Shaved Heads

[BA]

I just rebuilt a '95 F3 that we had lying around, and I put a shaved head that was given to us on top of it. This thing is shaved about .035" with a combustion chamber size of around 9.8cc. The exhast valves are at a safe .050" away from the pistons and the intake at .095" or so. I think the compression ratio is up to 14.3:1, but I may be calculating wrong.

I've never run with a shaved head and we're still breaking the engine in. I'm not sure what kind of fuel would be optimal for our higher compression engine, anybody else know?

Is there even any benefit to run with a shaved head on these engines? Are there any other tuning considerations that I need to take? I'd appreciate the discussion.

Brian Auer
University of Idaho
Vandals Racing (http://www.uidaho.edu/~racing)
FSAE 2003/2004

[BA]

I just rebuilt a '95 F3 that we had lying around, and I put a shaved head that was given to us on top of it. This thing is shaved about .035" with a combustion chamber size of around 9.8cc. The exhast valves are at a safe .050" away from the pistons and the intake at .095" or so. I think the compression ratio is up to 14.3:1, but I may be calculating wrong.

I've never run with a shaved head and we're still breaking the engine in. I'm not sure what kind of fuel would be optimal for our higher compression engine, anybody else know?

Is there even any benefit to run with a shaved head on these engines? Are there any other tuning considerations that I need to take? I'd appreciate the discussion.

Brian Auer
University of Idaho
Vandals Racing (http://www.uidaho.edu/~racing)
FSAE 2003/2004

[Chris Boyden]

I've considered using a shaved head. Here's what I've heard and have been thinking about.

On a restricted engine the dynamic compression ratio goes down. In order to compensate for this, you can raise the compression. And, one of the ways to do that is to shave the head.

Most of the 600cc class runs ~12:1 static compression ratio. I guess that when you restrict an engine and lower it's dynamic compression ratio, let's say to 9:1, you can compensate for it by raising the static compression ratio to 14:1 and thus raising the overall ratio. Sounds good to me...but I don't know, I've never done it.

In terms of fuel, I'd say go with the highest octane fuel used at competition which I believe is 100, because higher compression generally means that you're more prone to detonation which can lead to pre-ignition. Higher octane slower burning fuels tend to help with detonation.

In terms of other tuning considerations, I'd say that you should try advancing the timing 5 degrees from it's stock settings after the peak torque. This helps to extend the peak torque to a higher RPM, so I've heard.

Here's a snippet from MSD's website...
"Through extensive dyno testing, MSD engineers found that in certain engine applications, such as restrictor plate engines, by advancing the timing after peak torque has been made, even more torque can be achieved"

I've read similar info elsewhere.

The factory R6 has 55 degrees BTDC at 5250 rpm.
This works very well for the R6. I advanced things some more all the way up to 11000 rpm.

Anyway, it sure would be neat to see some real data on the subject, i.e. compression ratio vs. hp, torque, rpm. Then increasing the timing vs. hp/torque, etcetera, etc...

It would be pretty cool to see an 85-90 hp
restricted engine? Is it possible?

[BA]

We've been using 92 octane right out of the pump during our break in period with the shaved head and we haven't noticed any pre-ignition. Our engine manual says to use 89 octane on the stock setup, so I'm wondering if 100 octane is even needed. Are there any benefits to using a higher octane fuel other than being less prone to detonation?

Brian Auer
University of Idaho
Vandals Racing (http://www.uidaho.edu/~racing)
FSAE 2003/2004

[gug]

on the top hp, delft got 85bph (from this page of their brochure (http://www.dutracing.nl/html2/Folder/1024Folder_Page3.jpg)) in 2001, and 91bph (from this page of their brochure (http://www.dutracing.nl/html2/Folder/1024Folder_Page4.jpg)) in 2002. of course, this is from a brochure and is not at the wheels i would guess.

- if it isnt coming, you need a bigger tool.

[Marc Jaxa-Rozen]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Are there any benefits to using a higher octane fuel other than being less prone to detonation?<HR></BLOCKQUOTE>

If you're able to run aggressive timing without pinging, no. On the other hand, a fairly common misconception is that higher octane fuel automatically has a slower burn rate, which will in turn hurt power. In some cases this is true, but it varies so much depending on the fuel blend that it's not really a factor.

One of the reasons for this is that octane has a higher flash point than heptane (which is the other reference molecule for octane rating, 87 gas being equivalent in terms of detonation properties to a 87/13 isooctane/heptane mix). Thus the high-octane fuel would theoretically burn at a hotter temperature which, when applied to the Carnot efficiency equation, means better thermal efficiency.

But again, it depends on what's actually in the fuel, so I wouldn't worry too much about it.

Marc
ENAF1

[Frank]

fact 1

most engines run optimally with peak cylinder pressure at 12 deg after TDC

fact 2

the timing you use will (provided knock, a product of excessive compression ratio for a given fuel and VE, does not occur) cause the peak cylinder pressure to occur at 12 deg ATDC

fact 3

large timing advance is caused by low VE, high rpm and poor combustion chamber shape

comment

excessive timing (ie power dropping off.. something like 3-5 deg after max power) which is not causing knock, certainly indicates a possible gain available by increasing compression ratio (for that fuel and VE)

[Chris Boyden]

I've heard similar numbers...

The main goal with timing is to produce peak cylinder pressure 14 degress ATDC.

But, where in the hell do you put the pressure
transducer???? http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Frank]

you dont need a pressure transducer

you need a dyno with a sensitive measurment

[Chris Boyden]

Yea, I know, I was joking.... http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Frank]

hehe http://fsae.com/groupee_common/emoticons/icon_smile.gif