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Compression ratio

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Big Rhino:
Hello,

I am in the final stages of my BBC build(582), last thing is the pistons.  I plan to run a mechanical stack injection, methanol and occasional 30% nitro.  What is the compression ratio I should be looking for?  Will the Nitro like 15:1 or will I have to lower it some?  or is higher better???  Thanks for any help.

Spud Miller:

 Methanol will like the compression. The nitro will NOT like it. Running some toluene in your fuel will help the nitro be more tolerant of the compression. A quart of toluene in a 5 gallon jug of mixed fuel will be a big help.
 
 I would plan on a strategy that will get you an easily adjustable compression ratio. Get pistons that will deliver 15:1 with the thinnest head gasket you can possibly run. Then figure the thickest gasket you could run. For good "squish" in the chamber, I'd not go thicker than .060". Thicker than that can actually promote detonation. Hopefully, you can shed a full point of compression this way.

 Another relatively cheap/easy way to lower compression is to grind the valves. A brand new set of valves can have a HUGE margin on them. An old set of valves can be ground down to nothing on the margin and that will really sink the valve in the head a bunch lowering your compression significantly. Valves are probably cheaper than pistons and could be swapped out quickly during a head gasket change.

 So, a gasket and valve change and you might drop a couple of points and that's going to be plenty good enough for 30%. 13-14:1 will be fine using toluene as a stabilizer.

 Another thing to consider is the cam. If you're running a short duration, short overlap "stump puller" type cam, the nitro will tear things up. It will hate it. Your "dynamic" compression ratio will be very high. A long duration cam, with tons of overlap will lower that for you and the high static compression ratio won't be such an issue.

 Spud

Big Rhino:
Thanks a lot Spud.
I appreciate your advice on this.  Here are my  cam specs (294/300 @.050, .779/.717 lift, 112 LSA (Crower PN 01492)).
Does this look like it would work with the previously mentioned compression ratio if dropped down to 13 - 14:1 for the nitro?
Thanks again for the help,
Tom

H.G. Wells:
While I was not playing with nitro, I was very surprised how much changing the cam on my alky motor made the engine feel more compression.
I had a 10:1 flat top piston motor that had run on gas and put a stack injector on it running alcohol.  I had a cam built with little overlap so the motor would "see" more compression.  Makes sense that you can build one to "see" less also.

If you increase the overlap is there any detrimental affect on the valves?

Spud Miller:

 Hi Guys,

 Tom: That cam looks pretty good. I think it'll be fine. Down in the 260's @ .050 is trouble for a nitro application.

 Mr. Wells: As far as the valves are concerned, there's no problem on the extra overlap as long as they aren't hitting anything :)

 Spud

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