Performance expectations of Robert Davis 3.0l

[alaskajosh]

I don't get nearly the performance boasted by other users of the 3.0l Mercruiser conversion.

I'm hard pressed to maintain 55mph (3000rpm) on the flats and have to drop to 2nd on some hills. In other words about what a LR 2.25l might offer. The gas tank also seems to empty rather quickly, though it's been awhile since I've actually calculated it.

This is the non-cross flow model in a maybe heavier-than-average 109 (winch, winch bumper, extra fuel tank, etc). 3.8 R&P give me an engaged overdrive-like gear ratio.

I've done or checked anything I know to. Trying to capture more power through timing quickly results in detonation.

Compression was 140, 120, 140, 150. I poured a little 30 weight into that #2 cyl and it's compression reading increased to 170.

Are those compression numbers suspect? What other things might I consider?

Thank you for your time--

Kind regards, Josh

[LaneRover]

Josh

Let me start by saying I have absolutely no direct experience with the Robert Davis conver or the 3l Mercruiser.

I have a S1 107 SW that someone shoehorned a Buick V-6 into and with a crappy junkyard single throat carb of unknown ancestry it easily cruised at 70.

Is you motor carb or injected? Could you have jets that are too rich if it is a carb model? If injected, is it throttle body or direct injection? Are all the connections made correctly? Do you know this engine ran well beforehand?

I don't know enough about engine compression and adding oil to say that the difference you got was substantial. Was your engine new?

Best of luck, I am sure that you will get much better answers from others, all I can say is - yah that doesn't sound right!

[jopa]

Are you sure you have the 3 liter crank and not a 2.5 crank...and what cam are you using- was this one of his built engines or did you build this up yourself?
Many years ago I bilt one of those 3 liter engines for a 109 truck. It replaced a badly orchastrated 6 cyl 250 cu in conversion.
I was impressed- very similar driving characteristics- actually had better acceleration off the start then the 6 (which was tired) It handled highway speeds with no issues-65 to 70 mph was not a problem. It could have gone faster- but I didn't push it. I ran 3.54 gears. It was a working truck- towed many land rovers on trailers to my shop, hauled very heavy loads of fire wood, etc. I was quite happy with the converion.
Something does not sound right- it sounds like the engine is not putting out enough power- more on the lines of a stock 2.5 chevy 4...what carb are you using? What distributor?- J

[Leslie]

Not quite but getting close to a decade ago, at MAR one year, I hopped in the back of Robert's own Series as he was scooting around from campsite to campsite, checkin' in w/ various folks. There was *no* mistaking it for a stock Series by *any* stretch! I don't care if it was an 88 instead of a 109, that thing could move in ways a 2.25 can't even dream of.

No idea on compression, or build/tuning advice for that engine, but something's amiss, it should really make a difference.

Best o' luck...

[n6zt]

I have had two RoberT Davis conversions. The latest in a 109 Dormobile in which I replaced a 6cyl. It performed better than advertised. I had a Weber 2 bbl carb, an ashcroft transfer case, and could hold 60mph going over the Cascades here in Washington fully loaded. It easily kept up/ exceeded traffic everywhere; even in the mountains. I also installed disc brakes up front to keep stopping even with the speed increase. I was very happy. There must be some basic problem with your engine if it is not performing. They are very easy to diagnose.
Jim

[rovers2a]

another stab in the dark....did you get the engine from a boat that had twin inboards? cause some twins have counter rotating props, well i guess one engine is running backwards. i had this in my wellcraft. typical ignigtion timming and high tension lead install on one engine. then completly wackd out high tension lead install on the other engine. mind you these engines were big block v8s.... might also look into the gear ratio of the out drive of the boat. type of cam that was fitted and what not. i could be totally out to lunch but thought id mention it....

[rovers2a]

some info here https://www.fordmuscle.com/forums/all-ford-techboard/452983-differences-between-automotive-marine-302w.html

[rovers2a]

http://www.perfprotech.com/blog/articles/mercruiser-marineengine-timing im trying to look for counter rotation.. bear with me

[rovers2a]

here is the firing order for counter rotation. hope it helps if that is even your problem, scroll down a bit http://answers.yahoo.com/question/index?qid=20081127194325AA1yeK1

[adkrover]

I have a Robert Davis 3.0 in my '65 109 with winch and a heavy bumper and it is amazing. Plenty of power and amazing low end torque. I can set my gearbox in low and just let it crawl over logs and stumps. On the highway, I can cruise 60 with power to spare. Do you have a genuine Robert Davis or something else? Just putting in a Mercruiser 3.0 is not the same.

