I was considering selling my 850 wagon and upgrading to a V70. I was curious about one thing - why does the V70 require premium fuel? What changes were made to the engine or was it something else? Isn't the V70 base motor basically the same one from the 850?

 

Yes it's the same. I believe the 850 has the same octane rating. Recommended is 91, minimum is 87.

https://www.customers.volvocars.com/...4_1.html#pg4.2

 

For .02 cents worth--I've noticed better gas mileage and performance on higher octane. Mine has 188k (98 v70 awd turbo), and definately likes 91 better. I don't always put 91 in, but offsetting better mileage it does seem to be a wash to use the 91 vs lower grade. So I usually figure "why not?". Doesn't seem to be costing me more considering slightly better mpg... Now, the benefit between higher vs lower octane seems to disappear if you run the car hard. It seems to drink both octanes at the same rate if you are pushing it. But at lower highway rpms there is a difference. Anyway else?

 

If by base motor, you mean the N/A or non-turbo, then yes they are basically the same; compression ration is 10.5 to 1 which is relatively high, giving increased performance and efficiency.

So you should get a bit better performance and mileage with the higher octane because that's what the engine is designed for. But you can run regular fuel because these cars have knock sensors which allow the ECM to retard the spark enough to avoid knocking...

 

Hello, In the owner's manual you'll see the turbo models actually have a lower compression ratio (~8.5:1). Turbos always run hotter. Higher octane is needed so only the spark ignites the fuel/air mixture; not the high compression of a screamin' hot engine. Using lower octane gas and having your knock sensors retard the spark defeats the purpose of the turbo. I feel you'd lose not only the crisp operation designed into your engine but the cooler timing will bring your mileage down. What I don't know is the limits of spark retardation. You can stomp on the accelerator of a turbocharged car and rattle it to death going uphill on regular gas. If a $3 to $5 premium/15 gal. fill-up weighs on you then the conclusion is straightforward: You can't afford to own a turbo. No threat to anyone's macho here; it's just the way it is. Kira

 

GeorgeAndKira, Thanks for that elaboration. Makes sense. As a rough number off the car's avg mpg computer, I'm getting about 2 miles to gallon more on 91 plus octane. Definately didn't buy the car for gas mileage, though. It's not a 30mpg or more car, no matter how you drive it or what octane you put in it.

 

As for the 91+ octane... I've alway wondered about that and recently seen a car TV show with some guy with a hot rod (old vett) explain that issue. The thing is that some cars with smaller motors design them to preform better by raising the compression ratio. If you use lower octaine gas in a car that runs compression over 9.0:1 it runs the risk of knocking. What the guy said was that a lot of car companies, mainly the ones that produce preformance cars, have high compression, some over 10.5:1.

What happens is the fuel injector sprays fuel in the chamber and the fuel is waiting for spark to create the explosion. Well, as the compression in the piston gets greater, lower octane gas has a lower flash point and may ignite before the spark is given. If this happens the chamber will get a premature ignition and cause the piston to receive the entergy before the spark gives it causing a knee jerk reaction that is called KNOCK (some men experience this with their lady friend, not good for that situation either). Now Volvo's have 2 knock sensors and do their best to avoid this situation/problem. The US Gov website advises people to knotch up the octane if they suspect or feel their motor is knocking, one level at a time. Volvos do a good job at preventing knock, however, if one of those knock sensors malfunction and your motor starts knocking, you can cause problems with your motor. I can't prove it, but I bet some people that are complaining about their car missing or running poorly have knock problem. The thing is that they would have to pump 91+ and run out the 87 to figure it out.

Now, I always thought that it was more important for a higher preformance motor to use 91+ octane, like the turbo since it put out more hp. Well, the fact of the matter is that the N/A motor is actually a higher compression motor, it has to be just to make it peppy with no turbo. If you look at the Volvo site, you will see that the specs of the motors are as follows when it comes to compression:

2.3 T3: Compression ratio 8.5:1
2.5 T: Compression ratio 9.0:1
2.5 NA: Compression ratio 10.5:1

https://www.customers.volvocars.com/..._133.htm#pg135

The 960 is even worse. It runs Compression ratio 10.7:1. I put some gas in the car one day and it would not drive over 45 mph until I ran down that gas.

Of all the cars, the T5 has the lowest compression.

Your car, you can do what you like. I do run 91+ mainly because the owerns manual recommends it and I think I get better mileage with it. You may not experience the knock with low octane gas unless you put your foot in it, not sure if there is any immediate price to pay but you will end up with burnt valves and maybe even worse, bent valves. I've also read about people compalining about their timing being off. I wonder if they are knocking. I also see a lot of people complaining about their car not wanting to rev above 4k. Makes me wonder if they put some octane boost in the tank will it make a difference.

