Compression

[swampbuggy]

Here is a way to check static compression that i believe would be absolute. Lightly grease the top ring to ensure no liquid would pass by it. With the piston at B.D.C. and the spark plug hole straight up (facing ceiling) fill with a light (thin) oil such as 3 in 1, record the C.C.'s amount to the bottom of the spark plug hole. Drain oil over night. Next day bring piston to T.D.C. and fill to same level (bottom of spark plug hole). Due the division and there you have it, no wondering. Comments please. Mark H.

[af2]

Here is a way to check static compression that i believe would be absolute. Lightly grease the top ring to ensure no liquid would pass by it. With the piston at B.D.C. and the spark plug hole straight up (facing ceiling) fill with a light (thin) oil such as 3 in 1, record the C.C.'s amount to the bottom of the spark plug hole. Drain oil over night. Next day bring piston to T.D.C. and fill to same level (bottom of spark plug hole). Due the division and there you have it, no wondering. Comments please. Mark H.

[af2]

Larry I got lost and still come up with 17.what ever compression?

[MadBill]

Here is a way to check static compression that i believe would be absolute. Lightly grease the top ring to ensure no liquid would pass by it. With the piston at B.D.C. and the spark plug hole straight up (facing ceiling) fill with a light (thin) oil such as 3 in 1, record the C.C.'s amount to the bottom of the spark plug hole. Drain oil over night. Next day bring piston to T.D.C. and fill to same level (bottom of spark plug hole). Due the division and there you have it, no wondering. Comments please. Mark H.

The BDC operation is redundant if the bore and stroke are known, as they are 99.9% of the time.

Also, "spark plug hole straight up" can/should/could include flopping the engine this way and that in two planes while watching down the plug hole for air bubbles to appear, in order to ensure no air pockets are retained.

[swampbuggy]

Good day sir (Bill), have you figured your squeeze area this way before ? If so, did it show any difference than calculating on calculator/paper ? Thanks Mark H.

[F-BIRD'88]

17.94.1 cr

[MadBill]

Good day sir (Bill), have you figured your squeeze area this way before ? If so, did it show any difference than calculating on calculator/paper ? Thanks Mark H.

Used this method many times and it usually works well; sometimes reads a little high due to trapped air, but that's safer than reading low. Also, after getting the correct fill, I let it sit for a while to see if there's any seepage and then correct accordingly. (e.g. 0.5 cc/ in 5 min = 0.1cc/min., thus subtract 0.2 cc from the reading if it takes 2 min. to fill the chamber. I also keep the liquid a little on the thick side, say 50-50 ATF and solvent, to reduce such leakage.

[Protech Racing]

Measuring the CC at BDC takes into account/measures the piston dome. It is the exact way to measure compression.
measure again at TDC and you have it . I use ATF for fluid .
You can measure the dome with the piston down the hole also, but actual BDC CC is closer .and the math is easier . Takes a lot of fluid tho.

[6.50camaro]

I come up with 15.01 to 1. if the volume at .300 in the hole is 70cc the valve notches are 4cc larger then the dome .
825.2 cc = volume of cyl at bdc
+58.88 cc swept volume of chamber 46cc + h g volume 6.68cc + 2.2 cc piston .010 piston in hole +4cc valve notch
884.08
884.08 divided by 58.88=15.01
just my calculation Dan

[LoganD]

Yes,so let me get this straight, your shooting for a 17 to 1 compression on your 403 cid motor?

I am hoping you have more then 4 head bolts per cylinder?

Eh....why is this any different than running 25 psi of boost? As long as it doesn't go lean your peak firing pressure is just going to be similar to that of a boosted engine. Build it accordingly.

An NA engine at SCR of 17.0-1, will produce (at 20deg BTC) 230psi and 760deg F.

Lowering the SCR to 9.0 and running 25 psi boost will produce (at 20 deg BTC) 500psi anf 696deg F.
While the cylinder pressure under boost will be significantly higher, the temperatures will be somewhat similar.
And, if the boosted engine can be fitted with an efficient intercooler, inlet air temperature will be further reduced.

Inlet temperature at the begining of combustion is much more important than is pressure, except that pressure provides an
increase in air density.

We can always adjust boost as needed, but SCR is forever.

That's not what kills head gaskets or demands increased clamping load. Peak firing pressure with boost will be much higher than a high compression NA setup (assuming the same fuel), you can easily see this by the increase in power and EGTs boost provides. Heat and pressure over time = power.

This is why methanol is much easier on parts, the slower burn decreases peak firing pressure. This is why small bore engines are easier on parts than large bore engines of the same displacement, you're spreading the peak firing pressure over a larger area. This kills rod and main bearings, piston pins, etc. This is also why diesels need much larger bearings than SI engines, with compression ignition your peak firing pressure is much higher than SI.

I've never seen an SI naturally aspirated engine be over 100 bar peak firing pressure, but there's plenty of boosted factory engines runing around at 150 bar these days.

[MadBill]

Measuring the CC at BDC takes into account/measures the piston dome. It is the exact way to measure compression.
measure again at TDC and you have it . I use ATF for fluid .
You can measure the dome with the piston down the hole also, but actual BDC CC is closer .and the math is easier . Takes a lot of fluid tho.

Bingo. The smaller the diameter, the easier it is to get an accurate reading on a burette so I use a 100 cc one. For my 497.7 c.i. BBC it would have to be filled twelve times and accurately eyeballed at the zero and the 100 cc mark vs. twice for a chamber-only measurement. It's a simple matter to measure the piston set depth to three decimals and then do six key strokes on your calculator for the answer, so why mess around with another quart-plus of fluid?

[af2]

OK one more time checking.
@ .300" 70cc

4.135" x 3.75"

.010" in the hole

.030" x 4.160" Cometic head gasket

@ .300" 70cc means you have no dome volume and 3.98 cc's extra. As 4.135 X 0.3 = 66.018 cc's

15.26:1 does not include any top ring volume.

Stan

That is where i was messed up! Thanks Stan!!

[David Redszus]

OK one more time checking.
@ .300" 70cc

4.135" x 3.75"

.010" in the hole

.030" x 4.160" Cometic head gasket

@ .300" 70cc means you have no dome volume and 3.98 cc's extra. As 4.135 X 0.3 = 66.018 cc's

15.26:1 does not include any top ring volume.

Stan

That is where i was messed up! Thanks Stan!!

OK, so now we have an accurate Static Compression Ratio.

What will we do with it? Why is it useful?

[af2]

@ .300" 70cc means you have no dome volume and 3.98 cc's extra. As 4.135 X 0.3 = 66.018 cc's

15.26:1 does not include any top ring volume.

Stan

That is where i was messed up! Thanks Stan!!

OK, so now we have an accurate Static Compression Ratio.

What will we do with it? Why is it useful?

Because of the flow the pump has to be close. And I thank all that responded

[swampbuggy]