Let's talk high compression and pump gas

Wow guys, this is better than I expected. Great to see a real informative thread rather than "should I lower my '79 LTD?"

I remember reading about the Indian guy who made the grooved pistons and cylinder heads. His name is Somender Singh. Real brilliant guy and I remember me and some SAE teammates discussing his work and if we could apply some of his theories to our program. We never got around to it, but his website is well worth checking out.

www.somender-singh.com

 

I'd still shoot for 10:1, I like the idea of running colder plugs, heck I ran really cold plugs in my nitrous setup, and it ran fine on motor never fouled them out. I'd go 2 heat ranges colder than stock, run conservative timing, and go from there. I think running a good fan setup makes sense too.

 

Dyce colder plugs don't really work too well with pump gas anymore (supercharged mills being an exception). Newer fuel blends and heavily oxegenated fuels require hotter plugs to get a complete burn of the mixture.

The other thing is, many late model engines run higher CR's and higher temps for emissions and economy regs compliance,yet they do it on pump gas and with (as has been mentioned), efficent cooling and chamber design, ignition control and computer technology.

At least that what Ive learned.

Rat

 

Scotch and others . . . so what do you think about Hemi heads - Arduns, 331-392, 426, etc Obviously they are not 'quench' style heads . . . but they flow well. My guess is that they are much better suited to high flow and high RPM, than efficient low speed performance - my guess is that the design does not have much turbulance (???). Any and all throughts/comments would be appreciated

What about spark advance and the Hemi chamber? My guess is that they could use/require a lot more advance than other engines . . . due to lack of turbulance, no quench area, etc.. I know that the early 392 top-fuel pioneers would have liked to run more advance than they did - but the stock 92' cylinder blocks could not handle the increased pressure. When the 426 came out and the 92' boys got used to them, they figured out that they could throw lots more advance at them and make much more power than they could with a 92'. (Garlits has a great story on this!).

Flatheads don't require much advance due to the huge amount of turbulance the chamber design creates. Many racers will run really tight dome to head clearances - to increase the turbulance even more. On my blown flathead engines, I'd only run about 18 degrees of advance . . . ran well.

Even though I'm building some early Hemi stuff, I can't say that I know enough to compare cylinder head designs between the different Hemi engines - early, late, etc.. -- versus SBC early/late designs, etc.. Would love to hear from those who have actual race experience and know the facts and details. (There is always so much to learn).

Great thread!

 

My initial thinking is that the large open chamber heads of the hemi and arduns do not quench well. Like Scotch was saying, and similarly in the principle of Somender Singh's work, maybe a custom piston with a dome that mimics the shape of the chamber, recessed to not decrease the volume of the chamber and up compression, could create the same effect. This way, instead of getting the quench from the head design, you're getting it from the other end from the piston. The problem with this is obviously you'd need some seriously expensive custom pistons.

I've always heard that having sharp edges in combustion chamber, like the edge of a valve relief in a piston, is where heat will build up. Thus creating a hot spot and promoting detonation. Maybe rounding and polishing edges like this, along with ceramic coating like Scotch mentioned, is the ultimate solution?

Some serious shit

 

"So, when the piston travels up during compression stroke, some of the pressure/compression is bled off through the open valves."

I think you need a refresher on how an engine works...

 

No, that's actually exactly how it works.

edit: I just found this website. The top graph is actually very useful in illustrating the idea of overlap

http://www.rbracing-rsr.com/camshaft.html

 

This has to be one of the best technical posts in a while. I would like to add a little something on the hemi question.

I remember reading a magazine article many moons ago interviewing Dick Landy discussing max effort normally aspirated Chrysler hemis. He had designed a custom piston that effectively formed quench regions with the combustion chamber any where valve clearance was not an issue. The piston top looked like a ball cut in half with holes allowing for valve clearance. He noted that it help to control the combustion cycle as evidenced by really small timing lead numbers. On a side note though, it made an already heavy piston much heavier.

Of course if the "hemi" is supercharged, you don't have to worry about introducing more turbulence with quench.

