Install BP DOHC head on BP SOHC engine
August-13th-2003 | 08:46 PM
#16
Registered User
Joined: Jun 2002
Posts: 2,130
Originally posted by kcbhiw
It's just a safety factor. If you ever snap your timing belt, the valves and pistons will clash.
I stand corrected. I'd forgotten that the SOHC does, indeed, have 4 valves/cylinder. However, I'm not sure if they are the same size (doubt it).
Good luck with your head swap. Personally, I'd rather save myself the effort and just drop an entire BP in.
It's just a safety factor. If you ever snap your timing belt, the valves and pistons will clash.
I stand corrected. I'd forgotten that the SOHC does, indeed, have 4 valves/cylinder. However, I'm not sure if they are the same size (doubt it).
Good luck with your head swap. Personally, I'd rather save myself the effort and just drop an entire BP in.
August-14th-2003 | 01:01 AM
#18
The Horns of Jericho
Joined: Feb 2002
Posts: 320
From: Southern Californ-I-A (909)
You can use the existing harness. You just have to run a wire for the VICS.
This is something I was going to do, but now I'm building an engine that has diff pistons, and I'll be using a standalone. I won't have to worry about the wiring harness.
The wiring blocks that go into the ECU are the same between the SOHC and DOHC. You definitely need the DOHC ECU though. Engine won't run right if you don't do the swap. I'll double check the wiring diagrams if I can find the info again.
I believe the pully tensioners are different on the DOHC water pump. Oh. Yeah. IRC, the DOHC has two pully tensioners (one being adjustable), while the SOHC has only one.
You might want to look into installing an oil filter relocator and installing an oil cooler. I notice with my car when I drive on the freeway at 80-90mph for a bit of time, the engine has a more noticeable ...noise? ( I want to say ping, but it has nothing to do with ignition) ...when I get off the freeway. I'll sit there at the light and I can hear the engine running a bit louder than usual. I really dislike it. A good oil cooler would do a good job keeping the oil temp where it should be. It's something that will installed in my baby when I finish the engine and drop it in.
EDIT: I ment to mention it when I first typed the message, but the oil cooler that sits between the oil filter and the block doen't do jack. You run the engine hard, and that little cooler will do nothing for you.
Good luck on the swap.
This is something I was going to do, but now I'm building an engine that has diff pistons, and I'll be using a standalone. I won't have to worry about the wiring harness.
The wiring blocks that go into the ECU are the same between the SOHC and DOHC. You definitely need the DOHC ECU though. Engine won't run right if you don't do the swap. I'll double check the wiring diagrams if I can find the info again.
I believe the pully tensioners are different on the DOHC water pump. Oh. Yeah. IRC, the DOHC has two pully tensioners (one being adjustable), while the SOHC has only one.
You might want to look into installing an oil filter relocator and installing an oil cooler. I notice with my car when I drive on the freeway at 80-90mph for a bit of time, the engine has a more noticeable ...noise? ( I want to say ping, but it has nothing to do with ignition) ...when I get off the freeway. I'll sit there at the light and I can hear the engine running a bit louder than usual. I really dislike it. A good oil cooler would do a good job keeping the oil temp where it should be. It's something that will installed in my baby when I finish the engine and drop it in.
EDIT: I ment to mention it when I first typed the message, but the oil cooler that sits between the oil filter and the block doen't do jack. You run the engine hard, and that little cooler will do nothing for you.
Good luck on the swap.
Last edited by zenilder; August-14th-2003 at 01:05 AM.
August-14th-2003 | 12:29 PM
#20
The Horns of Jericho
Joined: Feb 2002
Posts: 320
From: Southern Californ-I-A (909)
I'll look for the info again. You'll have to run a wire from a certain pin on the ECU to the solenoid valve that activates the VICS.
I have a hayes book, but I'm not sure it'll show/tell which pin it is. I'll see what I can find.
I have a hayes book, but I'm not sure it'll show/tell which pin it is. I'll see what I can find.
Last edited by zenilder; August-14th-2003 at 04:34 PM.
