How much power is compression worth?

[chargd72]

Leaning toward building a 408 stroker in the future.  However, I'm indecisive with what I want for compression.  I will be using aluminum heads so I could go straight for a 10+:1 ratio but I later might want to throw on a supercharger.  So my question is, how big of a power difference will there be if I build a 408 with a 8.5:1 compression (supercharger ready) vs a 10+:1 ratio?  I'm asking because I would probably be driving it for a while until a supercharger goes on.

[Ghoste]

How long is a while?  Normally I'd say try and rebuild it once and have future mods in mind but 8.4 is getting pretty low.  I'm thinking you might want to put the 10's in there and beat the hell out of it since it isn't even a "for sure" on the blower and when the time comes go through the whole engine.

[chargd72]

How long is a while?  Normally I'd say try and rebuild it once and have future mods in mind but 8.4 is getting pretty low.  I'm thinking you might want to put the 10's in there and beat the hell out of it since it isn't even a "for sure" on the blower and when the time comes go through the whole engine.

I see where you're going with saying go for the 10s first but my concern is that stroker kits aren't the cheapest things out there.  And if I have to redo the entire engine, I want to only do it once.  The only reason I say 'for sure' is because I hear so many people that do incredible builds for power but are still left chasing after higher numbers.  Maybe I would be completely happy with a well built high comp 408.  I just don't know because I've never driven one.  Nor, heard much about them in big B bodies.  

Oh, and a "while" is however long it takes me to have the extra money for a $3k paxton and $750 blow thru carb after I spend all the money necessary for the 408 long block.

[HPP]

408, so your stroking a 360. Does your kit include a forged bottom end or is it cast steel? If cast steel, why not leave it as a 360, and save your stroking money for the blower.

[chargd72]

408, so your stroking a 360. Does your kit include a forged bottom end or is it cast steel? If cast steel, why not leave it as a 360, and save your stroking money for the blower.

Kit would be forged good up to 800hp.

Good point though.  I never thought about reversing the order    I guess I could just get a used 360 magnum long block and throw a supercharger on it.  Then I could stoke and strengthen the bottom end when the time comes.  

The only con with this approach is that I could build up the stroker while I drive the Charger with the 318 in the bay.  But if I drop in a unstroked 360, it will have to come back out later when I'm ready for the extra ci rendering the Charger immobile.  Hmmm....

[John_Kunkel]

The increase in power (or not) of a supercharger over high compression depends on how much boost the supercharger puts out...when you factor in the parasitic losses of driving a supercharger you might break out even.

[chargd72]

The increase in power (or not) of a supercharger over high compression depends on how much boost the supercharger puts out...when you factor in the parasitic losses of driving a supercharger you might break out even.

I have a hard time believing that.  Maybe it would even out if you added 1/2lb boost only.

[SRT-440]

Do the 8.5:1 and put a turbo on it...even running just 5lbs of boost u'd be putting down good power. 

[chargd72]

Do the 8.5:1 and put a turbo on it...even running just 5lbs of boost u'd be putting down good power. 

Turbos require too many modifications.  There are bolt on blow thru superchargers that can be put on in one day. Although, I do see why you're partial to turbos, nice GN!

[Ghoste]

There's something aout an old school blower whine too.

[Cooter]

While everybody's looking for some kinda cake and eat it too thing, you can't just stuff 10 LBs of non intercooled boost on a 10.0:1 engine and expect it to live..Especially on today's pump gas. Sure, if the tune up's dead nuts on every single time, and it's air to water intercooled, you "Might" get by with 10 Lbs on the street with 10.0:1 compression. Another thing you seem to not understand is a standard 360 Mopar block will never handle 800 Blown HP for very long, even though the "kit" says it is good for it. I see those Stroker "Crate" engines listed with 725 HP small blocks, normally aspirated, there's no way that thing will live under a heavy foot very long. I mean, Look how many 700 HP plus BB stokers have come apart here that have huge money in 'em.
I doubt your into O-ringing the block cause with that kind of power and compression, your gonna be blowing head gaskets for sure.

Whether it's boost, chemical, or Turbo, anytime you force more air in to the engine than it was designed to handle tripling sometimes, quadrupling the HP, your on borrowed time.