 

Mostly good post rspi but have to clarify above comment. Changing cams will not affect compression ratio; that has only to do with stroke, bore, and compression chamber volume in the head.

Guys go to NA cams on the turbo cars for better top end engine breathing (i.e. performance); they have slightly more lift and duration than stock turbo cams. More top end but a bit at the expense of low end torque and throttle response.

At the end of the day the engine's power has most to do with the pressure of the charge just before ignition; both higher compression ratio and turbo-charging add to that pressure. But of course knock happens when that pressure gets too high.

That's why knock sensors are critical on these engines; allows them to run (though not most effectively) on lower octane fuels and still achieve better performance when you do have higher octane in the tank.

 

Just wondering, if the T5 has lower compression(8.5:1) does this mean it's non-interference design?

 

The compression doesn't have anything to do with the basic design of the motor per-sae, it's more to do with the stroke and pressure that is built up on top of the piston as it raises to the top near the spark plug.

 

Is that yes or no to my question?
I don't know how lower compression ratio on T5 would be achieved without lowering the piston stroke.

 

Wheather a motor is a interference design or not has nothing to do with compression.

As for the T5 motor having lower compression than the NA motor, those are two different motors. Different displacements and stuff like that. Probably has different piston sizes, etc. The designed the NA motor to have higher compression to help it produce better. Which is not necessary with a turbo charged motor.

I'm not an engineer so if you need more information that this, you may want to read around on auto motor designed or something.

 

Using 87 octane in an engine that calls for premium and relying on knock sensors to retard timing richen the mix is asking for trouble. Use the gasoline recommended in your owners manual.
Both motors are interference motors. Lower base compression is needed in a turbocharged app so it can safely tolerate boost.

 

BPatrick,
You are sure the T5 is an interference motor? This is important information for T5 owners.

Compression ratio is not calculated from boost pressure or turbo or normally aspirated.., It's just ratio of volume displaced by piston from highest to lowest position. So if it's lowered from 10.5:1 to 8.5:1, either -
1. stroke stayed the same but crankshaft center moved down(highly unlikely)
2. stroke shortened, keeping bore the same size(most likely)
3. stroke shortened a little AND bore reduced(based on your feedback)

I don't have access to factory manual to ascertain any of this information.

If it's case #3, indeed it may still be interference engine. If it's case #2, I think T5 is a non-interference design.

Does anyone know if there has been valve clash on T5 models?

 

Tony, how many times do we have to tell you??? The 2.3L B52354T motor IS a interference motor. If you don't believe us, pull your timing belt off and turn the key to start the car and you'll be replacing bent and broken valves.

 

Ok. I searched for hours but could not nail down the specifics(did learn the T5 piston is different).

Don't mean to cause trouble here I accept the T5 is an interference engine.

 

OK trouble-maker (just kidding ) i will spell it out for you...

Compression ratio is really ratio of volumes in the cylinder: volume at BDC (bottom dead center) compared to TDC.

Not that the 2.4L N/A and 2.3L turbo engines are identical (as has been explained) in bore and stroke, but even if they were, there is the difference in the combustion chamber volume. Also consider (as you found in your research) that the tops of the pistons can be different. The higher compression piston will have a dome on it (displacing more volume into the combustion chamber in the head) than the lower compression piston that may be flat topped or even have an inverted dome.

I.e. it's mostly about the piston top shape. There are engines out there that have identical bore and stroke but have different compression ratios because of only differences in the piston top shape.

 

You have to give credit to the Volvo engineers for using the same design and varying just piston and cylinder sleeve to get different power configuration. And from what I see the basic design is a robust one with some generating over 350hp without issues. Kudos for good engineering.

I appreciate the feedback without hostility. It seems to be a good forum to learn and share.

BTW, sorry to OP - I seemed to have hijacked the thread.

 

Hello, "You have to give credit to the Porsche engineers..." Kira

 

Interesting discussion...One element not mentioned is the ethanol in gas. It significantly reduces engine performance hence mpg as well. Here in NC we can buy ethanol free gasoline. I always use 87 ethanol free and I do not nor ever had any knocking or lack of performance. I'm driving my '97 850 T5 Wagon and it loves the ethanol free stuff.
The difference in the way the engine performs between ethanol gas and ethan free gas is like night and day. If you can find it its well worth it IMHO. Check out this site for ethanol free gas stations nationwide. BTW at the bottom of the site page is every state that sell it...not all states do...sorry NJ...
Ethanol-free gas stations in the U.S. and Canada