Chamber size for iron head stroker

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Idaho

Idaho

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My project keeps changing. After being mothballed for a few years, I've gone from planning a "junkyard magnum" to doing a stroker. For those who want to see more detail on the project it is here with a post titled "The Frog Wants to be Stroked":

Project Frog Princess

I have RHS heads from IMM, 62 cc chambers to match original pistons (dished and .050" below deck). I cc'd them and got more like 61 cc.

I believe I can order a SCAT stroker kit with dished pistons (around 26 cc) to get me to a pump-gas friendly 9.5 compression ratio, according to the seller specs. I am not sure if those pistons are below deck? No spec given. Feeding the numbers into a compression calculator it seems they must be.

My limited understanding of quench suggests I may be more prone to detonation with a dished piston. My gut says it's not likely a big deal since the factory runs 0.50 below deck at 9.5 compression with leaner mixture than I will likely have, using a carb. Of course if all these strokers are well below deck, maybe quench is a non-issue.

Is there a performance loss? Again seems unlikely to be significant but I'm not sure.

If I were to sell the RHS heads I'd likely go with aluminum with similar flow, set up for a flat top piston with compression 10.5, and presumably better quench. My loss would be the discount for re-sold heads and the shipping cost since I think I can get aluminum (checked/prepped) for around the $1650 or so I paid for the RHS set with shipping.

I'd like to understand it better to decide on keeping the RHS heads.
 
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The compression height of the pistons and the actual deck height of your block will determine where the top of the piston is... call the seller and and ask what the CH is.

Anything else is just a guess.
 
I ran these imm-prepped rhs-x-heads before, they performed very good....mid 11s in my Valiant with a moderate camshaft (242/236° @ .050").

Regarding the pistons: there is a KB356 Piston, these do have a quench pad that must be machined when using closed chamber heads like the rhs. Ask scat about the piston type, then you can machine the piston exactly to deck height (that´s what i did).

Michael
 
If you are going with the forged assembly with the Icon pistons they have a 20.50 cc dish. That might be the piston reliefs .
 
Even the dished pistons will have quench effect if set up for a proper gap as either:
- the piston height can be set so that outer edge of the piston crown will close up close to the head's closed area surfaces
- or the area above the eyebrows will close up tight to the head like in a normal quench space.

Quench usually gives a bit better combustion efficiency, but the main benefit to seek is for improved detonation resistance. I always want that, with the way I like to set up engines for my uses (with DCR at 8 or a bit higher).

You can go on the SCAT site and go through their parts selector here: Rotating Assembly Product Search - SCAT Crankshafts

Enter 6.123 for rod length and it will give you all the SBM stroker kits. The Icon and KB dish/eyebrow volumes are all given as negative numbers. (Be aware that KB and Icon will list the same volumes as positive numbers.) You can deduce the piston PN's from that. Hypers will be KB's and forged will be Icon's.

The SCAT listings' dished forged at 20.5 cc's are very probably Icon IC984's with the dish and NO step on top of the piston crown in the quench area referred to in post #3, and so will not stick up above the stock Magnum deck height. These pistons have a littler lower compression height to work better with the .015" shorter deck height of the Magnum block. But they will not have quench unless you mill the deck OR you use .028" thick Mr Gasket 1121G head gaskets to get a quench gap of around .047".

Next choice is the IC745 with a .020" taller compression height than the IC984's. They will stick .003" + or - above the stock Magnum deck, and with a Felpro 1008 head gasket at .039", will give you a .036" + or - quench gap. A Felpro 8553PT head gasket will put you at about a .047" quench gap.

A 3rd choice is the KB416 which is like the IC745 with a bit larger dish. You can ask SCAT if this is an option.

Here are various combinations with SCR and quench gaps + or -. I say '+ or -' as the actual deck height will usually have some variations.
  • IC984 and 1121G gasket: 10.0 SCR and .047" gap
  • IC745 and 1008 gasket: 10.3 SCR and .036" gap
  • IC745 with 8553PT gasket: 10.0 SCR and .047" gap
  • KB416 with 1008 gasket: 9.8 SCR and .047" gap
  • KB416 with 8553PT gasket: 9.6 SCR and .047" gap
Keep in mind that the deck heights will vary across the block, so it would behoove you to measure this up front. You can get decent numbers for this with the stock crank, a stock rod and piston with rings removed(where you use the standard piston CH and rod length), and a dial indicator up on the deck.
 
