SL320 V6

[nickg]

I am thinking of buying a late '90s SL320. I have been told that the "V6" engine is the one to buy.
Can any of you tell me when the V6 came in, and what, if any difference there is between the V6 and the non-V6 - performance, reliability, desirability ??
Any help would be very gratefully received as I don't trust a dealer to give me a straight answer !!

 

[kes 300]

i have a 300 sl straight six i hit the pedal it goes faster .i dont find it  underpowered as some experts suggest but on the other hand ive never driven performance cars so i have nothing to judge it on.
i've been led to believe its eaiser to work on than a v6 the straight six was manufactured  from 90/92 i believe.  the advice i saw most often posted when looking to buy was make sure it has  fmb/sh mine did however check the repair list below  and that has all happend in the first 3months so i would say get one with a warranty and pay the extra to extend it warranty holdings have footed the bill for 90% of the work done on mine  ,hope  i answered  your question by the way despite the faults i still think its the muts good luck?

 

[nickg]

thanks very much for the info.
I drove the car on friday, bought it and am collecting it on saturday.
I can't say I found it underpowered - although I did test drive a sL500. That was a v quick car, though the 320 has enough power to satisfy, at least for the time being.

 

[Arnie]

The V6 engine is part of Mercedes "modular range". It's a good engine, but it's based on the V8 and as such has a 90 degree vee rather than the ideal 60 degree (for a 6-cyl V). Therefore, it requires (complicated) balancer shafts. The ideal configuration for a 6-cylinder is a straight-6, which naturally gives the least NVH. However, because a V engine is shorter it allows better packaging in the engine bay (shoter bonnet). Also, being a 90 degree design, allows commonality of parts with the V8. Essentially, it's cheaper for Mercedes to produce and, as V12s are out of favour these days, shorter bonnets = more interior space. A 90 degree vee also permits a lower bonnet height.

In purely engineering terms, there are only two configurations, which give a perfectly balanced engine (in terms of first and second-order moments). These are a straight-6 and a V-12 (=2xstraight-6).

Really, the V6 (90 deg) exists more because of design-economy reasons rather than engineering ones.

The previous Merc straight-6 was a great engine and it also had space for a variable, resonant-intake manifold, missing on the V6 and featured variable valve timing. Three valves per cylinder is also driven by reducing emissions on start-up rather than increasing performance.

Given the choice, I would go for the straight-6.  Certain reports I've read liken the Merc V6 to a 4-cyl in terms of smothness. That's why BMW will never shift from a straigbt-6.

Hope that answers your questions.

 

[steveperry]

Just out of interest Arnie, why does the optimum angle on a V6 have to be 60? Also I presume the optimum for a V8 is 90, is that correct?

Thanks,
S.

 

[Arnie]

Steve,

Yes, the optimum angle for a V-6 is 60 degrees (or 120 deg, which is not practical). This will balance out all the first order forces but leave some second-order forces. A cylinder will fire every 120 deg of the crankshaft and 60 degrees is a multiple of this so will give smooth, balanced rotation. (90 deg is not hence balancer shafts). A 60 deg V will have a higher profile than a 90 deg V, with less space between the two banks for air intakes etc, which is possibly one reason fo MB using 90 deg V.

It is also possible to have a flat, boxer-6, which will be perfectly balanced, like a straight-6 (no first or second order moments).

The optimum angle for a V-8 is, likewise, 90deg, with a cylinder firing every 90 deg.

[A very useful little book to have is the BOSCH AUTOMOTIVE HANDBOOK, which has all these details as well as other very interesting information.]

 

[Arnie]

Steve,

Yes, the optimum angle for a V-6 is 60 degrees (or 120 deg, which is not practical). This will balance out all the first order forces but leave some second-order forces. A cylinder will fire every 120 deg of the crankshaft and 60 degrees is a multiple of this so will give smooth, balanced rotation. (90 deg is not hence balancer shafts). Basically, the forces exerted by a piston moving downwards need to be balanced by those of an opposite piston moving up, all the way along the crankshaft such that there are no net moment forces.

A 60 deg V will have a higher profile than a 90 deg V, with less space between the two banks for air intakes etc, which is possibly one reason fo MB using 90 deg V.

It is also possible to have a flat, boxer-6, which will be perfectly balanced, like a straight-6 (no first or second order moments).

The optimum angle for a V-8 is, likewise, 90deg, with a cylinder firing every 90 deg.

[A very useful little book to have is the BOSCH AUTOMOTIVE HANDBOOK, which has all these details as well as other very interesting information.]

 

[steveperry]

Thanks for the info Arnie

I learnt something new.

S.

 

[Tony]

Very interesting and informative posting Arnie - I won't be able to look at my V6 in the same light again.

 

[Matt Crooke]

I wish i had a V6 to look at, period........sniff...sob.....!!

 

[fuzzer]

but you get more mpg with your 190

 

[Matt Crooke]

This is true

 

[Arnie]

Guys,

That's not to say that a Merc V-6 is a bad engine. It's just that as with most things these days certain aspects are compromised for economic and other reasons.

Certainly the balancer shafts help restore the smothness and, V6 or not, it's still a Mercedes.

I also suspect that Merc's biggest market focus has traditionally been the USA and here V8's and V12's are more the norm. Therefore, perhaps the V6's and 4-cylinders are not given the same importance. Although, this is probably changing now.

Perhaps also people these days are generally not so much concerned about the engineering under the bonnet, as long as a car handles well, goes well (reliably) and has all the modern goodies inside. The heart of any car is still the engine, but most modern engines are more than good enough in performance terms and it's easier to get distracted about the sat-nav inside tha car rather than look for chromed exhaust manifolds and shiny cam covers.

Engines, these days tend to have a lot more plastic parts, parly for heat insluating reasons, but more for cost. The engine-block castings and cam-covers on newer engines are also not as clean and well detailed as they used to be. But, they sit under a pretty plastic cover with a three-pointed star and most people tend to look no further. (Compare the details of a Merc AMG engine with those of a BMW M engine.)

So, straight-6 or V6? It really doesn't matter. At the end of the day they perform equally well. Cars these days are about much more than engineering purity.

 

[Arnie]

Guys,

That's not to say that a Merc V-6 is a bad engine. It's just that as with most things these days certain aspects are compromised for economic and other reasons.

Certainly the balancer shafts help restore the smothness and, V6 or not, it's still a Mercedes.

I also suspect that Merc's biggest market focus has traditionally been the USA and here V8's and V12's are more the norm. Therefore, perhaps the V6's and 4-cylinders are not given the same importance. Although, this is probably changing now.

Perhaps also people these days are generally not so much concerned about the engineering under the bonnet, as long as a car handles well, goes well (reliably) and has all the modern goodies inside. The heart of any car is still the engine, but most modern engines are more than good enough in performance terms and it's easier to get distracted about the sat-nav inside tha car rather than look for chromed exhaust manifolds and shiny cam covers.

Engines, these days tend to have a lot more plastic parts, parly for heat insluating reasons, but more for cost. The engine-block castings and cam-covers on newer engines are also not as clean and well detailed as they used to be. But, they sit under a pretty plastic cover with a three-pointed star and most people tend to look no further. (Compare the details of a Merc AMG engine with those of a BMW M engine.)

So, straight-6 or V6? It really doesn't matter. At the end of the day they perform equally well. Cars these days are about much more than engineering purity.