Speedpro H405 pistons, what to expect for performance?

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lee g

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I am rebuilding a 1972 dodge 360 to put in my 72 dart. I bought a rebuild kit 1 1/2 years ago which at the time advertised it included 9.5 to 1 hypereutectic pistons (Speedpro H405 pistons), stage 2 camshaft (can't remember specs). I have a set of J heads (small valves) to use for a while to save money, Edelbrock performer intake, Edelbrock 600 cfm carb. Well I am finally close to assembling my engine (many delays, work, family, my own health) and currently read a post on another website someone has almost the same set up and is only around 8.4 compression ratio.
Should I just put a set of thin head gaskets on it and hope it will put me closer to 9 on my compression? I am limited on how much I can invest in it. It will only be street driven. Any guess on possible horsepower or if it will be sluggish?
I don't really have the extra money to just buy a new set of pistons but if this is going to make it very sluggish I may have to reconsider.
 
thin head gasket will help, but it wont get ya from 8.5 to 9 to one......deck the block, mill the heads, or change pistons...only way to get compression up...

speedpro h116cp or kb107 are the pistons that will get ya there..
 
A new set of pistons kb 107 is around $200 which I am sure would be cheaper than a bunch of machine shop wwork. Do you all think the Speedpro H405 pistons will be OK for just a street driven car?
 
lee g said:
A new set of pistons kb 107 is around $200 which I am sure would be cheaper than a bunch of machine shop wwork. Do you all think the Speedpro H405 pistons will be OK for just a street driven car?
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ok so have the old pistons pressed off, have the thing re balanced, and then press on new ones... ring gap will change from one piston to the other as well... sounds cheaper...lol

i have speedpro pistons in my slant and race it all the time with no issues...
 
I guess they will work fine for just a street car. I guess I had it in my mind that I would at least be in the 9's for compression ratio. I did all the calculations and with my pistons being .040 over it puts me at about 8.7. Anyone know how much the thin head gaskets may bump up the compression?
 
lee g said:
I guess they will work fine for just a street car. I guess I had it in my mind that I would at least be in the 9's for compression ratio. I did all the calculations and with my pistons being .040 over it puts me at about 8.7. Anyone know how much the thin head gaskets may bump up the compression?
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what is the bore on your engine? and what head gasket did you figure with?
 
I had it bored for the. 40 pistons. Currently I have the standard felpro that comes in the kits. Just trying to get an idea if it's worth buying the thinner mopar performance gasket or mr gasket. I am just going to stick with the H405 pistons and continue with my restoration and play with it for a few years. So is it worth it to buy the thinner gasket? I know now I will not reach my goal of being in the 9's for compression without investing more money. With 4 kids I can't see being able to sink a bunch more into the engine without post ponning the completion for a while.
 
lee g said:
I had it bored for the. 40 pistons. Currently I have the standard felpro that comes in the kits. Just trying to get an idea if it's worth buying the thinner mopar performance gasket or mr gasket. I am just going to stick with the H405 pistons and continue with my restoration and play with it for a few years. So is it worth it to buy the thinner gasket? I know now I will not reach my goal of being in the 9's for compression without investing more money. With 4 kids I can't see being able to sink a bunch more into the engine without post ponning the completion for a while.
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its maybe about 200 to mill the heads...
 
I priced the Speedpro H116CP pistons. Right at $300 including rings and shipped. Then I would have to pay to get the old pressed off and new pressed on. This would put me about $350 and should put me in the mid 9's for compression ratio.
Just was hoping not to have to spend more but if I want to boost the ratio up I will have to. You all think I would have a hard time selling the H405s?
 
$280 or so for Speedpro H116CP pistons which would put me in the 9's. Do you all think I would have any problems selling my H405s brand new with rings they would just been pressed on and off of rods? Maybe $30.00 to $40 less than what I paid?
 
lee g said:
I priced the Speedpro H116CP pistons. Right at $300 including rings and shipped. Then I would have to pay to get the old pressed off and new pressed on. This would put me about $350 and should put me in the mid 9's for compression ratio.
Just was hoping not to have to spend more but if I want to boost the ratio up I will have to. You all think I would have a hard time selling the H405s?
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your not gonna sell them for anything becasue there is a huge chance they will wreak the piston getting them off... never heard of press on's working more than once...

mill the heads and be done with it...
 
The compression height on them kinda sucks balls. It is 1.637. So they are gonna sit in the hole a good bit. You can overcome that with milling the heads or block or runnin small chamber heads. I believe also too that, even though they are hypereutectic, that they do not require the special top ring gap that the KB pistons do. That might be worth a call to maybe the Summit tech department, but I am almost 100% certain about that.
 
you probably need to leave it like it is so you won't have to run the high grade gas since prices have gone up so.
 
StrokerScamp said:
The compression height on them kinda sucks balls. It is 1.637. So they are gonna sit in the hole a good bit. You can overcome that with milling the heads or block or runnin small chamber heads. I believe also too that, even though they are hypereutectic, that they do not require the special top ring gap that the KB pistons do. That might be worth a call to maybe the Summit tech department, but I am almost 100% certain about that.
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your right .i've used both only the kb's need more gap.the kb's are also a little lighter....
 
Lee,don't sweat it.The comp,works to 8.5 to 8.8 real world.Been around,a very similar build. The pistons survived 3 re rings( lots of N20....!)I would keep the pistons,go thin head gasket. Milling .030,040,would obviouslg help.Toss the cam,spend money on a small,quick ramp hi lift ,hydraulic.Somewhere around a 262,268 Comp.
 
I have used several KB pistons sets(although some years ago) without issue. I recently freshened a 360 for a customer and 1 Speed Pro hyper piston had very slight damage to the top from injesting a small object. After receiving 3 I could not get 1 that did not have out of round pin bores. I'd be very careful with these as they appear to be made by 4yr old girls with very worn out equipment.Yep, I put the damaged piston back in and ran it as I didn't have months to keep trying my luck with those! YRMV
 
Speed Pro ,is now unknown ,supply wise.Went south,about mid 2000's.
 
How about a set of magnum heads, 62 cc vs 72cc. Pick u a set do a valve job and you're pretty close to where you want to be. You won't have to get new pistons then.
 
These (magnum heads) are not a direct fit on an 1971 block are they?
 
Where would be a good source of information on what needs to be changed to make the magnum heads work
 
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