Preferred method to lower compression

[Philv6]

Given a short deck 9.50, 6.50 rods (7.147 to the top of the rod), and a 3.59 stroke crank. Add in the piston thickness @.250 with .020 rod/piston clearance and there is not alot of material that can be moved to lower the c/r. That being said, what would be the most preferred method? (need to order pistons)

- Thicker head gaskets .060 or more vs .039
- Larger gasket bore dia (4.015 cyl) 4.090 vs 4.020
- Drop the upper ledge of the piston .020 down in the hole or more if possible vs zero deck

What would be the advantage / disadvantage, or max to shoot for each method?
Head cc's not included, since that usually can't be changed much

 

[chevyII]

larger bores equal more comprssion and for max power you want to keep the guench to around .10 below the deck.

 

[Lee Thompson]

cr

My broken motor had a 9.500 deck with 6.5 rods and a 3.600 stroke. Had to order J&E with a support ring that goes in below the bottom oil rail. This will support the oil rail as the wrist pin is moved up into the oil groove. Is this what you are asking?

 

[Philv6]

Lee

I was thinking I would already have to do that . My calculations are,
0.647 Pin c/l to top of rod
0.020 Rod to underside of piston
0.250 Piston thickness
-------
0.917 Compression height

6.500+.917+1.795=9.212-9.500= 0.288

With a flat top piston this would give me a piston 0.288 down in the cylinder. The compression would be low @ 7.5 w/46cc head. But it would be too tight for a good ring pack and way too much deck clearance. As the outer diameter of the piston is raised to create a dish piston, increase ring land and get closer to a zero deck the compression goes up. Being unable to move the floor of the dish brings up the question. What is the preferred method to lower compression?

- Increase gasket thickness
- Increase gasket bore diameter
- Increase deck clearance

What method is preferred and what would be the max for each. Also what would be the advantage / disadvantage of each method?
I know some of this might be personal preference and opinion. But how have others done it? What deck clearance, bore diameter and gasket thicknesses are being used? I know there are a lot of excellent engine builders with way more experience than me. I would just like some direction on this. If I was building a pure race motor c/r would not be a major concern. But I want to build a motor that can run on street gas.

 

[TurboDave]

Are you sure about your C/R calculations with a flat top piston?? Reason I ask is, comparing your stuff to a stock configuration for example.

You said you are using a flat top piston. If memory serves (and my numbers might be a little off), the stock piston is dished at around 23-26cc's and the head is around 46-47cc's to obtain a C/R of around 8:1 or so. I believe the stock piston is something around .060-.065 down in the bore at TDC.

 

[Philv6]

Dave
I had to edit my calculations. I had pin c/l to top of piston. Should have said Pin c/l to top of rod.
Yes I believe my calculations are correct. No I don't want to use a flat top piston, I want to use a dish piston. But as the dish is added the cc's of the area go down. What I was trying to show is that with no dish (flat top only) the lowest I could go (piston top, floor of a dish) with the stack up of the stroke, rod, clearance and piston thickness. The power source book recommends a "piston deck to cylinder head clearance of min 0.045 max 0.060". To go 0.60 seems like too much, add to that another .040 for gasket thickness and it's like a mile inside a cylinder. I don't recall what the piston to deck height was for the stock set up. But I was figuring to get @ 30 cc's from the dish at the larger bore. The ultimate cc's are where I had to guess what would be the max diameter and depth (can only go so far, more dish less deck clearance, can't move the top of the dish) of the piston dish.

FYI I think the stock set up was.
1/2 stroke 1.7+1.850 comp ht + 5.965 rod = 9.515 - 9.535 deck ht = 0.020 in the hole

 

[TurboDave]

Probably not applicable to what you're doing, BUT. The stock parameters, according to Power Source:


(Stock motor bored .030)
Deck clearance: -0.070
Compression height: 1.825
Bore: 3.830 (only parameter different than stock)
Stroke: 3.4
Rod: 5.960
Piston dish: -23.6cc
Chamber: 48cc

C/R:8.1:1

 

[John Mason]

comp.

The .045 to .060 you're referring to is the TOTAL piston to head clearance. That's gasket plus deck height. On my non-turbo, aluminum rod Stage 2, I have -.020 deck and a .040 gasket for a total clearance of .060.
I don't know what comp ratio you want, but here's a nice calculator.
http://kb-silvolite.com/calc.php?action=comp
John