8.8 compared to 9:1 compression
Thread Starter
3rd Gear Member
Joined: Feb 2006
Posts: 664
From:
8.8 compared to 9:1 compression
well got the car running with my explorer intake and gt40(not p) heads. Sadly I was unaware the explorer gt40's are 8.8:1 until after I put the motor together. So I drove it today and am quite disapointed in it. No more power than it was before. So I guess it'll back off with the heads and get them cut down to be 9:1 compression. My question is, how much more power will it get by raising the compression that little bit. How much do Ihave to cut to achieve 9:1, I've read 30 thousands is 9.2:1
thanks
thanks
2nd Gear Member
Joined: Nov 2006
Posts: 205
From:
RE: 8.8 compared to 9:1 compression
First, you should know how to properly computer compression ratio. Good article here:
http://www.carcraft.com/techarticles...ression_ratio/
First use the method above to determine how much combustion chamber volume you need to lose to get the ratio you want.
One way to determine how much you need to mill is by first making a cast of the combustion chamber (plastar of paris works ok) with the valves installed. Make sure to use enough plastar so that the mold comes just above the machined surface of the heads (plastar shrinks as it dries). Let the mold dry. Next, sand the mold down so that it is even with the surface of the heads. Remove the mold from the heads.
Using a micrometer, measure the distance from the flat mold surface to the highest surface on the opposite side to within 1 thousandth (.001) of an inch. Record this number. Dip the mold in a small, graduated water container to find the volume of the combustion chamber. Record this number too.
Rub the flat side of the mold on a sheet of sandpaper. Try to keep the new surface as parallel to the old one as you can. Clean the mold off and dip it in the water again. Do this routinely until the mold is the size of the chamber you want.
Using the micrometer again, check the same width on the mold as you did earlier. The difference between this number and the one you mic-d earlier is how much you need to tell a machine shop to mill off your new heads.
http://www.carcraft.com/techarticles...ression_ratio/
First use the method above to determine how much combustion chamber volume you need to lose to get the ratio you want.
One way to determine how much you need to mill is by first making a cast of the combustion chamber (plastar of paris works ok) with the valves installed. Make sure to use enough plastar so that the mold comes just above the machined surface of the heads (plastar shrinks as it dries). Let the mold dry. Next, sand the mold down so that it is even with the surface of the heads. Remove the mold from the heads.
Using a micrometer, measure the distance from the flat mold surface to the highest surface on the opposite side to within 1 thousandth (.001) of an inch. Record this number. Dip the mold in a small, graduated water container to find the volume of the combustion chamber. Record this number too.
Rub the flat side of the mold on a sheet of sandpaper. Try to keep the new surface as parallel to the old one as you can. Clean the mold off and dip it in the water again. Do this routinely until the mold is the size of the chamber you want.
Using the micrometer again, check the same width on the mold as you did earlier. The difference between this number and the one you mic-d earlier is how much you need to tell a machine shop to mill off your new heads.
2nd Gear Member
Joined: Nov 2006
Posts: 205
From:
RE: 8.8 compared to 9:1 compression
Should be great on your setup. Looks like your engine breathes well on both the intake and the exhaust side. Increasing compression ratio will usually raise the torque peak, but leave it at the same rpm point. After the peak, the torque will not fall off quite as fast as you move further up the revs. This will push your overall horsepower up a fair measure.
Downside is that more compression will mean more heat. Make sure your cooling system is in good condition. It'll also want to idle a tad faster since each stroke takes more effort to compress. Also double check your piston/valve clearance. Make sure you have room for what you shave off.
Here's the thing: the cost is going to be the same no matter how much you mill off. So changing your compression to 9.45 will cost you just as much as milling them for 9.2. 9.5 is usually the safest ratio for iron heads, but I'd go with 9.45 for a little leeway.
Downside is that more compression will mean more heat. Make sure your cooling system is in good condition. It'll also want to idle a tad faster since each stroke takes more effort to compress. Also double check your piston/valve clearance. Make sure you have room for what you shave off.
Here's the thing: the cost is going to be the same no matter how much you mill off. So changing your compression to 9.45 will cost you just as much as milling them for 9.2. 9.5 is usually the safest ratio for iron heads, but I'd go with 9.45 for a little leeway.