[chargd72]

While everybody's looking for some kinda cake and eat it too thing, you can't just stuff 10 LBs of non intercooled boost on a 10.0:1 engine and expect it to live..Especially on today's pump gas. Sure, if the tune up's dead nuts on every single time, and it's air to water intercooled, you "Might" get by with 10 Lbs on the street with 10.0:1 compression. Another thing you seem to not understand is a standard 360 Mopar block will never handle 800 Blown HP for very long, even though the "kit" says it is good for it. I see those Stroker "Crate" engines listed with 725 HP small blocks, normally aspirated, there's no way that thing will live under a heavy foot very long. I mean, Look how many 700 HP plus BB stokers have come apart here that have huge money in 'em.
I doubt your into O-ringing the block cause with that kind of power and compression, your gonna be blowing head gaskets for sure.

Whether it's boost, chemical, or Turbo, anytime you force more air in to the engine than it was designed to handle tripling sometimes, quadrupling the HP, your on borrowed time.

If you read my entire post you will notice that I never once intended to stuff 10 psi into a 10:1 engine.  That would by my compression if I built a stroker and left it naturally aspirated.  I clearly said if I were even considering boost I would build it at 8.5:1.  And yes, I do understand that a 360 magnum block would need some work to handle that kind of power.  Once again, someone asked me what the rotating assemble was rated to and I simple answered.  I'm aware that some sleeve work and most likely even angling a four bolt main would be in order.  I will have help from people that have built some very strong engines that were built to last.

[Cooter]

 I will have help from people that have built some very strong engines that were built to last.

Good luck....

[chargd72]

 I will have help from people that have built some very strong engines that were built to last.

Good luck....

Thanks   

[HPP]

Changes in power based on changes in compresion ratio are not linear. A change from 7:1 to 8:1 will yeild more power than a change from 11:1 to 12:1. I've seen any number of changes quoted from 1 to 3 percent per point of ratio.

So, assuming no other changes other than simply a change in compression ratio, an 8.5:1 compression ratio 408 making 350 horsepower and an average increase of 2% per point, your 10:1 set up would then make 360 horsepower.

However, the low ratio engine with a bigger blower and higher boost will tend to be more sluggish off idle until the boost volume is in its more efficient range than a higher comrpession engine with less boost and a smaller supercharger that reaches its boost volume faster.

While playing around with all of this, also don't forget that cam timing and altitude have a trendous impact on the dynamic compression ratio your playing with. Simply putting a 10:1 piston in the hole does not assure you of a true, dynamic 10:1 ratio. Odds are with cam and altitude it could really be as low as 7:1 or may be somewhere around 8:1 or maybe 9:1.

[c00nhunterjoe]

You are comparing apples to oranges here. You can't just change pistons from 8:1 to 10:1 or vice versa for a blower and expect power comparisons. The cam grind needs to change as do the cylinder heads for flow. You need to decide whether or not your are building a boosted or naturally asperated engine and build accordingly.

[chargd72]

Changes in power based on changes in compresion ratio are not linear. A change from 7:1 to 8:1 will yeild more power than a change from 11:1 to 12:1. I've seen any number of changes quoted from 1 to 3 percent per point of ratio.

So, assuming no other changes other than simply a change in compression ratio, an 8.5:1 compression ratio 408 making 350 horsepower and an average increase of 2% per point, your 10:1 set up would then make 360 horsepower.

However, the low ratio engine with a bigger blower and higher boost will tend to be more sluggish off idle until the boost volume is in its more efficient range than a higher comrpession engine with less boost and a smaller supercharger that reaches its boost volume faster.

While playing around with all of this, also don't forget that cam timing and altitude have a trendous impact on the dynamic compression ratio your playing with. Simply putting a 10:1 piston in the hole does not assure you of a true, dynamic 10:1 ratio. Odds are with cam and altitude it could really be as low as 7:1 or may be somewhere around 8:1 or maybe 9:1.

Excellent write up!  This is the info I was looking for.  Thank you for the explanation.  I'm close to sea level so I shouldn't have much altitude issues.  Now I know the cam grinds can be specific for blower applications so you can get quicker valve lift but I was not aware that it had an affect on compression.

You are comparing apples to oranges here. You can't just change pistons from 8:1 to 10:1 or vice versa for a blower and expect power comparisons. The cam grind needs to change as do the cylinder heads for flow. You need to decide whether or not your are building a boosted or naturally asperated engine and build accordingly.

