Hemispherical cut heads

[tundrawolf]

Hello,

I am embarking on making a fuel efficient motor. I am running a small bore stroker for a square motor. I talked to a VW shop owner who builds VWs and a VW machinist and they both agree hemi cut heads adds to efficiency.

On another forum though, several people said it decreases efficiency. But what these people in the shops told me made sense, and I guess they have first hand experience.

I am running hydraulic lifters and a Web cams 91 grind high lift short duration camshaft, dual Weber ICT 34's, and SP heads.

Does anyone have real world experience of hemi cut heads?

[Dale M.]

IT could be the decrease in efficiency probably relates to if you hemispherical cut heads you dramatically increase combustion chamber volume severely reducing compression ratio... Solution to "problem" may be to severely fly cut heads to reduce combustion chamber volume.... Also so take in consideration valve position in heads, they have a lot to do with "flow" of gasses also, the fact that in VW head the valves are not in hemi configuration may have averse effect on full hemi efficiency of VW head ....

Yes most engine with hemispherical heads are very efficient and flame front of combustion spreads evenly across combustion chamber and piston top in stead of a wave offset from one side....

The old and new Chrysler "Got Hemi" is real....

Dale

[Ol'fogasaurus]

Like everything else there is a "cause and effect" and a trade off for good and bad. If you do too heavy of a mill job (and related machine work) you may have to do the push rod length change. You also could end up with sky high compression if you do too much also (been there done that).

Like Dale said, the location of the valves and ports is pretty important in a Hemi head which is one of the reasons the Mopar hemi heads are so big (the Ford Semi-hemi "Shotgun" head was also big). Pistons, and other related parts must match up too.

Be careful and good luck.

Lee

[Jadewombat]

I think a better example to understand valve and combustion area on a flat engine is Porsche, the 911 had a hemi-head design.



Lowering compression does decrease efficiency. The old skool 'keep that air-cooled engine compression low' is a bit outdated with knock sensing ignitions affordable and available nowadays (MS or Safeguard, etc.). But knock doesn't begin until 9-9.3 to 1 compression anyway. I had a Golf with 9 to 1 from the factory and it still ran like a top at 350K miles. I have 8.5 to 1 on my bus engine with no issues.

[Marc]

...knock doesn't begin until 9-9.3 to 1 compression anyway...

Depends upon many more variables than merely the static C.R.
A factory-stock 1585 will knock under load if given poor gas (especially on a hot day)...and they're only 7.64:1.
Cam specifications and timing are a major factor (obviously so is fuel octane) but there are other considerations such as rodlength-to-stroke ratio.
Mixture homogeneity and peak cylinder temperatures are much better managed in a fuel-injected waterpumper than they are in a carbureted aircooled, too - what's fine in one can be disastrous in the other.

The primary reason why hemi-cut heads aren't commonly used on VW engines is because they eliminate all "quench" effect - it's impossible to gain desirable mixture turbulence as the piston nears TDC when there's no head material there to form the squish area. VW never designed these engines to be "quench" motors, but the principle does still work on them. In searching for maximum efficiency it's common to run the minimum piston deck considered safe to maximize the effect - opinions vary on what's "safe" but few run less than .040". The benefits fall off rapidly over ~.060", and are nonexistent at any piston deck with hemi chambers.
This doesn't mean that a non-quench engine is automatically going to be inefficient; if there's good mixture management and intake charge velocity the contribution of the squish effect is trivial. But it is starting off on the wrong foot to use hemi heads if you're looking for resistance to knocking.

...Does anyone have real world experience of hemi cut heads?

1775cc (90.5x69) with Gene Berg stock-valve semi-hemi heads and "too much" piston deck (static C.R. set at 7.6:1). Old Sig Erson cam, similar to an Engle W-110 with a bit more lift and 106° lobe centers. Mild "mini-D" intake porting, stock rods. Used as a daily driver in a `69 Bug with Weber 34ICTs, running the cheapest Regular gas available, it got better mileage than a stock 1600DP - about the same as I was getting with a gutless 1600SP in the same car, same driving cycle, but with roughly double the horsepower it gave more grins per mile. IMO hemi heads don't present any problems when the intake tract is restrictive enough to create good charge velocity. The chambers/quench area aren't much different than original 36HP heads - how much more "real-world experience" do we need than hundreds of thousands of 36'ers?




http://www.chevyhiperformance.com/techa ... index.html

[Ol'fogasaurus]

Which brings up the discussion on back pressure of which small engines need some of when in non racing (and even then…) situations.

