Would too high compression and not enough octane cause a knock/clack sound? What should reving the engine do?

 

Check your timing,that will cause knock also.

T.

 

Well the engine is timed correctly...but it's making a sound like a deisel engine almost....we're thinking that the CC268H cam isn't right for such a high CR application (10.1-10.5) Gapless rings and it's making a bad sound....

 

Did you check piston to head clearance and piston to valve clearance?

 

Piston slap was questioned....but this was a running 350, crank, rods, pistons and heads are all the same, the engine was taken apart after 20k miles, heads fully ported, full rebuild. It was supposed to be putting out around 400HP with the old solid lifter cam setup, which was removed and replaced with the CC268H cam. The pistons were flat top single relief and around 10:1, it was running when it was pulled, the #8 cylinder had a bad valve which caused it to smoke, but other then that the engine was a purpose built 350 in a 62' vette only taken outfor a faster motor.

The engine and almost all the parts were existing and it was running, so mechanically this shoudl be a good engine. The heads had VERY bad shrouding (462 fuelie castings) but they are now completely ported. The bottom end is forged....it was rebuilt correctly so we're trying to figure out why it is making such a funny noise. We're thinking that maybe since the pistons were high compression to begin with that with the gapless rings it raised the compression even more which is causing problems....Every bearing was checked and measured, all journals....I mean...eveyr measurement was well within tolerances...this is a tight engine. Rod knock has been suggested but it doesn't make sense...it didn't have a rod knock before, the only thing changed was the cam/lifters etc. Mechanically the engine should run fine...is the 268H cam a bad compliment with such high compression?

Oh yea, reving the engine "seems" to slightly reduce the sound it makes, this isn't my engine, I just helped build it, my buddies really pissed off it's not working (hes about to torch the thing) I gotta get over there and hear it....we've had tons of mechanics and friends who build engines for a living come over and they have never heard this sound before...im told it sounds liek a deisel engine, with irrythmic clackity clack....it runs fine, it revs fine....but it just doens't sound right im told

 

Maybe a bad lifter?Sounds like rod knock.Change the oil and cut open the filter.If you find metal bye bye motor

 

I have an street 88' TPI Camaro stroked to 388 with a CR of 10:1 on 93+ pump gas, but I use alum. heads, for iron I would have used 9.5:1

if it was a running 350 and all you did was a rebuild and tolerances are within spec then piston slap shouldn't be an issue and even if it was it would go away after the engine warms up. Has anyone checked the compression? 93+ octane is plenty, but may knock with iron heads at 10:1.
Is it carbed, TBI or TPI, race, or street, does it have an emission system?
You said it's a 350, are you referring to the block or it's total volume?. You mentioned heads ported in the 1st & 2nd paragraph differently, I take that to mean that the combustion chamber and the valve ports had material removed an surfaces shaped & smoothed. The volume of the combustion chamber will increase by the volume of material removed, thereby decreasing the CR unless you've done something to compensate then your CR should be less than before. If you reduced the head volume by milling then valve to piston clearance becomes an issue especially if you put larger valves.
How much are the pistons in the hole?
What is the head gasket thickness?
What is the stroke?
What is the Rod length?
What is the piston compression height?
You really need to gives all the info on the engine spec's, mods and it's fuel/timing control.

 

........Have you tried a stethoscope to see if you could narrow down where the noise is comming from,ie. lower end,top end,etc., could even be comming from the timing area. If you don't have a stethoscope you can use a broom handle about 10" or 12"long,hold it to the engine an your ear. A nother old indian trick,just one of many,hehe!

T.

 

Dump all the gas in it and put in some high octane race fuel. Also make sure your fuel pressure is right and the regulator is working well. Then see if it still does that. If it does that then you dump the motor oil and put in the thickest stuff you can find. If it calms down, change motor.

 

did you make sure you guys degreed the cam before throwing everything together? if not a timing light will say its timed right but it wont be right and will cause the car to run weird.

 

is it a 268xtreme cam? If so then you do probably have to high of compression ratio. There was a really really good write up in car craft a few months ago about xtreme cams. The smaller extreme cams, 262 and 268 build up so much cylinder pressure that if you start with a 10:1 engine the engine actually "thinks" it has closer to 11:1, which will cause detonation.

Hope this helps.
Jason

 

his car has the standard 268H CC cam, no XE, i have the XE268H cam, with the summit rebuild kit 10.7:1 domed pistons....sounds like im gonna have problems.....i was told however that if pistons are 10.7:1 with 64cc advertised like summit does that you tend to lose almost a half a point of compression from when you "build"...i know ponykiller runs like 10.3:1 CR with the 274XE cam with excellent results, i wanted the 274XE but i was told it was too agressive and not streetable enough for a car i will be driving every day

i thought that the overlap on the XE268h cam would allow me to run high compression and make good power....i knew going in that the high comrpession might require me to back off my timing when it's warm out...but that in general that the cam w higher CR would yeild me in the 400HP range....

i just ran my specs thru a CR calc, not sure how well those work

4.03 bore
3.48 stroke
64 cc vortec head
.038 standard gasket
.0001 standard deck
.125 domed piston (advertised 10.7:1 by summit)
i got 11.12:1 CR....thats high....again, ive been told that you tend to lose some compression once it's built, so i figure im more around 10.5:1, which is still very high...however lots of "performance" shops that build hot motors have said that a 10.75:1 CR motor on 93 octane is very doable.....iron heads aren't helping me...im wondering about my cam now though