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Squish Volume and combustion efficiency
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journeyman
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PostPosted: Thu Feb 03, 2011 7:27 am    Post subject: Squish Volume and combustion efficiency Reply with quote

my only interest is in combustion efficiency, so building for efficiency is the logical starting point.

I read that squish volume should be minimized to decrease 'hot spots' and thereby increase combustion consistency from cyl. to cyl.

Quote:
A single side tight quench is a very important combustion enhancer that an engine builder can use to speed flame travel.

and
Quote:
Efficient engines must receive maximum power from all cylinders every cycle. The "all cylinders every cycle" is the hard part. Some of the biggest improvements in smog, power, and economy have come from our improved ability to make all combustion chambers burn at the same rate cylinder to cylinder and cycle to cycle.


Is this accepted theory? Is reducing squish volume really an efficiency booster?
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clay
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PostPosted: Thu Feb 03, 2011 11:06 am    Post subject: Reply with quote

I think the more equal combustion theory comes from the switch of carburetors to port fuel injection. Even with throttle body injection you are at the mercy of the intake design, pulses, etc. Port injection helps get rid of some of the variables. Every cylinder doing the same thing every time absolutely helps from a power / efficiency point. Are you talking about part throttle combustion effiicency or full throttle efficiency? Part throttle efficiency is a struggle to me because of the throttling effect. Cylinder filling isn't good because of the mostly closed throttle plate. Therefore there isn't a very dense charge in the cylinder and to me a small quench clearance can pay off more here than anywhere to me. As the charge gets more and more dense I think it becomes a little less critical. From what I have read, supercharged engines are more tolerant of a larger quench clearance without causing problems and I'm thinking this is because the charge is very dense compared to naturally aspirated. However another thing to think about is does the high manifold vacuum help vaporize the fuel better or is there not enough time for anything beneficial to happen? I think OE setups try to target the back of the intake valve to help vaporize the incoming fuel - it also helps to cool the intake valve but I don't really think that is necessary. Keep in mind I'm not a professional engine builder (hobbiest) and these are just theories I have read over the years that make sense to me. Clay
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journeyman
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PostPosted: Thu Feb 03, 2011 12:23 pm    Post subject: Reply with quote

Yes, I am especially interested in part throttle (n/a engine). Port inj does make a lot of sense in terms of equalizing.

Not sure if it is even possible to measure/evaluate how equal cylinders are to each other, but if one is begging for 32 deg. advance and another is only wanting 21, the compromise would be 25 or so one cyl is running too retarded (incomplete combustion), while the next is too advanced (predetonating). Neither one of them is running with an efficient timing.

Are 'hot spots' really that important? Is that a big issue for N/A engines in a partial throttle condition?

Is there any advantage to a larger 'squish' clearance? Seems like reducing it would always be advantageous, so why wouldn't all builders reduce it to minimums?
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PostPosted: Thu Feb 03, 2011 5:46 pm    Post subject: Reply with quote

direct inject it if you want efficiency

if you want to debate port injection vs carb you are splitting hairs
you can achieve relatively the same numbers if you tirelesly chase this goal...........carb will be cheaper every time

i state this before someone gets on my case of cost in d.i.

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PostPosted: Thu Feb 03, 2011 5:47 pm    Post subject: Reply with quote

ohhhh and what engine are we talking about anyway?????

what if you have an engine without squish?????

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af2
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PostPosted: Thu Feb 03, 2011 6:00 pm    Post subject: Reply with quote

journeyman wrote:

Are 'hot spots' really that important? Is that a big issue for N/A engines in a partial throttle condition?

Is there any advantage to a larger 'squish' clearance? Seems like reducing it would always be advantageous, so why wouldn't all builders reduce it to minimums?


I learned the hard way on N/A "squish" clearance in the early 80's.

I built a .030 over 427 with closed chamber Rectangle heads at just over 9.5:1 with a pretty big Isky solid FT. I did not have the block decked because it was within .004 of being square.

With the head gasket I was at .063 The thing would rattle until the total timing was 26*

I bottomed out 1 day and hit the pan not knowing
The next 7800 rpm burnout yielded a spun rod bearing.

Put it back together and did not have the $ for head gaskets.
Pops had a set of .015 steel shim and said put them on it will probably run better. I was thinking I would have to retard it even more with the higher CR.
The first outing was at 22 total and a total dog. After setting by ear and checking it was at 32. The thing ran night and day compared to the .063" and was hard to make it rattle.

The combustion chambers made nowadays are light years ahead compared to the bathtub designs that were used till the mid 90's

The squish is the mechanical part of the equation. The Quench is what kills the pre-ignition. The first part of the down stroke sends unburnt fuel into the mix to slow combustion along with the last few degrees on the upstroke. I should have swapped the up and down but all the same.
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PostPosted: Fri Feb 04, 2011 5:38 am    Post subject: Reply with quote

A lot of reading has left me with many more questions than answers. The Old One is some of the most intriguing info, but most of their work is focused on Honda engines. It is applicable to BBC (my app.), but I need to understand the principles to apply them correctly to big blocks.

