Buick v8 compression ratio help : Can get to 10.25:1

[dbizak]

Help! I'm in the middle of my engine build and just measured the combustion chamber of the rebuild and I'm getting a CR of 8.68:1 vs the 10.25:1 (or somewhere close) that I am looking for. This is my first rebuild and, in theory I have all the right components for 10.25:1 CR, but my measurements don't reflect it. What am I doing wrong? Here are the details:

Rover head: 35CC
Head Gasket: 6.4CC
Block with piston TDC: 17.1
Combustion chamber total : 58.538
Stroke 448.928
Means: 507.46/58.54 = 8.669

1) Using Rover Heads HRC2210 from a 3.9 liter rebuilt by Mark at D&D which come in at 35CC when using alcohol and a dropper. If I'm not mistaken, stock they are 36CC but may be coming in a little less because they have been decked.
2) Fel-Pro Head Gasket 7984 from kit FEL FS7984PT which, when compressed, should be .04 inches. This gives a conservative volume of 6.4CC assuming a 3.53" diameter (note engine rebored +.03). It's probably greater since the actual diameter of the gasket is greater than 3.53".
3) The piston portion of the chamber CC in at 17.5 using alcohol and a dropper. The backup math gets me pretty close to that (17.1cc) noting that the piston, when CC'ed by itself is about 7.5CC and the piston to deck gap is .06" which is about 9.6CC. The pistons are made by Egge and have a .05 cup. I've been assured that these pistons are for a 10.25:1, but, given the measurements, I'm having trouble believing this. Furthermore, the original pistons out of the motor had a much shallower cup (almost flat). I've been told that the original setup was likely 11:1 as a result. See pics of pistons below.
4) The stroke is 2.7992, or 448.9CC.
5) So Combustion chamber 58.54 in total means: 507.47 / 58.54 = 8.67 not the 9.8 to 10.1 I've been told it should be given the components.

Any thoughts about what I'm doing wrong from a measurement or a component standpoint? Any help would be great.

Dave

[mstemp]

No expert but I think overbore pistons are sometimes made for decked blocks. I know my silvolites were and the block had to be decked. Have you measured from pin to crown of the different pistons?

[dbizak]

Interesting comment. So the piston might be shorter.  I have not yet measured the pin crown yet.  I put in all the pistons before noticing this problem.

[dbizak]

 -- moved topic --

[Jim Stabe]

I put together an Excel spreadsheet to calculate compression ratios and this is what it shows for your engine. I assumed a 3.6" head gasket diameter. If you zero decked the engine it would only have a 10.15:1 compression ratio with a .040" gasket. If you would like a copy of the spreadsheet to play with PM me your email address and I'll send it to you (you can't attach anything but a .jpg ot .gif on here).

[BlownMGB-V8]

You will never get there 0.050" down in the bore. I built my 340 with zero deck, 20cc, I think maybe a 13 cc piston recess, zero deck (piston rock actually brought the crowns above the deck), a 0.050" head gasket and 300 SBB heads for a 10.6 cr. YMMV

Jim

[Dan Jones]

0.040" headgasket thickness is for a composite type headgasket.  You can
get a 0.016" embossed gasket from D&D. If I run the numbers for a Silvolite
1718 piston with 15.27cc dish and 0.016 compressed thickness gasket, I get
8.2:1 compression which is advertised at 8.8:1.  As a point of reference,
the 1980 Triumph TR8 had an compression ratio of 8.13:1 and the pistons look
identical to the 1718.  With a 0.04" thick headgasket, that drops to 7.8:1.
For your shallower dish piston, I get 9.27:1 with the thin gasket and 8.7:1
for the thick.  If you zero deck the block and use the 0.040" head gasket,
you'll get around 10.1:1.  That'll go up to 10.9:1 with zero deck and 0.016"
compressed thickness.

Note the Silvolite 9598 piston dish is 8.5 cc which is close to yours at 7.5cc.
They are advertised at 9.6:1 but to get that requires a shim headgasket and a
bit of clean up milling on the heads or block.

