Compression ratio with 250 B&M
03-25-2015, 12:22 PM
#1
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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?
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?
03-25-2015, 02:50 PM
#2
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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?
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?
03-25-2015, 03:42 PM
#3
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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.
03-25-2015, 04:08 PM
#4
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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.
03-25-2015, 04:30 PM
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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!!!!!!!!!
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!!!!!!!!!
03-25-2015, 04:34 PM
#6
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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!!!!!!!!!
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!!!!!!!!!
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.
03-25-2015, 05:05 PM
#7
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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.
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.
03-25-2015, 05:11 PM
#8
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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?
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?
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?
Last edited by MILD THUNDER; 03-25-2015 at 05:14 PM.
03-25-2015, 10:36 PM
#10
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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
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
