Octane & Detonation explained (Long)

MiG · Post by **MiG** » Mon Mar 05, 2007 9:47 am

Perhaps the fact that top fuel doesn't allow multivalve heads has something to do with it? IIRC, somebody in oz was getting good results with a three valve head but it got banned.
If you're crippled by having two valves then the hemi head allows the valves to be larger, but remove that restriction and suddenly nobody uses the hemispherical shape.

HemiDuty · Post by **HemiDuty** » Mon Mar 05, 2007 10:32 pm

Um, what?

Dude, if you have a personal dislike of Hemis that is cool, whatever. But the above is utter crap. Atleast you could have done some research before coming out with that drivel. The engine you are refering to is called the Sainty Engine. And guess what? The combustion chamber shape is basically the same as a Hemi!

I have a huge amount of respect for the Sainty engine, yet at the end of the day it never has run the numbers.

And yes, the Chrysler Hemi is a dated design. That's because it was designed in the 50s! Which just goes to show how good it was back then, that it can still make 7500+ BHP now. Oh and for your information, the Hemispherical combustion chamber shape has been used countless times in motorsport by many manufacturers, I will allow you to do your own research on that one.

BTW, ever actually seen a TF race? Only a very jaded individual can fail to be impressed by it.

Stretchy · Post by **Stretchy** » Tue Mar 06, 2007 1:03 am

I am with Hemi on this one, as well as what he said above there was a qaud cam top fuel engine that used to run but never worked well enough to be competitive.

In other areas of drag racing(IHRA), there pro stock class runs unlimited capacity and engine builders developed hemi heads for ford and chev engines just to be competitive.

Hemis were banned in other motorsports such as nascar as they were unbeatable, that is why chrysler pulled out of some motorsports for so many years.

BMW, Porsche, Peugeot, Jaguer and Alfa Romeo have all used hemi heads on cars, some even in GPs. The Chrysler corporation own the trademark for the term "Hemi" so you wont here it used from other manufacturers. They use some other bullsh@t marketing jargon for it.
Chrysler used them in such large numbers thats why people think only Chryslers have them.

A blown hemi does sound great but a high compression naturally aspirated hemi sounds absolutly amazing.

Stretchy · Post by **Stretchy** » Tue Mar 06, 2007 1:08 am

This one is just for Hemiduty.

Reckon you can find somewhere for it.

540ci Blown EFI Hemi - Daily drivable 1000+ HP on 93 octane pump gas.

HemiDuty · Post by **HemiDuty** » Tue Mar 06, 2007 9:29 pm

Cheers Stretchy, I got just the car for it too!

And yes aspirated Hemis are awsome, there used to be an A/MS Roadrunner up in NQ which had a 426 in it. Man that thing was insane. Class rules said single 4bbl only, so this thing had a 2600 CFM Barry Grant special on it. Ran a Jerico 4 spd, and man it launched and wheelied like you wouldn't believe. Man that car was cool.....

Stretchy · Post by **Stretchy** » Tue Mar 06, 2007 11:39 pm

Yeh i know the car, i got a few pictures of it somewhere i think.

Gosling1 · Post by **Gosling1** » Wed Mar 07, 2007 9:23 pm

There are 2 words for those people who think Hemi engines are shit......and those words are .....Keith Black. If you have to ask, you wouldn't understand anyway !

My experience of using a high-octane fuel in the Pearl was very positive. Using Mobil 100/130 octane fuel, straight out of the same tanker that filled light planes at the local airport, into a Z1000 motor with a static c/r of approx 11:1, the difference between avgas and pump fuel was like chalk and cheese.

It may have been a different matter using carbs, but the EFI system on the Pearl meant that jetting was simply not an issue. It started easier, idled smoother, had far better pick-up from down low (out of corners), and basically was much better on avgas than normal petrol. The c/r had been bumped up from the stock 8.5:1, but cams were stock, and ignition timing was also stock. It may have benefitted from a small increase in ignition advance.

MiG · Post by **MiG** » Thu Mar 08, 2007 1:19 pm

For fucks sake. All the hemi engines are either ancient or two valve only and competing in a class where more than two valves isn't allowed. Hence Keith Black aluminium big block engines aren't relevant to this argument.

I'm not saying top fuel isn't cool or that the hemi head (any of them) wasn't good in it's time.
But you guys have got that misty eyed wank going about how the hemi head is best ever. It's not.
Oh and for your information even the current production "hemis" aren't hemispherical.

Gosling1, you do know that avgas is leaded don't you?

Gosling1 · Post by **Gosling1** » Thu Mar 08, 2007 8:53 pm

absolutely - did I say anywhere that it was unleaded ??

Leaded fuel is heaps better in the old dungers, no doubt about it.

hoon · Post by **hoon** » Sat Mar 10, 2007 5:43 pm

I have the last of the 5 valve R1s,

They have 3 exhaust valves, so effectivly breathe better. more curtain area.

Good for turboing

I think they dumped the 3rd exhaust valve cos it didnt give any extra horse power. just extra drag in the vlave train hence reducing efficiency.

UnEven mixture in the combustion chamber is a common cause for pinging. When you get lean regions in the mixture inside the combustion chamber, these sections actually detonate at a lower pressure. Well designed cylinder heads swish the gas as it goes past the inlet valves, causing lots of eddy currents, evenly ixing the fuel.

I-K · Post by **I-K** » Sat Mar 10, 2007 10:57 pm

hoon wrote:I have the last of the 5 valve R1s,

They have 3 exhaust valves, so effectivly breathe better. more curtain area.

Um, no. Other way around. Three inlet, two exhaust. It's a lot less difficult to get the piston to push the waste gases out than to get it

Good for turboing

How do you figure that? Once forced induction comes into it, cylinder filling can take a back seat. If anything, valve curtains can get smaller to improve bottom end and midrange.

I think they dumped the 3rd exhaust valve cos it didnt give any extra horse power. just extra drag in the vlave train hence reducing efficiency.

This is a point of contention, actually. With a five-valve head, the valves can be smaller and lighter, thus requiring lower-rate valve springs. Whether the three weaker springs require less energy to compress than two stronger ones depends on the spring rates forbidden by the harmonic properties of the valve train.

The point of the increased valve area offered by the five-valve layout was to improve top end by increasing the valve curtain without going to cams with stupidly long duration, which makes the engine less efficient.

From the FZR1000 to the last of the long-stroke R1's, Yamaha never really had trouble building five-valve engines which did anything from hold their own against competing designs ('02-'03 R1) to completely pissing all over them (the FZR1000 kicking the oil-cooled GSX-R1100 despite its capacity handicap). There isn't much wrong with that layout.

It does, however, require four additional lobes to be machined on each inlet cam, four additional inlet valves and four additional inlet port pathways and inlet valve seats. Like anything else, the decision to ditch the 20-valve head for the R1 is likely to have just been made because of production costs.