Banned
Joined: Dec 2005
Posts: 22,374
From: Lil' Rhody
RE: 8.8 compared to 9:1 compression
ORIGINAL: MrMiracle
First, you should know how to properly computer compression ratio. Good article here:
http://www.carcraft.com/techarticles...ression_ratio/
First use the method above to determine how much combustion chamber volume you need to lose to get the ratio you want.
One way to determine how much you need to mill is by first making a cast of the combustion chamber (plastar of paris works ok) with the valves installed. Make sure to use enough plastar so that the mold comes just above the machined surface of the heads (plastar shrinks as it dries). Let the mold dry. Next, sand the mold down so that it is even with the surface of the heads. Remove the mold from the heads.
Using a micrometer, measure the distance from the flat mold surface to the highest surface on the opposite side to within 1 thousandth (.001) of an inch. Record this number. Dip the mold in a small, graduated water container to find the volume of the combustion chamber. Record this number too.
Rub the flat side of the mold on a sheet of sandpaper. Try to keep the new surface as parallel to the old one as you can. Clean the mold off and dip it in the water again. Do this routinely until the mold is the size of the chamber you want.
Using the micrometer again, check the same width on the mold as you did earlier. The difference between this number and the one you mic-d earlier is how much you need to tell a machine shop to mill off your new heads.
First, you should know how to properly computer compression ratio. Good article here:
http://www.carcraft.com/techarticles...ression_ratio/
First use the method above to determine how much combustion chamber volume you need to lose to get the ratio you want.
One way to determine how much you need to mill is by first making a cast of the combustion chamber (plastar of paris works ok) with the valves installed. Make sure to use enough plastar so that the mold comes just above the machined surface of the heads (plastar shrinks as it dries). Let the mold dry. Next, sand the mold down so that it is even with the surface of the heads. Remove the mold from the heads.
Using a micrometer, measure the distance from the flat mold surface to the highest surface on the opposite side to within 1 thousandth (.001) of an inch. Record this number. Dip the mold in a small, graduated water container to find the volume of the combustion chamber. Record this number too.
Rub the flat side of the mold on a sheet of sandpaper. Try to keep the new surface as parallel to the old one as you can. Clean the mold off and dip it in the water again. Do this routinely until the mold is the size of the chamber you want.
Using the micrometer again, check the same width on the mold as you did earlier. The difference between this number and the one you mic-d earlier is how much you need to tell a machine shop to mill off your new heads.
2nd Gear Member
Joined: Nov 2006
Posts: 205
From:
RE: 8.8 compared to 9:1 compression
ORIGINAL: AdderMk2
He really doesnt even have to go through all that. Simply tell the machine shop that you want 64cc or smaller chambers. they will mill the heads, then fluid test the combustion chamber to find the displacement. if it is still higher than 64cc, they will mill them further. and it doesnt make you do any extra work. Although it is a very good way of finding out how much to mill the heads, you really dont have to do it yourself.
He really doesnt even have to go through all that. Simply tell the machine shop that you want 64cc or smaller chambers. they will mill the heads, then fluid test the combustion chamber to find the displacement. if it is still higher than 64cc, they will mill them further. and it doesnt make you do any extra work. Although it is a very good way of finding out how much to mill the heads, you really dont have to do it yourself.
Machine shop might charge more for the extra work. You never know.
Thread Starter
3rd Gear Member
Joined: Feb 2006
Posts: 664
From:
RE: 8.8 compared to 9:1 compression
.038 or .039, I dont remember
I guess 15hp is alot when the head and intake swap(w/ 9:1 compression gt40s) would only yeild about 50hp anyway, so thats like a 30% hp loss.
I guess 15hp is alot when the head and intake swap(w/ 9:1 compression gt40s) would only yeild about 50hp anyway, so thats like a 30% hp loss.
Banned
Joined: Dec 2005
Posts: 22,374
From: Lil' Rhody
RE: 8.8 compared to 9:1 compression
ORIGINAL: 5speed GT
they are 64cc now, they need to be 59-60 to be 9:1
and no one has answered me about performance difference between the two
they are 64cc now, they need to be 59-60 to be 9:1
and no one has answered me about performance difference between the two