 

THe static or mathematical compression ratio means almost didley squat except to give reference. Why do we raise compression? To recover the compression lost when we install a longer duration cam. No compression takes place till we close the intake valve which is well into the beginning of the compression stroke with a perfomance cam. Tis a big subject. i wrote a lot about it a year i or so ago and it used to be archived. You cant get foolish for sure but you can squeeze the rules a bit. For instance I run a Max Wedge street only. it is 11 to one but has a hydraulic cam of 310 duration and 525 lift. I can run on premuim pump gas and have for 33000 miles . It doesnt ping or give trouble but i dont fool around with fuel either i buy the best I can (92 to 94 with 94 preferred. ) I am at the edge of practibilty to be sure but it has 355 gears and sees a fair amount of driving so i know it works ok.
The quench thing? I personally dont get to excited although the theory is sound and was a great help to the flat head. Hard to get quench in a hemi though. Swirl will help. Probably keeping around 10 to 10.5 with a perfomance cam is a good practice. Without a perfomance cam we are back to 9 to 1 or that general area. There is the mathematical compressiom ratio which is used for caculations and there is the ACTUAL compression ratio due to cam/piston design. The actual is what decides success or failure. The mathematical since it is not based on reality is only a general guide. It is though often better to error on the side of caution as long as you dont go beserk about it. Then it will always work . If you push the envelope you too far you can occasionally get in trouble.
The relationship between cam choice and actual compression ratio and piston choice is one of the big speed secrets that gets little ink.

 

Keep the information coming! One of the engines I'm building is a blown 392 Hemi - with Donovan 417 top-fuel heads. It does not have any quench that I can think of, but the blower helps out in that it first mixes the fuel/air charge, and it stuffs it in the dang cylinder. I find this thread interesting, given the many different types of engines that I've built. I only wish that I had more time, more engines and unlimited $$$ to chase the questions that come to mind.

The question about Hemi heads and quench was one that just sticks in my head - in that the hemi design is very different from any modern high-turbulance, quench design . . . yet we hear about how wonderful the Hemi design is. When I think of top-fuel motors, I really can't think of any combustion chamber design (other than the Hemi) that has ruled the roost in top-fuel in over 40 years. I believe (correct me if I'm wrong) the same is true for blown alcohol. I realize that one should not compare a blown top-fuel/alcohol motor to a street engine . . . but I'm still intrigued on the comparison between a Hemi head and all the others.

Keep the posts coming . . . this is very interesting to me. Who else has ideas and experiences to share?

Dale

 

Much of the success of using the hemi design in serious blown nitro and alcohol alcohol applications stems from the centrally located spark plug allowing the flame front to propagate evenly through the chamber. The other being that the laid back valves flow like crazy.

 

Yes, and in performance applications more-so than stock. When both valves are open at the same time (overlap) at higher RPM's the exhaust creates a scavenging or suction that helps pull MORE fuel/air through the open intake valve than just the piston's downward trip (the INTAKE stroke that occurs right before the compression stoke)is capable of doing. The results are better power upstairs, and that "dirty", rumpity rump idle that we love so much. It's the less than perfect combustion at idle that we sacrifice for higher RPM power. That's why we run loose torque converters and steep rear gears to get us to that higher RPM "sweet" spot quicker in those applications. Do I still need a refresher, or are you learning something now?
FWIW-when I was a kid, the cam specs for advertisements were, Lift-Duration-OVERLAP

 

Here is a photo of a VW piston with approx 2000 miles on it. One of our local city government officials had someone in Lubbock, Tx. build the motor. Near as I can tell, this piston was in a motor with a static compression ratio of 8 to 1. The engine was partially apart when I agreed to repair it, so I couldn't tell what the timing was set at. It had a stock cam. This thing "pinged" a hole you could throw a yeller dog through. It will have a 6.6 to 1 comp ratio when it goes back together, because it is the only way these little motors will live, street motor anyway.

I guess the point is, what works with one combination won't work with another. Gasoline isn't what it used to be, for sure. I've built motors ranging from little VW's to alcohol injected big blocks and they are all different. Alcohol will take an incredible amount of compression. On the street, small amounts of additional compression only slightly increases horsepower.

