Higher compression ratio??

[HellOnWheels]

I know that my compression ratio is above 8.5:1 but I don't know exactly how much, is there a way to measure the exact compression ratio while engine is still in car?

TIA
HOW

[TurboDave]

Not really. Here's the things you have to know:

Stroke, rod length, piston compression height, piston dish volume, piston deck clearance (+ or -), head gasket thickness, and last but not least, head chamber volume.

[bruce]

Originally posted by HellOnWheels
I know that my compression ratio is above 8.5:1 but I don't know exactly how much, is there a way to measure the exact compression ratio while engine is still in car?

Yes, but it's a PITB.
It involves, measuring the volume with the piston at BTC, and then at TDC. You might have to lift the engine off one mount to get it to where you can purge all the air out of the cylinder.

Some say slowly spinning the engine over, with a really good compression guage, and then divide by 14.7, will work.

NASCAR surely have something for doing that when the Bush series had the CR limit. They have an easy to use tester for volume, maybe a google for NASCAR, and inspections might reveal something.

[REDS HOT AIR]

why do you think your over 8.5 to one

whats your build ?

[HellOnWheels]

Originally posted by REDS HOT AIR
why do you think your over 8.5 to one

whats your build ?

Twice, both the block deck surfaces and heads have been "cleaned" up to improve sealing surfaces, and using only one steel shim head gasket.

[BoostKillsStres]

I would think if you googled you could find a compression calculator that had an input for HG thickness then you could at least change that value and get an idea of the amount of compression change you might be getting if not the exact value.

[wbrophy]

Every .004 in thickness + or - equals 1 cc or .1 in compression. PS: Got this info from a 30 yr. + headman. If you know how much your missing from the headgasket and machine work than this will get you in the ballpark.

[HellOnWheels]

Originally posted by TurboDave
Not really. Here's the things you have to know:

Stroke, rod length, piston compression height, piston dish volume, piston deck clearance (+ or -), head gasket thickness, and last but not least, head chamber volume.

Dave,

Its a stocker exception being .030 stock pistons, and single steel shim gaskets which measures .019", if memory serves.

What I don't know is how much was taken off when deck and head surfaces were "cleaned" up, and that's what I'm trying figure out the how's and where's to measure.
Is there a standard amount taken off when just cleaning the sealing surfaces?

Opinions welcomed from you 109 guru's.


Thanks,
Jerry

[TurboDave]

It depends a lot on who did the machining. If they're good, and somewhat paying attention to detail, then generally there shouldn't be more than a couple thousandths taken off the heads to clean them up. How much the block was shaved is anybody's guess, but shouldn't be much more.
He might have lost a total of 4-6 thou.

The steel shim head gasket in effect is at best, only half the thickness of a stock gasket, so there's probably been at least a 5cc loss in gasket volume. That change alone would, for example take a stock c/r from 8.14 up to 8.65 (assuming NOTHING else changed).

However, the total swept volume in the cylinder has changed because of the over bore. How much? Don't know. But that increase in swept volume is also going to increase the c/r by a good bit.

My guess, it's probably pushing close to 9.1:1 with just those two changes.

Did the new pistons change the deck volume? that's another part of the equation that could add or subtract from the result.

[chevyII]

nascar uses a whistler in the spark plug hole. The pitch tells the CR very close to what it actually is. Other than that get out the calculator and measuring tools.

[REDS HOT AIR]

an example is

a customer motor Im building is 30 over trw pistons deck was cut ~5 tho and heads were surfaced also not sure exact amont whatever champion shaves their iron heads

single steel shim ~23 tho unless you over tightened your heads

his motor came out just a hair under 8.5 to 1

If you cc a bone stock lc2 motor which I have done a couple and they have shown a stock compression of 7.9 to 1

my old motor was very slight surface , single steel shim , no head surfacing with factory pistons and came out 8.4 to 1

[HellOnWheels]

Okay, lets ASSume for a minute that the compression ratio is 9.0, how would that affect boost numbers, in other words, would 15 pounds of boost on an absolute stock motor be the same as if it were 9.0 compression ratio?

Thanks,
HOW

[bruce]

Originally posted by HellOnWheels
Okay, lets ASSume for a minute that the compression ratio is 9.0, how would that affect boost numbers, in other words, would 15 pounds of boost on an absolute stock motor be the same as if it were 9.0 compression ratio?

