Diff between SOHC and DOHC
 

Hello
I am a newbie looking to buy a 91 740 Non Turbo. In researching I see there is a big difference between a DOHC and a SOHC in that the DOHC is an interfearance motor. Is the DOHC easy to spot as I think it might be? I am a VW guy and when I see two valvle covers I would assume a DOHC.

Would the 740 of this year likely to be a red block? I havent looked at yet.

Again search here on the forum. Interference has nothing to do with SOHC & DOHC

Actually, there is some truth to it though. Most red blocks ( not sure if all ) were non interference engines. So if the timing belt would break driving down the road, you could replace the belt and keep driving the car. If the timing belt breaks driving down the road, you will bend valves and cause major damage because they are inference engines. Easiest way to check will be to see if it has 2 cam gears or one. All white blocks are dohc.

Yes that is correct. The reason for new cars being interference is that they are a lot more efficient, but then if the timing belt snaps you have very expensive paper weight.

That being said interference absolutely has nothing to do with SOHC & DOHC. And if I am not mistaken, the port design/numbers has a relation with the SOHC & DOHC of that particular engine.

All I am looking to understand is how to correctly identify a DOHC vs a SOHC. As I have researched this site I see the DOHC is an expensive fix when a timing belt breaks but the SOHC is no big deal; tow it home and replace the belt. Not having seen either motor close up, all I want to understand is how to best correctly identify so I can avoid the DOHC. I will keep researching the site further.

It's easy
one cam gear =single over head cam
Two cam gears= dual over head cam

The reason why I said search is because there are a bunch of people who pop in here with 1 or 2 posts - get their answers and never post back. Which in turn leads to a dozen threads being left incomplete because the OP will never show up. They need to put that into the forum rules. We are all here to help everyone out and share results.

Getting back to your question.You can have a SOHC with interference engine also. It's the interference and non interference that decides if timing belt breakage will cause bent pistons. Non interference is usually on older cars and breaking a timing belt will result in loss of power. But no other associated damage will happen.

The SOHC and DOHC is related to the number of valves, and how much pressure is present for the air+gases mixture going in and out and the related internal plumbing/passageway. Generally speaking though, DOHCs are present in higher output engines. And also, DOHCs can sometime be a bigger pain to work on compared to SOHCs (take a look at the Subarus- the '98 2.5L DOHC is horrible to work on while the '00 2.5L SOHC is much much better. Packaging remained more or less the same).

That being said, I don't think visually looking at an engine will tell you if it's a SOHC or DOHC.

Let us know when you buy the Volvo. Rest assured they are reliable and very simple to work on. Oh and very cheap too!

240-fan, you've spouted a bunch of wrong.

In the *SPECIFIC CASE OF THE VOLVO RED BRICK ENGINES USED ON 740/940*, the B234F 16V DOHC engine are interference, and the US model B230F 8V SOHC are non-interference.

the valve cover of the DOHC is very obviously different. Volvo called the B234F engines 16V rather than DOHC, btw.

16V:
http://www.vandsautodismantlers.com/...140_engine.jpg


the 16V engine revs freely to higher RPMs, and is mated with lower gears in the transmission to give better performance. its got counterbalance shafts so its smoother at these higher RPMs.

16V engine parts are harder to find, doing engine work on them can be more challenging, and if the timing belt dies you WILL bend valves and/or crack pistons.

Whoops I stand corrected. I was talking about the b230F SOHC that's on my car and I believe every late generation 240 and base 740s and 940s (atleast from what I have seen on my scavenging trips to the local junk yards).

Where did they use the B234F?

the B234F is the DOHC version, marketed as 16V, used on select 740/940 GLE models. got nearly as much HP as the B230FT turbo, with better gas mileage. Was generally mated with the A72L transmission, which is a variant on the heavy duty A71, with different ratios for 1-2-3, and a lockup torque converter.

And this is the SOHC B230F for the OP.

I am going to say that b230f is much more common to find and they last a while with very little maintenance.

and this is a B230FT from a 1992 740/940 turbo... note in 1992 they dropped the mechanical fan in favor of an electric fan, and increased the size of the radiator on the turbos.

http://pierce.smugmug.com/Misc/1992-...MG_5856-X3.jpg

this is the prior (1991) B230FT, from a 940SE (960 turbo) which my son used as a desert jeep, so its /filthy/

http://pierce.smugmug.com/Misc/1991-...MG_5860-X3.jpg

you can see the major differences in the radiator, turbo air plumbing, and fan... also old style coolant overflow tank.

Down the line, is it possible for switching to that 1992 bigger radiator plus the electric fan (in my '91 244)? How involving will the wiring for the electric fan be while switching from the mechanical fan?

I don't think the 1992+ 940 radiator will physically fit in a 240. you can retrofit an electric fan, of course. Its a 2-speed fan with a control relay (mounted up near the radiator), that gets inputs from high and low pressure-stats plugged into the A/C condensor manifold.

There's also a thermostat in the radiator (I believe up near the right-side hose connections) which is connected to the 'high speed' side of the relay.

In lieu of those pressure-stats, its probably OK to wire it instead so if your AC is on, the fan is on high, otherwise on low, but connect that thermostat to 'high' too. I noted the last replacement 240 radiator we got had the thermostat even tho the car didn't use it.

the fan relay has 5 pins. always-on power from the battery goes into 1A (with an inline fuse). 1C goes to the green wire to the fan (low speed). 2C goes to the red fan wire (high speed). 1B is wired to the low speed pressurestat which goes to ground. 2B goes to both the high speed pressurestat and the thermostat which both go to ground (so if either is 'on', 2B is grounded).

Thanks guys- this is the type of info I was looking for. From Pierces 1st picture I can see the 16V valve cover is significantly different from the standard 8V. This helps me know what to watch out for as I dont want the "Damocles" sword of a broken timing belt hanging over my head if I dont want to.

note that 'Damocles sword' is common to virtually ALL modern cars with high efficiency engines, be they toyota, honda, ford, anything really.