August-14th-2003 | 02:43 PM
#21
Salt addict
Joined: Aug 2002
Posts: 1,201
From: Niagara Falls Ont.
Originally posted by kcbhiw
You are correct. However, IF a head was used with pistons with improper valve clearances, then that all changes.
You are correct. However, IF a head was used with pistons with improper valve clearances, then that all changes.
August-14th-2003 | 04:33 PM
#22
The Horns of Jericho
Joined: Feb 2002
Posts: 320
From: Southern Californ-I-A (909)
Originally posted by midnightblue97
I can't see that, unless the block or head have been decked. But I might just not be understanding what you're getting at.
I can't see that, unless the block or head have been decked. But I might just not be understanding what you're getting at.
Now... Take a piston from a SOHC B8 with eight valves. Pistons in this engine will not have the same relief areas as the pistons from a 16-valve engine. The pistons will only have two relief areas in the top of the piston, and they'll be farther away from the edge of the pistons circle.
Now, if you look at the pistons in that picture and imagine that the relief areas are filled, and that new relief areas are inbetween where the old ones were, you would be picturing a piston from an eight-valve engine.
We go back to the hypothetical situation where the timing belt broke and put these eight-valve pistons in the engine. Some valves are stuck fully open, and the piston is coming up. Those valves are expecting a piston with four relief grooves, not the full height of the piston. Bam. The piston smacks and bends a valve and possibly damages other engine components.
August-14th-2003 | 07:11 PM
#24
The Horns of Jericho
Joined: Feb 2002
Posts: 320
From: Southern Californ-I-A (909)
Mazda made two SOHC engines. One with 16 valves, and one with 8. I know, because Mazda Recycling sent me the one that had 8 valves.
I was pissed at them, but that's besides the point.
Most cases, yes, the SOHC are the 16-valve versions.
I was pissed at them, but that's besides the point.
Most cases, yes, the SOHC are the 16-valve versions.
August-14th-2003 | 09:04 PM
#25
Registered User
Joined: Jun 2002
Posts: 2,130
Originally posted by zenilder
Mazda made two SOHC engines. One with 16 valves, and one with 8. I know, because Mazda Recycling sent me the one that had 8 valves.
I was pissed at them, but that's besides the point.
Most cases, yes, the SOHC are the 16-valve versions.
Mazda made two SOHC engines. One with 16 valves, and one with 8. I know, because Mazda Recycling sent me the one that had 8 valves.
I was pissed at them, but that's besides the point.
Most cases, yes, the SOHC are the 16-valve versions.
since we all know the search tool is flaky...
August-15th-2003 | 08:55 AM
#28
Thread Starter
Registered User
Joined: Aug 2003
Posts: 68
From: Montreal, Quebec
OK, I need to get this cleared up. If I install the DOHC head on my SOHC block, will my compression ratio be higher than the compression ratio on a DOHC engine.
On the Miata site they state that the SOHC pistons are dished therefore create a lower compression than DOHC pistons.
On the Miata site they state that the SOHC pistons are dished therefore create a lower compression than DOHC pistons.
August-15th-2003 | 02:55 PM
#30
The beige beast!
Joined: Sep 2002
Posts: 300
From: Mt. Pleasant, NC
Originally posted by Protoss
most of the compression ratio is effected by the piston.
Miata BP piston compression 10:1
DOHC pistons produce a compression of 9:1
SOHC pistons produce a compression of 8.9:1
if you put a DOHC head on a SOHC pistons, you will get a lower compression ratio, than that of the stock DOHC. therefore more clearance from the valves. but if you go the other way, you MIGHT, maybe have valve issues.
most of the compression ratio is effected by the piston.
Miata BP piston compression 10:1
DOHC pistons produce a compression of 9:1
SOHC pistons produce a compression of 8.9:1
if you put a DOHC head on a SOHC pistons, you will get a lower compression ratio, than that of the stock DOHC. therefore more clearance from the valves. but if you go the other way, you MIGHT, maybe have valve issues.