Don Terrill’s Speed-Talk

Are there any fundamental differences in the camshaft requirements for a wedge as compared to a hemi?

There is the notion that if the LSA is tight, there will be over scavenging in the case of the hemi. I'm not sure that I buy into that notion. And, even if it is valid, I suspect that at least some of the over scavenging could be mitigated with the header size/configuration.

Any ideas?

Thanks, Danny

I'd say mitigating any potential overscavenge via header compromises would be the wrong way to go. Better to tune the header do the best job possible and adjust overlap as needed to optimize power. Possibly though the header could be tuned for a slightly lower RPM range than it might be for a wedge and thus what might otherwise be overscavenge higher up would become optimum, with a useful boost in the mid range.

Sinking one or both valves, if it could be done without significant flow reduction, might also impede flow through.

Get a look at the current exhaust port arrangement for a SS/AH Hemi and it'll tell you what's being done to improve the Hemi scavenging issue and at the same time bring up the average power & allow enough exhausting to let the engine run well on top.

over scavanging IMO only causes fuel consumption to go up and can wreak havoc with tuning by instruments, rather than your gut and plugs.

Watch the LSA on a hemi or you will have valves running into each other.

I don't know what valve angles you are working with Danny, but I do have a little experience on the 35*/23* 426 style head. Back in the early 90's my pop had a gas/carbs/2.250'x1.940" valved 426 style hemi that was raced for fun in the ANRA shoebox circuit. With the guidance of a very knowledgable cylinder head guy at my then employer, I made some changes to the cylinder heads/manifold that showed incredible gains (especially in the mid-range) on the bench, and should have picked up the power on a typical wedge well into the high double digit region. We did not dyno, but did tune for it, and the car went about a tenth quicker in the 1/4. We later shrunk the exhaust valve to 1.850", and went a little quicker. The next step was to try a smaller exhaust lobe on the then single pattern cam, but the project was retired shortly after. We never finished what we started, but I'm thinking that when it comes to an oppossed valve engine, what goes in, might not necessarily stay in.

Thanks all

Just finishing up a pair of heads (Hot Heads early hemi) with the exhaust valve size reduced from the standard 1.800" to 1.650", and a corresponding reduction in the exhaust port CSA.

Planned on having it for EMC, but time's getting short. Trying to (more or less) mimic the 426 SS hemi's.

Danny

dan miller wrote: Just finishing up a pair of heads (Hot Heads early hemi) with the exhaust valve size reduced from the standard 1.800" to 1.650", and a corresponding reduction in the exhaust port CSA.

Sounds like a good move for that application.

Old post I found. Would a Hemi require a later IVO compared to a wedge because of the superior efficiency and low lift flow thus resulting in a smaller duration cam?

bigpoppapreston wrote: ↑Sun Feb 11, 2018 5:26 pm Old post I found. Would a Hemi require a later IVO compared to a wedge because of the superior efficiency and low lift flow thus resulting in a smaller duration cam?

If you look at most 2 valve, hemispherical chamber engine camshafts - be it a Harley, Chrysler, '70s era Japanese motorcycle engines, Porsche 911s - you typically see either the same duration on intake and exhaust, but slightly lower lift on the exhaust side, or you'll see a combination of lower lift and slightly less duration on the exhaust side. Sometimes you seen them cam'ed like a wedge head. I haven't seen any trends with IVO, but I've just casually looked. Mike will have some good input. A lot of times, it depends as even though a hemispherical chamber has great pressure recovery, it is sensitive to overscavenging and varying hemispherical designs have varying port efficiencies. It really just depends ultimately what was done to the head, it's architecture, and flow patterns. An modern offset hemi racing head won't be cam'ed the same way as a traditional inline hemi head.

I did a large simulation test of four camshafts on a small, single cylinder with a hemispherical chamber today. Expect a post in the coming days as I have questions for the village. With each change of camshaft design, I kept the intake centerline fixed, and varied the exhaust centerline (I:105, E:100,105,110).

My Mopar Perf cataloge lists 6 cams for the Hemi. LSAs vary from 104 to 108. The two smallest cams, re-issues of the Strret Hemi hyd & sol cams are both on 106.

BB wedge engines have much greater LSA variation, 115 to 106.

hoffman900 wrote: ↑Sun Feb 11, 2018 5:45 pm

bigpoppapreston wrote: ↑Sun Feb 11, 2018 5:26 pm Old post I found. Would a Hemi require a later IVO compared to a wedge because of the superior efficiency and low lift flow thus resulting in a smaller duration cam?

If you look at most 2 valve, hemispherical chamber engine camshafts - be it a Harley, Chrysler, '70s era Japanese motorcycle engines, Porsche 911s - you typically see either the same duration on intake and exhaust, but slightly lower lift on the exhaust side, or you'll see a combination of lower lift and slightly less duration on the exhaust side. Sometimes you seen them cam'ed like a wedge head. I haven't seen any trends with IVO, but I've just casually looked. Mike will have some good input. A lot of times, it depends as even though a hemispherical chamber has great pressure recovery, it is sensitive to overscavenging and varying hemispherical designs have varying port efficiencies. It really just depends ultimately what was done to the head, it's architecture, and flow patterns. A modern offset hemi racing head won't be cam'ed the same way as a traditional inline hemi head.

I did a large simulation test of four camshafts on a small, single cylinder with a hemispherical chamber today. Expect a post in the coming days as I have questions for the village. With each change of camshaft design, I kept the intake centerline fixed, and varied the exhaust centerline (I:105, E:100,105,110).

Great thread!!

Hoffman,

In other words, the still-born, "ball-stud", hemi-head won't perform quite as well as a traditional valve layout hemi-head?? Now discounting it's rather poor exhaust runner layout here due to head-bolt placement.

Right.

Please tell more.

Thanks,

pdq67

pdq67 wrote: ↑Mon Feb 12, 2018 5:36 am

hoffman900 wrote: ↑Sun Feb 11, 2018 5:45 pm

bigpoppapreston wrote: ↑Sun Feb 11, 2018 5:26 pm Old post I found. Would a Hemi require a later IVO compared to a wedge because of the superior efficiency and low lift flow thus resulting in a smaller duration cam?

If you look at most 2 valve, hemispherical chamber engine camshafts - be it a Harley, Chrysler, '70s era Japanese motorcycle engines, Porsche 911s - you typically see either the same duration on intake and exhaust, but slightly lower lift on the exhaust side, or you'll see a combination of lower lift and slightly less duration on the exhaust side. Sometimes you seen them cam'ed like a wedge head. I haven't seen any trends with IVO, but I've just casually looked. Mike will have some good input. A lot of times, it depends as even though a hemispherical chamber has great pressure recovery, it is sensitive to overscavenging and varying hemispherical designs have varying port efficiencies. It really just depends ultimately what was done to the head, it's architecture, and flow patterns. A modern offset hemi racing head won't be cam'ed the same way as a traditional inline hemi head.

I did a large simulation test of four camshafts on a small, single cylinder with a hemispherical chamber today. Expect a post in the coming days as I have questions for the village. With each change of camshaft design, I kept the intake centerline fixed, and varied the exhaust centerline (I:105, E:100,105,110).

Great thread!!

Hoffman,

In other words, the still-born, "ball-stud", hemi-head won't perform quite as well as a traditional valve layout hemi-head?? Now discounting it's rather poor exhaust runner layout here due to head-bolt placement.

Right.

Please tell more.

Thanks,

pdq67

In most cases they are closer to a "POLY" than a "HEMI"

Stan