View Full Version : 2F Head Question
04-15-2009, 09:24 AM
My 2F is down at Gunn's getting the full rebuild. I am also going to put an RV cam in it. I've heard that performance can improve quite a bit if you raise the compression a bit by milling the head. I dont want to push the envelope as far as how much I raise it, I don't want to have to run premium gas in it, still want it to be reliable, etc.
Any recommendations on how much I should have them mill the head to achieve this?
04-15-2009, 09:37 AM
I think you can remove .1000" but I could be mistaken... man that doesn't help you at all :lol:
04-15-2009, 02:25 PM
I just researched this extensively, and the Mark Whatley rule of thumb was .050 (conservative). The max is .100, as you said. I plan to have one milled this summer, and it'll be interesting to see what effect this has on compression. Some have done as much as .080. I would like to run no more than mid-grade, so I don't want to get totally silly with it.
Mark W's exact words are in the 2nd link below:
Good general head info: http://forum.ih8mud.com/40-55-series-tech/105385-2f-head.html
Mark's head-shaving info - 2/3rds of the way down on this page: http://forum.ih8mud.com/40-55-series-tech/87140-best-2f-10.html
04-15-2009, 06:26 PM
Thanks for the info. I also got feedback from another engine guru, JimC. Jim confirmed the .100 max and thinks going .060 on a later model 2F with a cam at our elevation is about right.
I also got confirmation that a 73-74 F.5 head can be used on a 75-80 2F block and it will raise the compression.
04-15-2009, 08:56 PM
I did 0.05 on my 2F and was pleased with the performance. Its probably worth having the distributor recurved though. I had a oddly random pinging that would come and go regardless of the octane fuel I ran in it. The only thing that seemed to fix it was monkeying with the vacuum to the dizzy.
04-16-2009, 08:46 AM
Did you put a vacuum delay valve in the line to your dist., or how, exactly, did you 'monkey' with it? I presume you can also back off the static timing a bit for chronic pinging.
04-16-2009, 09:00 AM
JimC installed a ported vacuum port on my carb and I will be running a DUI HEI distributor so we'll see how that works.
04-16-2009, 09:58 AM
If I remember correctly, I removed the vacuum delay on the advance so it would lose the advance at wide open throttle faster. It seemed to help but didn't entirely solve the problem. I was never clear if the centrifugal advance was additive to the vac advance or if the two systems provides the same amount of total advance. If the total was the same, then removing the delay valve probably didn't do much of anything.
04-16-2009, 12:19 PM
Thanks for the info!
Actually, the 'ported vacuum' sent to the advance module gives you *more* advance at full throttle. (If you had accidentally connected 'manifold vacuum' to the advance module, you'd have full advance at idle, and none at full throttle').
A vacuum delay valve could be added to prevent sharp transitions that might cause transient preignition, but it since the vacuum would 'catch up' eventually through the VDV, it wouldn't solve chronic spark knock under a static condition (prolonged full throttle, for instance).
Perhaps we need a vacuum line with a wastegate ;-).
My understanding of a 2F dist. (or most of that era) is that the mechanical advance provides the bulk of the total spark advance (perhaps up to 30-some degrees) and the vacuum advance just adds a few more. With a timing light you can do some experimenting (at least on an engine with real markings, unlike a 2F) and learn a few things. I know that when the vac. advance plate was rusted tight on my '78 2F, it was a real dog. A few smacks with a hammer and screwdriver, and the rusted-stuck advance plate broke loose and freed up some horsepower.
04-16-2009, 12:39 PM
What I found with a vacuum gauge was that the available vacuum drops as the throttle is opened. At WOT there just isn't any vac available to move the vacuum motor. Removing the delay valve allowed the vacuum advance to disappear rapidly as the cetrifugal advance took over. I figure it reduced the total advance enough to reduce the ping. It wasn't an elegant or even an informed mod, it just seemed to make sense at the time.
04-16-2009, 05:45 PM
(If you had accidentally connected 'manifold vacuum' to the advance module, you'd have full advance at idle, and none at full throttle').
You still get some vacuum at WOT, depending on RPMs. Higher RPMs=higher vacuum, but nowhere close to closed or partial throttle.
My understanding of a 2F dist. (or most of that era) is that the mechanical advance provides the bulk of the total spark advance (perhaps up to 30-some degrees) and the vacuum advance just adds a few more.
It's actually around 20 some degrees of centrifugal advance; vacuum advance will vary according to the year of the (stock) distributor, and is typically around 7°. This is info from various conversations with Jim C. over the years. My recollection is that vacuum plus centrifugal plus static is typically around mid-upper 30° max advance.
vBulletin® v3.7.1, Copyright ©2000-2015, Jelsoft Enterprises Ltd.