Over Bore Performance Decrease?

[87Turboid]

So a new question came up yesterday maybe you guys could shed some light. So in the rebuilding process we bore and hone our cylinders to a clean up any imperfections and true roundness. Now when we decide to bore we go with anywhere from 20-60 thousand over. Funny thing is a lot of European or Japanese motors go 20 over max then it’s time to resleeve. A lot of piston manufacturers start at 30 over and many rebuilders always start at 30. Now if you wanna save meat you need to pony up and pay the big dollars for top notch pistons that start at 10 or 15 thousand over or less with customs forged. Now with all this extra meat in the cylinders it is all good for rebuild after rebuild but the problem I see is the lowering of the combustion static pressure. Now you’ve lowered you compression ratio.

Racer except: “ The overbore penalty is a reaction from rule makers that are out of touch with reality. Most other sanctions allow a 30 thou over along with a 30 thou crank under with no penalty. When you increase the bore but do nothing to the combustion chamber volume, you decrease the static compression ratio. In talking to my machinist, they don't even do 10 thou on a street car. They go to 20 thou right out the gate on a 4 banger. Look at the after market parts guys. 20 thou is a standard. It's not like a a regular regional race you'll get a bore check anyway. You'll be lucky if tech knows the weight of the class or even has the scale calibrated. Unless you cheat the head and cam, your over bore is a performance decrease. I don't know what the agenda is for those that made the rule but it's clearly not in the best interest of the hobby racer.”

So with this info and all this over boring what are you guys doing to compensate for this? Has this question come up before?

 

[87Turboid]

Is this where deck shaving and head gaskets choice come in, has anyone done any combustion chamber resizing? Thanks for any and all info.

 

[marleyskater420]

This is why certain vendors, like Earl Brown, Cottons, and us- RPS, offer custom pistons that you can move the wrist pin to whatever height you need in order to achieve a desired CR.

 

[87Turboid]

Nice! Great info! So with small increases 10-20 overbore deck shaving is Ok? Now if you were at 40-60 over I could see moving the wrist pin. Correct?

 

[Jerryl]

The article is stating that an over bore with the same stroke and the same combustion chamber size decreases static compression ratio. Is my assessment of the writing correct?

One thing to remember is that any reduction in metal from the block is a penalty in block rigidity.

 

[87Turboid]

Yes Sir

 

[rag231]

The current trend in the LS world is going for the smaller bore. Smaller bore = thicker wall = more block strength. Remember when you over bore there are some things to consider 1. If I have a cylinder head that can benefit from the larger bore to unshroud the valves (for example GN1R heads) and 2. Now I have more cylinder volume to fill. The turbo that seemed super efficient in lb/min flow on a stock bore, will it perform the same or do I have to get an upgrade on a larger bore motor. Look at the trend on stage 2 blocks no one is requesting a very large bore, if you have a 4.060 you may be sitting on it awhile , but if your bore is a 4.00, gone in 60 seconds. WIth 109 blocks after 30 thousandths, piston selection gets very tough, and on your next rebuild you are probably seeking a stock bore block. Compression ratios have been debated also and I think the majority of the people will at the next head gasket change or rebuild opt for a little more compression. If a poll was taken now I would bet greater than 75% of "performance inspired" drivers wouldn't mind another point at least. Some would take 2 with todays fuel options. Remember also with deck shaving, cam lift , rocker ratio and head gasket thickness comes into play where you may have a valve get close to your piston or the edge of your cylinder wall. What kind of engine build do you have in mind?

 

[Jerryl]

Yes Sir

My brain can't quite follow that logic.

Regardless. . . . With the availability of custom pistons I would never bore our blocks bigger than it needs to.

 

[87Turboid]

The current trend in the LS world is going for the smaller bore. Smaller bore = thicker wall = more block strength. Remember when you over bore there are some things to consider 1. If I have a cylinder head that can benefit from the larger bore to unshroud the valves (for example GN1R heads) and 2. Now I have more cylinder volume to fill. The turbo that seemed super efficient in lb/min flow on a stock bore, will it perform the same or do I have to get an upgrade on a larger bore motor. Look at the trend on stage 2 blocks no one is requesting a very large bore, if you have a 4.060 you may be sitting on it awhile , but if your bore is a 4.00, gone in 60 seconds. WIth 109 blocks after 30 thousandths, piston selection gets very tough, and on your next rebuild you are probably seeking a stock bore block. Compression ratios have been debated also and I think the majority of the people will at the next head gasket change or rebuild opt for a little more compression. If a poll was taken now I would bet greater than 75% of "performance inspired" drivers wouldn't mind another point at least. Some would take 2 with todays fuel options. Remember also with deck shaving, cam lift , rocker ratio and head gasket thickness comes into play where you may have a valve get close to your piston or the edge of your cylinder wall. What kind of engine build do you have in mind?

This is basically a stock plus build, 15 thou over JE pistons Comp cam and springs and lifters, not sure of the lift yet will have to check, as this was a package buy with two blocks. Stock heads rebuilt may be ported havent checked yet, (still in storage) looks like stainless valves not sure of size. Still debating headers and turbo. So this is all good but with 15 tho over and having the deck surfaced to compansate for compression the most I'm looking to change are the pushrods, head gasket which I have to buy anyway. Depending on cam lift. Correct?

