Hey guys !!
Got the new 408 engine in the car today, the engine hasn't run yet, but out of curiosity, I gave it a couple of turns just to see the cylinder pressure
with a Equus pressure tester.

Cylinder pressure reads 195 lbs with RHS MA-X iron heads......I am very much afraid of pre ignition or pîng !!.....So is this really too much to run on premium gas or is it borderline ?


Also, what are the basics to prevent pre ignition in an engine ?

Dan

OK I might just do that......

Dan

Tips anyone ?

Things I know of you can do to help prevent ping short of running race gas (or E85)... roughly from easiest to hardest:

1. Start with conservative timing curve... delay "all in" to around 3000 RPM. 34 deg max total mechanical advance.

2. Cool air. Either a cool air intake or scoop

3. Increased cam duration to reduce "dynamic" cylinder pressure. Experienced help picking the cam would be valuable.

4. Are you starting with a "quench" engine? If so, a thicker head gasket will probably hurt by undoing quench. If not, a thicker head gasket to drop compression will help.

5. Machine chambers in head to drop CR (if possible) or change to heads with a larger chamber.

6. Rebuild with lower CR pistons (ouch)

Was the carb off/not installed, when you did the test? or was it at wide open throttle.

Have seen higher numbers when the throttle plates were shut.

195 is too high. try B and P 94oct with a 5-10% pure toulene mix. Even without the mix it should light up and run breakin with no load. Unless your timming is all goofy.

No carb or intake were installed during the test..

Pure toulene mix ??? what is it ? school me on this and where to get it..

Brand new engine, just rebuilt from the shop..

Dan

A few more specs on the engine may help. Cam, pistons, heads, quench. Also before doing anything radical, it would be worth a second test with a different and known accurate gage.

I have a '94 Magnum block
Hyd roller cam 230/238 @ .050
RHS 62 cc iron heads MA-X from Hughes engines
Forged dished pistons 20.5 cc
10.2 cr
9.3:1 dynamic cr
.047'' thick head gasket

Break it in then do a compression test hot. the cam is a little mild and your rings are likely covered with thick oil. I think they say 165 is the max with iron heads on 93 and 185 for aluminum. I have run 200 with aluminum on 94 and a 4000 stall the higher the stall the later it comes under full load. the higher the RPM the less octane requirement.

Do the cam break-in and a bit of street driving with race gas first, then go back and recheck it. If it's still too high, you can try retarding the cam a bit to lower the cranking CR.

Ok...
I appreciate all the infos here....

Dan

Dan, static compression with a never fired motor is really irrelovent. Assembly oil used and cam profile make it so. In fact, extra cranking of the motor just takes the lube off the cam you need at startup.
Don't worry, fire that thing up and enjoy!

OUFFFFF !!

I was afraid for a moment there....thanks !
Dan