Ideal compression?

[Anonymous]

Taken from a previous post regarding fuel mileage and compression, MarkP stated the following:

Like CZLN6 said: Raising compression WILL increase power and efficiency, resulting in better MPG numbers. The 200 underwent changes into the "3.3L" version of the late 1970s and later that lowered compression for 2 reasons: 1) lower octane fuel and 2) emission controls, specifically oxides of nitrogen. Compression ratios above 9.4:1 cause a peak heat during burn that creates nitrous oxide and dioxide. The introduction of EGR into the intake manifold is done to lower that peak heat about 10% or so: lowering the compression also reduces the peak heat and, with it, the efficiency. That becomes less MPG at the wheels.

The easiest way to rectify the situation with the 200 is to narrow the quench band while raising the compression. With a narrow quench, the oxides of nitrogen and hydrocarbons both go down and the tolerance to poor fuel octane goes up. In other words, like my engine, I got the quench band ("deck height") down to about .035" and milled the head to get the compression ratio up to 9.4:1 or so. The result was an extra 4 MPG, on average, across the board, with noticeably better acceleration and hill-climbing ability. It runs on cheap regular unleaded. It still passed emissions, too, even with advanced timing (+4 degrees over stock) and vacuum advance on the distributor.

Change the distributor advance distance, too. Stock ones usually give about 26-30 degrees total advance. You can weld up the holes a little to reduce this total to 22 degrees, then use an extra static advance to make up the difference. This is precisely what the new, computer-controlled engines do: more advance earlier with less overall. The overall advance for good MPG should not be more than 34 degrees: mine likes about 32 degrees total (static + full mechanical advance). It starts better when hot, too....

I know many of us have wondered, and probably discussed, about the topic of obtaining high compression ratios and still maintaining the use of regular gasoline (general 85 or 87). For many, including myself, have never seen a posted "formula", if you will, with regards to providing a sound solution to the never ending struggle for performance and fuel economy. I'm not saying Marks formula will work for everyone (especially for those at lower elevations) but I would think in general his example is most definetly worth considering.

So, all being equal, what is the "best" compression ratio without sacrificing fuel economy and low emissions? Has anyone applied this question mathematically. Is it possible. Does anyone else have similiar experience? Different results?

 

[mugsy]

There are so many variables to the "what compression to use" question, I don't think that there is a definite formula to use.

I have only heard of a "rule of thumb"as to CR. That "rule" is: the compression ratio should be 1/10th of the octane rating of the gasoline you want to use. Wanna use Regular, which is 87, then the CR should be 8.7:1. Like the good, expensive stuff, which is 92, then build the CR to 9.2:1.

I'm sure that this is a safe, conservative way to go. I bet that there are cars that could take more CR and I am sure that there are engines that ping like crazy based on this "rule". But that is the way "rule of thumbs" work.

tanx,
Mugsy

 

[6bangerbill]

It has been my experience that selecting a compression ratio is also related to cam timing. Specifically, the closing angle of the intake valve and measured by cranking a compression reading rather than exact compression ratio. For 93 octane gasoline at 700 ft of elevation the cranking compression should be no more than 190 and 200 psi. Ignition timing , engine coolant, and intake are temperatures must be optimized to avoid detination but its worth it for the extra hp attained. If detanation occures retarding the cam timing a few degrees will solve the problem.

 

[Anonymous]

10.5:1 is a good all around compression ratio for your weekend warrior if done correctly! 8) 8) 8)

Later,

Doug

 

[wsa111]

Doug, with your new engine with more camshaft duration do you still need the water injection?? Have you checked what the cranking compression is?? Just wondered. William

 

[Anonymous]

Bill,

No compression test and I havent had to use my water injection yet

Maybe the big cam is bleeding off the compression??

Later,

Doug

 

[xctasy]

From a while back, I started on using a standard chart to document the 11 factors which influence Road Octane Number for a given compression ratio. There is a curve for a Ford SOHC and one for a Mini A-series (both copy righted in David Vizard books), but none for a Ford I6....yet.

I am working on it. I am convinced the 11 factors are the main inputs to link to scaling the existing SOHC Pinto and A-series graphs. From here, you'll get a very safe way of calculating ideal compression from given details. Since the cold cranking pressure has been a focus point on tuning engines for the road, advancing anf retarding a cam must be a crtical input as it changes the effective timing. We are close the getting this one nailed, I'm certain!

We discussed it broadly a while ago, and I promised to spill the beans on data I had acess to.

