Compression ratio with 250 B&M
 

Running into a lot of issues with pistons...

Looking at replacements it seems that my choices for the stock 4.25 bore are very limited. I can go with a Speedpro L2399F to replace them, but I do not like the VMS-75 alloy for use with the blower. Replacements that I've seen are either way up there in compression or very low.

Example: Icon (Keith Black/UEM) makes a forged piston that will work with my Dart race heads (24 degree intake valve vs stock 26), but in either a 9.5:1 or 7.9:1 compression ratio (18cc dome or flat-top, respectively).

What's the better direction? Lower the compression and spin the blower harder, or crutch the blower with a lot more compression? 9:5:1 scares me a little. There is no option of boring the block .030 to get better pistons. The block was already prepped and made ready, and there's no reason to bore it other than piston selection.

What would you guys do?

Just a thought. Buy the dome Pistons and have a little taken off the dome to get you where you want to be. A competent machinist could make short work out of it with a Bridgeport. I would not use the speed pro Pistons regardles of the blower or not.

I had the same issue. I went with porting the chambers of my heads to get me to 9.0:1. But, I also called JE/SRP about fly cutting the tops of my pistons off. They said with the solid dome pistons that I had there was no problem with me cutting the tops off to get the compression down to 9.0:1. They also told me the density of the aluminum they used and how many grams/cc I needed to cut off.

Good advice... now I just need a piston that fits a standard bore AND isn't a Speedpro. With a couple of exceptions I've had no luck finding an off-the-shelf piston that's worth putting in the engine.

Order a set of JE or Carrillo custom to your specs. They are really not much more than a shelf piston. The benefits far outweigh the cost. The speedpro(TRW) LF 2399 is a fine piston for a budget build, 5 lb boost, 454.
I've said this to you before, this engine has been built 1000's of times by every builder there is. If your tune is correct the 2399 piston won't be your weak link. Yes it's a bit heavy and not as well made as JE, KB,Carrilo....., but it's a decent choice if your not looking to go custom. I've owned 10 pairs of engines with the 2399 in them , from 420 NA to 722 SC. NEVER had a failure do to piston!!!!!!!!!

Agreed. I'm probably just overanalyzing the whole thing (I'm famous for this).

If I could just find four 3999295 pistons (that use the more common standard ring) I'd be in good shape. Or eight that are all the same with the shallow groove. I just don't want mismatched parts in the engine.

If you decide to go with new, call Carillo and ask about their "bullet" line of Pistons. They are around 850 for a set but they come complete with the rings and can be ordered with pins that are .150, .180 or .200 wall.

With the rings included, 850 is a good deal.

Also. Wiseco and, I believe diamond will allow you to change one dimention on a set of Pistons for a nominal charge.

Tom, the 18cc dome piston, should not yield 9.5:1.. Even with the block zero decked, a .040 head gasket, and 121cc chamber , thats 9.25 ish . If the block was zero decked, I wouldnt have a problem going with a .050 or even .060 cometic gasket, bringing it down even further, to 9:1, 8.9, respectively.

Also, those race series darts would do well with a little chamber porting. Usually pick up a good amount of low lift flow, and Usually adding 4-5cc to the chamber depending on the work.

Theres nothing wrong with those speedpro/trw style pistons either, in this application, those pistons been used in these builds for decades. What rods are you using? The stock chevy rods?

Did you try Wisco pistons? Compression ratio depends on how long you want the engine to live and what gas you want to run

Thats a tough call . If the heads are iron and you have the keyed snout 250 and its availible pulley options i would
Gravitate towards low buck 8to1 flat top . You will never miss the spongy off idle tip in on boat app
If your head and block surfaces dead nuts on and or heads are aluminum , the world is your mls gasket oyster
Block and heads born for mls gasket duty should be able enter service with no head gaskets and only experience oil seepage.if thats not you and this thing is starting to get on your nerves , cut the new pistons down to the 9cc mini domes or shake down another set of early mag merc pistons . If the 250 rolls over and dies on ya you still have a foundation for some power . If you have the half spline 250 like me make sure you have the 3 different lower pulleys in hand you will need them

those #'s are with what thickness head gasket ? I would lean towards the lower comp ones and maybe play with head gaskets ? the merc 525SC were only about 7.5-1 or close...so if you could come up around 8-1 I think that would be ideal...

agreed... piston won't be the cause of a failure... will likely be tune related... J.E. Blower piston specifically built for the application would be a bit more resilient but not necessary at the additional price... my concern would be that if you're on that tight of a budget you may be in over your head when it comes to maintaining it... just sayin.. not bashin

Understood. Budget really isn't the issue (I've said it but it isn't the huge consideration that it is for some... it'll get done when it gets done... I do miss being out on MY boat though). My thinking is more around the idea that I had good parts to work with from the beginning AND that I didn't want to bore the block (which created a un-thought-of big hurdle). Good short block with less needed in that area meant being able to put money elsewhere (ex: I probably have another $800-1000 that still needs to be put into the heads... valves, springs, etc). In addition to the propulsion, the boat needs better instrumentation, and to be put back together from having the transom and stringers redone. Lots to do and spend on (maybe I should have bought another boat but I really love my 242).