I suggest the OP search the forums and read all he can on quench. No way am I typing that put again.

Way too much emphasis put on quench. Unless the chamber is horrible and the spark plug location is rediculous (think SBC or BBM) quench will do very little to suppres detonation.
 
Krooser said:
The compression height of the pistons and the actual deck height of your block will determine where the top of the piston is... call the seller and and ask what the CH is.

Anything else is just a guess.
Click to expand...
I think you bottom lined it.

360duster said:
I ran these imm-prepped rhs-x-heads before, they performed very good....mid 11s in my Valiant with a moderate camshaft (242/236° @ .050").

Regarding the pistons: there is a KB356 Piston, these do have a quench pad that must be machined when using closed chamber heads like the rhs. Ask scat about the piston type, then you can machine the piston exactly to deck height (that´s what i did).

Michael
Click to expand...
Impressive. Not a stroker, correct? I hope to avoid machining.

mbaird said:
If you are going with the forged assembly with the Icon pistons they have a 20.50 cc dish. That might be the piston reliefs .
Click to expand...
I'm fairly sure valve reliefs are under 10 cc. I should know more soon.

yellow rose said:
I suggest the OP search the forums and read all he can on quench. No way am I typing that put again.

Way too much emphasis put on quench. Unless the chamber is horrible and the spark plug location is rediculous (think SBC or BBM) quench will do very little to suppres detonation.
Click to expand...
I've read a fair bit including some of your comments. Enough to know it's not cut and dried. I also know your thoughts on aluminum. I'll keep reading.

nm9stheham said:
Even the dished pistons will have quench effect if set up for a proper gap as either:
- the piston height can be set so that outer edge of the piston crown will close up close to the head's closed area surfaces
- or the area above the eyebrows will close up tight to the head like in a normal quench space.

Quench usually gives a bit better combustion efficiency, but the main benefit to seek is for improved detonation resistance. I always want that, with the way I like to set up engines for my uses (with DCR at 8 or a bit higher).

You can go on the SCAT site and go through their parts selector here: Rotating Assembly Product Search - SCAT Crankshafts

Enter 6.123 for rod length and it will give you all the SBM stroker kits. The Icon and KB dish/eyebrow volumes are all given as negative numbers. (Be aware that KB and Icon will list the same volumes as positive numbers.) You can deduce the piston PN's from that. Hypers will be KB's and forged will be Icon's.

The SCAT listings' dished forged at 20.5 cc's are very probably Icon IC984's with the dish and NO step on top of the piston crown in the quench area referred to in post #3, and so will not stick up above the stock Magnum deck height. These pistons have a littler lower compression height to work better with the .015" shorter deck height of the Magnum block. But they will not have quench unless you mill the deck OR you use .028" thick Mr Gasket 1121G head gaskets to get a quench gap of around .047".

Next choice is the IC745 with a .020" taller compression height than the IC984's. They will stick .003" + or - above the stock Magnum deck, and with a Felpro 1008 head gasket at .039", will give you a .036" + or - quench gap. A Felpro 8553PT head gasket will put you at about a .047" quench gap.

A 3rd choice is the KB416 which is like the IC745 with a bit larger dish. You can ask SCAT if this is an option.

Here are various combinations with SCR and quench gaps + or -. I say '+ or -' as the actual deck height will usually have some variations.
  • IC984 and 1121G gasket: 10.0 SCR and .047" gap
  • IC745 and 1008 gasket: 10.3 SCR and .036" gap
  • IC745 with 8553PT gasket: 10.0 SCR and .047" gap
  • KB416 with 1008 gasket: 9.8 SCR and .047" gap
  • KB416 with 8553PT gasket: 9.6 SCR and .047" gap
Keep in mind that the deck heights will vary across the block, so it would behoove you to measure this up front. You can get decent numbers for this with the stock crank, a stock rod and piston with rings removed(where you use the standard piston CH and rod length), and a dial indicator up on the deck.
Click to expand...

Thanks! That's a lot of good info. It will help with tracking down the details. Sure would be nice if they gave piston specs.
I've measured deck height but need to do it more accurately on 4 corners. Why rings out?
Are those combinations using a 62 cc chamber?
 
