Guys,

I have a questions regarding the compression ratio. I am near the final assembly and have a big question regarding compression ratio. Rotating assembly has been installed, piston to deck height has been checked (.01") and now I am at the point for the heads to be attached to the block.

My question is I noticed the other night that the website for United Engine & Machine has updated their website and now the dish cc for my pistons has changed..... Previously it was noted at 10cc now they have it listed at 6.4cc...

Will I be ok?
Which head gasket should I use (listed below)?
How do I calculate my DCR?

Specs listed below, car is a 6spd with 3.45 rear end.

Block = 1994 LT1 4 Bolt
Bore = 4.03"
Deck = 9.0"
Crank = 3.48"
Rods = 5.7"
Piston to deck height = 0.01" (measured using straight edge and verified piston(s) were at TDC with dial indicator)
Pistons = 6.4cc (see pictures listed below).
Heads = 54.4cc (measured at machine shop)
Camshaft = LT4 Hot Cam kit
Head gaskets
A = Fel Pro 9966PT (.049" crush)
B = Fel Pro 1074 (.039" crush)

This is my first build and I want to play it safe while maximizing performance.

Thanks!

 

It's not to difficult to measure the above piston volume with liquid and it will be great exprience for you building engines. Since you have the block out of the car you just need to get the deck of the block level - with a level of course. Now you need the piston you want to measure at TDC. I use a degree wheel to find this and it's something you definitely want to learn when building an engine. If you don't have a 5"x5" piece of 1/4" plexiglass and a graduated cylinder Summet Racing sells a kit for like $30. The plexiglass does need a hole drilled in it.
With the piston at TDC push some grease in the piston to cylinder wall crevice. Now seal the plexiglass with grease. Finally add liquid to fill the above piston volume. I use over the counter alcohol with food dye. Also the pharmacist at Target gave me a handful of 1cc and 5cc syringes for free (w/o the needles of course).

Now you will have as accurate above piston volume as any professional engine builder.

Myself I no longer trust any published volume number for pistons or cylinder heads.

Hope this can help and good luck.

 

Wallace Racing has calculators to answer many questions. The extra .010 isn't going effect compression much.

 

In my opinion I would run a .030 compressed head gasket to get the squish at .040. That is more important when you are running high compression to control detonation rhen the minor compression change the gasket will provide. The compression ratio with the .039 gasket is 11.15 with a 6 cc valve relief and with .03 it is 11.35. That being said .040 squish is near perfect and .049 is almost completely ineffective at controlling detonation. Also when you have it tuned have you fans come on at 195 and off at 190.

 

Thanks for the responses so far.

Preliminary measurements are the following

Static compression
Felpro .039 crush = 11.2:1
Felpro .049 crush = 10.9:1

Dynamic w/ 65° ABDC (not sure if this is right).
Wallace Racing calculator = 8.96:1

Dynamic w/ 53.5° ABDC (again not sure if this is right).
Wallace Racing calculator = 9.65:1 (I believe this is concerning because it's in the "danger zone").







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Thanks for the help.

Regarding the fluid volume measurement, would the gap between the piston top and the block deck add volume displacement causing additional volume to be added to the actual piston volume? Say the piston volume is actually 6.4cc (for now until UEM decides to change the stated volume). How would I account for the difference between the piston and block? Could the entire volume number be used is a calculation?

 

Yes I see your trying to figure this out for yourself and want a truly accurate measurement/calculation. So I will try my best to help you with this.

Yes the volume above the piston will have a huge effect on compression ratio. Even the small area between the piston and cylinder wall needs to be included thats why you will find you want to wipe the grease down that you use to seal the piston so as to have minimum effect - not to difficult but will take a few practice tries. Also you may find your piston to plexiglass/deck clearance is so tight it's difficult to get your liquid to fill it. In that case I use a dial indicator to move the piston down the hole as much as 0.050" and subtract that 0.050" volume from my measured. Sounds like it's getting difficult but really not much and after just a few practice tries you should have it mastered. This method becomes very important it your pistons are out of the hole as it sometimes happens.

For piston eyebrow volumes it pretty easy just to block up the piston level (with a level of course) and fill the eyebrows with a measured amount of liquid - watch for bulging surface tension though (called stichen I think) .


And yes you answered your own question as you now have one measured volume to use in your compression ratio calcs which is much easier and much, much more accurate than using the published volume which are always suspect. So now you only have the above piston volume + cylinder head volume + compressed gasket volume = static volume for your compression ratio calculation. The swept volume stays the same unless you change the stroke.

