Bbc SC deck height
 

This has been discussed before however what is the magic number on the piston below deck? This is debatable question and can ask 10 different people and get 7 different answers so the question is more directed toward SC builds with hands on experience. At the same time what is considered a good target quench. I have my own experience with such however on lower hp builds and mostly NA builds. 555 SC 8.5:1 comp build.

NA I'd typically be btwn 005-008 in the hole with .040 gasket with a total of .045-.048 quench. seems like many engine builders try and end up with 0 to .010 deck height to help avoid detonation. SC being a little more forgiving??? Forced induction and chambers can change things up but how much?

Keep the Quench between 34-38 for less detonation, I just finished an engine that piston was .008 down in the cyl., used a .027 Cometic head gasket , I have build many engines, all NA., with Quench in this range, never had any problems.

i usually look for.050 but a little less will do.

That's seems to be middle of the road. I wasn't sure if an sc application with more turbulence so to speak would have an affect or not. Plus the chambers are going to be reworked and opened up regardless so would more help or not? Just a thought that popped int o my head. Jim v tried steering toward flat tops but just don't see any good way to get 8:5.1 or less without some kind of other potential risk. Personally I've always liked flat tops when able to use just cause of the even combustion. May not mean chit in the real world but always made sense to me. That's only when comparing to higher dome pistons though. Almost like side loading. Anyway I'm getting off track here.

as you know different builds will require different pistons.if i am building a pro charged deal i like more compression because the unit needs to be spun faster before it makes boost so it needs to make more power before boost.a roots or screw blower makes boost as soon as you start to open the throttle blades.the total amount of boost and the engine application needs to be be thought out before deciding on a c/r. on a pro charged 565 i built last year the c/r came in at 9.2 to 1,if i had that one to do over the c/r would be closer to 10 to 1.

Ya I'd like to have my static mid 8's + or - but I was more so wondering about the quench and the deck height. Looks like I would end up with 20 -22cc dish regardless so I'm not going to be concerned with deck height at this point.

Just for reference, merc 1075's use a flat top piston just over .100 in the hole

the best is 0 if you can get the compression where you need @ that

I like them around .010 in the hole with a .040 gasket. Has worked well for me

with a new block i almost always go .010 in the hole,that way if it needs to be resurfaced down the road for some reason you still have some metal to machine on and won,t have the piston out of the hole.

so here is a tricky question,should a builder go with less compression and more boost or more compression and less boost?

That's interesting merc sets up 1075's .100 in the hole. I've heard and read of builds close to that but just goes against everything I was taught.

In regards to above post I guess once again I could look at what merc does. My initial conversation with Valako was to keep the compression down around 8.5 or even 8 and throw the boost at it. I'm open for suggestions on what makes the most sense. This is a learning experience for me and not afraid to be up front about it.

Tks guys
John

Here is my opinion. The more power that an engine makes N/A, the more power it should make under boost, so therefore higher compression and lower boost should be better. However, most blower pistons have a dish in them which will equal lower compression, requiring more boost.

I am going with B. I want the best pistons that I can afford for the application. True blower pistons. Thicker tops and lowered rings. Not some flat tops that weren't really designed for supercharging.

what supercharger are you going to use?

Hey snap. Just wondering if you have real world experience. I'm not questioning at all just real world data. Personally I think what boost etc could dictate what pistons you end up with in particular build. If your running 15 plus pounds of boost then that would change the whole picture. Now if your running a 8.5:1 uild with 5-7 pounds of boost then that would change things up. Good high comp/boost pistons weigh a ton but built for 1200 plus hp. Just asking.

Unless I misunderstood Valako he said get pistons that can handle 1000 hp plus without concern. Just curious what are they using in the 900-1075 sc engines. Probably the best without a doubt but I do know Jim ran blower engines with well over 1,000 hp on blown 540's pump gas engines with your basic run of the mill 2618 pistons with no issues. Not over boosted though by any means. Evidently he doesn't like heavy pistons. Just a guess..

614 Build , .060 in the hole , .060 head gasket , 22cc dish , 120cc chamber. Compression is 8.0:1

871's. I'm not looking for any record breaking hp numbers but reliability etc. will be run in cold water with 850's and similar cam to 1075 sci but have another cam for back up on dyno that made over 1k hp with 540 builds but still involved raising the ports.

What did smitty use for a blower?

You can get flat tops that are blower pistons. But, I do agree. A blower piston will have the Top ring down .300+, thicker piston, etc. Also, the wrist PIN. Most forget about the pin. I have JE blower pistons, with upgraded .180 wall , series 52 High Carbon steel through hardened pin. I believe the standard JE pin, is a series 51, low carbon steel pin.

Wondered when you'd be around.

What about the deck height and gasket height with over all quench scinereo? I know you have researched this and have info that could potentially help. Amazing how much you can research this and still be lost. You've obliviously got a good thing going on with your builds so why not use them as an example of what's proven to work. Btw both blocks are identacle.

I personally wouldn't get too wrapped up in it. Fel Pro's MLS gaskets, are avail in .041 and .053 if I recall. Cometics, you can go much thicker if desired, but the price also goes up a good amount. If you're truly .010 in the hole, I'd go with the .041. IF you're 0 decked, I'd go .053.

Its not nearly as critical as a N/A engine, and even N/A engines, its not do or die like some make it out to be. Some guys will chase a super tight quench, thinking anything over .050 is horrid. Theres tons of marine BBC's running around with pistons .02 in the hole, with a .040 fel pro gasket, just fine.

I'd prob shoot for .050-.060. A little more, little less is fine.

