LMAO!!!!!!

 

Since we’re here…. I just used a calculator, and milling 0.016 should be good for half a compression point. It was good to read that won’t create VTP issues with a mild duration cam.

 

A thinner gasket will NOT help with detonation! A THICKER one will. But any time you increase compression, you also increase the chance of detonation! Sometimes 1 range colder on the sparkplugs heat range is all thats needed...

 

Rule of thumb for compression is 3% to 4% increase in power for a full point of compression.

 

For the cost of milling the head, that’s about as cheap as hp comes 👍

 

Thinner gasket, Tighter Quench, reduced detonation

https://help.summitracing.com/app/an.../engine-quench

"

• Don't run more than 0.060 in. quench trying to lower Compression Ratio.
•  
  • This will slow the combustion process and could cause Detonation."

 

I think raising compression is a terrible idea for gasoline.
its my opinion.

1. Higher compression is harder to crank, harder on the battery and electrical.
2. Less tolerant of low octane fuel, especially bad gas.
3. Less tolerant of low rpm mediocre tuning under varying load conditions (more load than when it was tuned originally)
4. Harder to tune under load in general.
5. Smaller tuning window for timing on gasoline
6. Less tolerant of high temperature situations, heat soaking
7. Less tolerant of high oil and coolant temperature
8. Less tolerant of certain cam profiles
9. Increased potential for rod bearing stress/wear during cranking

Do what you want but I think its dumb for a 1 or 2% increase in torque, too much negative consequences and potential risks for such a tiny gain.

Aside,
Factory skyline, silvia, supra engines are 8.5:1 compression, go 250,000 miles+ do 28-32 miles per gallon 3000lbs and handle 500-700rwhp on gasoline fuels without the stress of high compression cranking. Factory Supra engine is already 450rwhp with zero upgrades, that engine from 1995. Turbo is the answer, not V8 high compression garbage.

 

I have my reasons, and stated what experience has taught me. I won't comment on this any further....

 

king, how much do you figure cranking compression will increase when going from 10.2 → 11.0? i figure not by much.

 

I'm not saying the difference will be enormous. Only that it doesn't make sense to walk in that direction on purpose. Its like a room full of unpredictable animals guarding a small pile of multi vitamins, why go closer than you have to for such a minimal prize. You are fine where you are.

And there is more than just the cranking compression to look at but lets just examine that real quick since we are here.

Cranking Discussion
First, remember the compression pressure peak is not instantaneous nor is it regulated strictly by cranking RPM or VE. During cranking a spark creates higher than cranking compression, so really we could be looking at a difference of (100)'s of PSI not just 10 or 20 during a sparkless cranking compression cycle. The pressure peak of a cylinder is what may wear rod bearings and bend rods, it is the most abusive type of stress to place on an engine in my opinion.



Next we should understand the difference between instantaneous and integral stress. If we pick a specific point on the graph above for example 750psi peak pressure, we can determine the stress placed on the piston/rod/bearing/oil film etc... However this is only 1 specific point of stress and does not include any of the other points leading up to that peak. The other points when taken together are called an integral of pressure, or the area under the curve of cylinder pressure or if we want to call it the area under the curve of pressure over some area of a piston surface which relates directly to the stress applied to the rod and rod bearing and so forth (down the drivetrain). So lets say we increase the cylinder pressure 1 psi at every point- 1psi by itself instantaneously is insignificant of course. But when taken together we have an extra 1psi across the board for the entire duration of a revolution through the compression cycle, this extra shear stress applied constantly to the oil film is acting as an integral function, or a 'accumulative' or 'summation' style function which is going to have a much greater overall impact at the end of the rotation than if we just increased the pressure at a single point in the curve for a split .0001 second.

Next, oil flow must be compared to these stress forces,
The affect is cumulative as oil flow is not immediate. The engine will take something like 800rpm for lets say 5 to 10 seconds for oil to make its way from the back of the engine to the front. Your oil pressure sensor pegs out at say 40psi in the first second or two but the oil does not reach the cylinder #1 rod bearing for another 5 to 10 seconds or whatever. The lifters take another 5 to 10 seconds over that as well, they are even slower to receive the oil.

Summary
-The pressure rise in the cylinder due to small changes of compression may be 'small' while it is enhanced via combustion
-Pressure and stress are taken as an accumulative integral function applied to the fluid film protecting rod bearings over many revolutions
-The engine may experience (800rpm for 10 seconds) around 130 revolutions without oil pressure while an additional 100~psi (for a slight compression increase) is applied to the rod bearing of cylinder #1

And there is also the rate of fluid flow into the bearing compared to the rate of it leaving over time. Some oil is pumped in while some leaves based on applied shear stress the oil is squeezed out of the bearing. As RPM increases generally more is leaving and less is entering due to losses from various sources mostly friction I guess. At some point there is enough oil leaving the film is weak enough to encourage bearing wear. This point will be sooner the more pressure is applied to the bearing due to cylinder pressure. Higher RPM engines will survive more easily with reduced compression.

