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oklowride 02-16-2006 03:23 AM

compression calculator
 
im installing a 327 into a 75 chevy truck and my friend (the owner) his friends are telling him its going to ping all day long....
the specs are 1967 327 block .030 over bore with TRW L2166N pistons .125 dome forged with crate 350 76cc heads(1.94int), as far as i can tell it will end up with approx. 9.0 compression and 332 CU. his redneck friends keep telling him its too high compression. i say theyre smokin too much hay. the short block was a barely used engine from a friend who ran it with 64cc heads and the cam looks to be(i measured with a dial caliper) .3 at the cam so that would be .45 total lift at the heads, i say it will make a good strong engine much better than a stock 350. any one done a combo like this?? or input on compression for such an engine.

SPIIRIT 02-16-2006 04:40 AM

Compression Calculator
 
Going strictly by your numbers and build, 9:1 CR is a little much for supreme pump gas. The problem is the .450 total cam lift vs. dynamic compression ratio. The more lift/duration of the cam, the less CR. the engine sees. A real strong cam might have a .750 total lift and corresponding duration along with a 1-1 1/2 point drop in compression ratio. Then at 8-8 1/2 ratio any problems could be offset by "Total Ignition Retard" say from initial setting of 36* to as much as 3* less (33*). Furthe reduction would hurt the power too much in my opinion.

In any case, check the static compression by using a known good pressure guage and fresh battery. Block the throttle wide open, crank the engine about 5 compression strokes. For the street, in no case should the readings exceed 170 pounds (max for premium pump gas).

From what I know, this should give you an engine that will have a reasonably long life and run near or at the peak of it's potential at the same time.

Until you know the engine... read those plugs for metallic flakes or blistering. You know that the best read is to see some color up inside the plug with a little white on the porcelin tip, however that is cutting it very close, perhaps the tan tip is best here.

This subject has nany facets. I strongly suggest that you research and triple check all the info you can... to build a dependable engine!

camaroman7d 02-16-2006 09:42 AM

9:1 with iron heads should not be a problem. 10:1 is the "rule of thumb limit" for iron heads and pump gas. If your quench is tight you should have no problems with 9:1 on pump gas w/iron heads.

Royce

curtis73 02-16-2006 11:12 AM

A thousand factors come into play here, but 9:1 should be no problem at all on that engine. You'll need to run the highest octane you can find. Here in L.A. the best we can get is 91, but that should do fine with a careful ignition curve.

The domes on the piston will slow flame front propagation which will require more ignition lead for peak power which you may not get to experience on pump gas. Since its just a .125 dome, I don't think it will take you out of the realm of tunability on premium pump gas though.

mstngjoe 02-16-2006 11:23 AM

Quote:

Originally Posted by SPIIRIT
...In any case, check the static compression by using a known good pressure guage and fresh battery. Block the throttle wide open, crank the engine about 5 compression strokes. For the street, in no case should the readings exceed 170 pounds (max for premium pump gas).

This method will not accurately determine static compression ratio.

And just as a side note, my last 2 engines had cranking pressure of 188 and 200. Ran great on 92 octane.

mstngjoe 02-16-2006 11:25 AM

Quote:

Originally Posted by camaroman7d
9:1 with iron heads should not be a problem. 10:1 is the "rule of thumb limit" for iron heads and pump gas. If your quench is tight you should have no problems with 9:1 on pump gas w/iron heads.

Royce

I would agree with this.......and quench is a very important factor.

oklowride 02-21-2006 12:06 AM

i get the 9.0 from a sellers website showing the pistons at stock, 10, 20, and 30 over and with 64 and 76cc heads it was the most complete info i could find. we started it up yesterday and it definately sounds like it has alot more balls than the stock 305 it replaced. we did not get to do a road test thanks to mothernature droppin snow and leaving ice all over the roads for days.... my friend that i put this together for just wants a tire smoker hes not going to race it hes too low budget to spend real money so i came up with this budget combo out of the used shortblock from one guy and heads from a swapmeet, it also has the previous goodies from the 305, hedman headers straight duals with IMCO? race mufflers edelbrock rpm intake and 1405 carb, stock '75 HEI, and TH350 with a new B&M shiftkit.
any idea on where i should set the timing? i just did a quickie by ear timing and plugged in the light and its like 14BTDC. i have yet to tune the carb its out of whack and check everything else out


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