Choosing pistons

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Evan Dutch

Evan Dutch

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Hello, I need some help deciding piston type. I currently have a stroker kit for a 408 which includes; forged crank, forged h beam rods, and forged flat top pistons. I plan to run boost in this engine via single turbo. My question is; will I be ok with the forged flat tops? How much boost could I safely run with the flat tops? Or should I sell the flat tops and invest in forged dished piston? Thanks.
 
What type of "boost" ?
Either way, the choice of compression ratio depends on many things. EFI? Carburetor? Intercooler? Placement of turbo? Amount of "boost"?
It may seem simple and boring to just build a regular carbureted engine, I get it. The "boost" you are speaking of really complicates things. I'm not against the idea but you'd benefit from checking out a race forum where there are guys with hands-on experience. A turbo requires some real fabrication experience, welding, etc.
 
Kern Dog said:
What type of "boost" ?
Either way, the choice of compression ratio depends on many things. EFI? Carburetor? Intercooler? Placement of turbo? Amount of "boost"?
It may seem simple and boring to just build a regular carbureted engine, I get it. The "boost" you are speaking of really complicates things. I'm not against the idea but you'd benefit from checking out a race forum where there are guys with hands-on experience. A turbo requires some real fabrication experience, welding, etc.
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Carburetor, single turbo, will have an intercooler, 15-20lbs if boost.
 
Typically stroker kits run a kinda high compression ratio, especially the 408. This might be too high for a boosted application using pump gas, are you running E85?

If not a piston with a -30 to -40cc dish will drop the CR to be useable with pump gas. You might be able to keep your current pistons if you plan on low boost and W/M injection.

I'd recommend a 2618 alloy forged piston. All forged pistons aren't exactly boost friendly, the 2618 is your best bet.

I'd run full groove main bearings to provide full time oiling to the rod bearings in a boosted application too.

Best of luck on your build.
 
MiradaMegaCab said:
Typically stroker kits run a kinda high compression ratio, especially the 408. This might be too high for a boosted application using pump gas, are you running E85?

If not a piston with a -30 to -40cc dish will drop the CR to be useable with pump gas. You might be able to keep your current pistons if you plan on low boost and W/M injection.

I'd recommend a 2618 alloy forged piston. All forged pistons aren't exactly boost friendly, the 2618 is your best bet.

I'd run full groove main bearings to provide full time oiling to the rod bearings in a boosted application too.

Best of luck on your build.
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I'd rather not run e85 due to its low availability in my area. But I will if I have too. I plan on running thick head gaskets and large combustion chamber heads to further lower my compression ratio.
 
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Give's you an approximation of "boosted" compression ratio.

Aside from that. There's lots you can do to run boost with higher compression ratios on pump gas. Less timing. Keep it under certain RPM. Improved spark. Boost retard. Detune. Etc etc. But honestly these are all "band-aids". And are done to make a race gas engine run on pump gas, usually done by the racer guys who wanna take the car out around the town once and a while. An engine designed for race gas and detuned for pump gas WILL make less power than an engine designed for max effort on pump gas.

You mention you will run "large" combustion chamber heads. By "large" do you mean open? Open compression chambers actually INCREASE your chance of detonation. A closed combustion chamber only allows the combustion itself to occur in one controlled spot. Open chambers can have combustion occur in two or more spots. Especially if they are prone to hot spots. This is detonation.

Intercooler is a great way to reduce intake charge temp. Which will help you run more boost on pump gas. The cooler everything enters the chamber. The better.

I wouldn't run too thick a head gasket to take away compression. The thicker the gasket the better the chance of blowing it out. Dish pistons/pistons below deck are better, a combination of the two is best. Ideally you want to create a closed combustion chamber with the CR you want, while still having good quench and head gasket size. If that makes sense. For a headgasket look at something like a cometic. They make custom metal gaskets that hold up well to boost.
 
Lustle said:
View attachment 1715016135

Give's you an approximation of "boosted" compression ratio.

