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MG MGB GT V8 Factory Originals Technical - Decking Block to Increase Compression

Can someone with more knowledge than me please help. I've been discussing with a friend the subject of decking a 3.5 block to increase compression while still using stock 8.13 pistons. The engine in question has low mileage and is in excellent condition. We want about 10:1 compression. I say that we will run into all sorts of problems from pushrod lengths to the intake manifold not fitting properly if this is done or even if the heads are milled. He says that this has become an accepted method among some Rover engine builders and it's a no-problem affair. I have a set of good used Buick 10.5:1 pistons that would probably be a better option but my counterpart also swears that Buick pistons are junk. Does anyone have any info? Thanks.
David
David

Someone please correct me where I'm wrong.

You would need to deck .020 off the block for every point of compression. If you end up milling .040 off the deck, you'll need to shim the rockers, or get shorter pushrods to offset the difference. The rover rockers are (obviously) not adjustible, and you can't just fudge .040. It probably wouldn't be a good idea. I have buick 10.5:1 compression pistons in my rover 3.5 and haven't had a problem with them. We'll find out when I take out the driveline this winter and see what all is there.

Justin
Justin

The object of block decking (besides to equalize the 4 corners and to get a true 45 degree crank/cam centerline split) is to obtain the optimum quench area between the piston crown and the head,.040" is the recognized number.If you need to take a lot off the decks to obtain this distance with the pistons you have then it is best to use a piston with a taller compression distance and keep the decking to a minimum.Otherwise the above mentioned problems with intake fit and pushrod length will need to be addressed.
Dale

Decking the block can also cause an ill fit between the head intake ports and the manifold. The manifold might have to be machined for fit and port alignment.
Dann Wade

David
Best to speak to someone who does this all day every day for a living - I believe 9.75:1 is generally thought of as the best solution for european fuels - but Simon at Progress engineering (and many others here ) will give you chapter and verse. The firm routinely got 350BHP from road-going versions of the Buick'Rover V8 before BMW re-engineered the RR.

If it's power that really concerns you - careful choice of exhaust system and cleaned up heads will transform performance.

Simon is at (+44)1622 687070
Roger

Justin,
Thanks for the input and info. Have you run into any pinging problems with your 10.5 engine?

Dale,
I appreciate your input as well. You make very valid points.

Dann,
It's good to see that you and I agree on the manifold issue. I hope all is well with you and your family and that your venture is progressing. I'll give you a call in the next few days and chat.

Roger,
I'll give Simon a call and see what he says. Power is an issue, but I don't need a finicky or high-strung engine. The motor in question is a US spec SD1 piece and I'm trying to gain as much with as little cash outlay as possible. It's destined for a fairly stock and conservative rubber bumper GT. As always, many thanks.

David
David

If it pings, I've never noticed it. I bought my pistons MANY moons ago from Ted Schumacher, and as I said, I haven't had any known problems. (knocking on my wooden desk profusely)

I kinda eluded to this in a v6 thread a few days ago, but when you want more power out of your car, will a 3.5 suffice? 3.9's are starting to get plentiful, and cheap. Maybe a better starting point?

Justin

Just as a side note on compression ratios,the theoretical published spec for a given engine and the actual compression ratio are often 2 completely different numbers.The variable is most often the combustion chamber volume,followed by deck heights.OE machining has a lot of room for improvement.As an example, although an extreme one,I decked a 340 Chrysler once that had decks measuring the same at the front,one bank got .020" taller at the back,the other bank .020" shorter for a total .040" difference.Most V8 blocks do vary at least some but that one was particularly memorable.The same can be said for heads.Sometimes heads are machined so as to produce chambers that get progressively smaller or larger one end to the other.A very common Chevy head that we see a lot of has a published spec of 76cc but is often closer to 78cc.Big difference when calculating CR.We see a lot of "11.0:1 motors" that in reality are 9.0:1.This explains why they could run them on 87 octane fuel.My point is that if you are planning an engine build and want a specific CR do not rely on the advertised CR that a particular piston is supposed to give you.Measure the piston height and the chamber volume and then you will know for certain exactly what you have.Especially important if you are choosing a cam or other components based on your CR.
Dale

I was talking to my partner in crime concerning my original post and he said that Glenn Towery has built several low compression Rover engines utilizing 8.13 pistons and decking the block 40 thou to increase compression. Glenn, are you out there? What's the scoop?
David
David

I would suggest changing pistons rather than extensive machining, especially since you already have a set to use.

My first 3.5 Buick was an advertised 10.75:1 compression, but was never actually cc'd & calculated. It was running very well at 270,000 miles when I broke a rocker shaft drag racing a new Camero, & then retired the engine. Pump gas- 93 octane-was fine with no ping.

See Dale's earlier comments about actual compression ratio vs. advertised.

With a low milage engine, careful measuring of the bores will tell you if your pistons are suitable in diameter. If so, deglazing the cylinders may be all the machining required. At this point an appropriate cam such as the Crower 260 or 272 or your favorite other choice would be a good idea to compliment the increase in compression. Now you can use the 4 bbl intake with a 500 cfm Edelbrock or Carter carb.

