01 Hp500 Efi Performance Upgrade HELP!!??
 

Hey Guy's I Know That There Tons Of Info On These Motors,i Just Cant Seem To Find All The Awnsers To My Questions-the Motor Is Stock And Has About 300-350 Hrs On It-bravo Xr Drive -question Is Its Time To Tear The Heads Off(lots Of Leak Down)whats Best Bolt On Stuff To Increase Hp? From What I've Seen Dart/afr Heads,741 Cam,throttle Body,efi Remap-looking To Have About 600hp And Be Very Reliable -- Can You Guys Give Me Some Ideas On Whats The Best Reliable Set Up And To Reduce Future Reversion
Thanks In Advance Everbody!!!

Wow Nobody's Got Any Good Info For Me?

Wow Nobody's Got Any Good Info For Me?

Reversion should not be an issue with a 500efi. You need to find a reputable engine builder that has a proven dynoed package.

As an example, I just did 3 for a customer with a tripple engine Sonic. We did change the heads since the stockers were rusted away to nothing. They had spent 11 years in salt. I went with a set of Merlin cast iron heads since they were about the closest to stock. I didn't change the heads for performance but because I had to. That did allow me to bump the compression a little to 9:1. I also changed the cam to something a little larger, installed monoblade throttle body and milled the intake to match the monoblade. That's all we did and they made 600 hp at on the nose at 5400 and over 650 ft lbs of torque. We obviously reprogrammed the mefi 3 as well.
It was some very simple upgrades that really woke them up. They owner is on OSO so maybe he will chime in. Let me know if i can help.
Eddie

Eddie is right it woke the boat up BIG time. I will have to reprop, and will do so after Eddie does the retune in the water. It still idles and sounds like stock 500. Throttle response is great with the mono blade. The tune was also done on 91 octane fuel, and everything went great.

I Pulled Off The Cmi Headers And Sent Them To Cmi For Repair- They Were Leaking Like Lawn Sprinklers Hopefully They Can Be Repaired

Thanks For The Info And Opions Please Keep Them Coming As This Is My Winter Project And Want To Get It Right! Thanks In Advace Everybody

I Would Like To Have At Least 600 Solid Hp How Can I Acheive This With This Motor I Saw That Cp Performance Sells A Hp600 Kit -any Experiance With This?

We did a set of500 efi that was upgraded to around 620hp ,afr cnc heads,cam upgrade ,mono throttle boddie,milled intake,55lb injectors,upgraded fuel system,i found on the dyno that the stock injectors and fuel system was having a hard time supporting the 600hp engine,can be done but the injectors were real close to going static(which can cause overheating and sticking)and the fuel pump was dropping the fuel pressure below the 38lb i had it set at,we also have a custom tune in the ecm engines run great and have been very reliable

I just refreshed 3 stock 500 EFIs. Dyno'd all 3 and they all made right at 510 HP at 5300 each. Not 5 hp variation. So, you picked up 90 HP basically with a cam change and monoblade throttle body with no rpm increase? I don't think the stock Merlin heads outflow the merc prepped 500 heads, unless you tweaked them. Do you have a base number for a stock 500 efi on your dyno? Do you run full wet exhaust on your dyno? If not that could account for some of it. Mine runs water all the way out the tips. A while back I prop shaft dyno'd a couple of AZM 500efi /540s with afr heads and monoblade TBs. Big cams, but I don't have the specs. They made 550 at the prop, which would be low 600's at the flwheel. What are your thoughts? Maybe the Merlin heads are better than I thought? Just trying to understand. I know your data is accurate.

Bob Lloyd
Full Throttle Marine

I think that the Merlin heads contributed a lot to the power increase. Just after I did those 3, I did another just like it with Dart Pro 1 345's. I knew the head was big. The customer bought them used at a good price and wanted to eventually run a Procharger, so I decided to use them. Everything else was identical to John's 3 engines. It only made 550 hp with the aluminum heads. I had a hell of a time even getting that much out of it. Obviously, the heads were just to big. I still didn't think it would hurt it that much.
I can't run wet headers on the dyno so it is def. helping with power. I would absolutely love to run them wet, but can't figure out a way to do it without the neighborhood *****ing. We did run all of the accessories, including the p/s pump.
The boat has picked up quite a bit of mid range and top end speed, so they are definitely making more power. I am about maxed out on injector as well. At the stock pressure, the injectors are good for just over 600 hp. With a higher base pressure, you can squeeze 650 hp from them.
Bottom line is that I didn't think the heads were doing that much for power, but they must be. The raised exhaust port and larger 11/32 valves are making a bigger difference than I thought.
Eddie

