High Compression VS Low Compression Opinions

[BEATAV8]

To get compression that high, you'll have to use the stroker crank. (or heads with a really small combustion chamber)

Yeah I ran the numbers through one of those compression calculators a while back and came up with like -3 or -4cc dish volume for a 3.4 stroke / 3.840 bore. Pretty much a flat top.

 

[earlbrown]

Yeah, a single valve relief is 3 or 4 by itself. I made a set for a Ford 400 I'm building and they were 3ccs from just one half moon and the rest of the slug is flat as a board.

 

[jasjamz]

I would love to see three cars with the same long blocks with the only variable being compression a point apart. 9:1, 10:1, and 11:1. All on the same fuel and all use a 6766 turbo and 17 blade PTC. Same cams, same timing, a/f target, and dyno. I know there are cars like this floating around.... we just need them to chime in.

 

[earlbrown]

The 'same cam' part would kinda make it apples and oranges. Duration and degrees overlap are tied to compression (when done right). That test would probably show the 'best compression' as which ever one works best with that exact cam (and combo). Same with the timing. Changing the total combustion chamber at TDC and keeping the same lead-in time would show the 'best compression' at which ever happens to match that exact spark curve.


Sort of on the same line of thought as how people say E85 is junk and give bad mileage when they pour it in an engine designed around 87 gas. If 14:1 was the 'norm', E85 would be god and gas would be known as 'shit fuel that blows up engines'.

 

[marleyskater420]

The 'same cam' part would kinda make it apples and oranges. Duration and degrees overlap are tied to compression (when done right). That test would probably show the 'best compression' as which ever one works best with that exact cam (and combo). Same with the timing. Changing the total combustion chamber at TDC and keeping the same lead-in time would show the 'best compression' at which ever happens to match that exact spark curve.


Sort of on the same line of thought as how people say E85 is junk and give bad mileage when they pour it in an engine designed around 87 gas. If 14:1 was the 'norm', E85 would be god and gas would be known as 'shit fuel that blows up engines'.

I do not have your engine building experience, but from what I have read by the great cam builder Dema Elgin, everything you are saying is correct.

For those who don't know, Elgin was the mastermind behind Duttweiler's cams in the 90's. The man is a true genius.

 

[earlbrown]

Here's another little tidbit you don't know.... I've forgotten more about this stuff that most people will ever know... and I am CERTAIN I don't know shit!

When it comes to cams, there's one steadfast rule: Once you know a lot about that subject, all you know is that there's a LOT you don't know!


Tyler, basically it works like this. If you take an optimized engine and but a huge 'rump rump' cam in it, you just made a lazy dog. When you put a fat ass intake lobe in there the piston is halfway up the bore before the intake closes, THEN (and only then) can you start building cylinder pressure. A huge lobe in an 8:1 N/A SBC, just turned into a 4:1 ''hot rod''.

So you have to bump up the static compression to end up with the same cylinder pressure you had before. The upside of doing that is you now have an engine with much less cylinder dilution and less 'dead air' space during overlap. That means even though you given up some of the stroke for squishing the A/F charge, you know have the ability to suck like a Thai hooker during overlap and get more A/F in there to squich (without going over the octaine limit of the fuel).


...now here's where it gets good.... All of that above example is WOT. And most of the time that's all anybody talks about.|

In real life we drive under vacuum most of the time. The static and dynamic are artifically starved during cruise. A tank full of 87 could live just fine in a 14:1 engine if you never went full throttle. As long as you never let it get full, it won't detonate itself to death. The trick is to have an engine that's not a dog while being 'starved' at cruise, but really gets on the clock at WOT.
You can build the most efficient, super neat, awesome WOT combustion even ever known to man..... ....and still have a POS if it can't act right at the other 99 points under the curve.
That's where higher compression shines. When 'starving' the engine of a full charge, the extra squish allows it to still move the car with very little throttle opening (non-car people call that 'gas mileage' and 'throttle response'). Then when you crack the throttle the engine springs to life on that exact intake stroke. In the case of a turbo motor, that means you start spinning the wheels right then, and spinning them harder due to the stronger exhaust pulses.

Now what's really cool about out turbo engines, is we can adjust the exact cylinder pressure we want by turning a screw. N/A guys don't have that option so they have to guess low (and hope they got it right). Granted you can't make as much boost with higher compression, but you can achieve the same thing with less boost, load, time, inefficiency, etc.. Bragging about boost levels is for ricers and bench racers.

Boost is a measure of how much air you couldn't get in the cylinder. In a way that's like paying a prostitute more than the asking price, and not getting laid.