Assumptions:

 5.66" rod length
 3.6" diameter headgasket bore
 3.53" bore
 2.8" stroke
 35 cc heads
 8.96" deck height undecked
 8.903" deck height when zero decked
 Silvolite 1718 pistons
  1.843" compression height
  2.812" piston dish diameter
  0.15"  piston dish depth
  15.27 cc's dish volume (8.5cc for 9.6:1 Silvolites)
 Silvolite 9598 pistons
  1.843" compression height
  2.68" piston dish diameter
  0.094"  piston dish depth
  8.5 cc's dish volume

I wrote a program to calculate compression.  If you'd like I can forward it
to you.

Dan Jones

[roverman]

David, When you say "dropper", I hope you mean beauret ? Did you cc' the piston installed at tdc ? Did I read .050" down from the deck ? If so, this plus .040" compressed gasket renders the squish band as useless. This is why zero deck works by letting the gasket thickness determine the squish height.With steel rods and the aluminum block, .020" min., should be adequate. I wouldn't be too concerned about piston rock, as your pistons have offset pins and they will not rock when warmed-up. Good Luck, Art.

[BlownMGB-V8]

I don't know that I would trust less than 0.040" of squish regardless. Tagging the head with the piston is never good.

Jim

[Jim Stabe]

I agree with Jim B. Mine is set at .039" - .040" and I made sure to check everything a couple times. I have heard of some NASCAR types running under .030" but you better really know what you are doing. Some have claimed to have small witness marks in the deposit on the head surface. You have to really want that last horsepower

[roverman]

Ok, relevancy ? How much does a rover deck grow at operating temp ? Nascar uses a 1.850" crank pin, iron block  and consistantly turns 9,200-9,500 ? I suspect they are stretching things a bit ?  roverman.

[Jim Stabe]

Art

I'm squeemish enough at .040", you can put together an engine at .020" and tell us how it works. Might create some conversation pieces for the mantle.

[BlownMGB-V8]

If all those parts were truly rigid then no worries mate, but cranks flex, big ends deform, the beam stretches, piston skirts flex, the crown rocks ever so slightly, and even the block and wrist pin deflect. It all adds up. What are you comfortable with? I've heard stories about witness marks too. Although 0.020" would seem to be enough, judging from common lore from expected reliable sources it is not. Who will be the one to find out by trying it, at a cost of... several thousand? Sorry, it isn't going to be me. Conventional wisdom says anything under 0.040" is going to be a high risk venture. Is the payoff worth it? Doubtful.

Jim

[Moderator]

I'm thinking that the first step for boosting David's compression ratio is to sell the Rover 3.9 heads and purchase a set of Rover 4.0 heads. My local junkyard always has them, and they're not expensive. That's good for about 1 point of compression ratio. Going from 35cc to 28cc nominally takes the ratio from 8.67:1 to 9.71:1 - but obviously those numbers (35 and 28) are before shaving heads. At that point, changing from a composite to a steel head gasket brings compression ratio right to 10.25:1 without a change of pistons.

[roverman]

Curtis, Sounds like a plan, and don't the later heads "flow" slightly better ?  roverman.

[Moderator]

I believe so.

Using the search box at the top of this page would probably provide David all the detailed info he might want. For example, we've had pretty good threads on checking lifter preload and shimming rocker pedestals. Several of us have switched to 4.0 heads on our Buick 215s.

[Dan Jones]

> I don't know that I would trust less than 0.040" of squish regardless.

When I pulled the Ford 5.0L HO from my 1987 Mustang GT, I found the forged
pistons were actually above the deck at TDC.  The Ford factory graphite gaskets
are 0.040" compressed and my replacement 3428SG Victor Reinz head gaskets are
0.042" compressed. It's certainly less than 0.040" but I'm not sure by how much.

> I'm thinking that the first step for boosting David's compression ratio is
> to sell the Rover 3.9 heads

Since his heads have already been rebuilt, an alternative to replacing them
would be to mill them to get the desired compression ratio.  You could also
mill the block deck and/or use a steel shim headgasket.  When I was building
my first Buick 215, I was given a new set of the dished Silvolite pistons on
reconditioned rods.  I called D&D and they recommended milling both the block
and heads which I did, along with the steel shim headgasket.

