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Expert 5K+
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Location: So. Cal | I have some std 392 hemi pistons number 1732540 which I believe are 1957 9.25:1. Can anyone verify this part number as being 1957? What is the part number for '58 pistons?
Anyway, I have a .06" over 354 motor that I am thinking of putting these in. If I were to use hemi heads on it, it would likely drop the compression down to around 8.5:1, but I am thinking of using poly heads instead. Does anyone have an idea what the resulting compression ratio would be?
Alternatively, does anyone know the combustion chamber volume of a poly head? My poly heads are mounted to my running motors at the moment so I don't have the ability to measure them. I beleive the 354 hemi head is 100cc, and the 392 head is 110cc, is that correct? When did they change to 110cc on the hemis? |
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Elite Veteran
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Location: Peoples Republic of Oregon | ...random thoughts...
Combustion chamber volume varies from 108 to 112 cc. These are heads measured in the shop, never tracked volume vs casting number.
I won't guess on the resulting c/r from the 392Hemi piston and Spitfire head combo since I don't have a Spitfire cc available. I'd be most concerned with piston clearance to the head and valves; you will need to modify the piston for valve clearance I am quite sure and I worry about dome clearance....
Take one of each piston, place them on the same wrist pin, make a visual comparison. |
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Exner Expert 19,174 posts. Neil passed away 18 Sep 2015. You will be missed, Neil!
Posts: 19146
Location: bishop, ca | There are several 'easy' ways to check for valve head clearance, including putting a layer of putty on the pistons, but here's a REAL
Old School method...using one of those ubiquitous wrecked hemi heads that are lying around, everywhere (back in the 3/68 H.R.)
Using the Poly heads with Hemi pistons will raise the c.r. significantly over the 9.25(?) 1957 Hemi-rating.
You may have to use Premium gas on it, but the higher c.r. will increase both MPG and MPH, and, will give you one HELL of
a sleeper engine!
And, don't forget to have the distributor be re-curved; otherwise, you are wasting your time and money on hopping-
up the engine.
Edited by d500neil 2014-06-04 6:20 PM
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Location: N.W. Fla. | Powerflite - 2014-06-02 10:45 AM
I have some std 392 hemi pistons number 1732540 which I believe are 1957 9.25:1. Can anyone verify this part number as being 1957? What is the part number for '58 pistons?
Anyway, I have a .06" over 354 motor that I am thinking of putting these in. If I were to use hemi heads on it, it would likely drop the compression down to around 8.5:1, A 9.25 392 piston in a 354 hemi drops C/R 1 point, so apx 8.25. |
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Exner Expert 19,174 posts. Neil passed away 18 Sep 2015. You will be missed, Neil!
Posts: 19146
Location: bishop, ca | No....it's got the POLY head on it....taller (Hemi) piston, in a SMALLER combustion chamber.
Maybe 9.75:1 c.r.; maybe even 10.:1 .
(Too lazy to check on the Hemi/Poly crankshaft differences, but the Hemi crank may well be needed, too).
Edited by d500neil 2014-06-05 1:20 AM
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Location: Springville, CA | There are No differences betwen a poly or hemi crank. |
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Location: N.W. Fla. | d500neil - 2014-06-04 9:05 PM
No....it's got the POLY head on it....taller (Hemi) piston, in a SMALLER combustion chamber.
Maybe 9.75:1 c.r.; maybe even 10.:1 .
(Too lazy to check on the Hemi/Poly crankshaft differences, but the Hemi crank may well be needed, too).