....then bragging about it.



My advice is to learn as little about cams as possible. It's just easier that way. The 206/206 is an awesome piece that can get the job done.

Oh on an interesting parallel, my piston engineer was Duttweiler's piston guy in the 90's. That made working with him and building my plans were so much easier (once I convinced him that we don't really bust slugs like the average dumbass. We're above average! )

 

[marleyskater420]

Here's another little tidbit you don't know.... I've forgotten more about this stuff that most people will ever know... and I am CERTAIN I don't know shit!

When it comes to cams, there's one steadfast rule: Once you know a lot about that subject, all you know is that there's a LOT you don't know!


Tyler, basically it works like this. If you take an optimized engine and but a huge 'rump rump' cam in it, you just made a lazy dog. When you put a fat ass intake lobe in there the piston is halfway up the bore before the intake closes, THEN (and only then) can you start building cylinder pressure. A huge lobe in an 8:1 N/A SBC, just turned into a 4:1 ''hot rod''.

So you have to bump up the static compression to end up with the same cylinder pressure you had before. The upside of doing that is you now have an engine with much less cylinder dilution and less 'dead air' space during overlap. That means even though you given up some of the stroke for squishing the A/F charge, you know have the ability to suck like a Thai hooker during overlap and get more A/F in there to squich (without going over the octaine limit of the fuel).


...now here's where it gets good.... All of that above example is WOT. And most of the time that's all anybody talks about.|

In real life we drive under vacuum most of the time. The static and dynamic are artifically starved during cruise. A tank full of 87 could live just fine in a 14:1 engine if you never went full throttle. As long as you never let it get full, it won't detonate itself to death. The trick is to have an engine that's not a dog while being 'starved' at cruise, but really gets on the clock at WOT.
You can build the most efficient, super neat, awesome WOT combustion even ever known to man..... ....and still have a POS if it can't act right at the other 99 points under the curve.
That's where higher compression shines. When 'starving' the engine of a full charge, the extra squish allows it to still move the car with very little throttle opening (non-car people call that 'gas mileage' and 'throttle response'). Then when you crack the throttle the engine springs to life on that exact intake stroke. In the case of a turbo motor, that means you start spinning the wheels right then, and spinning them harder due to the stronger exhaust pulses.

Now what's really cool about out turbo engines, is we can adjust the exact cylinder pressure we want by turning a screw. N/A guys don't have that option so they have to guess low (and hope they got it right). Granted you can't make as much boost with higher compression, but you can achieve the same thing with less boost, load, time, inefficiency, etc.. Bragging about boost levels is for ricers and bench racers.

Boost is a measure of how much air you couldn't get in the cylinder. In a way that's like paying a prostitute more than the asking price, and not getting laid.


....then bragging about it.



My advice is to learn as little about cams as possible. It's just easier that way. The 206/206 is an awesome piece that can get the job done.

Oh on an interesting parallel, my piston engineer was Duttweiler's piston guy in the 90's. That made working with him and building my plans were so much easier (once I convinced him that we don't really bust slugs like the average dumbass. We're above average! )

Earl (is that your actual name? I am not being facetious) I really do appreciate you explaining this to me.

I've been looking for a good source for custom pistons for the stroker kits we sell...would I be able to give you a call and discuss purchasing pistons from you on an as-needed basis?

I won't lie that I enjoy reading about various parts of engine building. Until I am able to obtain the real-world experiences you all have, I do whatever I can to try to educate myself and make educated decisions. As I grow older, I am gaining this knowledge piece by piece, but if there is one thing I have learned from the beginning, is that I "still need to learn", as it is written in my signature on this very forum.

Whether it matters to you or not, I want you to know that I truly enjoy reading your posts and the wealth of information you have.

I believe it was Socrates that said "The only true wisdom is knowing you know nothing."

 

[earlbrown]

I guess I'm smarter than him then. I know a smidge more than nothing.

On the pistons, you can. Only thing is you can't make 'stroker kits' and have them be custom at the same time. That's kind of the niche I created. Obviously I can't stock every single bore size for ever single block height with every single stroke.... in bolt alloys. I have been thinking about ordering a bunch of ''zero deck slugs'' for common ('common' being the same as 'wild ass guess') bore sizes based on published deck heights. Not really sure how that would work since I like working the people direct with the exact block that's going to be built.


I have been thinking about hitting every pull-a-part around to grab all the 109's before they get crushed. Then making slugs for each block and doing the main caps, cam bearings, freeze plugs etc, then moving them for the end user to finish. That'll have to happen before it gets hot as balls in GA though.... So I have about 6 hours to make it happen.

