Volvo 8.1 Gi

imported_Tpeck · Mar 2, 2007

Anyone know why the Volvo 8.1 Gi engine (375 php) would have a max RPM of 4600 compared to the 8.1 GXi (420 php) of 5000? With the same gear ratio, wouldn't the GXi perform a lot better? As a comparison, the same 8.1 liter engine in the Mercruiser 496 (375 php) and 496 HO (425 php) don't have big differences in max RPMs as the Volvo do they?

QC · Mar 2, 2007

Re: Volvo 8.1 Gi

Well a couple of things to consider here:

1) The simplest way to get more horsepower is more RPM with the same fuel flow per firing event. Just make that event happen more often i.e. more RPM

2) I believe each (Merc and Volvo) did their own fuel mapping (the actual fuel rates that the injectors squirt when you tell them to). Merc may have had a bigger budget for the higher rating, so they spent more time on it.

3) They really aren't that different. The Merc 375 is rated at 4400 - 4800 and their 425 is rated 4600 - 5000

4) This is prop-shaft hp and maybe the Volvo drive is less efficient to turn internally (requires Duoprop) so it eats a little power requiring a higher RPM to get 5 less php . . .

What's the specific concern? Longevity? Fuel? Curiosity? Other?

imported_Tpeck · Mar 2, 2007

Re: Volvo 8.1 Gi

Thanks for your thoughts QC. I am getting a boat with the 8.1 Gi 375 php that has a recommended RPM range at wide open throttle of 4200 - 4600. At the low end of this range I am a little concerned about longevity. In addition to being curious too, I wonder if performance is affected. Maybe it's an incorrect perception, but it seems to me that a boat with a motor turning lower RPMs will be slower out of the hole than one with the same horsepower, but with higher RPMs.

Don S · Mar 2, 2007

Re: Volvo 8.1 Gi

Would sure help to answer your questions if some idea of the year, full model number or SN of the engines were.

I don't recall ever seeing an 8.1 of any kind with a 42 to 4600 WOT.

QC · Mar 2, 2007

Re: Volvo 8.1 Gi

Tpeck said:
it seems to me that a boat with a motor turning lower RPMs will be slower out of the hole than one with the same horsepower, but with higher RPMs.

That is true for prop selection purposes. But as far as comparing two of the same engines, with the same hp rating, but one rated at a slightly higher RPM really doesn't tell you anything about the hole shot. The one with the most torque at the point that the boat has to struggle to get up and out, will have the better hole shot. This assumes all other things are equal, boat, weight, drive, props etc.

imported_Tpeck · Mar 2, 2007

Re: Volvo 8.1 Gi

It's a new boat (2007). The Volvo operator's manual on page 31 lists it as an 8.1 Gi-H(F) motor with a WOT RPM range of 4200 - 4600. Engine serial number is 4012210765.

QC · Mar 2, 2007

Re: Volvo 8.1 Gi

Most here will disagree with me, and most have more experience with Marine applications, but based on the words in the manuals that I have read along with my understanding of wear after 27 years in the engine biz, propped @ 4200 WOT with your max load would probably be the longest lived as she will be turning less RPM at all speeds as well. That's my take on it . . .

With that said, IMHO unless you are in a commercial application, longevity is not really even a discussion if she is maintained properly.

Don S · Mar 2, 2007

Re: Volvo 8.1 Gi

I can't say why for sure, you might want to call Volvo and ask them. But one thing I did find out is the Gi is SAV rated and the GXi is not. Could be the engine had to be slowed down and detuned a bit to meet the European SAV emission standards.
I know SAV has to do with emissions standards in Europe, but that is about all I know about it.
Personally I would prop it to 4600 or as close as I could get.

QC · Mar 2, 2007

Re: Volvo 8.1 Gi

Don said:
Personally I would prop it to 4600 or as close as I could get.

I agree for flexibility. I was trying to be specific as far as the question of longevity is concerned. I mentioned that most would disagree . . . Most mention valve seat wear and/or tuliped valves and holes in pistons. If that's true, then I don't understand why OEMs would put the low end of the range in print. I think one thing that you and I agree on Don, is that the manual(s) is your best friend . . .

Not looking for a debate here. I just wanted to 'splain myself.

imported_Tpeck · Mar 2, 2007

Re: Volvo 8.1 Gi

I agree - sounds like good advice. I will try to prop it close to 4600. Thanks guys.

yamamarinetech40 · Mar 3, 2007

Re: Volvo 8.1 Gi

You do realize that the final drive will decide which prop will spin faster and load on engine....and I'll say it again, the Volvo uses a cone clutch instead of dog....much better

Volvo 8.1 Gi

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