Once you have a handle on using liquid measurement you will want to measure the heads also as that even much easier. But you will have to use the the gasket manufactures compressed height to calculate that volume. Also make sure you convert everything to cubic inches or else everything to CC's before you crunch the numbers.

That's all I can think of, good luck.

 

Anyone know the intake closing ABDC value for the Hotcam?

Is .050" perfered or .006" (spec sheet above).

Thanks

 

Your GMPP camshaft spec sheet says duration at 0.006" = 272 degrees.

1/2 of 272 = 136 degrees to add to lobe centerline of 109 degrees will be 245 degrees ATDC. 245* - 180* = 65* ABDC.

So you should be right with what you already used in post #5.

 

Cardo0,

Thanks for the guidance and information, much appreciated. I plan on measuring the overall volume tomorrow with the piston TDC using the mentioned alcohol, grease and plexiglass.

My question at this point is using the recently provided ABDC value of 65 and the presumed ~11.244:1 static compression my Dynamic compression is estimated to be 8.95:1.

Is dynamic compression of 8.95:1 a safe number for street driving with 91 octane and a standard (but new) GM LT1 waterpump?

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As measured today

5ml per eyebrow
12ml total overall cylinder
2.015" radius
.01" height
Volume = π×2.0152×.01 = 0.12755573031922 inches3
multiply the volume value by 16.387
2.0893507ml + 10ml = 12.0809cc

Static compression = 10.752:1
Dynamic = 8.571:1

Sometimes I am my own worst enemy, its the engineer side of me.

Side note - I received different values with the Wallace Racing calculator (mobile site provided the 11.2:1 values....).

Thanks!

 

Congratulations I believe you have accomplished what few others make the effort to do.. And I hope you can share with us it wasn't all that difficult.

I think you have the information you need but I'm not sure how you are crunching the numbers correctly.

If you sealed the piston and measured liquid volume above the piston to the plexiglass then you don't need to measure the eyebrows.* Just use the above piston volume + gasket compressed volume + head chamber volume.* This is the static volume.* I don't understand the 2.015" number use as that sounds like the final cylinder bore radius which would be included in the above piston volume.* Maybe your trying to calculate the head gasket compressed volume?* That has a significant larger diameter/radius than the cylinder bore - gasket manufacturer will provide hole diameter and compressed height of their gasket under their part number.* Also it looks like you only included 2 valve reliefs but your pistons have 4 reliefs correct?

As far as dynamic compression ratio goes the number I can recall for 91 octane gas is less than 8.3.* But let's get the static compression nailed first.* There are many dynamic calculators available but some are way more complicated than what you need and you really need to try several until you find one you feel comfortable with - at least you can fully understand it.

Nice effort here and please send us updates - it helps us all.

 

At first I attempted to use the plexiglas method but quickly realizing that the hole I drilled wasn't large enough causing erroneous values due to air trying to vent and over-spill (it was similar to the game "operation"). In order to maintain sanity I ended up filling each eyebrow (this piston set has more of a uni-brow) separately (5ml each) and then continued to fill the remaining "gap". The surface tension of the alcohol comes into play as does the overall "height" of the fluid, but in the end I really only needed to verify the stated piston volume.

Continuing to learn a lot regarding engine builds.

Thanks again.

 

That reads you are using your own ingenuity - cool. I have to admit I couldn't give all the instructions that may be needed in every situation over the internet and really the owner has to repeat the tests enough that he feels its reliable enough. I know I had difficulty with the tiny last bubble under the glass and after enough tries came to realize how much volume the little bubble accounted for. I have to laugh when I read of measurements down to tenths of a cc as there is just to much error involved to make those claims. Unless you have a highly polished piston, chamber and maintain a clean room environment - but I believe some professional race builders may do/have that while few amateurs will.

The important thing is we get you a accurate static compression ratio for you to make decisions. Now if your dynamic compression is to high there a couple of things you can consider. With the heads off you can reshape and polish the head chambers as well as the piston eyebrows for more volume. Choose a cam with later closing intake event or even just time the cam you have to close the intake valve later (retarding the intake valve centerline). Or possibly a thicker head gasket can get you where you want to be.

Good luck then and I hope this has been a help to you. I believe you will have a much better calculation now rather than using published volume numbers let alone internet hearsay.