Aside from that. There's lots you can do to run boost with higher compression ratios on pump gas. Less timing. Keep it under certain RPM. Improved spark. Boost retard. Detune. Etc etc. But honestly these are all "band-aids". And are done to make a race gas engine run on pump gas, usually done by the racer guys who wanna take the car out around the town once and a while. An engine designed for race gas and detuned for pump gas WILL make less power than an engine designed for max effort on pump gas.

You mention you will run "large" combustion chamber heads. By "large" do you mean open? Open compression chambers actually INCREASE your chance of detonation. A closed combustion chamber only allows the combustion itself to occur in one controlled spot. Open chambers can have combustion occur in two or more spots. Especially if they are prone to hot spots. This is detonation.

Intercooler is a great way to reduce intake charge temp. Which will help you run more boost on pump gas. The cooler everything enters the chamber. The better.

I wouldn't run too thick a head gasket to take away compression. The thicker the gasket the better the chance of blowing it out. Dish pistons/pistons below deck are better, a combination of the two is best. Ideally you want to create a closed combustion chamber with the CR you want, while still having good quench and head gasket size. If that makes sense. For a headgasket look at something like a cometic. They make custom metal gaskets that hold up well to boost.
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By larger heads. I do mean more open. I will be running an intercooler as I would most likely be driving this car more street than track. I would prefer the compression lower so that the car is more "streetable" (if that makes any sense). From what I've seen in the replies to this post I will most likely go with the dish pistons.
 
Evan Dutch said:
By larger heads. I do mean more open. I will be running an intercooler as I would most likely be driving this car more street than track. I would prefer the compression lower so that the car is more "streetable" (if that makes any sense). From what I've seen in the replies to this post I will most likely go with the dish pistons.
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You don't need to reinvent the wheel here. Talk to some builders with experience building boosted small block Mopars. The kind of boost you are talking about will require a race block and race gas to last long, as it should put out well over 600 hp. Electronic EFI with methanol injection would help a lot.

Why do you want to throw so much boost at it? 7 to 10 psi would be much more street friendly and dramatically less expensive to build a motor for. What transmission are you going to run? Are you going to back half the car to fit bigger tires? Just curious.
 
jbc426 said:
You don't need to reinvent the wheel here. Talk to some builders with experience building boosted small block Mopars. The kind of boost you are talking about will require a race block and race gas to last long, as it should put out well over 600 hp. Electronic EFI with methanol injection would help a lot.

Why do you want to throw so much boost at it? 7 to 10 psi would be much more street friendly and dramatically less expensive to build a motor for. What transmission are you going to run? Are you going to back half the car to fit bigger tires? Just curious.
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After thought, I would like to keep the boost below 12. Car will be tubbed and springs relocated. I would like to run a 46rh trans.
 
Evan Dutch said:
After thought, I would like to keep the boost below 12. Car will be tubbed and springs relocated. I would like to run a 46rh trans.
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Those take significantly more power to spin and are bigger. You will have to cut the floor.
 
jbc426 said:
Those take significantly more power to spin and are bigger. You will have to cut the floor.
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Yeah I know. Modify the floor, new trans cross member, and shorter drive shaft. Didn't want to be limited to three gears.
 
Icon pistons part# IC982 Has a -34cc dish and will result in a 8.8CR with 65cc heads.
It's a forged 2618 piston. Get the skirts coated.
 
Evan Dutch said:
By larger heads. I do mean more open. I will be running an intercooler as I would most likely be driving this car more street than track. I would prefer the compression lower so that the car is more "streetable" (if that makes any sense). From what I've seen in the replies to this post I will most likely go with the dish pistons.
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Just be aware that open chambers can run LESS psi and are more prone to detonation.

Evan Dutch said:
After thought, I would like to keep the boost below 12. Car will be tubbed and springs relocated. I would like to run a 46rh trans.
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As a rule of thumb for boost. Every 14.7 psi is about double horsepower (I say about cause things like air temp, tune, etc etc can decrease that). Because that's atmospheric pressure. A 300 horse 408 small block will make 545 horse at 12 psi. And it's preeeety darn hard to make a 408 stroker with only 300 horse. So keep that in mind.
 