I avoid high compression engines. I've tuned a few of them from the 90's to recently.
There is almost always some issue with cranking which is why mentioned it. The battery needs to be bristling with energy and the cables need to be really well done and its really draining to have a no start condition while diagnosing something. First thing I ask for is a battery charger. I cringe when I hear them turning over without fire especially in the 12:1 and 13:1 range. I always talk the owners out of that eventually and we go turbo if possible. Nice 9:1 easy cranking and twice the power output using cheaper fuel. No brainer.
Besides cranking, I don't like the the tuning window. They act like forced induction engines using cheap fuel no matter how good the fuel is during peak torque region the pressure can get very high and requires some really careful timing adjustment and small change in temperature can have dramatic influence on temperament (behavior). I know how to do it but I think it is ridiculous to go through all of that for an engine that puts out so little. And I do not think many people understand how to properly tune it. Two different timing approaches can yield the same rwhp number but one of them is chewing the bearings up.

The other thing about the heads is the distance and angles from the deck to the valve cover is changed. It can screw with the valve geometry creating extra friction or unnoticed stress that leads to failure over time. If I have any engine I want to reach 200k miles you bet your *** I won't be adjusting the distance of the head to the deck by shaving and I won't be changing much in the valvetrain either. On some engines it can affect the manifolds, sealing and bolting them or whatever. On some engines it can even influence the cam timing and oil orifices. You guys have to keep in mind I tune every kind of engine, not just V8 and LS but also every kind of engine 1.6L 1.8L 2.0L 2.2 2.4 2.5 2.6 3.0 etc.... I tune them all. So my generality and precautions and intuitions are adjusted or acclimated to offering advice that will work for any engine, its like a safety net for everyone to leave stuff alone in general keep the OEM configs and work with how it comes. If the OEM is any good this is always a good rule of thumb.

 

My friend must be a complete idiot, running 10.5:1 compression and 10+ psi of boost. Must be a completely stupid moron....

 

you can run 15 psi on 11:1 static in modern engines.

king, nice write up. of course it all depends on the engine in question. a classic 8.5:1 engine will benefit some from 11:1 compression with the effects you mention as a trade off and head shaving being not such a good idea. when going from 10.5 to 11.5 the effect will be barely measurable. im also i friend of boost, but often n/a cant be avoided, be it because of rules or philosophy.

in general people get worked up too much by oil pressure, most components (except e.g. squirters) dont need pressure at all, just the presence of oil, that for the first revolutions is given. cranking usually is much shorter than you mention.

you briefly toutch on valvetrain geometry. while cam specs get discussed ad nauseum (as if +/- 2° would matter) the way the motion gets translated to the valves is neglected. being somewhat of a "black art" that realy can only be modeled through numerical analysis its a difficult topic, but imo the reason why some engines run better than others.

 

The LS6 engine is 10.5:1, compared to the 10.0:1 LS1 engine. Could you offer some insight into the GM engineer’s decision to increase compression by half a point, given your conclusion above?

TIA!

 

a ferrari has a compression of 13.5:1. obviously higher compression increases efficiency, with quickly diminishing returns above 10:1 however.

here we were talking about modifing an existing engine, no? i would like to venture the opinion, that if you take a stock ls1 and increase the compression to 10.5 you wont be able to reliably measure a difference given a repeatability of +/- 2%.

 

Damn good question, @vette4fl

 

Never said it was stock, lol.. Anyway, my question is if the difference isn’t measurable, then why would GM engineers, in your opinion, make this choice? The Z has a mild cam, so?

If you don’t really know, that’s fine, I’m just trying to achieve a better understanding of this subject, since all the builders in my area are heavily in favor of 11:1 or 11.25:1 on pump gas. So I ask myself, why are all these racers, who have dyno and track results to compare, on board with the GM engineers? What am I missing here? Appreciate your help on understanding this better…

 

you aim at as hight compression as you can get away with given all other parameters. a "hotter" cam will bleed off in-cylinder pressure and dilute the mixture throughout most of the powerband. thus a higher compression ratio is possible. it shouldnt be difficult to find a calculation showing the thermo-dynamically possible gains. these theoretical values then have to be "degraded" to reality.

edit: a better head design allows for higher compression as well.

 

There is more to burning fuel efficiently than compression but lets put a pin on that for a sec and get something clear

1. The more power an engine is intended to make the lower the compression needs to be
If shooting for 2500hp then perhaps 7:1 or 8:1 is ideal with alcohol style fuels as ethanol or methanol
If only wanting 1000hp then perhaps 10:1 is fine using the same fuel
Alcohol will tolerate much higher compression than gasoline. But alcohol is more expensive and you stop for fuel more often, all the extra compression does is bump off-boost performance slightly at the added expense of using a much higher quality fuel and stopping for fuel more often and taking away the ability to use a lower quality, cheaper fuel when you need to, in some circumstance or situation where saving money or being stranded without alcohol might surface.

