Is it true that milling a low comp 350 head and block by .040 can bump cr to 9.5?

I read that milling low comp heads and decking block by .040 increases comp ratio rom 8.5 to 9.5. Is this true and whats it worth in terms of horsepower/torque and fuel requirements/? peace

 

It would be closer to going from 8.0 to 9.0. But lets assume that you did this and gained a point of compression.

280 hp -20% = 224 hp net SAE (this is your starting point)

1 point increase in compression yields about 4% more horsespower

224hp increased by 4% = 232.96 hp

In other words all this work would gain you another 9 horsepower.


So keep your CR low for now in case you want to add in a turbo set up later or just keep using regular gas. It is going to take that or the addition of a 455 for the most economical big jump in performance. (I am excluding NO)

 

Why do you say closer to 8. I had one member tell me that it was actually less than 8.

Im curious, isnt CR simple math? How could GM publish a wrong compression ratio. What is the compression chamber size on a 73 buick 350? what is the volume of the space to be compressed into the compression chamber?


(bore x stroke + compression chamber )/ compression chamber right?

all 350 compression chambers are the same correct?, pistons are dished by how much on 73 350 pistons?

lets figure it out exactly.

 

The main reason compression is increased is to run bigger cams. Raising compression with a stock cam will yield a smidgen more power, but it's definitely not worth all the work to do it. My new philosophy is if its not knocking, leaking, or smoking, don't freaking touch it! That's just me though.:laugh:

Looking at Jim's post I have to agree with him saying the more realistic compression increase is closer to 9:1 . First time I ran ported heads, they were milled .040 and I used a Fel-pro gasket,it ran on 87 Octane gas with no problems. Second time I ran ported heads, they were milled .050 and I used a MLS .020 gasket compared to a fel-pro .040 gasket, and the engine required 93 octane gas.

 

Always mill the block first.
Block milling allows the piston to come closer to the head. This increases combustion efficiency and decreases detonation tendancies.
Head milling leaves the piston the same distance from the head and only produces an increased comp ratio as well as increased detonation.

If you see a nice 1928 model A Ford for sale, don't buy it. Drive on down the street till you see a 29 model. For the 1929 model year, the factory milled the block for increased compression, more power, better gas milage, and still run on cheap gas.

All good HP crate motors will have the piston less than .060 from the flat of the head. Usually .040-.050.
My friend milled his 455 block to zero deck, (with a .040 gasket), and went from 12.50 to 12.10 ET on pump gas.

 

The factory advertised compressions are aproximate and not consistent. The only way to find out you actual compression ratio is to take the engine appart and measure the depth of the pistons in the block and head chamber volumes. The piston dish we know, the gasket thickness we know but the depth of the piston varies, as well as the head chamber volumes.

Like Cason says, the biggest reason to up the compression is for a larger cam.

 

Compression ratio is:
TOTAL volume (chamber volume+bore/stroke volume) divided by CHAMBER volume.
Chamber volume is: combustion chamber + gasket + piston to deck clearance + piston dish.
Tighten the gasket and deck clearance first to bring the head down toward the piston, (to about .040 .050 piston to head clearance), before cutting the heads. It squezes the gasses from the outer edges into the center of the chamber, creating turbulence for efficient burning.
Having done this first, you can mill the heads to reach the comp ratio you need.
This is far more power efficient than head milling first, leaving the piston .070-.080 from the head, and just adding to detonation problems.

 

Jim, you say that upping compression by 1 point will only yield ~ 4% increase, yet when Buick dropped the CR on the 350 in 71 rated HP decreased by 30 HP or about 10%.

 

That rating game was so arbitrary that it cannot be taken seriously. Another issue is the rpm used by year to "Rate" the horsepower.

In fact as I recall this is a nonlinear curve and the higher you go in compression numbers the less percentage increase there is to yield in horsepower.

 

Look at this illustrative graph:

http://blogs.hotrod.com/6253885/editorials/hot-rod-engine-tech/index.html

 

I think net ratings started in '72.

That aside, I doubt the compression drop was the only change for the '71 model year. I wouldn't be surprised if ignition curve and fuel metering was recalibrated, maybe even cam timing as well.

Devon

 

I think if you look closely at the graph you will see that I actually overestimated the hp increase he would likely achieve if he did nothing else. As you increase compression however you can change the cam profile and change the equation somewhat.

But back to the original issue. In the real world look at the graph above. If he went from 8.5 to 10.5 he would go from regular gas prices to premium gas prices and get less than 10 hp more! You can hardly detect a 10 hp increase by the seat of the pants. Is it worth it? I would say not and I suspect the owner was thinking he would be getting a lot more grunt for the trouble and cost in machine work and fuel octane change.

 

Very interesting, I think alot of people like myself thought that bumping compression was one of the easiest ways to up performance. It would be interesting to get a dyno pull with a stock low CR 350 (CR would need to be measured) then bump up the CR 1 pt and do another dyno run without changing anything else. Is it possible that a change in CR affects a long stroke engine more than a short stroke?

 

Marc--I find myself constantly dissenting over a lot changes people commonly think will (or has changed) their car into a rocketship. A lot of it is due to years of cleaver advertising by manufacturers--high performance oil, STP, bigger gage spark plugs wires (painted red), high voltage coils, giant cam, boring cylinders, electronic ignition and the list goes on and on.......

In general it takes a significant number of Franklin brothers to go fast because the whole package often has to be modified. Naturally owners hope there are cheap shortcuts to getting their cars to really perform but those are hard to find.

 

do you guys know of any 13 sec cars running 8.5 cr?

 

Well, Jim, at least there is some improvement in HP with increased comp not like the other things.
Here are some useful calculators (including HP vs CR) http://www.camaroz28.com/calcs/index.shtml

 

JW built John Looney a 500 Hp engine with a small cam that is great for the street. It has a 8.5 CR or less and it uses regular gas we think. Search above under the word "sneaky".

 

When I was at the BPG nats, someone asked Denny Manner what the HP difference was between a 1970 Stage 1 and a 1971 Stage 1. He stated the difference, not factoring in the larger '71 carburetor was around 15 HP. That is very much in line with what Mr. Lore stated earlier.

Ken

 

He showed us the factory dyno sheets once to back it up.