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And with all that boosted power, 3 gears will be plenty. In fact you don't need more than 2 gears, cuz first gear is gonna hit 65mph PDQ, and then you just stick it in Drive,lol.
I have Zero turbo experience. But I can hear. And I hear guys on FABO, as well as elsewhere say things like, to load up their turbo, and get moving sooner and harder, they run much smaller rear gears than what one might think to be normal. So 3.23s to 2.76s are the norm. Well Running a 727 will get you a 2.45 low gear, and that with a say 2.94s and 28 inch tires, will get you 65mph at 5600 plus TC slip.As a streeter you are done, right? If not ,then 5600 pus slip will get you 109mph at the top of second, and drive is theoretically able to hit about 160, still at 5600+slip . You think 3 gears is limiting your style? Normally aspirated small blocks need more gears, but not 600PLUS, turbocharged, blow-everything-to-smithereens, 408s,lol. 85mph cruise will be a tic under 3000 with zero-slip.65 is a tic under 2300ns.

With 2.76s and 28s, the speeds at 5600plus slip, are;69,117,169, and 85 is 2816
 
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Hot damn. Y'all have given me some great information. Wasn't aware of how much power such small amounts of boost can make. After reading some of this I think I'll still go with the dome pistons. And decrease the head size. Thank you all!!
 
Evan, dome pistons and small chamber heads are backward for a turbo

The less Scr you have, the more boost you can run. The more boost you run, the more power you can make.
Sooner or later, the looooow Scr will become a problem off boost.
So for a streeter,the trick is marry enough off-boost Scr to still be able to start and drive the beast on a beer-run. The engine will then be the limiting factor for boost cuz sooner or later boost will bust the block.
I gotta tell you tho, that a well-tuned 360 with 400/450ponies, in a street A-body with appropriate supports, is an awful lot of fun. There are NO tires that I could install in the stock wheelhouses that will withstand that power in first gear at LEAST.With enough gear or rpm,I can spin them to over 60mph.
So I gotta ask the obvious; Since you are building a 408, you already have enough torque to do that in an Imperial! So what's up, that you want to boost it?

Getting back to those chambers. With boost, you can run almost any chamber, and use the top of the piston to be a part of it.
A D-cup is commonly used, cuz that's what is commonly available,for supercharging.
Commonly;for Normally Aspirated engines; Domed pistons are for racing, Quench-domes are for streeters with open chambers, and Flat tops for closed chambers;Commonly.
 
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AJ/FormS said:
Evan, dome pistons and small chamber heads are backward for a turbo
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No, not small chamber heads. Small in comparison to what I was previously speaking about. I said I wanted to run larger cylinder heads. But based on the information from this thread I will no longer be doing that.
 
AJ/FormS said:
Evan, dome pistons and small chamber heads are backward for a turbo

The less Scr you have, the more boost you can run. The more boost you run, the more power you can make.
Sooner or later, the looooow Scr will become a problem off boost.
So for a streeter,the trick is marry enough off-boost Scr to still be able to start and drive the beast on a beer-run. The engine will then be the limiting factor for boost cuz sooner or later boost will bust the block.
I gotta tell you tho, that a well-tuned 360 with 400/450ponies, in a street A-body with appropriate supports, is an awful lot of fun. There are NO tires that I could install in the stock wheelhouses that will withstand that power in first gear at LEAST.With enough gear or rpm,I can spin them to over 60mph.
So I gotta ask the obvious; Since you are building a 408, you already have enough torque to do that in an Imperial! So what's up, that you want to boost it?

Getting back to those chambers. With boost, you can run almost any chamber, and use the top of the piston to be a part of it.
A D-cup is commonly used, cuz that's what is commonly available,for supercharging.
Commonly;for Normally Aspirated engines; Domed pistons are for racing, Quench-domes are for streeters with open chambers, and Flat tops for closed chambers;Commonly.
Click to expand...