 

[bison]

I would love to see three cars with the same long blocks with the only variable being compression a point apart. 9:1, 10:1, and 11:1. All on the same fuel and all use a 6766 turbo and 17 blade PTC. Same cams, same timing, a/f target, and dyno. I know there are cars like this floating around.... we just need them to chime in.

Ka-boom!


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[bison]

Watch out for some of the advice in this thread!!!


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[jasjamz]

I will stay in the 9.xx range and crank up the boost. I have recently learned the prices of built 109 short blocks are a bit more expensive than they use to be.

 

[BEATAV8]

11 to 1 works in my combo with E85.

Neal

Is your IVC kinda late?

 

[750H.P.V6]

Is your IVC kinda late?

I'll dig out the cam card and shoot you a PM.

Neal

 

[turbo89]

I would love to see three cars with the same long blocks with the only variable being compression a point apart. 9:1, 10:1, and 11:1. All on the same fuel and all use a 6766 turbo and 17 blade PTC. Same cams, same timing, a/f target, and dyno. I know there are cars like this floating around.... we just need them to chime in.

You cant handicap all 3 setups like that .. Why are we limiting anything .. if your going to go one direction fully commit..
no questions on compression its simple .. higher compression wins if all your after is power production.

How much is ideal compression ?? Simple ... as much as you can run for how much you want to spend ...
There is nobody that can give a set # on ideal compression.. and if someone is asking as to what number that is.. they need to keep researching as there are far more important compression numbers to worry about than static compression.

I run another turbo application @ 12.5 :1 static .. so what though ... you can have a lot more cylinder pressure with a 10.5:1 setup.

Currently my buick is @ 10.5:1 and its a destroked motor ... I don't have track times yet .. but it's significantly quicker than my old stock compression , stock bottom end 3.8 that ran 10.5x 's

 

[ITSAV6]

Theory and IMHO are just that. I know the wrong cam choice can make or break a combo quick. Got a friend who built his 77 Bandit TA with a 455 and used 400 heads not realizing he made the compression over 11:1 and spark knock galore. He had spent a but load on the heads and ended up with a custom cam. Sounds awesome now and will run on 93. Don't go out on a limb! Listen to Bison,Earl, and the others that know their stuff!

 

[jasjamz]

You cant handicap all 3 setups like that .. Why are we limiting anything .. if your going to go one direction fully commit..
no questions on compression its simple .. higher compression wins if all your after is power production.

How much is ideal compression ?? Simple ... as much as you can run for how much you want to spend ...
There is nobody that can give a set # on ideal compression.. and if someone is asking as to what number that is.. they need to keep researching as there are far more important compression numbers to worry about than static compression.

I run another turbo application @ 12.5 :1 static .. so what though ... you can have a lot more cylinder pressure with a 10.5:1 setup.

Currently my buick is @ 10.5:1 and its a destroked motor ... I don't have track times yet .. but it's significantly quicker than my old stock compression , stock bottom end 3.8 that ran 10.5x 's

It's not a handicap. It's a way to see the difference the change in compression makes with all other things being equal. I doubt someone wants to swap pistons twice and I bet there are some very similar builds here that have diffent static compression ratios.
What can you learn comparing a 10.5:1 destroked motor to a 9.5:1 stroker with a different cam...? Little to nothing because you cant say what makes the or loses HP/TQ.

I have 3 iron headed strokers that I was allowed to make the compression what wanted, pick the cams, converters, and turbos. Wonder what I will learn....?

 

[No disintegrations]

But you said "same timing". They can't safely run the same timing.

 

[jasjamz]

But you said "same timing". They can't safely run the same timing.

And just how much timing did I say? How much boost did I say? Since you want to "Quote" me...
The point was to establish a baseline and move forward from there. Anyway good luck to you gents.
I was also told I'd blow my engines up using E85 in 2012.... but in 2014 it was going 10.30@135 in E85 in Bowling Green.

 

[1952ray]

I will stay in the 9.xx range and crank up the boost. I have recently learned the prices of built 109 short blocks are a bit more expensive than they use to be.

I know this is old school, however Red Armstrong use to say he only wanted enough compression to be able for the engine to crank. He would make up the power with boost. Richard Clark somewhat believes the same.

 

[No disintegrations]

I don't understand why someone with an 11 to 1 engine would run the same spark timing as a guy with 9 to 1. Just for shits? Not matter how low or high it was.