Somebody suggesting 15psi of boost on 10:1 "Oh I must be crazy" Is completely neglecting to mention what fuel and what application.
They aren't thinking like a tuner or engineer, just a consumer and copy cat "what I see people do so it must be right" I will quickly explain what I mean.
Of course modern engine can use 12:1 compression with E85 fuel and 15psi of boost or more.
However, the philosophy of this combination only fits specific type of application range: Non-Drag racing setups which must use high quality expensive fuel, stop for fuel more often, and will not tolerate lower quality fuels to some extent, and with no fuel savings benefit yielded from the higher compression.
For example, 8:1 compression engine can use 87 octane fuel with minor boost. 8:1 or 9:1 could also use 93 octane with significant amount of boost, say 25psi to 30psi of boost I've tuned on 8.5:1 even 9:1 compression. That means, the engine during cruise is sitting on 93 octane $$ which means nearly maximum miles/dollar ratio, the vehicle is essentially or could be a street car with economical value, economical cruise with gasoline while having 25 or 30psi of tire shredding boost on standby.

Now lets see what happens if we change the 9:1 to a 11:1 engine. A minor increase in compression- what is the result?
Now the engine is more efficient. However it will no longer tolerate 93 octane fuel with much boost. If you want boost now, the engine needs alcohol fuel. Now you have ruined the economical value of the vehicle, it is no longer an economical cruise vehicle. Therefore the additional efficiency gained by the compression increase actually ruined the efficiency of the vehicle on a whole.
On the other hand, this is the ultimate street machine now. An alcohol burning, boosted high compression engine offers superior off boost response and transitional response. You pay extra money for the fuel, the vehicle is strictly a 'toy' and no longer economical to drive, its a bit harder to tune and more susceptible to maintenance issues as clogged fuel filters tend to arise when using alcohol fuels, the alcohol is a bit corrosive and there will be more maintenance issues with fuel system, there is extra fuel pumps needed for the dramatic increase in demand for fuel volume, but there will be a nice increase in torque before the boost kicks in for that split second- was it worth it? I think its crazy to give up all that economy and have to deal with so much extra maintenance and provide more current to more pumps and have to watch all these things carefully just for a small bump in torque off boost which could be neglected by simply choosing the correct torque converter and rear gear ratio. But thats my opinion!

Now lets look in terms of drag racing
Cylinder pressure or stress applied to the surface of the piston due to instantaneous pressure is related to compression, whereas power output is related to integral pressure and engine frequency.
Drag racing engines with forced induction tolerate the most boost and highest peak output power with the minimum compression ratio.
There is no need for efficiency because these drag racing vehicles do not need to cruise economically.
There is no need for response and off-boost response because drag racing vehicles dial their boost pre-launch and may include nitrous.

Therefore, drag racing vehicles have no use absolutely no reason to increase compression. It is against the philosophy of drag racing in general. Of course it depends on the engine design and piston materials and rod strengths and so forth- in some cases with very expensive parts the extra stress is negligible and the fuel quality is so high it doesn't matter but even in those cases we are still looking at a small percentage increase in power and no need for the economy feature or response character the extra compression provides.

Now finally, increase the compression in a natural aspirated engine.
Of course this is feasible to some extent, getting higher compression helps with economy cruise. This is likely why the engines over time get higher and higher compression wherever they can find ways to offset poor behavior from gasoline fuels. For example DIrect Injection allows a higher compression for most engines with modern chambers. Improved cylinder and piston cooling. Improved valve events and metal materials science. Each year I assume the EPA puts a harsher requirement and so the engineer will look for any little way to increase economy that tiny bit- if they need to worse case scenario bump the compression I suppose that would be an easy quick fix to get the ball rolled out the door.

To put this into perspective Lets go back to the first sentence of this post. There is more than just compression to consider.
Chamber design for starters. If we compare small block chevrolet to LS style chambers you will see an enormous difference. Computer modelling has allowed the LS chambers to more efficiently sequester and propagate expanding gas/flame of chemical reaction resulting with a completely different tuning style such as spark advance and optimal air fuel ratio and chamber swirl character and other details. This means more compression is possible using the same fuel quality. Another change is fuel injection strategy, port to direct injection and injection timing that high pressure spray of fuel properly can mean more compression compatibility with the same fuel quality. It isn't fair to say "this compression is only good to XYZ amount of cylinder pressure (boost or otherwise)" when we have all these variables. If the engineers improved the DI strategy and improved the metal materials or cooling of the cylinder components and altered the valve events to help cool the cylinder or changed how airflow moves from outside to inside or altered the exhaust gas pressure or dynamical momentum for fluid flow in the exhaust etc--- all of these things may allow the engine to increase it's compression slightly using the same fuel for a slight edge to economy and maybe a couple hp.

 

Well, since this isn’t going anywhere, I’m just gonna stick with the local knowledge base for some proven results.

 

i feel you are getting excellent answers. or what is it you want to know exactly?