Paid for 10.25:1 but got 7.22:1 (or why you have to do the compression ratio mat

Started by Dan Jones, May 29, 2026, 05:03:11 PM

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Dan Jones

May 29, 2026, 05:03:11 PM
Hagerty Media recently posted a Rover 3.5L V8 engine rebuild time lapse video:

 https://www.hagerty.com/media/videos/tired-v-8-in-jason-cammisas-rover-sd1-gets-an-overhaul/

On their first try, they ordered Egge L2152 Buick pistons which are advertised as 10.25:1 but, once everything was assembled, they ran it through Summit Racing's compression ratio calculator and got 7.22:1.  Given the magnitude of the difference, I thought they made an entry mistake but the "high compression" pistons really did yield only 7.22:1.  They fixed the issue by substituting L2153 flat top pistons (with 4 valve notches) meant for Oldsmobile 215 V8s.  I've got several sets of Buick 215 and Rover 3.5L pistons here that I measured and and went down the compression ratio rabbit hole.  In the attached picture, the pistons are, from left to right:

 1. Hepolite TR8 and Rover SD1 (U.S. market)
 2. Silv-O-Lite 1718 (advertised at 8.8:1 or 9:1 depending upon source)
 3. Buick Alcoa (advertised at 10.25:1 in 1962)
 4. Buick Bohnalite (advertised at 11.00:1 in 1963)

The U.S. versions of the Rover SD1 (and TR8) had Rover 3.5L V8s with a claimed 8.13:1 compression ratio using deeply dished Hepolite pistons.  The specifications on those pistons quote a dish volume of 15.27cc.  As a check, I measured the dish depth and averaged the upper and lower diameter of the dish chamfer which yields 15.04cc.  Finally, I got out the burette measured the dish volume at ~16cc so that all checks out. 

 Hepolite dished pistons (Triumph TR8 and Rover SD1 3.5L)
 8.13:1 advertised compression ratio in 3.5L
 1.875" compression height (Hepolite)
 0.2" piston dish depth
 2.80"/1.96" (OD/ID) piston dish diameter
  20.18 cc OD, 9.89 cc ID, (OD+ID)/2 = 15.04 cc
 15.27 cc quoted (16 cc dish volume as measured)
 1/16", 1/16", 3/16" rings
 full round skirt
 no valve reliefs

The nominal deck height for a Buick 215 and Rover 3.5L is 8.96" inches, rod length is 5.66" and stroke is 2.8".  The compression height of the Hepolite pistons is 1.875" so the nominal deck clearance is 0.025":

 deck clearance = deck height - (pin height + rod length + stroke/2)
 nominal deck clearance = 8.96 - (1.875 + 5.66 + 2.8/2) = 0.025

If I run the numbers on the original engine, I get 8.06:1 compression which is close enough, given the assumed head gasket dimensions:

 3.50" bore diameter (0.020" overbore)
 2.80" stroke
 36 cc cylinder head volume
 15.27 cc effective dish volume
 0.025" deck clearance
 0.045" compressed HG thickness
 3.56" HG diameter
 215 CID
 8.06:1 compression ratio
 
If you jump ahead to 7:00 minutes in the video, a computer screen displays Summit Racing's compression calculator showing:

 3.52" bore diameter (0.020" overbore)
 2.80" stroke
 36 cc cylinder head volume
 18.6 cc effective dish volume (as measured by Davin Reckow)
 0.062" deck clearance
 0.045" compressed HG thickness (no diameter specified)
 217.98 CID
 7.22:1 compression ratio

The engine builder (Davin Reckow) initially chose Egge L2152 pistons which, in the past, were listed as 10.25:1 replacements for the high compression 4 barrel equipped Buick 215 so, as mentioned previously, the 7.22:1 compression ratio surprised me.  As I check, I calculated the compression, assuming 3.56" diameter head gaskets and their measured 18.6 cc dish volume, but got the same number.  From the D&D catalog, the usual rebuild pistons for the Rover 3.5L and Buick 215 V8s are:

 Silv-O-Lite 1718 9.00:1 compression for Buick & Rover 3.5L
 Silv-O-Lite 9598 9.60:1 compression for Buick & Rover 3.5L
 Egge 2152         10.25:1 compression for Buick & Rover 3.5L
 Egge 2153         Oldsmobile 215 flat top with 4 valve notches

The Silv-O-Lite 1718 are replacements for the low compression 2 barrel equipped Buick 215 V8 and the Silv-O-Lite catalog lists them as 8.8:1 compression.  The Silv-O-Lite 9598 are a higher compression version of the 1718 with a shallower dish but are no longer available, though D&D may still have sets on the shelf.  At 7:10, the Hagerty video shows L2152 pistons with a dish at least as deep as the low compression 1718 pistons and the diameter somewhat larger. By comparison, the Buick 10.25:1 pistons have a very shallow dish so I contacted Egge to see what the dish volume is supposed to be.  They couldn't answer my question but sent me their piston drawings:

 Egge L2152 per blueprints:

  1.844" compression height
  2.812" piston dish diameter
  0.060" piston dish depth (OD of chamfer)
  4.22 cc dish volume (OD 6.11 cc, ID 2.33 cc ==> 4.22 cc average)
   
Nominal deck clearance = 8.96 - (1.844 + 5.66 + 2.8/2) = 0.056". 