[Marc]

Years ago my little circle-track race team fielded a couple of Mini-Supers and three Street Stocks, all ACVW powered. All used merged-collector "competition" header systems; the "big" engines got 1-5/8" systems and the milder, limited-1600s all used 1½". We only carried one spare header to the track, a 1-5/8". As luck would have it, one day one of the Street Stocks got stacked up in an accident which demolished its exhaust system so we had to fit the big one to it for the main event. Imagine my surprise when it turned lap times .2 seconds faster than it ever had with the 1½" system (that's majorly significant on a ¼mi oval track, if you didn't know). Admittedly this is a special case scenario, where engine RPMs were mostly between 4000 and 5500, but the difference was unexpected and astounding. I still wouldn't advise going overboard on exhaust size for a "street" engine where 2000-3500RPM torque improves overall driveability, but if you're going racin' don't be afraid to uncork it!

[supaninja]

You sound pretty serious about this efficient motor build, so why baby webers instead of EFI and computer controlled ignition?

I got 1971 in my notch, one of Jake R's cams (lil more agressive then a 110), .028" deck height, 8.5:1 CR and I'm getting 30-35 mpg religiously. No hyper miling, 70-75 on the freeway, and driving a dickhead every chance I can.

I really really like the controllable ignition, cruise about 45-46 deg, WOT 32-34. It was a hot hot hot summer here in houston and I never had any issues at all with running hot. Intake air temps saw over 140F a few times.

[tundrawolf]

You sound pretty serious about this efficient motor build, so why baby webers instead of EFI and computer controlled ignition?

I got 1971 in my notch, one of Jake R's cams (lil more agressive then a 110), .028" deck height, 8.5:1 CR and I'm getting 30-35 mpg religiously. No hyper miling, 70-75 on the freeway, and driving a dickhead every chance I can.

I really really like the controllable ignition, cruise about 45-46 deg, WOT 32-34. It was a hot hot hot summer here in houston and I never had any issues at all with running hot. Intake air temps saw over 140F a few times.

I am serious. I am trying to duplicate John Karceys engine as best I can. He got a real world 55MPG without babying it, as tested by Popular Mechanics. He used Weber ICT 34's.

I am also running a 76MM stroker crankshaft with 1300cc pistons and cyls. I am running 1300cc SP heads. I like the smaller intake/higher velocity.

One thing my machinist does when he hemi cuts heads, is he deepens the bore in the heads so that the valves sit closer to the piston to increase compression ratio. I am still learning a lot when it comes to this sort of thing.

Can someone explain "quench" to me?

[supaninja]

This article explains it better then I can and even mentions "hemi's" lol. http://www.rbracing-rsr.com/squishcalc1.html

ninja edit- he did that back in the early 80's, you could do a lot better with EFI/ign. Square motors are the hot ticket. my honda has a 86x86mm and short rods, redline is 8,000 and it gets over 30mpg pushing a 2800lb car with 197 bhp's. That motor loved being turbo charged.

[Ol'fogasaurus]

Be cautious on some of those old claims.

Lee

[Eaallred]

I am serious. I am trying to duplicate John Karceys engine as best I can. He got a real world 55MPG without babying it, as tested by Popular Mechanics.

IIRC that ghia was netting 55mpg while driving something like 50mph on the islands of Hawaii (correct me if i'm wrong, it's been a few years since I've read the article). If driving along at 50mph to get max MPG isn't babying it, I don't know what is.

The Berg semi-hemi cut is nonsense. It's a way to drop CR back down after flycutting too much. It kills the quench area of the head and makes a very inefficient burn. The proof of that is the documentation of needing to add another 4-6 degrees of ignition timing afterward to get it to run correctly. The more efficient an engine is, the less ignition timing it requires. The less efficient the engine is, the more ignition timing is required. To quote someone from Cal-look a number of years back regarding the "semi hemi" chambers, "the unique bucket shape works well for holding a lot of carbon buildup".

The "real" Hemi heads are done like they are so that you can fit larger intake and exhaust valves in the head for the cylinder bore. By tilting them up, you can fit bigger ones in there. Keep them flat (like in our heads) and you're limited to what size you can fit. Berg calling it a "semi-hemi" was just marketing to make it sound like some V8-world crossover innovation. It wasn't.