[alaskajosh]

Thanks for all the great responses. I'll answer what I can.. and that's not much...

This is a Weber 2bbl carb 32/36(??). It's a typical GM HEI cap. Remember, it's NON-crossflow which I've heard is not as quick.
Aside from that I can't tell you much else. It was all previous owner stuff but it's my understanding that he (the PO) dealt with RD directly.

As far as identifying the crank or cam I wouldn't know where to begin aside from tearing the engine down and looking for casting numbers.. which doesn't seem like the "next step" at this point. At any rate I feel comfortable enough that this was a RD built engine that I doubt those things were spec'd or installed incorrectly.

Rovers2a: Very interesting point about the possible reverse firing!! I haven't visited those links yet (my next stop) but I sure appreciate your efforts, sir.

Kind regards, Josh

[n6zt]

BTW, both of my engines were non-crossflow, and both have gobs of power. I built one, and RD built the other. I did switch to 36/36 Weber as I did not like the progressive; seemed like it did not have enough gas with the 32, and the 36 was late kicking in. Also, Webers are picky about fuel pump pressure.
Jim

[adkrover]

I think it would be well worth your time to contact Robert Davis yourself and get the info on your motor. I have spoken with him several times about mine and he is always very helpful. I am pretty sure he can still remember every engine he has ever built. The first time I spoke with him, he wanted to know who I purchased my truck from (previous owner installed the engine) because there are apparently engines out there that people claim were his build but really are not. That's why I ask if you are really sure that he built it. One sure way to find out would be to call him. If it is a genuine RD then I am sure he can help you get it running strong again.

757-421-3504 was the last number I had for him. maybe someone else has one.

[zayante]

Carb setup is critical. I once had an FJ55 with a 2-bbl Holley and had to spend a lot of time playing with jets and the accelerator pump to get good power without flat spots.

Compression may be a contributor. The rule of thumb is that a 10% or greater difference between high and low cylinder is the beginning of concern.

Cam or cam timing might be an issue. I don't know what cams are supplied with either Mercruiser or 3.0 forklift engines, but there may be some that are not optimum for road driving.

It could also be something random, like an intake manifold leak. Let us know what you find out.

[zayante]

You might also check for exhaust restriction. An intake vacuum test might help pinpoint problems. See:

http://www.v8sho.com/SHO/IntakeManifoldVacuumTest.htm

[daveb]

willing to bet your throttle is not opening fully. BTDT

[alaskajosh]

willing to bet your throttle is not opening fully. BTDT

Thanks for weighing in, Dave.
I haven't driven it since summer so my memory is hazy but I'm sure my rpm's on the hwy were up around 2700+... shoot, I'd better not say for certain. I just don't remember. But suffice to say things seemed to be opening up.. Is that what you mean?

Thank you for your good advice, Zayante. I think I'll give that vaccuum test a shot--

What would you guys do with those compression numbers? 140, 120, 140, 150. That was a cold engine, BTW. I agree it must be at least a contributing factor.. Does it warrant a rebuild? What would you do?

Thank you, gents!

[yorker]

I would exhaust all other possibilities before I even considered doing the engine. Check the throttle linkage and make sure you have the full range of motion when you press the gas pedal. Make sure you don't have any issues with the timing. Make sure there is no crushed section in your exhaust. Check your fuel lines and make sure there are no issues there. If those are cold compression readings I wouldn't be too worried yet. Do a leakdown test yet?

[alaskajosh]

I thought I'd follow up on this issue as it unfolds and resolves.

Investigating, I did find a pretty bad exhaust leak from a badly cracked manifold. That wasn't helping matters.

Then, long story short, I upgraded to the carb that RD recommends. The Weber 38/38 in place of the 32/36 the po had installed. I've only driven it a little since but I think I can say I'm well satisfied at this point. It's pretty quick and only bogs down a little on the steepest, longest hills which I don't hold against it.

With my 3.8 diffs I run approx. 3000rpm at 60mph. It sounds happier at around 2800, 58mph, which is a fine speed for a vehicle of this nature. It would easily do 70+ if I wanted to listen to it scream, which I don't.
It jumps off from a stop with plenty of zest.
Bear in mind mine's much heavier than the average 109 with a winch bumper, camper top, and more.

Haven't been off-road since the upgrade/tune but imagine it will only be better and it was never bad.

I'd enthusiastically recommend this engine upgrade. As a GM family engine, it's understood and accepted at you local parts counter or junkyard and a neat fit.

Edited to add: I spoke with RD today and he said there is a lot more power there to be tapped.