I do not have a problem with the GM 3999295 or older TRW 2399 piston (2618 allow vs VMS-75). If I can find another set of the GM pistons, problem solved for the most part.

Joe, still using the stock rods. Nothing was wrong with them and they were well within spec. Hell, the entire rotating assembly was perfect with exception of the one piston that needed to be replaced... it probably could have been cleaned up and reused but I was being a quite the stickler in regard to what I was putting back in the engine (that lead to the others being changed too). Had the issue with the rings (and bearings) not come up, the short block would have been done weeks ago (btw... King really came through on the bearing side).

Regarding compression with the bigger dome (18cc vs 14cc), I get a measurement of 9.2:1 which is based on:

4.251 bore
4.0 stroke
-18cc dome
120cc chamber

Same parameters give me 8.9:1 with the 14cc (stock) domed pistons (there's probable a smidge of loss with the valve relief on both, but probably a negligible amount).

For conversations sake, is that too much compression for the blower at say, 7 lbs but with aluminum heads? The tune is very important... I know that. Just curious on the thinking is all.

a flat top "zero" deck 4.25 bore x 4.0 stroke with a .040 compressed head gasket and 121cc chamber with net you around 8.2 compression in my opinion its a safe place to be... it may not be what "Joe" has but Joe isn't a first rounder, we all gotta start somewhere, and safe for the first time around is in your best interest...

If the blocks havent been decked the pistons are prob down in the hole a bit. Have you measured that?

I wouldnt drive that blower harder than 80% range. Which is about where the 3" top pulley would put you. That should net about 7psi on your build. If you went with a flat top, if you ever went with a bigger blower, like a 8-71 , 6-71, or 420, you can take advantage of it with the lower compression.

I measured the deck height before the block went to the machine shop and it was at .010 (dial indicator to gauge piston at TDC, machinists ruler across the bore at the pin, feeler gauge). A little material was removed during the cleanup of the block to freshen-up the deck surface. I have not measured since it has come back so the .007 is a guestimation. It's probably closer to .009. It definitely was not down in the hole like others I've seen. The ratio numbers I posted are very close.

The pulley I have on the blower now is a 2.70 which is too small. I'd like to find a 3.0... the search is on. I have a 3.65 also, but that pulley probably won't do a lot (though it can really tame it down nicely).

Regarding compression ratio with the 250 (and smaller blowers), there's a lot of talk about running them with more compression in order to crutch them up some. Honestly, I think this starts to cause some confusion when trying to put together an engine with one of these blowers. It would seem that it would be far easier to just go with a larger blower, and setup the engine traditionally. For my boat, this isn't an option... I would not be able to fit a 420/6-17/8-71 under my hatch. Just saying is all.

I know I can get the flat-tops readily, but using a 3cc number with my head and with the piston where it is, I come up with 7.9:1 which I think is too low of a ratio for the 250.

Jegs and summit have those pulleys. 155192 is the 3", 155193 is 3.34" if I recall. The 3.34 made about 5psi on mine, and 155192 about 7psi.

If you end up going low compression, the 192 pulley would be my pick. 9:1, I'd run the 193 pulley.

If the heads are bare, and need assembly, how about a flat top with some milling of the heads? Those race series heads have thick decks, and can be milled a bit more than the pro 1 series. Down to 105cc /.070

.010 in the hole would drop your ratio to 8.1 and that is fine

+1

All things being equal, I'd be willing to bet the 8:1 engine with 7psi will make more power than the 9:1 engine at 5psi.

Crutching the blower with more compression works great with a 177. Simply because 7psi isn't an option. Well, you could overdrive it engough to make 7psi, but its past the blowers efficiency , and a waste of time . so 9:1 and 5lbs works best. the 250 will be OK at 7psi, but that's about as hard as I'd push it. I wouldn't run 7psi with 9:1 and the 250. I've done it, but its not what I'd recommend to a buddy.

Thanks for the info Joe!

Regarding the heads, they're already done in terms of porting, etc. I need to change out the springs and retainers (originally setup for a HUGE solid roller... sad that the titanium retainers have to go) and switch to Inconel valves. I guess I could have them shaved since they're coming apart anyway, but that wasn't really in the game plan. Not much work though.

That sheds a lot of light on things, Joe. Almost like a light bulb going off!

Yes. 5 psi should be good for 34% more power, 7psi for 47%. The 1 point of compression is generally worth 4%. So maybe 9% more power with the low compression high boost scenario?

I have a set of standard pistons marked 14097021 if they will help you out. They are stock out of a 454 mag and still have the rods attached.

I hate dipping into this stuff so late at nite but i swore the merc bbc blower builds had quite a dish in the piston to hit that 7.5 with off the shelf heads .could almost do a good shot of whiskey out of one . If it was clean .

That is correct. Here is a pic of one[ATTACH=CONFIG]539079[/ATTACH] But on second thought, this is a pic of a 502 blower piston, not a 454.

I believe the 525sc has a very similar piston to that one cannot remember the chamber volume but they were also set to 7.5 to 1