cam requires at least 10.5. When I figured out I might have less than 10-1, it kind of worried me.