Dan Jones

[bigaldart]

While the head change and the thinner gasket will help with compression, be careful with the squish clearance, it looks like you will be just about ok at 0.076" but that is getting closer to the detonation zone than I like, It also won't do a lot for power, reducing deck height to get that number nearer to 0.040" will make for a lot happier engine even at the higher compression. Efficiency will be improved, a cleaner burn will result and more power, after all that is why we do this.

Alan

[Motor-vated]

New Member resurrecting an older thread because I have a very similar situation with a '62 Buick 215 block and stock Buick heads.  I bought the motor with clean block and balanced crank, rods, and new cast dished pistons and rings. Block has been bored 0.030" over.  Measured the deck height at 0.060".  Piston dish measured at about 15 cc. Total "squish" including piston dish to top of block is about 30 cc.  Head chamber volume measured at about 35 cc.. Stock 2.8" stroke running in 3.53" bore.  Feel-Pro head gasket #7984PT is metal fiber composite with uncompressed thickness of 0.050".

I'm calculating compression ratios in the range of  7.6 - 7.8 depending on assumed head gasket thickness.  I have a new moderately aggressive street cam in it (getting spec fron PO) and will be tracking and Vintage Racing the car with this motor.  I  was hoping that the motor would be set up for a compression ratio of at least 8.5, and even better if it was in the 9-10 range.

From this older thread, it seems that the new 0.030" dished pistons may be the problem.  They may be too short, resulting in the large deck height, and both the large deck height and large dish are the cause of the low anticipated compression ratio.  Does anyone know where I can get a set of 0.030" over flat top Pistons that may be taller than the ones that are in there now?  If they aren't readily available, and I have to use the existing pistons, I can get a compression ratio of about 9.7 with a zero deck height, which I don't  think is reasonable. I calculate a compression ratio of about 8.2 with a deck height of 0.040" and the existing pistons. I'd really like to get rid of the dished pistons if possible.  

All comments and suggestions welcome.

[Dan Jones]

In the United States, there are four commonly available cast pistons for the Buick 215/Rover 3.5L/Oldmobile 215:

 Silvolite p/n 1718
  1.843" compression height
  2.812" piston dish diameter
  0.15"  piston dish depth
  15.27 cc's dish volume
  Advertised as 8.8:1 but similar pistons were rated at 8.13:1 in Rover 3.5L as installed in a Triumph TR8

 Silvolite p/n 9598
  1.843" compression height
  2.68" piston dish diameter
  0.094"  piston dish depth
  8.5 cc's dish volume
  Advertised as 9.6:1

 Egge L2152
  1.844" compression height
  0.062" piston dish depth
  Advertised as 10.25:1

 Egge L2153
  Flat top replacement piston for Oldsmobile 215
  OEM Olds 215 piston did not have valve notches
  Illustration for L2153-8 on Egge website shows 4 valve notches
  1.875" compression height

Sources for the pistons are D&D, Ted at TSI Imported Automotive and Woody at the Wedge Shop.  It appears you have the Silvolite 1718 pistons.  Back in the day, installing OEM Olds 215 flat top pistons in a Buick 215 was supposed to yield approximately 12:1 compression ratio but those had no valve notches.  When I calculated compression for a Buick 215 with flat top pistons, I could only get 12:1 if I assumed a zero-decked block.  Also be aware that re-sizing connecting rods can shorten them slightly, reducing compression ratio.  You might want to contact Egge and get the dish volume for L2152 and the valve notch volume for L2153, then calculate the compression ratios.

Dan Jones

[Motor-vated]

Thank you Dan.  That's exactly what I need to know.

Mike S.

[pcmenten]

What about swapping to the Chevy 5.7" rods? Aren't the Buick/Olds rods 5.66"?

[Dan Jones]

> What about swapping to the Chevy 5.7" rods? Aren't the Buick/Olds rods 5.66"?

Yes but you need the small journal size version and then they need to be narrowed and the pin diameter is different.

Dan Jones