Venturing into performance upgrades for an AQ271C

[Scott Danforth]

Have an 88 AQ271C with a DP290 drive. now the quest for more power. going to take this in phases: update stock, simple bolt-on mods, then ??? plan is to get the boat to 50+

starting to stock-pile a few parts.

first things - simple carb go-thru. new power valve, gaskets, and maybe play with the vacuum diaphram springs and the jets.
after that - 1.6:1 rockers on stock heads. Anyone know how close the current volvo cam brings the valves to the pistons? would hate to violate the fundamental first law of physics. If not, dial indicator time.

this winter the plans are to source a decent set of heads, a different cam, and a different intake. May look into a set of headers from Custom Marine. Just the weight savings alone there may make the boat faster by 2MPH

Any other suggestions?

last resort is to build a 383.

 

[Maclin]

Re: Venturing into performance upgrades for an AQ271C

Your best bet for a first time at bat home run is the 383 with the same cam and heads in my opinion, see my caveats below. The AQ271C engine is already pretty "hot" as far as camming, probably right at the max before you hit driveability and reversion issues already. My 22 foot cuddy can hit 50+ with B7 props in "stock" specs. Your boat (assuming this is for the Rogue) would best be served with just more low end and midrange grunt which a 383 would automatically provide without any special camming. Then you should be able to prop up some from where you are now.

I am assuming your engine is the one with the Holley 4bbl on it. If so, then in my opinion the cam and topend (heads, intake, carb) from the engine you have now can feed the 383 just fine. The carb is a 735cfm and is pretty rich if still in stock specs. These same carbs were used on the '65 or '66 Shelby 427 Cobra engines. The secondary vacuum control spring is the key for it being able to keep up with the 383.

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

thanks for the comments, however it did not address my first question on valve to piston clearance. If no one can answer, then I will pull out the dial indicator and pull the valve springs on one cylinder and simply measure.

The stock intake is also a bit restrictive so that is on the short list of items to get replaced.

I do have a set of heads coming from a friend of mine that races (have to love end of season). the stock GM heads from the mid to late 80's were not the greatest things in the world. Vortec's are better, however are limited on cam lift with a stock head. Aftermarket has a bunch of options, which are now available cheap. Reversion is easily overcome with thru-hull exhaust if I need, however if I keep an eye on the lobe separation, one can usually go much "hotter" on the cam than the stock AQ271C.

330hp is easily achieved with the existing engine with bolt ons.

I talked to Keith this afternoon at Cruisers and now have knowledge of what the hull can do for top speed, so I have a goal which is still to optimize the 5.7, then go to a 6.2 or maybe even an LSA

Again, thanks for posting.

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

found the specs that GM used for the 260hp 5.7's

202 duration intake
214 duration exhaust
395" lift intake
404" lift exhaust
112 lobe separation.

have not found the specific cam yet for the 275hp AQ271.

however looking to go with a comp cam

either the CL12-236-3 - Comp Cams 'Xtreme Marine' Hydraulic Flat Tappet

Operating Range: 1300-5500 RPM
Duration Advertised: 262 Intake / 268 Exhaust
Duration @ .050'' Lift: 218 Intake / 224 Exhaust
Valve Lift w/1.5 Rockers: .462'' Intake / .477'' Exhaust
Lobe Separation Angle: 112

or the CL12-232-3 - Comp Cams 'Xtreme Marine' Hydraulic Flat Tappet

Operating Range: 1300-5000 RPM
Duration Advertised: 256 Intake / 262 Exhaust
Duration @ .050'' Lift: 212 Intake / 218 Exhaust
Valve Lift w/1.5 Rockers: .447'' Intake / .462'' Exhaust
Lobe Separation Angle: 112

The cam, coupled with a set of vortec heads and vortec intake should get me where I want to be for now.

 

[Maclin]

Re: Venturing into performance upgrades for an AQ271C

Scott, your knowledge of these engines is at a higher level than what most understand so I apologize for my "restrictive" posts. No disrespect intended here to any other readers, but the typical "rookie" hotrodding threads posted here start out with big cam, more timing, compression,etc and end up with a very hot engine that would not work well in a boat, and that is the audience I posted "at".

I also would be concerned about the octane of the fuel required, that is another reason I post to caution would-be performance enhancers. Once the engine is "smiled on" then it can get finnicky and run hot, ping, all that. Things that out on land may not noticeably affect driveability but with the restricted space and harsher load on the water driveability becomes an issue. Something as simple as finding the right heat range spark plugs can get hairy. Ignitions can be outpaced as well. Simply going with more cubes that can run on "pump gas" may get similar results and end up less expensive in the long run.

Just any old thru-hull headers by themselves do not eliminate the potential for reversion. The better designs have the rise and higher tech water flow that reduce the potential at the expense of taking up more room and, well, the expense!

I can tell now that you ARE ready to do what it takes, so I wish you well brother! Sounds like a fun learning adventure. I like how you post all the tech data also, that is good stuff for future researchers on this subject.

 

[gbeltran]

Re: Venturing into performance upgrades for an AQ271C

You are not going to get anywhere near the piston with the valve with either of those cams. I had a 1986 vintage 350 260HP engine that I ran a Comp cam with .488 lift and it still had loads of room. You are correct in that you will have to carefully check the retainer to guide clearance. I doubt either one of those cams will give you reversion issues with a free flowing exhaust, if you don't go with such a setup the smaller cam will probably serve you better. Be careful though, you can increase the cam only so much over stock without doing any other work, you can go too big to where your dynamic compression suffers (think of it as cranking compression). 1988, isn't that about the year roller cams started showing up in the GM engines, might want to check to see if that's what you have. Can't go wrong with a 383 though. Can be mildly built and still have enough power to turn a larger prop.