The motor will be more prone to pre-ignition/ detonation.

A search for the phrase Abnormal Combustion, will get ya a link for an in depth easy read about those two terms and what they mean.

[HellOnWheels]

Originally posted by bruce
The motor will be more prone to pre-ignition/ detonation.

A search for the phrase Abnormal Combustion, will get ya a link for an in depth easy read about those two terms and what they mean.

Bruce,

I believe you are right, and as you know, I've been fighting a knock retard issue since day one, even after installing Razor's PAC system squirting Methanol, and using one of Eric's (TurboTweak) 20 degree alky chips, using 93 octane, anything over 12-15 #'s of boost would set off the knock gauge so I'd have to pull 4-6 degrees timing out (Trans +) before the car would do a wot w/o getting knock!!

I made contact with Eric and he too commented that the excessive knock could be due to the higher compression, and stated that it could possibly be that 15 #'s of boost on my engine could be the equivalent of 18-19 #'s on a stock block that compression ration was still factory stock?

Again, thanks to all who responded, I do appreciate!

HOW

[REDS HOT AIR]

I donnot think it has to do with compression .. I have run these motors at over 9 to 1 and 15# boost should be no problem

Im sure you have direct scan ... sent me a file to evtb@cox.net

sounds more to me like your taking on oil in the intake tract somewhere ... be it pcv valve or turbo itself

send me that file and I'll be able to tell more

[TurboDave]

Originally posted by HellOnWheels
Bruce,

I believe you are right, and as you know, I've been fighting a knock retard issue since day one, even after installing Razor's PAC system squirting Methanol, and using one of Eric's (TurboTweak) 20 degree alky chips, using 93 octane, anything over 12-15 #'s of boost would set off the knock gauge so I'd have to pull 4-6 degrees timing out (Trans +) before the car would do a wot w/o getting knock!!

I made contact with Eric and he too commented that the excessive knock could be due to the higher compression, and stated that it could possibly be that 15 #'s of boost on my engine could be the equivalent of 18-19 #'s on a stock block that compression ration was still factory stock?

Again, thanks to all who responded, I do appreciate!

HOW

Could be several reasons. Maybe the fuel octane where you're at isn't up to snuff.
For example, we have good quality 93 octane fuel here, which allows for a little more boost, all things being equal.

You also might want to try a lower timing chip. I have my Extender cut for 18° of timing in 1st and 2nd, and 16° in 3rd and 4th.

That, coupled with the 93 octane fuel, allows me to run an honest 21# of boost in 1st and 2nd, and I only have it pulling back to 20# of boost in 3rd and 4th. and suffer NO knock retard.

So, what does all that mean? Try a lower timing chip, AND you may be able to jump up to 17# of boost or so, assuming you do in fact have a 9:1 C/R.

Also, MAKE SURE THE KNOCK SENSOR IS PROPERLY TORQUED!!!!!!!! 14 lb/ft, no more, not less.

[HellOnWheels]

Originally posted by TurboDave
Could be several reasons. Maybe the fuel octane where you're at isn't up to snuff.
For example, we have good quality 93 octane fuel here, which allows for a little more boost, all things being equal.

You also might want to try a lower timing chip. I have my Extender cut for 18° of timing in 1st and 2nd, and 16° in 3rd and 4th.

That, coupled with the 93 octane fuel, allows me to run an honest 21# of boost in 1st and 2nd, and I only have it pulling back to 20# of boost in 3rd and 4th. and suffer NO knock retard.

So, what does all that mean? Try a lower timing chip, AND you may be able to jump up to 17# of boost or so, assuming you do in fact have a 9:1 C/R.

Also, MAKE SURE THE KNOCK SENSOR IS PROPERLY TORQUED!!!!!!!! 14 lb/ft, no more, not less.

Dave,

I have a 20/17 degree chip by TT and even then I cannot run it or the extender without taking a least 4-6 degrees of timing out in order to have 0 kr. Now when the timing is pulled back 6 degrees I can run a solid 20#'s boost-plus if the higher compression ratio of my engine that 20 #'s may actually be 23-25 #'s of boost????

Well, I would assume you and I probably have pretty much the same fuels that advertise "93" octane, I'm just up the river in an adjoining state from where you are located.

I bought an inch pound torque wrench just for the purpose of setting the torque on the knock sensor.

Thanks,
Jerry