Mathematical Question

Kev · Post by **Kev** » Tue Jul 17, 07 9:23 am

One for all you Vordermans! What sort of difference to a Compression Ratio if the compressed thickness of the head gasket is 5 thou thinner. (3.91" bore)

Anonymous · Post by **Anonymous** » Tue Jul 17, 07 9:30 am

Provided the reciprocating assemblies stay ecxactly the same it will go upsidaisy.

Pete · Post by **Pete** » Tue Jul 17, 07 10:06 am

Iit will go up, but not much - a lot depends on bore size. The Hemi made .2 CR difference for a 11 thou change in gasket.

Kev · Post by **Kev** » Tue Jul 17, 07 10:07 am

Thanks!

Pete · Post by **Pete** » Tue Jul 17, 07 10:09 am

Hemi was 4.3750 bore....so more impact thna yours.

Ivor · Post by **Ivor** » Tue Jul 17, 07 10:17 am

I've looked closely at the relationship between the thickness of the head gasket in direct comparison to the effect that has on compression ratio and performed an experiment to make a calculation on the possible increase in efficiency and therefore, performance.

By taking a close look at the configuration of heterodyne interferometry with the fiberoptic sensor. A frequency stabilized He-Ne laser, with a wavelength λ of 632.8 nm and an output power of 1 mW, provides a linear polarized beam for the measurements. The AOM system for heterodyne interferometry produces two beams. In this experiment, the frequency of the first beam is shifted by 80.100MHz, and that of the second by 80.125 MHz.

The stability of our AOM is within 0.02ppm.These beams meet at the polarized beam splitter and create a beat frequency of 25.0 kHz. After the polarized beam splitter, the beam is split into two by the half mirror.

One beam is detected by a photo-transistor as a reference signal; the other is used for modified Michelson interferometry. The reference signal beam passes outside the combustion chamber and is reflected by a mirror.

The beam used for modified Michelson interferometry passes through the test section, is reflected by a mirror, and then passes back through the test section again.

The result was a drastically improved flame front in relationship to the .00355479 % increase in compression ratio, by using the thinner gasket on the small black Mopar engine.

Anonymous · Post by **Anonymous** » Tue Jul 17, 07 10:17 am

What is the standard thickness of the head gasket?

Pete · Post by **Pete** » Tue Jul 17, 07 10:29 am

Time to up the medication dosage again Ivor; or should I say "Igor"????

Ivor · Post by **Ivor** » Tue Jul 17, 07 10:34 am

The day I'm having Pete, I need a very large glass of medication!

Anonymous · Post by **Anonymous** » Tue Jul 17, 07 11:08 am

Ivor, my boss tells me that an engineer should be able to do for a quid, what any old fool can do for a fiver.

Surely you could have just said " the compression goes up" Kev didnt ask "by how much, why, and how did you know"

You are a one.

Dave-R · Post by **Dave-R** » Tue Jul 17, 07 11:20 am

Not sure what sort of answer Kev wanted but the actual amount would be different from engine to engine.

Anonymous · Post by **Anonymous** » Tue Jul 17, 07 11:26 am

Since I am still officially a student engineer this is free

Assuming:
- 3.91" Bore
- 3.31" Stroke
- 0.039" Compressed gasket height
- 4.180" Gasket bore
- 64.5cc Chamber

Your compression ratio will be 9.97:1

If the gasket is 5 thou thinner (compressed) then your compression ratio will be 10.1:1.

I would be more worried about the crevice between the head and deck created by the oversized bore of the gasket though. That is a nice area for burnt gases to collect and then spoil the fresh mixture coming in.
Incidentally if you could match a gasket of exactly 3.91" bore then your compression would be 10.1 at the standard height and go up to 10.2 for the 5 thou thinner gasket.

Kev · Post by **Kev** » Tue Jul 17, 07 12:30 pm

Great answer, James! The standard Fel Pro is 0.050" the Rol Extreme is 0.045". Know what you mean about the oversize gasket

Curse of the 318!

MilesnMiles · Post by **MilesnMiles** » Tue Jul 17, 07 2:45 pm

Thats interesting Bratfink, I went down to .027 (0.29?) from Felpro standard head thickness, wondering what effect it would have on compression.

MilesnMiles · Post by **MilesnMiles** » Tue Jul 17, 07 2:51 pm

Although Kev asked about the difference in compression rates, james, surely to assess actual compression rates you would have to know if the block was decked and whether the piston was flat/domed/had cutouts?
You can tell i'm on summer hols...