As far as the static compression goes, on a traditional roots build, I like between 8 to 8.5:1 for the most part. If youre blower limited, with a small blower, then you want to keep the static up, and the cam small (minimal overlap). If you have enough blower, the lower static, more overlap, and more boost, will walk all over a small cammed , low boost, higher compression deal.

Meaning, an 8:1 engine, with a healthy cam, and 10lbs of boost, will generally outperform a 9:1, 5lb boost, small cam setup on the big end. Big torque numbers in the low end from short cams and high static, are nice, but also leads to big cylinder pressures that can cause detonation. Lots of ways to skin the cat.

High static compression, can make high PEAK cylinder pressures. But high PEAK cylinder pressures, aren't what make supercharged engines make big numbers. It high AVERAGE cylinder pressures that get it done. Its why a 565 thats 12:1 NA, still wont make the power that an 7.5:1 565 will make, with 10lbs of boost. Although the final compression ratios both may be 12.5:1, and both be limited to a particular octane level before detonation occurs.

I'm surprised that piston to wall clearance, piston material, bore size and rpm aren't talked about in this thread when considering compression distance. Its all a factor.

i was going to give you some info from builds i have done with 8,71s but it looks like joe has you covered and i am not to good putting it in type.

I agree, keep in mind a 600hp na motor with 14.7lbs of boost should theoretically see a extra 600 hp minus what it takes to turn blower , any other parasitic losses that occur from makng the extra hp, assuming cam, heads etc are up to the task. Thats 40 hp per lb of boost
So would we want a 650 hp motor with 5 lbs of boost/200 hp minus parasitic or would we rather have 600 hp with 10 lbs of boost, ie a extra 400hp?its actually not quite that simple though as parasitic loss goes up exponentially.
Personally i ran 8.5-1 , 10 in hole w .042 head gaskets and 14 lbs boost (blown 540).
I did switch once to diamond pistons that were pretty much zero decked w head gasket compressed to .039 i had a few pistons touching heads enough to knock ring seal out of motor (.0065 wall) Im surprised to see another poster saying he successfully ran .027s .010 in hole blown. If i had zero decked pistons i would run .053 minimum head gaskets after my last experience,fwiw, Smitty

some guys don,t think about carbon buildup or rod stretch because they are to bent about quench.the longer the stroke the faster the piston changes direction.

forget about quench... not important in boosted applications... blower will create more turbulence than the engine will know what to do with...the answer to your question depends primarily on the style/design of your piston

its not a tricky question, lower compression (static) and more boost will yield a higher gain and less prone to detonation IMPO... boost/time relationship

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Here is a little 540 that I built many years ago.
8.2-1 compression, 10 pounds of boost. ran for years

My current 509 is going on 230hrs without having the bottom end apart. Makes a little over 700hp with 5 pounds of boost.

Currently building a pair of 555's with blowers. The one thing that they all have in common is that they all have JE -20cc blower pistons

try that on a heavy single engine boat with a pro charger,like i said,every build is application specific.so yes it is a tricky question.

Like I said piston design is critical to this question... I prefer a flat top with thick crown, low top ring placement as opposed to the partial dish with quench pad

true Mike, every build is different.. always an exception to every rule

Thats a good point . Thats one of the reasons a Screw/roots, centrifugal, and turbo, all require different cams.

Sticking a cam in that works great in a roots/screw combo, might make the procharger engine, feel like a turd. Or worse, a roots blower cam, in a turbo engine.

Lets say one was building a 540ci, with a roots. You put in a 248/256 114 LSA cam in it, where the intake valve closes at 53* . You nail the throttle, thing makes gobs of torque and puts you in the seat. You take the roots off, put a procharger on it, and all of a sudden, the engine becomes a slug. Because the late closing intake valve , and low static compression, means that off boost, she cant make any cyl pressure whatsover.

You'd want to get that static up, and close the intake valve sooner, than you can build some cyl pressure at low engine speeds.

Thats one of my favorite thing about a positive displacement blower. You can change the dynamic compression, or effective compression really, with the push of the throttle, or , with a pulley change.

Doing a comparison of an engine model I was working on last night, I decided to do a comparison, of what my previous post states.

I first had a 502ci, using a roots. Then switched to a V7 vortech centrifugal. The cam, was a roots style blower cam. The BMEP (brake mean effective pressure) numbers are as follows. Keep in mind, both combos set up to deliver 10psi of boost at 6k rpm.
Vortech/roots 10-71
3000=150/206
3500=170/219
4000=185/227
4500=204/237
5000=216/244
5500=218/246
6000=219/242

As you can see, the 8.5:1 engine, with roots blower typical cam, simply makes more cyl pressure. Now, to even the playing field, and get BMEP numbers closer, I had to knock out 10 degrees of duration, advance the cams ICL 3*, and raise the static from 8.5:1, to 9.5:1. The intake valve is now closing at 43*, rather than 51* with the roots setup.

So, while a roots blower may do well with more duration, a centrifugal, will NOT like the same setup. The numbers are even further apart, below 3k rpm, which in a street car, would be very important.

If you try to build a centrifugal engine, like a roots engine, you are losing. If you try to build a roots engine, like a centrifugal, you are losing. IMO, this stuff, plays a much larger role in the final product, than simply arguiing over a particular cyl head port volume. Like say, a 335 vs a 315, or 325 vs 305.

My Cat 572s are .050 in with a .080 gasket... at tad over 8:1 with my chamber...

U using the 252/257 SR cam

I think he's a little bigger than that :coolcowboy:

he wouldn't believe you if you told him... so don't as that's one of my secret weapon cam grinds!!! and it works nails!!!

my pistons are .020 in the hole with .041 gskt. with 8.25 static comp. 8-71s 6% under driven making 5.5 of boost
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