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

Maclin - I run 91 octane premium simply to NOT use any fuel with ethanol in it so I am covered with higher octane than most people run. I do understand the difference between boats, cars, trucks, tow vehicles, generators, pump motors, drag motors, and race motors. each one requires a different recipe. However it is always easier to start with a clean sheet build than to modify the unknown.

gbeltran - I cant go too radical on the top end of the existing engine without looking at new slugs, rods, bearings, and exhaust. while the land based '88's may have come with rollers, I am certain that the industrial engines from GM (and that includes all the marine engines) still have flat tappet cams, however I have been wrong before. once I pull the intake, I will know. If I have a true roller setup, then I will look at a roller cam.

I dont want to get too radical on the cam as I may still want to pursue 1.6:1 rockers if needed.

I plan on keeping the existing stock exhaust initially and then working to a set of headers/manifolds from either Custom Marine or Hi-Tek (which will cost more than the heads, intake, cam, gaskets and beer combined)

I also never said I would go mild on a 383 . they can be made to run pump gas with a set of AFR heads and ceramic coating to keep detonation at bay and to get well over 450 hp. of course I still keep my eyes open for the one good deal on an LSA knowing that the Marine version starts at 425 from the factory and goes as high as 557 with proper tuning while getting excellent idle quality.

 

[gbeltran]

Re: Venturing into performance upgrades for an AQ271C

Only reason I said mild is because I think 400HP out of that engine when I say it, and it would still have a decent idle. I have a 383 in my jet but it probably closer to what you were thinking. I've got a much bigger cam with water injected headers and no reversion issues. Don't have to worry about the idle quality so much with that, don't know that an outdrive will tolerate the 900-1000rpm in and out of gear for a long time, although volvo outdrives are pretty durable. Hope you get lucky with a roller cam, my friends 89 mercruiser had one, cheers.

 

[Maclin]

Re: Venturing into performance upgrades for an AQ271C

(which will cost more than the heads, intake, cam, gaskets and beer combined)

I would definitely hit the "like" button on this part....

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

in the bilge this past weekend running heavier wiring to the cuddy for the radio, etc and I noticed a crack in the edge of the motor mounts and what appears to be weeping from the crack. so I am 90% certain the motor mounts have moisture. so the motor will come out this winter.

with the motor out, engine mods will be a bit easier than hanging upside down in the bilge. will also upgrade the oil pump while I am at it.

the question I have, the AQ271 stock risers and stock y-pipe do not lend themselves to silent choice/captains choice exhaust. any suggestions from the Been There Done That crowd on adding selectable exhaust to an AQ271C? I looked into marine silencers and got scared. the price is more than a pair of stainless headers

 

[Maclin]

Re: Venturing into performance upgrades for an AQ271C

Scott,

In the product bulletin and also the owner's manual for a 570A, which is the next version of and very similar to the AQ271C, the pictures show a "POWER PLUS" exhaust system as standard, it has a silencer with thru-hull exhaust but uses what look like stock manifolds and risers. The bulletin claims "low back pressure and thereby minimal performance losses". The Y connector exhaust-thru-the-leg exit is listed as optional!

I do not know if the components are available or not, I just remembered this from when I first downloaded all my manuals from the VP site. I tried to attach it here but the pdf file is too large. It may be available as one of the manuals in the stickies, I have not looked there.

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

not sure if it was the power plus or what, however, I found a factory stainless marine muffler thru transom unit, looked at the price, shuddered violently, and decided to go silent choice AND marine headers to save money

 

[Maclin]

Re: Venturing into performance upgrades for an AQ271C

Glad you found what you wanted.

This is a capture from the brochure of factory setup I was referring to...

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

yes, that looks like the $2k muffler that I found. much more than a pair of $1300 headers and the $600 silent choice.

 

[Maclin]

Re: Venturing into performance upgrades for an AQ271C

Cool, and

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

found a factory Volvo-Penta dual alloy aluminum vortec intake on Ebay. $300 shipped May have found a low$ set of vortec heads. progress has been slow, as a few other things decided they needed my money.

next, looking to make an a-frame to pull the engine while its in storage, unless I can talk my prior boss at Cummins into using the shop there on some weekend.

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

scored a set of vortec heads with stainless valves, guide plates, screw studs, etc. all for $400 from a racer friend of mine.

 

[Maclin]

Re: Venturing into performance upgrades for an AQ271C

Good deal. That should keep the smile going in your beer head foam!

 

[PiratePast40]

Re: Venturing into performance upgrades for an AQ271C

in the bilge this past weekend running heavier wiring to the cuddy for the radio, etc and I noticed a crack in the edge of the motor mounts and what appears to be weeping from the crack. so I am 90% certain the motor mounts have moisture. so the motor will come out this winter.

Might want to head on over to the restoration section with this issue so you can get a good handle on what's involved with that repair. Would be frustrating to put all the time and effort into the powerplant only to find out the stringer issue needs more attention than originally anticipated.

 

[Scott Danforth]

Re: Venturing into performance upgrades for an AQ271C

Might want to head on over to the restoration section with this issue so you can get a good handle on what's involved with that repair. Would be frustrating to put all the time and effort into the powerplant only to find out the stringer issue needs more attention than originally anticipated.

Not my first older fiberglass boat. Thanks for the post though. the issue is not the stringers, however the blocks fiberglassed next to the stringers for the motor mounts.