 

Here is a picture of an Yamaha R6 cylinder head that we used in our University of Delaware Formula SAE car. We CNC milled the head for copper O-rings, and by deleting the head gasket, compression went from a factory 13.5:1 to an absolutely absurd 14.5:1 static ratio. This motor ran on 100 octane because we were allowed to run it, but the truth is it survived and continues to run on 93.

Look at the design of the combustion chambers. A huge portion of the cylinder area is quench. Though I don't have a picture, the piston shape mimics the combustion chamber. I'd also imagine that piston speed helps allow more compression here. This motor idles at 3000 rpm, and when it's being run is no lower than 6000 rpm. Peak power was 64 wheel hp at 11,600 rpm, redlining at 14,500. Not bad for 600 cc, with stock cams, through a 20 mm restrictor (about the diameter of a highlighter). If I recall correctly, this took around 45 degrees of advance like a champ

 

I have a 355 Chevy in my other car, Keith Black pistons, supposed to be 10.3 static, Comp 280 Magnum flat tappet hydraulic with Edelbrock Performer RPM heads. No issues with 91 octane.

 

I mentioned the Edelbrock Vara-Jection unit I ran on a Mustang commuter.
This was a '79 302 with the 4th OD standard shift on a 75 mi. round trip per day commute.
When I got it, leaded regular was available still, here in GA and I don't get smog inspection, so you can guess what my next moves were. Like remove cat, egr valve, opened up gas neck, remove restrictor(s) in vacuum line to vacuum advance, etc. It all worked great!
Then along came removal of the leaded regular sales anywhere! Had pinging problems on unleaded 87, and it was a commuter car, and cheap gas is king!
So I bought and installed an Edelbrock Vara-Jection and it worked like a charm. Detonation gone, and when I found a deal on methanol and ran it 50/50 with water, it actually seemed peppier!
At 150K, it shucked the teeth on the plastic cam sprocket and bent 12 valves and 15 pushrods. Teardown for repair revealed comb. chambers with almost no carbon!
Edelbrock no longer shows it in their catalog, printed or online. I'd buy one for my '31 hiboy that I'm building if they still sold it.
Dave

 

Panic,

Rather than rag on the guy, why don't you take the time to explain to us why the cam /valve overlap theory does'nt work?

I'm sure a lot of the guys would like to know as well....

Rat

 

Panic,

Rather than rag on the guy, why don't you please take the time to explain to us why the cam /valve overlap theory does'nt work?

I'm sure a lot of the guys would like to know as well....

Rat

 

The reason Panic said that is that BOTH valves are never open during the compression stroke - overlap doesn't come into play during the second of the 4 strokes of the cycle. With that said, the intake valve DOES close after bottom dead center - because the velocity of the intake charge continues, even as the piston is starting to come back up. Holding the intake open increases the amount of fuel filling the cylinder.

Overlap is the number of crankshaft degrees that both the intake and exhaust valves are both open. This happens at the END of the 4th (exhaust) stroke --> where the intake is opened BEFORE TDC and the exhaust is held open AFTER TDC. There are a couple things going on: 1) the exhaust system helps start the intake charge by "pulling" it in. Also, given the extreme high speed of the whole process, opening the intake a bit early helps get things moving and increases the amount of fuel/air charge in the system. At idle and low speed, high overlap isn't beneficial . . . but helps the car sound cool (rumpity-rump ) . . . at high speed and WOT, it reallly helps peak HP. When the cam grinders started experimenting with overlap (I believe it was in the late 20's, early 30's . . . people like Ed Winfield, etc) - they found they increased horsepower . . . and the trend has continued to this day.

Hope this helps . . . Dale.

 

Thanx B&S.... I kinda already knew what you posted, but Panic always seems to have a similar kinda approach to other members...or is it me being paranoid.

We're all here to learn from one another no?