It still isn’t doen yet, but I;m very close. It is as significant as

a) flame travel due to plug placement (its most often reduced with hi-po pistons),
b) valve shrouding (often increased, but is mitigated by gas flowing with a famous SuperFlow flow bench!),
c) piston dome masking (often, the dome is relief cut to suit flame travel and valve opening).
d) Effective compression (cam related octane demand). Doesn't always go up with increased compression, as the whole reason for it is usually to raise opening duration to increase effective compression. When a racer bolts on a 320 degree cam, he can loose a huge amount of effective compression, which reduces the octane demand.
e) lastly, quench is often increased as at the 10 to 14:1 level, people go to alloy heads with closed chambers or welded iron or alloy production heads.
f) intake heating
g) fuel distribution
h) head material (iron, alloy)
i) Inertial Ramming (Carburation, efi, vee engines have better thermodynamic properties)
j) piston deck impinging , flat or recessed.
k) blue printed to 0.1 c/r or C/R balanced on all cylinders accept the detonation prone ones. (If they are reduced by half a point, you can go up another 0.5 points on the other cylinders)

There are characteristic curves I’ve seen for most combustion chambers, so you don’t have to run these factors into a program. The two curves I have are for the Pinto 2000 and Austin A-series engine. These have very accurate additional data about the 11 items listed above.

There are many chambers. I haven’t listed the huge range of vintage flat head and inlet over exhast combos.

Steep included angle Mopar Hemi’s, Jag XK 6, Alfa Romeo
Steep Wedge Porcupine Chevies, there is very little postive alteration to a) to e), and the results are counter productive has you always have to compromise on pistons popping up into the chamber.
Steep Reverse Wedge the Pinto and Lima fours,
Shallow Compound Vetrtex Hemi
Shallow included Pontiac’s, Holden’s, Olds, Wedge, Chev, Ford’s ohv non Clevo, Lima
Shallow porcupine 335/385 Martel Fords, Geelong built X-flow
Flat bathtub Weslake A-series Austin/Mini I4's and the , there are only positives.
Flat Heron head Kent, XKE 12, Y-block.
Cosworth narrow Angle Pentrooof (Sierra, Toyota 4AGE, Ford BDA, etc)[/quote]

 

[6bangerbill]

I have noted that many of the newer drag engines notably the sb2 chevy and even some canted valve engines are using very shallow chambers ( like 30 cc) with additional combustion volume in a shallow piston dish. I have have cogitated the subject and concluded that perhaps the comustion process is inhanced when squish gasses are squeezed above the bottom othe chamber rather than the bottom to acheve better mixing and complete combustion. Do you have any information on this subject? I am thinking of angle milling my ls1 alum heads and using dished pistons if I can confirm this theory. This modification would also provide more room for my distributor cap under the intake manifold.

 

[ASMART]

XTAXI: You seem to have a handle on what it takes to run higher compressions with that list of factors. Could you elaborate on each one and dfefine terms some of us may not be familiar with? I have always liked you postes. They are imformative and to the point.

Thanks

 

[xctasy]

Oh golly, terms are something I get a little befuddled with.

The basic gist of it is that, like algebra, the total sum of the Ideal Compression ratio, is based on lots of seamingly irrelevent terms.

The 11 terms a, b,c, d, e, f g, h, i, j and k are best defined by me giving good giving examples and then a bad example.

That takes some time, and I'm not there yet.

One thing is that a great engine for a high ideal compression is the DFV/DFX Cosworth. A bad engine which demands a low ideal compression ratio is the stock 250 I6.

Once I've locked down the terms a little better, I'll bring it out of the closet.

Meantime, look at the graphs from Page 215 on Modifiying A-series Engines by David Vizard,

and pages 123, 139, and 37 of Modifying Fords SOHC four cylinder engine by the same author.

I'm bound by copywrite not to disclose these here, but if you'll find the issues, you will know as much as I do.