Correct.  Along with compression, the rest of the engine will be prepped accordingly.  That's why I'm digging for answers now.  It will probably be a year until this build starts so lots of reading to do.

[HPP]

You are comparing apples to oranges here. You can't just change pistons from 8:1 to 10:1 or vice versa for a blower and expect power comparisons. The cam grind needs to change as do the cylinder heads for flow. You need to decide whether or not your are building a boosted or naturally asperated engine and build accordingly.

Why not? Would it be optimized, no, but it is certainly possible. Beside, that wasn't the exact question. He wanted to know how much power he would give up if he built and 8:1 engine that would later have a blower compared to building a 10:1 engine right out of the gate that would have no blower. My answer to that is 10 horses give or take a few.

I'm also assuming just any old cam and nothing specific to a boosted application. Yes, cams impact compression ratio. The closing of the intake valve in relationship to piston position can increase or decrease dynamic compression ratio.

play with this one some: http://www.rbracing-rsr.com/comprAdvHD.htm

[Brass]

This is an interesting table: 

http://victorylibrary.com/brit/compression-c.htm

[c00nhunterjoe]

The way I read the origonal question was that he will be driving this engine for a little while then putting the blower on, he said that twice in the 1st post. In my opinion if you build a "blower ready" engine, as he stated he is looking at doing, whether its 8:1 or 10:1, you will have a total dog driving it without a blower.

10:1 is not common for a blower but it has been done. There is a local guy to me (cecil county md) with a 289 powered mustang with 10.5:1 slugs topped with a blower. Its been thrashed for years reliably. There was lso an article a few years ago with a 289 with 11:1's (I'm pretty sure) that was a blower car.

The bottom line is you need to decide if you are building a blower car or a N\A car. Pick one and build accordingly

[Cooter]

The way I read the origonal question was that he will be driving this engine for a little while then putting the blower on, he said that twice in the 1st post. In my opinion if you build a "blower ready" engine, as he stated he is looking at doing, whether its 8:1 or 10:1, you will have a total dog driving it without a blower.

Well, lookie there, I wasn't the only one that read it that way then.....

[HPP]

That is how I read it as well. My response to that is that there is a 10 horsepower difference between identical set ups with no change other than compression. Will it be a dog, yes. Anyone who has driven a mid 70s 8:1  "muscle car" will tell you they were nothing like golden years with 11:1 engines.  In my own experience, I changed an engine from 11:1 to 9.5:1 and the change was very noticeable in the seat of the pants feel. It is tolerable given the application, but it is noticeable. I also read that the original poster does want to only build once, that is why he is asking how big of a difference is there bewtween building a blower set up vs non-blower setup. As I also pointed out, higher static compression with less boost may be a significantly better street approach than ultra low compresion with massive amounts of boost.

IMO, the original poster needs to do a whole lot more reading and talking than what can simply be found on DC.com and his favorite machine shop.

[chargd72]

I also read that the original poster does want to only build once, that is why he is asking how big of a difference is there bewtween building a blower set up vs non-blower setup. As I also pointed out, higher static compression with less boost may be a significantly better street approach than ultra low compresion with massive amounts of boost.

IMO, the original poster needs to do a whole lot more reading and talking than what can simply be found on DC.com and his favorite machine shop.

Yes and yes.  I like the idea of starting with a higher static and running less boost. This was the goal of my thread.  Now that I have any idea of what would make a good street machine, I can continue with educating myself more on the subject.  Thanks to all that gave pointers and opinions 

[c00nhunterjoe]

Another thing to remember: 10:1 with boost will be race gas only

[mauve66]

You are comparing apples to oranges here. You can't just change pistons from 8:1 to 10:1 or vice versa for a blower and expect power comparisons. The cam grind needs to change as do the cylinder heads for flow. You need to decide whether or not your are building a boosted or naturally asperated engine and build accordingly.

that's what i was thinking, blowers use their own cam grinds i thought, and if you start with a thin head gasket on the lower end of 10.0 CR then go to an o ringed head or those fancy head gaskets from cometic(sp) in a thicker version on a larger flowing head (for the blower) then your CR would drop closer to the blower CR you want and then your bottom end would still be good, right??