 

[earlbrown]

The reason you seem most manufacturers offering only .030, 040 and .060 is just to keep the part numbers down. (small engines use 1/4, 1/2, 3/4, and 1mm) It'd be ridiculous to make a 005, 010, 012, 018. 022, 030, 036, 041, etc.. for every single engine. So they just do it in steps. And due to that, most machinists think like that. If you have a STD bore block that's tapered you automatically fire up the boring bar and go to 'the next step' offered by the piston manufacturer. Most of the time, that's straight to .030". If a .040" block comes in with scratches, ti's time to his .060" and "the block is used up" after that. Boring isn't a performance thing, it's a tolerance thing.

That's why with my slugs, I tell people to measure the worst spot on the worst hole, hone it clean and give me an exact number. Not only does that leave the maximum amount of meat on the cylinder, but it also keeps the customer from having to pay for a bore job. That savings alone pays for about 1/2 the price of the pistons. (not having to deck the block pretty much pays for the other 1/2).

As far as static compression going down in all cases that's an overly simplistic statement and not true. In some cases it could go up. And it most all cases, it's so inconsequential it doesn't matter one way or the other. The deck height variance can change the static compression ratio more than removing .005" of meat of a cylinder wall.

 

[87Turboid]

Great info Earl, so with boring 15 thou over do you suggest shaving the deck at all? So maybe on this next block I will check out your pistons. This present block could of been honed maybe 5 over and used your pistons. Lets see now... JE pistons, Block boring would still have to hone, yes thats is getting close to custom pistons. Boy I would love to see some pictures of those slugs of yours and prices. So these custom pistons of yours are milled on a CNC machine or how are they made? or is this a secret process.

 

[rag231]

I personally wouldn't shave the deck unless I was correcting factory tolerances or someone oringed the block and fouled it up somehow. Same goes for the heads.

 

[Ttype6]

When you increase the bore but do nothing to the combustion chamber volume, you decrease the static compression ratio.

Increasing either or both the bore and/or stroke of an engine and not changing the combustion chamber size will always result in an increase in compression ratio.

 

[bison]

I’ve gone from .005-.035” over. Increasing bore without changing anything else will increase compression ratio. Pistons should be ordered as needed for application


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[earlbrown]

Great info Earl, so with boring 15 thou over do you suggest shaving the deck at all? So maybe on this next block I will check out your pistons. This present block could of been honed maybe 5 over and used your pistons. Lets see now... JE pistons, Block boring would still have to hone, yes thats is getting close to custom pistons. Boy I would love to see some pictures of those slugs of yours and prices. So these custom pistons of yours are milled on a CNC machine or how are they made? or is this a secret process.

There's a few different reasons to shave the deck. One is if you're pistons are too far down in the hole to give quench or desired compression ratio. Another is if all four corners of the deck are out of whack and you're building something other than a grocery getter (very common). And one where decking is flat out mandatory is if you want to run MLS gaskets. When I first built my 4.1 the only choice were Federal Mogul hypers. I had to deck .035" off my decks just to get .060" quench and 9:1. (and since the smallest set you can get is .030" I had to remove more metal than needed.
That's the reason I started my piston project. I busted two pistons within 100 yards of my driveway... rebuild the engine with a new set... and busted one within a 100 miles. Parked it until I could finish the project. Now instead of having to remove meat all over the place, I can just move the piston up.

For example, on my BBC jetboat, I measured all four corners of my block. Found out which end was the shortest, put the block in the square decking fixture I built and started shaving until just a smidge of that corner was dirty. Took that measurement, flipped the block and made the other side match. On the bore, I measured the worst hole where the top ring stopped at TDC. Honed that hole until it was clean and made the others match. Ended up .008" over and I took another .002" with the fine stones for my moly rings. So I got a set of .010 over slugs with a CH that's perfect for my exact deck height. (020" in the hole to go with my .020" gaskets. Never had to lay hands on the boring bar and only removed the minimum amount of meat from the deck to have a smooth surface instead of the horrible factory cutter marks (and high spots around the head bolt holes).

My slugs start with one of two USA made forgings, then got a few different CNC machines using diamond tooling, the hand deburred before the final inspection.

 

[earlbrown]

These are my small dome boat slugs that started this whole ordeal...
https://turbobuick.com/threads/what...ton-for-the-average-build.395533/post-3223057

 

[87Turboid]

Thanks for all the info Earl and all the pictures, you are a wealth of knowledge sir. So correct me if I’m wrong but the piston starts out as a forged block of aluminum and is milled from there.

 

[87Turboid]

Also nice touch with your name on them, very cool. The domes are very cool as well. I don’t know why but I was thinking it was more like the movie “ The Fastest Indian” when he was forging or was it casting his pistons in his little shack with the boy next door looking on. Man could you really make pistons that way all by hand? That was a great movie.

 

[earlbrown]

Yes, they start out as a chunk of aluminum that was heated and pounded into a piston looking shape thing. Then after I give the specs I want, they start getting turned, faceded, broached, honed, milled, and deburred into something that can squish and bang.

Machine pr0n ahead....<iframe width="560" height="315" src="

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LOL, no. I do all my machining manually. Can you identify the part sitting on what's about to be a bad ass square deck block truing fixture. Oh, and a future mouse murdering shop cat.

 

[87Turboid]

So Earl you are a machinist first, mechanic second or both at the same time? I think that part is my intake your working on right? Just cleaned mine up. Check out those intake ports, some ruff casting.