 

[ASMART]

XTAXI

We can discuss here if you like.

a) flame travel due to plug placement (its most often reduced with hi-po pistons),

I understand how flane travel affects efficient burning of the a/f mix. I also understand that placement of the plug relative to the center of the combustion chamber promote cleaner faster burns since the distance the flame has to travel is the same in all directions. I am not sure how plug placement can effect ability to take higher compression.

b) valve shrouding
I under stand the effect on flow but what is the effect on abilty to take higher compression

c) piston dome masking
I understand piston dome shape can present obsticles to the flame front and also provide hot spots to cause detonation. What else am I missing.

d) Effective compression
I think that is obvious. The compression ratio is not really a good measure when figuring detonation problems. Actual pressure is what counts. Cam overlap and duration will change actual pressure and that will vary as RPM changes.

e) lastly, quench
I am not sure I understand quench. Any explanation would help.

f) intake heating
obviously, the more heat involved the more likely detonation will occur
g) fuel distribution
This is obvious also, since a/f ratio may vary between cylinders, detonation may happen in on chamber and not another since the a/f ratios would be different
h) head material (iron, alloy)
This is also obvious if you remember that aluminum conducts heat betterthan iron. I assume aluminum helps reduce detonation.
i) Inertial Ramming
More air, higher pressure, more prone to detonation
j) piston deck impinging , flat or recessed
I am not sure how this plays into the detonation issue
blue printed to 0.1 c/r or C/R balanced on all cylinders accept the detonation prone ones.
I understand that one also.

 

[xctasy]

XTAXI

We can discuss here if you like.

a) flame travel due to plug placement (its most often reduced with hi-po pistons),

I understand how flame travel affects efficient burning of the a/f mix. I also understand that placement of the plug relative to the center of the combustion chamber promote cleaner faster burns since the distance the flame has to travel is the same in all directions. I am not sure how plug placement can effect ability to take higher compression.

The flame front must travel from the intake mix to the exhast, not towards the intake valve. Get this wrong, and the octane demand for a given compression goes up.

b) valve shrouding
I under stand the effect on flow but what is the effect on abilty to take higher compression.
If a Mays head, or high swirl compustion, the designers claim the partially shrouded intake valves increase mixture motion, and can reduce detonation and allow a higher compression for a given fuel. This is in fact incorrect. These engines are timed to mean best torque on the ignition, and then retarded for total timing to create excellent part throttle economy an dclean burning. i have seen not a scrap of evidence that shrouding produces redced octane demand. In the 12.5 and 11.5 :1 HE Jags, the shrouded valve gave better power and perforamnce and economy only becasue the C/r gave the power. The igntion was timied right back at maximum torque. 86 EFI 5.0's and 86-93 XF Falcons ran HSC heads, and they required very low compression ratios to run 87 to 91 octane fuel. On classic bathtube heads, shrouding requires a lower octane rating. In early closed chamber Clevleand heads, the octane demand was very, very high compared to open chamber heads, and the closed chamber heads carried less compression than open chambers before detonation set in.

c) piston dome masking
I understand piston dome shape can present obsticles to the flame front and also provide hot spots to cause detonation. What else am I missing.

The Chev Big Block and Mopar Hemi's had a high degree of piston squish for a given compression ratio. It's better to compromise on the dome (make it shallower), and give it room to propagate a proper flame travel. Ford SVO pistons for A3 and NASCAR legal heads has a flame front channel forged into the pop top 12.5:1 pistons. Any piston which fails to allow for flame front propagation hursts the octane demand.

d) Effective compression
I think that is obvious. The compression ratio is not really a good measure when figuring detonation problems. Actual pressure is what counts. Cam overlap and duration will change actual pressure and that will vary as RPM changes. Applying a retard or advance scenario of cam timing, eg 104 , 108, or 112 degrees, has a huge bearing on an engines ability to run on pump grade gas. The cold cranking compression varies with mintue change the cam installation. The exhast closure point is the be-all and end all of cam bleadoff, and as such, a poor cam can be made passable by changing the installation advnace or retard. Jeremy Diamond posts the theoretical effect of changing the effective compression. In my opinion, this is the most important thing a cam selection, as a cam is the heart beat to the igntion event. If the effective compression is low, the chamber filling is poor, and detonation due to lean out is nore likely.

e) lastly, quench
I am not sure I understand quench. Any explanation would help.