It seems that the piston/head chamber geometry must be accurately matched to the cam profile to obtain results better than traditional engine building results.

I would like to see significant increases in efficiencies (less waste heat produced), and it seems like Larry Widmer is one of the only ones that has ventured beyond traditional approaches. I sincerely hope that I am wrong in this!
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PostPosted: Fri Feb 04, 2011 5:41 am    Post subject: Reply with quote

af2 wrote:
journeyman wrote:

Are 'hot spots' really that important? Is that a big issue for N/A engines in a partial throttle condition?

Is there any advantage to a larger 'squish' clearance? Seems like reducing it would always be advantageous, so why wouldn't all builders reduce it to minimums?


I learned the hard way on N/A "squish" clearance in the early 80's.

I built a .030 over 427 with closed chamber Rectangle heads at just over 9.5:1 with a pretty big Isky solid FT. I did not have the block decked because it was within .004 of being square.

With the head gasket I was at .063 The thing would rattle until the total timing was 26*

I bottomed out 1 day and hit the pan not knowing
The next 7800 rpm burnout yielded a spun rod bearing.

Put it back together and did not have the $ for head gaskets.
Pops had a set of .015 steel shim and said put them on it will probably run better. I was thinking I would have to retard it even more with the higher CR.
The first outing was at 22 total and a total dog. After setting by ear and checking it was at 32. The thing ran night and day compared to the .063" and was hard to make it rattle.

The combustion chambers made nowadays are light years ahead compared to the bathtub designs that were used till the mid 90's

The squish is the mechanical part of the equation. The Quench is what kills the pre-ignition. The first part of the down stroke sends unburnt fuel into the mix to slow combustion along with the last few degrees on the upstroke. I should have swapped the up and down but all the same.


Interesting... I was actually considering replacing my stock (probably .050 - .065) headgaskets on stock 454 with thin variety... Seemed like there was no downside. Now I wish I would have.
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clay
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PostPosted: Fri Feb 04, 2011 10:41 am    Post subject: Reply with quote

Quote:
Not sure if it is even possible to measure/evaluate how equal cylinders are to each other

I'm thinking it can be done - just not by the average person however. I'm thinking I have read NASCAR has been doing studies with pressure sensors in the combustion chamber to try and do exactly what you are talking about - maximizing every cylinder.

Quote:
t seems that the piston/head chamber geometry must be accurately matched to the cam profile to obtain results better than traditional engine building results.

I think camshaft needs to be matched to the induction and exhaust systems more so than chamber design. This is how I look at it - once the burn really starts to happen and chamber design comes into play, both valves are shut so what difference does the cam profile, rocker ratio, etc. make? Now chamber shape does play a role in getting air in and exhaust out. I think there is a lot to be gained tuning the cam events to take maximum advantage of every pulse that is available to help get air in and exhaust out. One theory on this is to have two different profiles to suit the different runner lengths on a dual plane intake. Also you could take it further and have two different primary lengths on the headers. Other things come into play also - engine temperature is one example. Hotter here is better. Not so much for a outright power perspective, but from an efficiency viewpoint, I think the hotter the better. Here is my thought on this. Once you get the charge burning you want everything to be turned into expansion of the hot gasses. Any heat that is lost is bad. So theoretically the last thing we want is heat being transferred to the head, piston, cylinder wall, etc. If water jacket temperature is higher, then less temperature difference exists so heat transfer should be a little slower - hence more fuel converted to mechanical energy. I'm sorta like you - the more I read, the more there is to think about and I feel like the more confused I get. Internal combustion engines are a fascination to me - they are a mechanical marvel. What is strange is the more you learn, you realize the less you know. Clay

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PostPosted: Fri Feb 04, 2011 5:38 pm    Post subject: Reply with quote

You can measure cylinder pressures using a PSIplug and an encoder on the crank. Then you can compare the differences between cylinders. This would allow you to see cylinder pressures at crank angles and be able to optimize timing. Last time I checked PSIplugs were going for about $1000 each but they may have come down since then.

Setting a tight quench will really mix up the air fuel mixture providing a consistent fuel density and therefore increase overall efficiency.

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journeyman
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PostPosted: Fri Feb 04, 2011 6:33 pm    Post subject: Reply with quote

clay wrote:
Internal combustion engines are a fascination to me - they are a mechanical marvel. What is strange is the more you learn, you realize the less you know. Clay


I sense a kindred spirit here.
Power is good... efficiency is great. Few focus on the long game of efficiency for the easier target of power.