AJ/FormS said:
Evan, dome pistons and small chamber heads are backward for a turbo

The less Scr you have, the more boost you can run. The more boost you run, the more power you can make.
Sooner or later, the looooow Scr will become a problem off boost.
So for a streeter,the trick is marry enough off-boost Scr to still be able to start and drive the beast on a beer-run. The engine will then be the limiting factor for boost cuz sooner or later boost will bust the block.
I gotta tell you tho, that a well-tuned 360 with 400/450ponies, in a street A-body with appropriate supports, is an awful lot of fun. There are NO tires that I could install in the stock wheelhouses that will withstand that power in first gear at LEAST.With enough gear or rpm,I can spin them to over 60mph.
So I gotta ask the obvious; Since you are building a 408, you already have enough torque to do that in an Imperial! So what's up, that you want to boost it?

Getting back to those chambers. With boost, you can run almost any chamber, and use the top of the piston to be a part of it.
A D-cup is commonly used, cuz that's what is commonly available,for supercharging.
Commonly;for Normally Aspirated engines; Domed pistons are for racing, Quench-domes are for streeters with open chambers, and Flat tops for closed chambers;Commonly.
Click to expand...
well, to be honest I've always been told that you cannot have a higher hp engine (500+) and have it be streetable. That's where the boost comes in, still streetable and I can have power when I want it. Also I've always heard that dish pistons are the best for boost. Also my parts supplier said that I can run small amounts of boost with the flat tops I currently have. I guess I just don't want to take any chances with anything going wrong. Would really suck to have everything together and then something goes wrong. Ya never know.
 
Ok here's something else; you said
I would most likely be driving this car more street than track, and,Car will be tubbed and springs relocated
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My 367 goes Zero to 93 mph in 7.92 seconds with a 2.2 60ft. The speed limit around here is 100kph or a tic over 60mph.Guess how quick this car might be to 60 mph, if I spent some money to 60ft decent? I'm guessing that would be a low 5 seconds.
Now try and guess how quick it might be with 500hp.
Well, I'll tell you; if you snooze, you lose. The turbo will make a difference in drag-racing for sure, and if set up right, can turn a very impressive 60ft. But as a streeter, with street tires and street suspension, here it comes; IMO, whether you have 450NA hp or 500NA hp is as good as moot.A win here can be as simple as rear gear and a tight tranny split.Even if you can harness the tremendous torque boost of a turbo, you still will be only a fraction of time quicker than the 450NA 360, pound for pound.
So here you are carrying around an extra buncha weight, with, IMO, no practical daily application.
See, I have had three different cams in my engine, from 270 to 292 degrees, and guess what; in first gear they were all capable of smoking the tires to well past the speed limit. And guess what else, the smallest was my favorite.
From 270 to 292 is a difference of 22 degrees, an easy 3 to 3.5 cam sizes difference. or perhaps 35 to 40 hp.And my butt-dyno was happy with the little guy.
If you want to better your ET, NO2 is a more viable idea.

Or better yet, turbo a teener. Build it up to 270 hp and and jam 12 psi into that guy. Now you have the street world by the tail. 18/22mpgs,or even 25Plus with overdrive, 87 gas off boost, and 500+ hp, unbelievable passing power, and with this combo a 4 speed auto would make sense.Tubs optional.
Look, the end result is the same, namely 500+hp. But because of the softer hit, monster tires are not required.Monster tires don't handle all that well. The 270hp build is cheap and easy.87 gas can save you thousands of dollars over the life of the engine.And the fuel economy means you can drive it everywhere,all the time.An overdrive would be icing on the cake.
Run it on 7 psi/400 hp as a DD and then crank it up on the weekend!
Oh wait, this is not your dream; it's mine.lol
 