Mark P describe quench and 'squench (squish and quench)' as the anular segment which the chamber wall forms. Its a progessive ramp or even a semi sharp ledge which is designed to cool off the heated fuel air mix, and create power via added compression. On a 302 Boss, it is huge. On a later 351 SCJ, its non existant. The 57 cc chanber 302Boss and the 75cc chanbers of the last 351C 4v SJC and HO engines have the same valve and depth, but the quench is supposed to provide a controlled squash of the mixture on the 302, while providing good compresion. In my observations, quench doesn't work unless it helps produce a good flame front becasue of a problem in plug placement. Detonation was the main cause of damage to the bottom end of these engines. Octane demand on closed chamber Aussie 2v 302C heads is far higher than open chamber 2V 351c . David Vizard added a metal quench area on high compression racing Pinto engines, and found it helped because of the compression increase and poor plug placement and restricted mixture flow. In so doing, he created a dam which herded the flammed mixture into the correct area for efficent exhast flow. In some instances, this quench can solve a problem, but in itself, it should be avoided. Modern LSx Chevies use a small quench area to create mixture motion, and it works. Im note sure if this tecnically quench, but in additon , on the block ledge, or anular element around the ring land, there is a heated region.
f) intake heating
obviously, the more heat involved the more likely detonation will occur

There is a specific graph which shows you to see just how lousey adding an exhast heated separated intake is on a Mini. A minimum RON requirement goes up 8 points with it!!!!To my thinking, the mono manifold Falcon I6 is even worse.
g) fuel distribution
This is obvious also, since a/f ratio may vary between cylinders, detonation may happen in on chamber and not another since the a/f ratios would be different.

This is a massive problem on I6 engines, and why I advocate any kind of method of reducing the time of concetration from the carb's air fuel mix to the outer cylinders. A six is over 20 inches long, and the outer cylinders run lean, the inners rich, and if the centres see 12.5:1 at full power, the outers see less than 13.5:1 It's a recipe for disaster if your running an engine which is old and worn, has blow by from the rings and intake and exhast valve seals, has poor ignition, and the other 10 factors listed here are crook or less than ideal. The solution is to use staged 3 bbl Offy intakes or getting a better Argie or Aussie intake, or doing a triple SU swap. Both the time of concentration, a response time issue which removes power from the outer cylinders, and the wetted permiater, the collective amount of interanl surface area the fuel has to travel over, are the reasons for the dispartiy. It varies with rpm, air speed, and with temperature.

h) head material (iron, alloy)
This is also obvious if you remember that aluminum conducts heat betterthan iron. I assume aluminum helps reduce detonation.

Sure does. But its more than the material, its the finish. The microtecture of the suface changes detonation. Smooth, polished chambers can ruin detonation resitance. Hig pressure, japanse style Alloy heads as cast have a roughness average which is ideal, but cast iron can have more mixture lumps and ramps, and people end up plishing them, ruining mixture motion, and inciting detonation. Engineers have noticed that Brake Specfic Fuel consumption is best with stock intake ports, and worse when polished. Ultimate power may go up 2% with plished heads, but BSFC drops dramatically. A similar dynamic can happen in the chamber. It's best to clean up the head, rather than polish it. The material used changes the thermodynamic conductivity. Iron heads have trouble passing head out, and tend to need another 4 to 5 RON of fuel octane to do the same job as an alloy head. The primary constituent of alloy is silica, and it allows the head to tranmitt heat back into the chamber, and remove it via convection to the block with a lot more efficieny than the iron head.

i) Inertial Ramming
More air, higher pressure, more prone to detonation

This is very important. The mean best torque advance has to be wound back when your engine is more efficently filled with fuel air mix. A set of triple DCOE 45'Ss will inertially ram, or push slugs of fuel into the cylinder in a super critical speed. The igntion advance on a 4-bbl engine with 600 cfm may be 36 degrees, but the Webers might like 32 degrees becasue the chamber filling is so much better.

j) piston deck impinging , flat or recessed. I am not sure how this plays into the detonation issue

The example is on a 250 Ford, where the piston at tdc is 110 thou below the deck of the engine, and maybee more with an aftermarket piston. The top of the ring land parks 275 tho below the 110 thou short fall, and the heated zone is effectively 385 thou. The reduction of this unburnt zone is a good idea for detonation reduction. This makes a heated ledge which becomes a sharp detonation zone which spoils all the hard work Ford put into the quech side of the chamber. Shaving or zero decking the block so the piston parks at the top is very important.

Chev LSx's are actually postive deck engines with nicely shaped pistons with a flame front freindly design


k) blue printed to 0.1 c/r or C/R balanced on all cylinders except the detonation prone ones. (typo mine)
I understand that one also.

It is very important becasue the engine is only the sun of its componets. If a developement engineer sees No1 is failing due to detonation, then he will turn the whole engine tune down just to make sure that cylinder is looked after. On a handbuilt engine, the C/R of the offending cylinder should be knoked down, and the others brought up. The engine as a hwholw must produce the best power. Number one piston on our sixes runs way too cold, so No 1 is both lean, and cool. The engine under heat will never detinate at Number 1, it will be No 5 and 2. These cylinders should get a C/R drop, and the others raised for a huge reduction in octane requirement for the whole engine.