I like The Old One's practice of sky high CR's and limited cam to produce dynamic CRs that will burn pump gas and still win races.
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PostPosted: Sat Feb 05, 2011 2:06 pm    Post subject: Reply with quote

Squish is a moot point with a big block. It has a 121 cc open semi-hemispherical combustion chamber designed that way to unshroud the valves for better breathing. The flat top piston is 0.20 inch down in the hole and is generally not flat. Hemi engines where designed to use spherical pistons to take advantage of the reward the Otto cycle offers increasing the compression ratio. A spherical piston has no one point on it's surface that would promote a hot spot to form (which is generally along all edges and at any sharp points). Unfortunately the EPA bans 15.0:1 compression engines negating the advantage of the big blocks combustion chamber design.

As an example of the difference consider the small block 400 compared to the big block 402. Both are nearly identical in displacement bore and stroke (either engine's piston can be installed in either block once you cut valve reliefs). The small block easily makes over one horsepower per cubic inch yet the big block struggles to meet that mark. The big block has the advantage in port design with 230cc intakes to the 170 cc intakes used in the small block. but the small block's combustion chamber allows the small block to more than catch up.

You can easily increase the small block's intake port size to match the big block, but the big block has nothing outside of a Big Chief or similar aftermarket head to compete with the small block (and the aftermarket head's larger intake ports hurt the lower and mid range performance).

Big Dave
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PostPosted: Sat Feb 05, 2011 3:04 pm    Post subject: Reply with quote

Big Dave wrote:
Squish is a moot point with a big block. It has a 121 cc open semi-hemispherical combustion chamber designed that way to unshroud the valves for better breathing. The flat top piston is 0.20 inch down in the hole and is generally not flat. Hemi engines where designed to use spherical pistons to take advantage of the reward the Otto cycle offers increasing the compression ratio. A spherical piston has no one point on it's surface that would promote a hot spot to form (which is generally along all edges and at any sharp points). Unfortunately the EPA bans 15.0:1 compression engines negating the advantage of the big blocks combustion chamber design.

As an example of the difference consider the small block 400 compared to the big block 402. Both are nearly identical in displacement bore and stroke (either engine's piston can be installed in either block once you cut valve reliefs). The small block easily makes over one horsepower per cubic inch yet the big block struggles to meet that mark. The big block has the advantage in port design with 230cc intakes to the 170 cc intakes used in the small block. but the small block's combustion chamber allows the small block to more than catch up.

You can easily increase the small block's intake port size to match the big block, but the big block has nothing outside of a Big Chief or similar aftermarket head to compete with the small block (and the aftermarket head's larger intake ports hurt the lower and mid range performance).

Big Dave


So EPA aside (let's just consider pre-smog year vehicles for the moment), raising the piston dome (growing up we used to call these 'pop up pistons' although they were not really 'dome' shaped) would not only raise the compression in a BBC, but also improve the overall combustion efficiency by reducing squish area/volume? And could this even be accomplished in the realm of the pump gas world?

On the other hand, are there any other chamber designs for the BBC head besides the hemispherical?
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clay
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PostPosted: Sun Feb 06, 2011 2:08 pm    Post subject: Reply with quote

Personally I have had a bad experience with a high dome engine in the past. My best guess is the high dome was interfering with flame travel by shrouding the plug and just basically being in the way. I look at a dome as a wedge that is trying to seperate the fuel / air charge into two areas. I don't know if you have ever heard about these heads but this article should be an interesting read. Clay
http://www.hotrod.com/techarticles/engine/hrdp_9902_mark_vi_chevrolet_502_engine_build/intake_and_exhaust_valves.html

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PostPosted: Sun Feb 06, 2011 9:42 pm    Post subject: Reply with quote

Hemi engines were the result of aircraft engine reseach conducted during WWII to increase the piston powered planes overall speed and power.

Yes if you filled the cylinder with a spherical dome you get lots of power and the efficiency of the engine is maxed out (so a 402 BBC with a hemi head would indeed blow away the SBC 400). Will it run on pump gas? Not at 15.0:1 compression ratios it won't.

And yes the Dart's Big Chief 14 degree head has a fairly small 74 to 85cc heart shaped combustion chamber chamber (74 as cast 85 cc when fully CNC machined) for use with a flat top or mild dome piston designed to accept the angle of the splayed valves. Brodix's PB1200 Man Eater 12 heads have 68-72cc chambers. You can pick up a set of either for around $4,800 a head complete or $2,500 for a bare casting if you have your own CNC five axis mill (like a lot of the pro's do) and make your own heads with your own flow characteristics.

The big block head was designed in 1962 with a slide rule and was reengineered again in 1967 based upon information that William T. Jenkins provided the design team with, but the engineers where still using log books and a slide rule. Modern aftermarket heads from most manufactures are relying upon CAD CAM CNC to bring a design from the computer to the shop flor and out the door.

Big Dave
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