My comment was on his comment with hemi heads |
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Location: Alaska | I was under the impression that the 392 piston had a shorter piston pin to deck height than the 354 because of its longer stroke and therefore would give you less compression when used in the smaller engine. I have both a 331 hemi and a 354 poly and am planning on making the 354 a hemi with these parts. It seems it is easier to get 392 parts than other sizes so was considering boring out the 354 and using 392 pistons rather than buying an expensive set of custom pistons but I would like a C.R. closer to 9:1 than 8:1. |
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Location: N.W. Fla. | 58coupe - 2014-06-05 9:17 AM
I was under the impression that the 392 piston had a shorter piston pin to deck height than the 354 because of its longer stroke and therefore would give you less compression when used in the smaller engine. I have both a 331 hemi and a 354 poly and am planning on making the 354 a hemi with these parts. It seems it is easier to get 392 parts than other sizes so was considering boring out the 354 and using 392 pistons rather than buying an expensive set of custom pistons but I would like a C.R. closer to 9:1 than 8:1. When putting 392 pistons in a 354 there is a reduction of 1 point from the advertised C/R due to reduced swept volume of the shorter stroke. 10:1 392 pistons will give you 9:1 in the 354. 354 bored to 392 std (with sonic check) is considered to be max overbore. If I could get away with .030 instead. I would do that myself. |
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Location: Springville, CA | By my calculations the 392 pistons have a taller compression height by about 0.028". Remember the 392 has a taller block deck height than the 331/354. My info shows the 392 to have a 10.870" deck height versus the 331/354 deck height of 10.370". |
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Location: Alaska | So what is the correct answer? Mopar1, have you actually measured this or what is your source of information? 56Fanatic, what is your source? |
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Expert 5K+
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Location: So. Cal | Using 392 hemi pistons in a 354 hemi will decrease the compression ratio as Mopar1 and I have stated for exactly the reason that 58coupe mentioned. It is misleading to compare deck heights without considering the stroke as well. The longer stroke of the 392 gives you more swept volume of gas to compress. But I stated this compression drop in my first post as a fact; because it is, and I don't wish to debate it here. Even Mickey Thompson piston manufacturer states this to be the case with their product.
But can anyone answer:
1. What is the 354 poly head combustion chamber volume?
2. When did the hemi head volume change from 100cc to 110cc?
3. What is the part number of stock 392 pistons for 1958?
On number 2, I can say that the 331 heads I have seen have 100cc chambers and the 392 heads have 110cc chambers. I just don't know about 354 heads because I don't have any. Also, Hot Heads modeled their heads after the 1955 331 head and retain the smaller 100cc chamber.
Also, I agree with Mopar1 about boring out the 354 to 392 specs. I wouldn't do it. The only reason I am doing it is because the block I have has already been bored out to that level (with sleeves - go figure). If you bore it out too far, you will be stuck with having to sleeve it the next go-round which can be sooner than you would like it to be.
Edited by Powerflite 2014-06-05 6:04 PM
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Location: Chestertown, NY ( near Lake George) | No way to bore out the 354 as the 392 larger displacement included a longer stroke so just boring even to the limit will not get you 392. ( 354/3.625 stroke vs 392/3.906 stroke)
Edited by Shep 2014-06-05 8:16 PM
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Location: N.W. Fla. | Shep - 2014-06-05 7:06 PM
No way to bore out the 354 as the 392 larger displacement included a longer stroke so just boring even to the limit will not get you 392. ( 354/3.625 stroke vs 392/3.906 stroke) I probably should have said "bored to 392 std bore diameter" actual CID is about 360. '57 car std bore piston part #1752265, casting # 1732540 |
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Location: Alaska | I have a 57 354 poly but it is still together so can't measure combustion chamber easily.
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Location: Chestertown, NY ( near Lake George) | Mopar1 - 2014-06-06 5:15 AM
Shep - 2014-06-05 7:06 PM
No way to bore out the 354 as the 392 larger displacement included a longer stroke so just boring even to the limit will not get you 392. ( 354/3.625 stroke vs 392/3.906 stroke) I probably should have said "bored to 392 std bore diameter" actual CID is about 360. '57 car std bore piston part #1752265, casting # 1732540 |
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Location: Springville, CA | <p>Well, my sources are HotHeads, Victory Press, The Complete Book of Early Chrysler Hemi's, Gary Beck and Arias pistons and four hemi engines built. Anyway I stated my specs thinking that people who were familiar with hemi engines would realize you can't put 392 pistons in a 331/354 hemi without machining the piston crown or changing the con rod length since the compression height of an unmodified 392 piston in a 354 would give about a 0.028" positive deck and almost certain piston to valve interference. With all that said, there are intriguing possiblities with using BBC rods and offset grinding the 354 rod journals to BBC size and gaining about 0.040 stroke. Of course, you might have to have custom pistons to pull that off which totally negates using 392 pistons or custom rods!</p>
Edited by 56Fanatic 2014-06-06 12:18 PM
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Expert 5K+
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Location: So. Cal | I have taken a picture and made measurements for you-all. On the left is a stock '57 392 piston with 9.25:1 compression and on the right is a stock 354 hemi Marine piston with 8.2:1 compression. The lower compression 354 piston is taller. It is difficult to see in the picture I took; but after careful measuring, the 354 piston is .03" taller. So using a 392 piston in a 354 hemi will definitely result in a compression loss. This should put this question to bed once and for all so we don't have to endlessly debate it.
