Pro's and cons of milling 2.5 heads to 33cc's

[Johnny Venomous]

I am thinking about milling my heads down to 33cc's for some extra top end speed, and I'm hoping for some more rev's too. The application is on a 30 Cobra Terminator Cat with twin 2.5's carbed versions, 7 petal reeds, Offshore mids, 4 hole CLE's and 28 DAH Cleavers. Motors are only revving to 6500 and I want them up at 7500. Some questions I have are do I have to mess with jetting or timing after I do this. Will the motors run hotter because of the extra compression. Can I expect more revs out of the motors. Will I be changing the shape of the chambers in the heads and will this cause detonation. I still want to run pump gas( 92-93 octane). What head gaskets would work best. If I have to change timing or jetting, where should I be at. Any feedback would be very helpful.
Thanks in advance.
Johnny :confused:

[Jay Smith]

Johnny,

We cut a lot of heads at JSRE and I don't suggest any tighter than 34cc and even that tight I suggest the # 44 head gaskets
( Drag motor thickness ) .The problem with getting on the edge with pump gas is that the fuel is NOT consistant. Fuel will vary station to station and day to day, if your on the ragged edge and something is not correct (fuel inconsisity , jetting , timing , altitude , humity , etc... )you can slip over that edge quickly and without you being aware and detonate a piston . As far as top end the rise in compression is not going to do a great deal to increase top end speed, it normally builds torque and grunt on the bottom and mid range only ! We have found in our race engines that VERY compression ( 240 ish + ) will acuatually hurt the top end numbers as the motor dead heads as it trys to buck the big compression , also high compression play havoc on bearings and causes prmature wear to the aluminum lower middle and upper middle main saddles in the block as the pistons buck the high compression the crank is being applyed with a very high pressure and the aluminum block simply can't take that sort of abuse long and QUICKLY wears out......

Stick with a 35 cc ( with thin head gasket ) or 34 cc ( with a #44 drag motor gaskest ) and you will reap all the benifits on the increased torque and not harm the engine running Super Unleaded ....


Good luck !