Remember, an engine is as strong as its weakest link, and like the human body, it can perform to a level almost beyond the sum of its components if all parts are working towards total performance. It's like a row boat. You provide the brains on how to make the members haul there collective butts!

 

[ASMART]

Thanks for the explanations!

 

[Lazy JW]

XTAXI

Engineers have noticed that Brake Specfic Fuel consumption is best with stock intake ports, and worse when polished. Ultimate power may go up 2% with plished heads, but BSFC drops dramatically. A similar dynamic can happen in the chamber. It's best to clean up the head, rather than polish it. The material used changes the thermodynamic conductivity. Iron heads have trouble passing head out, and tend to need another 4 to 5 RON of fuel octane to do the same job as an alloy head. The primary constituent of alloy is silica, and it allows the head to tranmitt heat back into the chamber, and remove it via convection to the block with a lot more efficieny than the iron head.[/b]

So for best fuel economy on my 240 it would be best to leave the ports with the stock, as-cast finish?
Joe

 

[xctasy]

Yes.

It's easier to sell a polished head than one that has just been mildly cleaned up.


When the day to day economy is bench marked, things change with polished ports and chambers. In a stock 'as cast' port, becasue the 50 to 60 thou atomised fuel droplets tend to get smashed up by the coarse microtecture, the fuel is burnt better. Remove the sand castings marks, and it might increase peak power, but it'll certainly reduce BSFC. They are moulded with green sand. It is a mixture of uniformly graded fine silica sand, bentonite, and organic binders that is used to make molds for castings in gray-iron foundries, and America does more of it than anyone. Why ruin a work of art?

There is some compelling evidence that a smooth combustion chamber works very well on a race engine, where peak power is everything, and you can pit stop on the safety flag rather than have to budget your fuel use.

In terms of long term durability and resistance to carbon build up, it's also a plus.

The peak flow of the gas is restricted by surface roughness (term is microtexture, or, if your a plumber, the e value), but the fuel must stay in suspension when being ignited for economy. It shouldn't condense or wet the chamber wall.

I'd say 90% of the work we by polishing and expanding the post size for power is just going to hurt part throttle fuel economy unless you can do dyno runs to prove otherwise. Generally, the exhast can be polished, and the exhast valve enlargened, and the guide clearance reduced with 30 degree bullet chamfer, and a better high silica bronze guide fitted still exposed to the exhast gas flow. On the intake can have the intkae valve gude cut right back to the port wall, and the stock port used. The chamber should stay the same unless there is significant shrouding og the exhast valve. Cam can go up to 270 degree gross duration without hrting things, and a medium size exhast with low restriction mufflers will help.

Econo section, David Vizard books, none is my idea.

 

[Lazy JW]

In a stock 'as cast' port, becasue the 50 to 60 thou atomised fuel droplets tend to get smashed up by the coarse microtecture, the fuel is burnt better. Remove the sand castings marks, and it might increase peak power, but it'll certainly reduce BSFC. They are moulded with green sand. It is a mixture of uniformly graded fine silica sand, bentonite, and organic binders that is used to make molds for castings in gray-iron foundries, and America does more of it than anyone. Why ruin a work of art?

So is it the sand casting marks causing turbulence that helps keep the fuel in suspension, or is there someting about the roughness that keeps fuel from clinging to the walls, or perhaps a combination of both? I find this to be very interesting, thanks for the info.
Joe

 

[Anonymous]

Is it a valid cleanup process then to use sand as a blasting media to clean the chambers? Assuming that surrounding surfaces get masked off.
DaveP

 

[Anonymous]

The speed of the fuel/air determined by the size of the runners would certainly have to make a significant difference to the fuel staying in suspension (smaller is generally faster without being too small which would then restrict peak power).

 

[benwantland]

So is it the sand casting marks causing turbulence that helps keep the fuel in suspension, or is there someting about the roughness that keeps fuel from clinging to the walls, or perhaps a combination of both? I find this to be very interesting, thanks for the info.
Joe

I don't think the casting flash is desirable, as it hurts the flow more than it makes up for with increased charge mixing. I would guess it is almost always advantageous to grind down said flash.

The general rough texture of the ports, however, does comparatively little to impede flow, while helping to jostle the air/fuel charge around just enough to help a lot in keeping it mixed properly.