 3.52" bore diameter (0.020" overbore)
 2.80" stroke
 36cc cylinder head volume
 4.22 cc effective dish volume
 0.056" deck clearance
 0.045" compressed HG thickness
 3.56" head gasket diameter

If I run the numbers using the L2152 drawing specs, I get between 8.9:1 and 10.4:1 compression:

 8.91:1 compression ratio with 0.045" thick head gasket
 9.52:1 with 0.020" thick shim type head gasket
10.39:1 with zero deck and 0.045" thick head gasket

The pistons clearly marked as L2152 in the video at 7:10 do not match the drawing.  It appears someone changed the pistons without changing the part number or drawing.  I'd be an unhappy customer if I ordered high compression pistons only to end up with 7.22:1.  Davin Reckow substituted Egge L2153 pistons (shown at 7:12 in the video) which are Oldsmobile 215 flat tops with 4 valve notches.  The drawing for the L2153 doesn't have all the information needed to calculate the valve notch volume but I estimate it at 1.9 cc and the L2153 have a taller pin height for higher compression:

 nominal deck clearance = 8.96 - (1.875 + 5.66 + 2.8/2) = 0.025

 3.52" bore diameter (0.020" overbore)
 2.80" stroke
 36 cc cylinder head volume
 1.9 cc effective dish volume
 0.025" deck clearance
 0.045" compressed HG thickness
 3.56" HG diameter
10.10:1 compression ratio with 0.045" head gasket
10.89:1 w/0.020" steel shim head gasket
10.87:1 zero deck and 0.045" HG
11.85:1 zero deck and 0.020" shim HG
11.83:1 with 28cc heads (Rover 4.0L/4.6L), 0.045" HG
12.99:1 with 28cc heads (Rover 4.0L/4.6L), zero deck, 0.045" HG

Back in the day, the OEM Olds 215 flat top pistons were said to yield between 11:1 and 12:1 so that checks out.  Today, the Egge L2153 Olds 215 pistons look like the only choice left for high compression off-the-shelf pistons.  I was thinking of sticking a set of the high compression Buick pistons in the Rover 3.5L out of my TR8 as a backup engine but I've also got a Buick 215 with forged flat top pistons (and forged crankshaft and lightened forged rods) that displaces 225 cubic inches.  That ones has nicely ported heads (by Mondello) that I need to cc to see what sort of compression ratio that one is.

Below, I've summarized some of the piston info for future reference.

Dan

Silv-O-Lite 1718 pistons
 1.843" compression height
 2.812" piston dish diameter (OD of chamfer)
 0.150"  piston dish depth
 12.5 cc advertised dish volume (10cc measured)
  15.27 cc if calculate from OD
  10.21 cc if calculate from ID
  (15.27 + 10.21)/2 = 12.74 cc (2.53 cc delta due to chamfer)
 0.875" pin diameter center
 Advertised as 8.8:1 in Buick 215 (by Silv-O-Lite and 9.00:1 by D&D)
 5/64", 5/64", 3/16" rings
 full round skirt
 no valve reliefs

Silv-O-Lite 9598 pistons
 1.843" compression height
 2.68" piston dish diameter
 0.094"  piston dish depth
  8.69 cc if calculate from OD
  8.5 cc dish volume quoted by Silv-O-Lite
 Advertised as 9.6:1 in Buick 215 (by D&D)

Egge L2152 pistons
 1.844" compression height
 2.812" piston dish diameter
 0.060" piston dish depth (OD of chamfer)
 4.22 cc dish volume
 Advertised as 10.25:1 in Buick 215 (by D&D) 
 18.6cc as measured in Hagerty video
 Dish is visibly much deeper than either Buick 215 high compression piston, looks close to 1718 so no way near 10.25:1.
 Note: Egge drawing doesn't give chamfer radius but appears similar to 1718.

Egge L2153 pistons
 1.875" compression height
 flat top with 4 canted valve reliefs
 1.9cc's effective dish volume (estimated)
 Valve notch is 0.100" deep and there are 2 pairs so treat each pair as constant 0.1" depth
 but it's not a full circle, treat has half the volume (on high side)
 1.72" diameter by 0.1" deep / 2 = 1.905 cc (similar to Bohnalite but taller pin height so higher compression)
 
Alcoa Buick 215 high compression pistons (p/n 1348001)
 10.25:1 advertised compression ratio
 1.860" compression height
 2.80"/2.45" (OD/ID) piston dish diameter
 0.033"  piston dish depth
  3.33 cc OD
  2.55 cc ID
  2.94 cc (OD+ID)/2
  3 cc dish volume as measured
 0.875" pin diameter center
 Advertised as 10.25:1 in Buick 215
 5/64", 5/64", 3/16" rings
 full round skirt
 no valve reliefs
 
Bohnalite Buick 215 high compression pistons (p/n 1353005)
 11.00:1 advertised compression ratio in 1962-1963 Buick 215
 1.860" compression height
 2.84"/2.70" (OD/ID) piston dish diameter
 0.031" piston dish depth but raised circular pad in center
  3.22 cc OD
  2.91 cc ID
  3.06 cc (OD+ID)/2 less the raised pad
 <2 cc dish volume as measured
 0.875" pin diameter center
 Advertised as 10.25:1 
 5/64", 5/64", 3/16" rings
 full round skirt
 no valve reliefs

MGBV8

#1
May 30, 2026, 01:01:09 PM
That  "4. Buick Bohnalite (advertised at 11.00:1 in 1963)" piston looks just like what is in my '63 215 JN engine.

Now, if you could just find a decent piston for my 3.9, Dan.  :)
Carl

MGBV8

#3
May 31, 2026, 11:01:57 AM
Thanks.  I was hoping for a lighter, hypereutectic piston.  A set for my 4.2L crankshaft would be fantastic.  :)
Carl

mgb260

#4
May 31, 2026, 11:22:07 AM Last Edit: May 31, 2026, 12:23:00 PM by mgb260
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