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Mon Sep 02, 2013 5:07 pm

Hi can i please get your guys thought about hemi heads. I have talked to a few people that like idea ( one reason that i was told was you can run higher compression like 10-1 and still run pump gas ) on the against side i was told they run hotter and over time can get blow by because its speading the combustion chamber over the whole piston. And 2 builds i talked to said make sure with hemi heads dont run over 9-1 cr so please help out thanks

Mon Sep 02, 2013 6:12 pm

A true hemi head would give you a very low compression ratio on an over square engine like the VW. The original Hemi (Mopar greatness) was quite undersquare and it's performance was very good for the time. These days technology has improved and if you were going to the trouble of relocating the valves and rockers a compact wedge would suit our engines better anyway.

If you mean Berg's "semi-hemi" it again tends to produce low CR and a reduction in quench/squish, but could be very good on a turbo setup. The hotter and blow by theories sound like internet chatter.

Mon Sep 02, 2013 7:30 pm

The problem with hemi cut heads in the VW world is the head was never really designed to be that by design with the valve, chamber design (not a wedge) and spark plug location. Sure you can cut a dome in the head, but you still have a flat top piston. Well then you could put domed piston in it, but you still have an offset spark plug and poor valve location to make it flow effectively. So really without a bunch of time and money invested in redesigning the wheel, you whined up with a less efficient combustion chamber than you started with. A properly set up hemi chamber can be efficient, but the expense to get there from where you would start from is not. Likewise a properly set up quench type chamber can be equally efficient. From my experience hemi cut VW heads require more timing lead and richer jetting to make them run correctly. What that means is the combustion chamber is less efficient than it was originally. Also they tend to be detonation prone, and generate less power basically due to the chambers now less efficient ability to cool. It's a cheap quick way to lower compression in a VW engine, but it's not very effective.
Again if you want to re-engineer the wheel hemi chamber heads can be the bee's knee's if done correctly, but IMO it's just not worth the investment for so little return. Honestly nothing more than a bunch of crap in a VW engine.

Mon Sep 02, 2013 7:42 pm

The HEMISPHERICAL head was a technological leap in its day. Its shape allowed for bigger valves and therefore better breathing and more power. But there are drawbacks to the hemispherical design.

Today's technology is leaning torward compact shaped combustion chambers. A compact shaped combustion chamber vs. a long spread out shaped combution chamber, like hemis, has the advantage of shorter flame distances from the sparkplug to the edges of the combution chamber. It also allows for quench areas on the edges of the combution chamber that can be set up for engine quench if you run a .060" deck highth or less. The shorter flame distances and engine quench speed up the flame burn time allowing for less ignition advance and more efficiency and therefore more power and fuel economy. That's why (water cooled) hemispherical heads can see over 45* mechanical ignition advance whereas small compact combustion chambers of the same CR can't withstand that much ignition advance. Tuners find themselves increasing and increasing ignition advance on Hemi head engines because the slow flame keeps showing more and more power with more advance until suddenly knock sets in and you blow the engine.

If you want a higher Compresion Ratio the anti-knock properties of compact combution chambers with quench areas allows for a higher CR than hemi heads. Of course this will quicken flame speed which in turn causes the engine to not tolerate as much ignition advance, which is why some who had pinging problems actually due to too much ignition advance "solved" their problem by using hemi heads. But it's best to use quench heads and retard the ignition advance.

You also have the problem of heat saturation. A hemispherical head has a larger surface area that will assorb more heat into the head. Small compact combustion chambers have less surface area and therefore less area for heat to assorb into the head at TDC.

Hemi heads will naturally give you (and tolerate) less compression. You can increase compression by using dome pistons, which would make a banana shaped combustion chamber. But the theoretical best would be a small compact combustion chamber with a dish piston so the flame has a very short distance to travel until it reaches the edges of the combustion chamber.

So my suggestion is that if you want more power, efficiency, compression ratio, etc, look into quench heads and pistons, not hemi heads.