Spoke to Austin at SCAT. Very helpful.
Icon 983 (forged) has a 27 cc dish - very close to the 26 I thought would be needed.
CH = 1.445
With zero deck at 1.462 (Magnum block 9.585) this puts it 0.017 below deck.
With the deck cleaned up by 0.010 it comes in right at 9.5 CR. (unfortunately I forgot to ask what gasket is in the equation :() Edit: 0.040 gasket.

For this combo he suggests the cast crank for $1600 shipped, no tax. Forged would be $2100.

We spoke about quench. He sees no issue with detonation, and does not think performance will be harmed by the dished piston. He has many happy customers running that piston.

I'm going to check the piston depth at 4 corners and cc both heads to verify actual of 61 cc. I'll find out what gasket is in the equation and might use a slightly thicker one. Edit: 0.040 gasket.
Of course I'll have the block work done before ordering.

I'm comfortable with Austin's advice. I'm guessing some may disagree. Feel free to comment, or offer any other tips.
 
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I am running the Scat Forged kit with Icon but I dont recall the pistons being dished like my stock Magnum pistons.
 
Just a shot of how I did the chamber cc.
60 ml syringe
3 ml syringe
1/4” plexiglass scrap
Alcohol
Vaseline
I’m just holding it down. Best to have 2 people.

552BED52-2B58-4272-A9D8-E537A24CCDB7.jpeg
 
I am running W2 closed chamber race heads with 68 cc chambers.
I have yet to drive the car other than into the street , turn around and back into garage .
Ran good that far though ! Lol

Idaho said:
That kit has a dish of 20.5 cc.
Do you recall your head volume? Iron or Aluminum?
Are you happy with it?
Click to expand...
 
Idaho said:
I think you bottom lined it.


Impressive. Not a stroker, correct? I hope to avoid machining.


I'm fairly sure valve reliefs are under 10 cc. I should know more soon.


I've read a fair bit including some of your comments. Enough to know it's not cut and dried. I also know your thoughts on aluminum. I'll keep reading.



Thanks! That's a lot of good info. It will help with tracking down the details. Sure would be nice if they gave piston specs.
I've measured deck height but need to do it more accurately on 4 corners. Why rings out?
Are those combinations using a 62 cc chamber?
Click to expand...



LOL. Where did I say cut a dry? I NEVER said that. Maybe read a bit closer.

Again, queen his highly over rated, unless you are an emissions freak.
 
Idaho said:
Why rings out?
Are those combinations using a 62 cc chamber?
Click to expand...
'cuz it is just easier.... no need to compress the rings! Ideally, you would accurately measure the rod length and the piston CH that you are using for deck measurements for best accuracy. Piston CH is easy when assembled wiht a pressed pin; rod length is a bit tricker. Otherwise, you just have to go on faith that your rod length is actually 6.123"..... probably so from the factory, but a reman rod can be short.

The numbers I gave were with 61 cc chambers.
 
yellow rose said:
Way too much emphasis put on quench. Unless the chamber is horrible and the spark plug location is rediculous (think SBC or BBM) quench will do very little to suppres detonation.
Click to expand...
Could well be.... no sense 'leaving it on the table' IMHO. The bigger benefit may well be just keeping the overall chamber more compact around TDC.....
 
good point nm
compact chamber looking from both the front and the side
nice to have every point equal distant from plug or however the flame front is going to pattern
chambers that look like a fried egg like the BBM and Early SBC are just terrible
later heads like the Magnum with two quench pads take less timing for a very good reason
and late 37-360 roller motors
that's one way to tell a good chamber- takes less timing to make it work
so (this is IMHO) a dish under the plug is better than a piston down the hole and a long way away from the plug to the far side of the chamber pinging and detonation starts there
so do not agree with yr 100% to many variables like compression and fuel and torque build vs bracket racing build
I've made more hp with open chambers (not lately with modern heads) when the revs were kept above the torque peak and good gas
 
Wyrmrider said:
I've made more hp with open chambers (not lately with modern heads) when the revs were kept above the torque peak and good gas
Click to expand...
Which makes a lot of sense.... the combustion process goes no faster at 6k rpm versus 3k, but the piston moves away from TDC a lot faster at 6k, and so the peak pressures have no chance to build as high when you get the rev's up. So less chance for detonation from that particular factor.... which jives with common observations.

