hemi cut heads

[nicolas]

i have a set of hemi cut heads and wanted to know what CR i can use with these. i did some searches on the net and came up with real low CR's (7) and quite high CR's (10), but my question is, what is correct. does CR affect the 'working' of the hemi cut.

on the 2007 i plan to built i have 68cc chambers now. with a deck of 1mm i should have 7.3-ish CR. a bit low for my taste, but what is recommanded. BTW i think these were intended for a bus, hence the big chambers.

[Diederick/DVK]

hemi cut heads and a "bowl shaped" combustion chamber right?
i wouldn't know the answer to your question though...

[Lee.C]

i have a set of hemi cut heads and wanted to know what CR i can use with these. i did some searches on the net and came up with real low CR's (7) and quite high CR's (10), but my question is, what is correct. does CR affect the 'working' of the hemi cut.

on the 2007 i plan to built i have 68cc chambers now. with a deck of 1mm i should have 7.3-ish CR. a bit low for my taste, but what is recommanded. BTW i think these were intended for a bus, hence the big chambers.

Hmmm interesting - I'd like to know more too 

[BeetleBug]

I have seen extremely high CR on a semi hemi head and both engines worked VERY well.

BB

[Donny B.]

I am running 8.1 to on my 2165 and it works well.  The only thing is you need to use a lot of advance on the timing for them to work properly.  I run about 38-39 degrees total advance.

[bugnut68]

I am running 8.1 to on my 2165 and it works well.  The only thing is you need to use a lot of advance on the timing for them to work properly.  I run about 38-39 degrees total advance.

x2

I had a set of these heads done by Competition Engineering that I bought from Rick Mortensen on my last 1776... stock valves, port and polish, Engle 100, 1.25 VW rockers, dual Kads... great engine.  Ran awesome!  Like Don said, additional advance is needed, I think I ran between 36 and 38 with my setup.

[bugnut68]

Forgot to mention, I was running 7.8:1 compression.

[Joel Mohr]

 I've been experimenting with the "Hemi Cut" for years, and just yesterday I crossed a new threshhold.... I built a 2275, CB wedgeports with a hemi cut, I set it up at 11-1 and it purrrrrs like a kitten... and WANTED 35 degrees total....and it made 190 hp! K8 with 1.4s, 48 IDFs....91 octane UNION 76 PUMP GAS! I'm taking it to the dunes tomarrow,  I'll let you know how much fun it was! My street 2442 has been at 10.3 to 1 for a couple of years, and it's NEVER deseiled or pinged...valve adjustments have always been normal. And just so you know...there are a few modern motors running as high as 14 to 1 on 91 octane, and the ones I've seen the chambers from all use some kind of "dished" chamber. I think we've only just begun to understand what REALLY goes on inside of these things...

[1sickvw]

I ran a 2007cc with a set of berg 865 heads, 7.5 to 1, web cam 86b, ida's and ran 12.99@ 100mph at sac. With that combo we drove it all over the place.

[Zach Gomulka]

11:1 and 35 degrees... I bet that can be a bitch to start! Joel, those motor specs are pretty cookie cutter (except for the chambers)... The same basic engine will make as much, or more power with less compression and normal timing. Just look at the HotVW's 2276 for example.

I'm not sold on hemi cut heads in the least bit. The fact that they need all of that advance tells me that it's a shitty design.

[-Alex-]

I dont like either semihemi cut heads. I am not an expert, but why to try make it hemi type head, even the valves are at the wrong place. Flame needs to travel further.
Tight deck and good squish sounds a lot better. Bergs does/did have lot of good parts, but semihemi and 6.8:1 compression ratio sounds stupid with angry +300 degree cams and +13mm lift.

[Donny B.]

...may sound stupid, but it still worked.

[Torben Alstrup]

The whole idea behind those low comp engines were to detune them as much as possible, so they did´nt overheat. - only problem is, that most of the oil temps comes from rpm, not power, as long as youre in the cruising rpms. Cylinderhead temperatures comes from the power used and the (in)efficiency of the combustion (Dynamic compression) The other thing is, that when you use Hemi cut heads, you run the advance high and the CR low (As long as you use them in the traditional way) That means that you actually get the ignition to be in the area that VW intended it to be, (only with a modified 009 ) in the driving rpms. That again results in the fact that you can lean it out, keep the heads cooler and get good fuel economy. Then when you stomp it you simply have to richen it up a tad at WOT to let the fuel help cool the heads. - BINGO!  

WRT the Hemi heads. That is one of the places where you can NOT trust a flow bench. - Well, you can trust the numbers, but they do not compute with real life, because of the fact that the flame travel is significantly longer and thus you need more ignition advance as menthioned earlier. and more ignition advance actually robs some of the power at WOT.

But I´ll give it this much. - when youre living in Southern CA, Arizona, New Mexico, June 7. noon  it gets ungodly hot (from my point of view) in the summertime. So at the time it seemed like a good idea to promote this way of doing things. Claiming that it yielded more power compared to conventional Squish was........ well, ..... a sales trick

Today we know so much more. And have found ways to lurk more power out of even Hemi heads because of innovative people all over the Globe, thinking outside the box

[Bruce]

My back-to-back tests made me never want to use semi-hemi heads ever again.

First test was a 2 liter street engine at 8.0:1, Engle 125 cam.  This engine required 89-91 octane to prevent pinging.  If I gave it 87, I didn't need to look for hills to make it ping, it would ping on the flat.

Second test swapped the head for conventional VW squish chambers.  Instead of 8.0:1, I went up to 8.5:1.  This engine only gets 87 octane, and it never pings.

More power using cheaper gas.

[Joel Mohr]

I've got an auto-stick starter, and it cranks like a stocker.... remember that the more duration in the cam, the less cylinder pressure at low rpm....Yaris has a 3 cylinder that runs 87 octane and is 13 to 1, a modern 4 stroke dirt bike runs 91 octane and runs 14 to 1... the envelope exists, we just need to find VW stuff that will produce the same effects.... tell me, when were vw chambers designed??? maybe some of you should go to a V8 shop, and start looking at heads....and did you know a top fuel motor runs almost 50 degrees of advance on NITRO?

[Zach Gomulka]

I've got an auto-stick starter, and it cranks like a stocker.... remember that the more duration in the cam, the less cylinder pressure at low rpm....Yaris has a 3 cylinder that runs 87 octane and is 13 to 1, a modern 4 stroke dirt bike runs 91 octane and runs 14 to 1... the envelope exists, we just need to find VW stuff that will produce the same effects.... tell me, when were vw chambers designed??? maybe some of you should go to a V8 shop, and start looking at heads....and did you know a top fuel motor runs almost 50 degrees of advance on NITRO?

As I've read (old Don Garlits stuff), the old 392/426 Chrysler Hemi's ran 70-80 degrees on nitro. Correct me if I'm wrong, but nitro burns slowly, and thus works well with the hemi head.

[Lee.C]

11:1 and 35 degrees... I bet that can be a bitch to start! Joel, those motor specs are pretty cookie cutter (except for the chambers)... The same basic engine will make as much, or more power with less compression and normal timing. Just look at the HotVW's 2276 for example.

I'm not sold on hemi cut heads in the least bit. The fact that they need all of that advance tells me that it's a shitty design.

Very good point's Zach - And it seems like your backed by other on this one