Wed Sep 04, 2013 2:10 pm

Anyone else ? I'm interested on this also. I talked to Joel Moore a few times and he loves hemi heads and a few others say they are ok. But I had a few say they suck and wouldn't use them. I have no idea either way

Wed Sep 04, 2013 3:05 pm

Steve pretty much covered it. Either you thump the Berg Bible or you don't, which means you either believe in 70's tuning wisdom or modern technology. We know now that the T1 engine doesn't need a lower compression ratio to live on the street and make decent power, so to me the days of semi-hemi and 6:1 ratios were gone before I was born. I don't care what you read, if it works for a Chevy it will work for a VW. Big CC's, high compression, big cam. It'll make power. Keep the deck tight, and detonation won't be an issue on pump premium.

Wed Sep 04, 2013 3:29 pm

How do you guys feel about my situation if I were to have my heads hemi cut. Its turbo'd but due to the 35cc chambers I am forced to stack spacers to get even a remotely low CR. I havent fully tuned for my new stuff yet but Im having a little det and if tuning/timing cant solve it would I be worse off with a hemi cut to lower CR?

Wed Sep 04, 2013 4:05 pm

In your case, the "semi hemi" cut is not a bad way to consistently open the chambers up for lower compression. If you look at chambers that have been seriously opened up to lower compression you will note they often don't have much squish area left anyway. Smooth off the edges when it is done, too. Being cut to 35cc there won't be a lot of the original chamber left, but you should be able to achieve your goal if there is enough aluminium around the chambers.

Wed Sep 04, 2013 4:13 pm

Head designers are like fashion designers. They go back and forth with designs. Chevrolet made an engine called a W engine (turbojet). The pistons had a quench area instead of the heads. On a canted design ;like fords cleveland/modified , you get some extra space for a huge valve without the extra expense of the hemi. Ford also made a 'semi-hemi' dual cam head.

How are you going to 'cant' the valves in a vw head for a hemi head? The porsche 914 2.0 heads look like they rounded the chamber out to unshroud the valves and they tend to crack around the spark plug hole because there is not much material there because of this. I guess you actually need a more 'open' chamber than a closed chamber head.

Wed Sep 04, 2013 4:17 pm

I run Berg semi-hemi heads. They are P&P, and the engine has power before the turbo starts boosting.

Even with the semi-hemi heads, I needed the barrels shimmed. Got the size needed, again from Berg, and didn't need to stack shims.

Running only 7:1 CR, it would have needed several shims. I prefer one spacer.

.

Wed Sep 04, 2013 7:55 pm

well i might sale my steve tims stage 2 heads that have extra welding for strength and Joel moore said he sent them to Gene berg for hemi cut hit me up if your interested

Wed Sep 04, 2013 8:50 pm

2332turboSF wrote: How do you guys feel about my situation if I were to have my heads hemi cut. Its turbo'd but due to the 35cc chambers I am forced to stack spacers to get even a remotely low CR. I havent fully tuned for my new stuff yet but Im having a little det and if tuning/timing cant solve it would I be worse off with a hemi cut to lower CR?

running a lot of deck on a turbo engine just puts more heat in the cylinder increasing the chance of warpage and cracking. (theoretically)

That being said there have been plenty of 9 and 10 second turbo drag cars running over .250 deck with small cc heads that were formerly on high compression drag motors.

making the power is not going to be a problem.

Thu Sep 05, 2013 8:13 am

2332turboSF wrote: How do you guys feel about my situation if I were to have my heads hemi cut. Its turbo'd but due to the 35cc chambers I am forced to stack spacers to get even a remotely low CR. I havent fully tuned for my new stuff yet but Im having a little det and if tuning/timing cant solve it would I be worse off with a hemi cut to lower CR?

Had the same issue with heads.
Had a nice set of Super Flo heads with 47cc chambers I wanted to use on my sand drag buggy.
Had Ross make some dished pistons, 22cc dish to be exact.
Keeps the piston high in the cylinder but got my compression down to a manageable level.

Thu Sep 05, 2013 3:53 pm

there great on a hemi!!! a blower works nicely with them too when preped for it. but I would not do it to lower the cc,bad idea.