AJ/FormS said:
Ok here's something else; you said


My 367 goes Zero to 93 mph in 7.92 seconds with a 2.2 60ft. The speed limit around here is 100kph or a tic over 60mph.Guess how quick this car might be to 60 mph, if I spent some money to 60ft decent? I'm guessing that would be a low 5 seconds.
Now try and guess how quick it might be with 500hp.
Well, I'll tell you; if you snooze, you lose. The turbo will make a difference in drag-racing for sure, and if set up right, can turn a very impressive 60ft. But as a streeter, with street tires and street suspension, here it comes; IMO, whether you have 450NA hp or 500NA hp is as good as moot.A win here can be as simple as rear gear and a tight tranny split.Even if you can harness the tremendous torque boost of a turbo, you still will be only a fraction of time quicker than the 450NA 360, pound for pound.
So here you are carrying around an extra buncha weight, with, IMO, no practical daily application.
See, I have had three different cams in my engine, from 270 to 292 degrees, and guess what; in first gear they were all capable of smoking the tires to well past the speed limit. And guess what else, the smallest was my favorite.
From 270 to 292 is a difference of 22 degrees, an easy 3 to 3.5 cam sizes difference. or perhaps 35 to 40 hp.And my butt-dyno was happy with the little guy.
If you want to better your ET, NO2 is a more viable idea.

Or better yet, turbo a teener. Build it up to 270 hp and and jam 12 psi into that guy. Now you have the street world by the tail. 18/22mpgs,or even 25Plus with overdrive, 87 gas off boost, and 500+ hp, unbelievable passing power, and with this combo a 4 speed auto would make sense.Tubs optional.
Look, the end result is the same, namely 500+hp. But because of the softer hit, monster tires are not required.Monster tires don't handle all that well. The 270hp build is cheap and easy.87 gas can save you thousands of dollars over the life of the engine.And the fuel economy means you can drive it everywhere,all the time.An overdrive would be icing on the cake.
Run it on 7 psi/400 hp as a DD and then crank it up on the weekend!
Oh wait, this is not your dream; it's mine.lol
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Thanks for the info. Oh and one question, the turbo 318 idea, Wouldn't you still have to run higher octane fuel? Boost will raise the compression ratio yes? Lower octane fuel under high compression will cause detonation right?
 
Yes and yes;the post was edited while you were typing.
But you can probably run up to 360hp in the 8.0 factory holes, on 87.
I have always wanted to run a teener on dual,staged turbos, a little one up front,to get things rolling and a bigger take-no-prisoners kindof guy remote-mounted,out back under the rear seat. I would set it up to run on 87 with the little guy for as long as possible, maybe to 330hp. I would plumb the primaries with 87. I would plumb the secondary bowl with 91. I would try to run an anti-detonant when on boost that leads to detonation;eg water injection. I think I would call it done at 425/450 hp. I would try to not run an intercooler. Our igloos don't melt until about late June,lol. That's my dream.
Edit 360 hp on top of 270 is about 5 psi of boost.If you look on the chart above,(post#8), you will see that 5 psi and 8.0 Scr makes an equivalent to 10.7Scr, which I run all the time on 87 with aluminum heads.
That's my dream.
 
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AJ/FormS said:
Yes and yes;the post was edited while you were typing.
But you can probably run up to 360hp in the 8.0 factory holes, on 87.
I have always wanted to run a teener on dual,staged turbos, a little one up front,to get things rolling and a bigger take-no-prisoners kindof guy remote-mounted,out back under the rear seat. I would set it up to run on 87 with the little guy for as long as possible, maybe to 330hp. I would plumb the primaries with 87. I would plumb the secondary bowl with 91. I would try to run an anti-detonant when on boost that leads to detonation;eg water injection. I think I would call it done at 425/450 hp. I would try to not run an intercooler. Our igloos don't melt until about late June,lol. That's my dream.
If you look on the chart above, you will see that to make 360 hp is about 5 psi, and with 8.0 Scr, this makes an equivalent to 10.7Scr, which I run all the time on 87 with aluminum heads. That's my dream.
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Damn that sounds awesome, will definitely subscribe to that thread if it arises. Oh, also I planned on using cast iron heads at first until I can get a good set of aluminum heads. Anything wrong with doing this or should I wait until the aluminum heads?
 
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