I personally would not recommend a Holley unless you have the time & inclination to get it set up correctly. Well done, they are great, less than well set up, they are pigs.
Jim Stuart

Dale and Jim,
Thank you for taking the time to respond and for sharing your knowledge. Lots to think about and good advice on the advertised vs. actual CR's. I had the head and pistons/deck on my crossflow 'B cc'd when it was put together late last year and found just what Dale described. I'm glad that I didn't just hope for the best because more than a little work was needed to get things right. I'll check my bore sizes and then measure the Buick pistons and go from there. Jim and Justin, I'm encouraged by your good experiences with high compression, too. That's had me worried. I have a few Buick 4 barrel manifolds in my attic. I guess that one of those will be adequate for an Edelbrock.
David

Guys,
I'm ignorant of these things but wouldn't it be easier to skim the heads & shim the rockers?. Easier than Machining the block and must be cheaper and easier than new pistons. Mines a new low comp' motor under heads with about 50K on them (bought it that way). Although I don't necessarily miss the addittional 5 BHP, at some stage I'll be doing some work on the heads and that might be the time to bump up the compression ratio.
Any thoughts?
Peter

Peter wroet:

"I'm ignorant of these things but wouldn't it be easier to skim the heads & shim the rockers?."

I'm also ignorant of these things, but it seems to me that you would NOT want to shim the rockers after skimming the heads, even though that seems to be the common knowledge on the subject. Skimming the heads does not change the rocker arm/valve stem gometry at all, only the rocker arm/pushrod geometry. If the heads aren't skimmed by much, there may be enough adjustment to compensate for the head skimming. When I had the head milled n my TR6 many years ago (I forgot how much I had it milled, but it was significant), I was able to get by with just an adjustment.

OTOH, wouldn't skimming the head have the same impact as decking the block? The rocker arm/puchrod geometry would change the same, as would the intake manifold/head interface.

Milling the heads for a performance gain is a routine operation, so I know the problems can be solved, I just don't know exactly how it is done in regards to the intake manifold problem.

I also know that decking the block is a routine operation as well, not for compression ratio gains, but just to ensure the deck is parallel to the crank centerline, as part of a "blueprintng" operation. This eliminates the problems mentioned by Dale.
Dan Masters

Were do I start? I have read a lot of the hot rod wright ups that have been put out by Phil Backer, Vic Elderbrock Sr. & it is known that the G.M. comp. ratios were off by 1/2 a point (8.8 was realy 8.3) We cked. a rover 3500 with the 10.5 & it was realy 10.1. the low comp. piston made 135hp. & the high comp. pistons made 185. If you deck the head you make a small hole smaller, if you deck the block you make the BIG hole smaller & with cutting .040 of the block you will get 1 point higher comp. If you cut more than .040 you might have to egg file the holes that hold the intake to the heads to get the bolts to start & get the intake ports to matck & push rods. The buick pistons are a junk cast piston & there is no way I would put in a USED set of G.M. pistons were the rover are a forged piston & are 100 grams heaver. A 9.13 rover motor rung good & can run on 89 otc. & make a good 180-185 hp. Put a alu. flywheel on & a buick dizzy with elect. ing clean up & match the ports & it will run good
Glenn Towery

Soo,
Pistons seems to be easiest after all,
Thanks for the replys
Peter

Glenn,
Thank you very much for the reply. In conclusion, I can take an 8.13 engine, deck the block .040 and end up with a 9.13 (or thereabouts) CR utilizing stock pistons. I should have no valvetrain problems using the stock pushrods, unshimmed, nor should I have a problem fitting the intake manifold. I should use a light flywheel and a Buick distributor (w/electronic conversion) and I'll have a pretty slick motor that'll run on mid-grade fuel. Oh yeah, I will need to port match the intake due to the machining of the block.
David
David

I'll throw in my bit of experience here as I went through all this mess around 6 months ago. I was confused when I did the math on my engine. I bought silvolite 1718 pistons advertised at 8.8CR. My math worked out to 7.775:1. I knew the thicker felpro gaskets were part of the problem, but couldn't figure out where the rest of the CR went to.

I asked around to see if there was any difference in the compression height of the various pistons. I never got any hard numbers. Glenn T said there was no difference in stock rover vs silvolites (correct me if I misquoted you Glenn) While Dan over at D&D said there was a difference.

Anywho, with the headgaskets at .045" and a deck height of around .060-.065, I was running over .1" of squish height. I heard that big of a squish height was going to leave me with a poor running engine. So I stripped the engine down and had the block decked .055" That pushed me up to 9.0:1

I did have to get custom length pushrods made up. The guys over at smith brothers had reasonable prices (www.pushrod.net) The intake manifold fit is so-so. I'll take it off and get it machined down later on.
Michael Hartwig

The best way to increase your compression is with pistons. when decking a block or the heads, the head gasket and block loose some integrety than can cause problems later, unless the block has extra material like the HD Bow Tie Chevy blocks.
Gm pistons are cast but not the cast you may think.
The cast use by GM is call Perma Mold the the alum is injected into a mold making it free of defects and creating a very strong piston that can be use in a turbo charge engine, which GM did.
GM also change the high of the wrist pin thus increasing the compression with the same block, gasket, and head. This type of piston is call by GM HO etc. Diferent applications diferent compression.
Kenne-Bell has the best knowledge when it comes to Buicks and parts.
Bill Guzman

I see Glenn has added a comment that I was going to bring up,that of you get more by decking the block x amount than you do by decking the heads the same amount.The cross sectional area of the Rover/Buick chamber is quite small as compared to the bore diameter,so it takes 3 bowls of Raisin Bran to equal one bowl of Total.Additionally the benefit of a proper quench area achieved by decking the block has been proven in various tests.An engine with proper quench can run a higher compression without detonation as compared to the same engine with less compression and the piston down the hole.I think Michael Hartwigs experience with CR and piston recession is quite normal and he fixed it the right way in my opinion,by getting the quench corrected.Normally anytime more than .040" is removed from a block or heads the intake will be tight and should be cut at least the same amount,or more depending on the intake angle.
Dale

This thread was discussed between 08/10/2003 and 14/10/2003

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