I appreciated Wayfarer's comments about the valve reliefs for using these in a poly motor. That is likely my biggest issue, but hopefully, I can find some shop willing to cut them for me.
Edited by Powerflite 2014-06-09 12:43 PM
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Location: N.W. Fla. | Assuming a "modern" rebuild with decent C/R Hemi & Poly pistons don't interchange. On low comp engines like truck engines there are reports of miss matched pistons( hemi in polys & vice versa)being found in virgin engines. |
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Expert 5K+
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Location: So. Cal | I was digging through the stash in the garage and found some '58 392 pistons that I didn't know I had. So I am listing the casting/part numbers of them here for future reference.
They are: 1827804 |
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Location: Alaska | According to my sources, the 58 392 (not 300D) was rated at 345 HP but the 57 392 was rated at 325HP, was the difference C.R. or camshaft? |
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Elite Veteran
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Location: Peoples Republic of Oregon | ...¾ point of c/r will do that.......
So, Mr PowerFlite, what became of the project?
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Expert 5K+
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Location: So. Cal | I agree. It was probably mostly due to the compression change.
Projects changed and I didn't end up doing it. |
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Location: Alaska | So, if i bore my 354 out .060 and run 10 to1 392 pistons I will end up with about 9 to 1. That is about what I want. In Alaska, we don't have gasahol, but I believe the 10% alcohol actually might help to reduce pinging in an engine although it has other harmful side effects. |
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Elite Veteran
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Location: Peoples Republic of Oregon | Which 392 "10:1" piston? I admit that I have never measured the actual c/r on a stock assembly (but I have checked Egge pistons and they are dismal),
so, how accurate do you expect the oem numbers to be? And, since the 354 was 9:1 from the factory, what is the gain???
Yes, if you happen to have a small-ish combustion chamber you might get by.
Oh, be sure to sonic check the cylinders before you do any cutting....some of these blocks have ugly core shift..... |
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Location: Alaska | I am using a 57 354 poly and the hemi parts from a 55 331 so I have to change the pistons. The 354 I have is still together and appears to be in original condition so, not sure yet what the cylinders look like. I would rather not bore it .060 but was wondering how hard it is to get 354 hemi pistons in about 9 to 1. |
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Expert 5K+
Posts: 9649
Location: So. Cal | Stock replacement 9:1 pistons are pretty cheap $345 in any bore size for the 354 hemi motor:
http://hothemiheads.com/pistons_rods/pistons_cast_replacement.html
Or you can get custom forged Ross for $859:
http://hothemiheads.com/pistons_rods/pistons_custom_forged.html
And I believe that custom Venolia are a little cheaper at around $750. But 9:1 sucks for a hemi motor. Maybe the 331 hemi heads will give you something closer to 9.3:1? Hemi motors can take more compression without pre-detonation because of the position of the spark plug. I would go with at least 10:1. And because you are pretty much forced to go with custom pistons if you want more than 9:1 compression, just pick up a 392 crank and offset grind it down to BB chev rod size to stroke your 354 hemi to 4.1" to give you 406 cubic inches. You can do it with just the cost of the crank if you end up going with custom pistons anyway. The new BB chev rods will be cheaper & stronger than reconditioning your originals. |
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Location: Alaska | Remember where I live, 392 cranks are not just laying around and shipping is a killer for something heavy. I believe the 331 hemi heads are just about the same as 354 hemi heads but; what I have are the "so called" 555 triple nickel heads. I think there is a slight difference in the locating pin size (not a problem) but I think I have to use the head gasket to fit the 55 head not the 57 block. Wayfarer, do you know the right combination? |
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Elite Veteran
Posts: 888
Location: Peoples Republic of Oregon | Some random thought...
Minimize the overbore.
We custom order forgings so virtually any combination of specs is available. We routinely use 10:1 forgings in all EarlyHemi engines. Never have had a complaint from any customer. We sell AutoTec for $800.
Dowel pins can be removed/swapped.
There are three distinct head gaskets for the 331; 51-54, 55, 56. This is due to the change in location of a coolant passage. You can plug either the head or the block and drill the opposite to match which ever gasket you use. Lay the chosen gasket on the head/block and check alignment.
As for the crank swap, the counterweights must also cut down to match the 354 but it does make a nice package. |
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392 Hemi Pistons in 354 Poly