Rat

 

BTTT. This is a good thread

 

You're right on a few counts. Hemi heads work well due to several important reasons. Not least among them is the flow capability. The intake and exahust ports are spread far away from each other, and identical in design. It's much more like four single-cylinder engines per-side than, say, a small-block Chevy (with its siamesed exhaust ports) or a big-block Chevy (with it's differing-length intake ports on adjoining cylinders). The angled valves help a lot too - they improve flow vs. more typical applications.

Quench-wise, it's a different approach. While flat surface quench doesn't exist in a vintage Hemi, what does exist is a nice shape for combustion. The central spark plug and piston dish force the air/fuel into a nice tight shape, and once lit, the flame doesn't have to travel very far to acheive a complete burn. However...

The reason the Hemi made the impact it did on the track was mostly due to it's volumetric advantages. This is especially true on nitro, where massive quantities of the fuel need to moved into and out of the engine. No other engine design could move (or be modified to move) as much through the engine as the Hemi, and live. The hemi's beefy shaft-mounted rocker arms withstood higher rpm use than comparable stud-mounted rockers too, and the combination of flow quantity and rpm made for a legend.

But, on the street, hemis weren't as dominant as they were on the track...especially in stock or near-stock form. Street racers will tell ya that a 440 was a hotter ticket on the street. Street cars had to perform at low rpm, and couldn't use the huge hemi ports unless there was a blower involved.

On the right fuel, and at the right rpm, the design advantages of the Hemi come into play. But for most street guys running pump gas, there are more efficient ways to make power.

The new LS-series of V8s from GM are about as good as it gets right now. They make huge power on pump gas and more performance is there with a few bolt-ons. Look to the LS for inspiration, and know that vintage engines are capable of more efficiency with some new-school mods.

Oil control, heat control, and consistency from cylinder to cylinder are big parts of that, and these are things any hot rodder could do to a vintage engine and see some real performance benefits on pump gas.

~Scotch~

 

BTTT for Tech Week

 

I agree with scotch, I recall that the 440,s were more popular [and cheaper] than the Hemi,s on the street. came to the conclusion that the wedge was better on gas, but the Hemi, with big valves and ports, blower and FUEL you hav a top fuel dragster!

 

Yes, that's right.

Everyone is entitled to their opinion?
Certainly, if it's an opinion.
Whether both valves are open during the compression stroke isn't an opinion. If yours are, take your cam out and beat the manufacturer to death with it.
Repeat after me, 100 times: overlap has no effect on compression.

 

I think what you need to explain is the difference between compression, which is a static ratio of swept volume + chamber volume / chamber volume and cylinder pressure which is the actual p.s.i. the cylinder sees at a given rpm.

 

A buddy of mine runs a 540 in his race car.... 14:1 compression and it's on pump gas (93). I'm not a techie on this stuff, but he did say that cam profile (overlap, duration and lift) all make it work. 9.30's all day out of the box 3100 lbs.

 

Exactly! One is relative to the other. (compression/cylinder pressure)

 

Cam overlap may not affect static compression, but it certainly does on dynamic compression.

Please prove otherwise. This is an intellectual discussion, please provide evidence

 

Valve Overlap has nothing to do with the calculation of DCR (dynamic compression ratio) - what does is the intake valve closing event and/or degrees ABDC (after bottom dead center) of the intake stroke. The reason that some associate valve overlap with a lower DCR ratio is that longer duration cams tend to have more overlap . . . and also tend to close the intake valve LATER (more degrees ABDC) - which reduces the DCR. Again - overlap has nothing to do with it -- when the intake closes does.

The calculations for DCR end up with a fixed number (for a given engine package) - which is ALWAYS less than SCR (static compression ratio). Once you know your DCR, it does not change - as it is based on how far the piston is coming BACK UP the compression stroke when the intake valve is actually closed. Another way to think about it is "less cylinder volume - being squeezed into the same combustion chamber".

Cylinder Pressure is another thing altogether - as it is influenced by many things -- manifolds, headers, cam, etc.. Cylinder pressure does change given changes in RPM, etc..

Hope this helps . . . I may create a small DCR calculator applicaiton and post it (when I have some spare time!).