It has been going on 3 days looking for information on how to figure out what octane is needed based off compression ratio and what type of heads (ie aluminum or cast iron) with no luck. I have came across forums with different figures such as running 10:1 on 89 octane and no ping issue cause hes using a EGR valve which is reducing the combustion chamber temp (I personally dont buy that unless he went with a egr valve for a bigger engine).

So I am looking for a site with information or a calculator on the said subject.

If it comes to it I can type out the planned specs of this engine as well as the specs on the camshaft if someone wants tot ake a crack at what octane I would most likely have to run.

Thanks in advance.

 

Without extensive testing on each engine design, you won't be able to get clear octane requirements. Generally, start on 87 and if it pings go up.
Aluminum heads and EGR will help, also a larger quench area will help. If you have flat top pistons with a large combustion chamber like the FEs, you can have detonation problems. Something like a small chamber twisted wedge will run on a lower octane.

 

Option I am leaning towards is stock Ford 351 heads (I assume they are 351 heads) with 1.94"/1.60" valves and 3 angle valve job, dished pistons and 9.5:1 compression. I am thinking I would need 93 Octane with the cast iron heads. I was thinking about going with bigger dished pistons to hopefully drop the compression down to 9:1 so that I would hopefully get away with 87.

Other than that all I can really say is this about the build

351W block 4.040" bore x 4.10" stroke (420.5 cubic inches)
Dished pistons cut for larger valves
stock ford heads with 1.94"/1.60" valves 69cc chambers (stock heads on my 78 351 are listed as 73cc @ 8.5:1 compression)
6.20" connecting rods
206/212 @ 0.050" - 0.461"/0.474" - 110 lsa camshaft

Other than that I just have this feeling with cast iron heads and 9.5:1 compression I would need 93 octane at a bare min, then again my 63 ran off regular fuel (what we call 87 today) with 9.2:1 compression.

Guess only option would be to do as you said and start off with 87 and see how it runs and go from there.

 

Well besides all that now, now this guy has me thinking bout the trq, he says 500 - 520 ft lbs of trq is too much for 2.50 gears in a 4,000 lb car that I would sit there and spin the tires on launch. Personally I dont see why it would do that and why going with 3.30:1 axle ratio would improve the launch and reduce tire spin on the launch when it would make it easier for the tires to spin.

 

I dont know Ford engines well, but aircooled, such as VW, Lycoming, etc I do. Octane requirement is so much more than CR. The most important things are bore size and quench area. Spark plug position is also an issue, and of course ignition timing and mixture distribution. Dished pistons and/or open chamber heads wont work if you give up quench in order to lower CR.

Detonation occurs when mixture in at the edges of the combustion chamber light off before the flame front, starting at the spark plug, reaches it. This can be due to radiant and/or compression heating.

Preignition is a different issue, but the two are often confused, or worse I have read the false term "predetonation" on internet forums. Preignition is most often caused by glowing carbon deposits or too hot of a spark plug heat range.

Jim

 

Well my plans have been altered since my last post in here. Compression ratio is going to be around 9.1:1 with 60cc heads, bore is not the same as above anymore as well. Roughly 4" x 3 1/2" bore/stroke. I am thinking since its pretty much a stock 351W I should get by with 89 octane. I even tracked down a company that makes 4V EGR valve spacer plates, that would reduce any detonation issues when at rpm and moving. That is taking into consideration 8.5:1 compression on my stock 351W and im able to run 87 easily. I should be able to run 89 octane maybe even get away with 87.

 

The EGR part is interesting. Do you give up more hp to mixture dilution than you would from less total spark advance, for instance?

Jim

 

At cruise EGR is great. It actually makes up upwards of 30% of the mixture in the cylinders. You don't really lose any power, it's a really close balance since you don't need as much throttle and have less pumping losses. It does have an effect on WOT power, but it would be fairly simple to lock it out when you step on the throttle.

 

The way how I understood it when it was explained to me is the EGR valve just pumps exhaust gasses into the cylinder off idle as a "space saver" so you have less fuel in the cylinders resulting in a cooler combustion chamber to prevent the formation of NOx. In this sense I would be using it to prevent detonation based off cylinder temperatures when at speed. I will try it first with it disconnected (no vacuum line hooked up) and if it has detonation then I will try it with it hooked up. If there is no change then I can simply just plug off the vacuum line and just leave the EGR valve there for looks. Wouldnt cost me any money seeing as I have a fairly new one on the car as of now.



Wouldnt the EGR valve close when you go WOT since it uses vacuum to open the valve? Atleast I thought when you went wide open throttle vacuum instantly goes to zero for a moment.

Aside from that what kind of effect would it have on WOT power? I wont mind if im just looking at 5 - 10 hp loss with it at WOT, I dont think I would notice it that much of a power loss considering the first gear ratio of the transmission is 2.39:1. Then again as I was thinking the EGR valve would be of great help at preventing detonation off idle when driving if I encountered it. But surely 9.1:1 compression should run off 89 if I am getting the car to run off 87 as of now with 8.5:1. But if I did deside to go with it regardless and wanted to lock out the EGR valve at WOT how would one go about doing that?



Going to get alittle off topic though but I am thinking go the "sleeper" look for the most part. I found some nice chrome valve covers that look fairly stockish that will work with the stock PVC inlet tube on the passengerside.





and since the factory valve covers uses a twist in breather cap that the PVC valve snaps into I desided I would go with this by Zoops



The aircleaner will be the stock unit off my 2V carb, I measured the opening of the aircleaner to the carb and it measures just over 5" and the 4V aircleaners on summit is listed as being 5 1/8". I would have to get some some sheet metal to form a small box around part of the passengerside header for the hot air tube to connect to. Shouldnt be hard to do though.

Then I would be using a Edelbrock Performer Intake manifold (been thinking I might would paint it oem Ford Blue)


Which I belive comes with the choke block off plates as well as the 4V EGR Valve Plate

 

Detonation is only damaging at high power, WOT or close to it. If EGR cuts out at WOT, how is it going to help reduce octane requirement? Just asking.

While the EGR valve itself is pulled open by vacuum, there needs to be a control valve to tell it when to open. If it was just straight manifold vacuum, the engine would die at idle. I had this condition with a failed (open) EGR control valve. The engine would cut out at decel, closed throttle and high vacuum.

Jim

 

The carburetor is the control valve. You plug it into ported vacuum not manifold vacuum. manifold vacuum is vacuum all the time, ported vacuum is only when the throttle is opened. Like wise the vacuum hose runs from ported vacuum to a thermister (i belive that is the name for the temp sensitive valve) that cuts vacuum to the EGR valve when the engine is cold and opens vacuum to the EGR valve once the engine is warmed up.