Keep a good pad area, and run a tight deck (0.40 to 0.60) and you'll be leaps and bounds ahead of what you hope to gain with a semi-hemi cut if shooting for efficiency.

My 2276 drag motor, not built for efficiency, netted me 40 miles per gallon at 70 miles per hour this summer. Wanted to double check the MPG figures my datalogging was showing me to see if it was accurate. Turns out, it was, lol. All speed and distance measurements done with GPS so yes, it is VERY accurate. That's real-world freeway speeds, and was running on regular 91 octane gas from the pump. EFI with ignition control is a beautiful thing, carbs and distributors can't touch it. Not bad for a 12 second 1700 lb street legal bug.

The datalogging shows I was hitting 52 MPG at 60 MPH and 70 MPG at 50 MPH (in the areas where it really steadied out), but I never wanted to double check that figure as I don't think I could stand driving that slow for 60-70 miles in that car. But the datalogging was dead-on for my 70mph test (60 miles worth of highway driving).

Imagine what I could do if I built an engine and car with the sole purpose of netting the highest MPG I could get? My 50mph figures would be insane, I have no doubt. Just got to get that money tree to start growing..... It wouldn't be with no semi-hemi heads, I'll tell you that.

[tundrawolf]

http://books.google.com/books?id=f9kDAA ... CCAQ6AEwAA

What are the specs of your 2276 motor?

[Eaallred]

What are the specs of your 2276 motor?

FK89 cam, Scat 1.5:1 rockers, 48x38 Comp Eliminator heads, 12.5:1 CR, 1 7/8" merged header with 3" muffler, custom home-built intake system. Wasn't built with economy in mind at all, but the MS3 v3.0 crank fire with coil on plug ignition controling fuel and igntion i'm able to tune it for it. 91 octane from the pump too.

What Jon did back in the 80's was pretty incredible, I don't want to take that away from him. But thirty years later we have people getting huge praise for hitting mid 30 mpg figures and I don't get it. It's not 'that' great, we're capable of so much more. Wonder if I need to develop a 1600 solely for mileage, and sell it as a turnkey complete engine. Maybe that's where my focus should be instead of getting my car ready for the track next season......

[Marc]

...Can someone explain "quench" to me?

I guess you missed the link I posted above - it's explained pretty well in layman's terms there.

http://www.chevyhiperformance.com/techa ... index.html

Let's set aside the marvels of modern engine management for a moment and get pragmatic. If you're committed to a 77x76 with singleport heads, ICTs are the best carb choice I can think of. Have you done a mockup assembly to see if the 1300 piston skirts will clear the 76mm crank? You'll be needing some thick cylinder base shims to keep the pistons from poking out at TDC, on the order of .130"+ for a target piston deck height of .045". That'll be a wide engine - not so much that it won't fit in the car or that the exhaust can't be made to fit, but it will probably need longer-than-stock pushrods.
Unless you've found some 1300 heads you'll want to step-cut 15/1600 heads to the smaller diameter of the 1300 cylinders. Around .060" deep should reduce the chamber volume to ~45cc, which with .045" deck will yield right around 8:1 compression. If you opened up the chambers by hemi-cutting they'd need to be flycut about twice as deep to end up with the same volume. Advantage there would be that the engine would be close enough to stock width that off-the-shelf pushrods should fit, but that'd be the only justification for doing the extra machining - and as has been covered, you'd lose all quench effect.

I would expect an engine like Karcey's to return at best 40MPG overall in real-world driving. That's not bad, over 40% better than a stock 1600, but a far cry from 55MPG. It'd be relatively cheap to build, though.

I drive 12,000mi per year commuting to work. At 27½MPG that takes 436 gallons at a cost of ~$1600. 40MPG would save me $500 a year; 55MPG would "only" save another $300. Over the ~100,000mi life of an engine there'd be a savings of over $6000 at today's gas prices (and things'll no doubt get worse) - for that kind of budget someone should be able to develop an ACVW engine (using state-of-the-art management) capable of 50+MPG if they set their mind to it...problem would be finding VW owners who'd both appreciate and be able to afford it.

One of the projects on my back burner is a 1416. But instead of SP heads, I'll be using 33x30 1600DP heads from an L-Jet engine. That'll make it relatively simple to convert to fuel injection if I'm not satisfied with the ICTs.