I'm gradually learning that you have to look at things differently in the 3k and lower RPM range versus the higher RPM ranges. Things like this, and intake breathing and cylinder filling mechanisms, change a LOT from low to high RPM ranges, and your thinking needs to change if you emphasize one vs the other. The broad RPM range engines that I like have to be a compromise of factors.

Reading a lot of sources of info, both gearhead and research papers, and hearing the ideas of others here, have been invaluable to me getting a better grip on this stuff. I really appreciate the ideas expressed, even if they challenge my thinking.
 
If you think about it all motors (street and strip can benefit from sub 3500 rpm power.

When I drop the clutch @ 5000 rpms on the strip the rear wheels do not immediately turn
450 rpms . (Unless I am spinning )
So I dont understand why people ignore things like vacuum advance or quench.

Ismy thinking flawed ?
 
yellow rose said:
LOL. Where did I say cut a dry? I NEVER said that. Maybe read a bit closer.

Again, queen his highly over rated, unless you are an emissions freak.
Click to expand...

I think you are misinterpreting. The majority emphasize quench. You don't. My comment is saying you could be right.
Emissions? I plan to emit :)
 
IDK mopar went open chamber for better emissions, dropped the compression, kept the hP cams with 114 lcas and long lazy ramps- ruined to great engines

UDHarold wrote 2007

PS---You are correct. Wider LSAs generally give lower emissions, particularly at idle. These cams are very efficient and pretty clean-burning. I do not know if Holley has had them certified or not.

and in the very next post
You have to be careful going with too wide a lobe center. It does cut down on the amount of raw fuel going through at overlap, but it can also cause the NOX to go up.
I believe California still measures for that.
Mike Jones
Jones Cam Designs
Mopar stroker cam selection? Please help! - Don Terrill’s Speed-Talk
 
360duster said:
I ran these imm-prepped rhs-x-heads before, they performed very good....mid 11s in my Valiant with a moderate camshaft (242/236° @ .050").

Regarding the pistons: there is a KB356 Piston, these do have a quench pad that must be machined when using closed chamber heads like the rhs. Ask scat about the piston type, then you can machine the piston exactly to deck height (that´s what i did).

Michael
Click to expand...
Michael there is alternative part number for a version that's meant for closed chambers. Kb 416 it needs no milling.
 
Wyrmrider said:
IDK mopar went open chamber for better emissions, dropped the compression, kept the hP cams with 114 lcas and long lazy ramps- ruined to great engines
Click to expand...
Everyone dropped compression ratio... to lower chamber temps (and pressures) and thus lower NoX emissions. It certainly was not for better performance...

Had a 114 LSA Crane HE cam in my 315C.... Good economy while cranking out 325-350 HP. And would shock the AAA guys running the courtesy emission test in the mall parking lot by passing HC and CO emissions, but the 10.3 SCR made passing NoX a no-go.
 
Egr puts heat into the chamber. Heat burns more. GM went crazy in that and ran all their temps way up in over 200-230 ish

Dont take and railroad the thread with emissions talk.
Quench is great, smaller compact chambers burn faster, less timing squeeze it all central to the plug and watch the power go up.
As long as you can unshroud the closed chamber.. it's not really behind the open one..its the flow that is slightly hindered from valve shrouding of closed chambers.
Really small down-to-earth example would be a 273 head closed chamber 920 casting. They flow around 170 CFM. If you unshroud that chamber all the sudden they flow around 183 CFM, allowing more signal to the straight side of the port ,therefore carrying the flow up higher in the lift than would normally carry. Same for all heads. While some chambers ARE so compact it's hard to unshroud them enpugh... there in lyes the idea getting a better port attached to that chamber to realize the advantage. Though there may always be a trade off...you just have to exploit the flow potential to offset that when comparing heads. Regardless..
It's all just planning and more or less work depending on the path you take. I pushed 91 octane california gas on heavily ported open chamber j hesds to 188 cranking psi /high 8's dynamic, I used kb 356 milled them about .046 off the quench pad...and milled the open chambers about .050ish for a depth of .030 and 60cc.. so the piston took up the gasket and the chamber gave the clearance. Heads flowing a hair better than those/op's.. a beast that rpm'd just fine and would just leave behind any car at speed...hardly, if at all, hooked from a light.
Just do it right and it wont matter either way.
 
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