Forgot to add that since i don't really do any stock engines, I don't have a baseline for a stock 500 efi.
Eddie

Good info. I built a huge aluminum box that I dump the wet exhaust into, then mufflers out the top and water drains out the bottom. I measured 85db on a 900HP Whipple motor standing about 10 feet from the box at WOT vs 120 db open, so it works pretty good and didn't hurt power at all. We are in the country so we don't really have any restrictions to worry about. The added benefit is I have no heat in the dyno room. Even when I run dyno headers I dump them into the box and just plumb enough water to keep everything cool. About 99% of everything we do is marine. I do believe running wet exhaust costs a significant amount of power over dry tails. One of these days I'll have to actually do a comparison test. I just have to figure out how to run them dry on my setup.

Bob Lloyd
Full Throttle Marine

My problem is that it isn't my dyno. I use it, dyno my own stuff, and come and go as i please, but at the end of the day it isn't mine. They do mostly street and drag race stuff, so they have no use for wet exhaust. The problem is trying to make it work both ways. That's what I haven't been able to figure out. I would also love to see a back to back comparison to see the hp difference. There is no doubt that there is a difference. I'm just not sure how much. If you ever find out let me know. Thanks
Eddie

Thanks guys can anyone recommend specific info on which heads afr-dart cam 741?, who sells throttle body and who can remap efi box?

Lets get a few things straight: a STOCK hp 500 makes 505-510 hp at the crankshaft and that is using the sae J-1349 dyno correction factor. Every performance shop except for most oems uses j-607 which will typically give you hp numbers that are more generous to the tune of 5 %, to the average guy this sounds real technical and confusing BUT what it really means is that 510 hp that merc gets magically turns into 535 hp on most dynos using the older, more generous J-607. Add some dry headers which should be worth 25 hp that hp number now becomes 560 hp, so a fresh stock merc hp 500 with dry headers on a dyno is now a "560 hp" motor even though NOTHING has been really changed. Add a better cam, bigger throttle body and a set of Merlin heads , which by the way ONLY flow about 1.3% better at .600 lift intake/4% better at .600 lift exhaust (un-ported): (merlin Grumpy Jenkins 310's vs GM 990 317's))
http://users.erols.com/srweiss/table...hevy_Big_Block
and I wouldn't be surprised if that 560hp could become 600-610 hp. Now, if a guy had some high hour hp 500's and they weren't putting out the hp like they did when they were new , yes, the boat would pick up a decent amount of speed but without investing in a seriously performing head don't expect a ACTUAL 100 plus hp change, Smitty





"You need to understand that Mercury Racing has been for a long time establishing for them selves an engine horsepower rating on the Racing engines only that represent approximate horsepower at the prop or less drive losses. When others look at Mercruiser engines ,ie the MPI's, EFI's, 496's and now 502HO's etc. they are using engine crankshaft horsepower as their advertised horsepower rating, no drive losses or horsepower at the prop.
On the Mercury Racing HP500efi the engine really makes about 505-510 HP at the Crankshaft and about 470-480 at the prop with its Bravo Drive losses.
The Mercury Racing hp525efi makes about 550-560HP at the crankshaft and this results in about 525HP at the prop in a Bravo XR ( bigger drive losses in an XR)
When Mercruiser 496HO's for example are measured at the prop they have shown to be about 390-400HP at the prop in a standard Bravo 1 or 1X drive.
The differences here are mostly due to the way two different divisions of Mercury Marine choose to rate and measure their engine offerings horsepower and this has obviously created a lot of confusion for customers who view them all as Mercury Marine engines.
Kind of a simple way to remember:
If its Mercruiser black its at the crank
If its Mercury Racing Blue its at the prop

Also keep in mind that most aftermarket marine engine manufacturers like ourselves measure our horsepower at the crankshaft on dyno's with most using corrected outputs with all accessories running on the dyno.

Hope this info helps with some of the confusion.

Best Regards,
Ray @ Raylar"

If you are looking for a real noticeable hp gain invest in the best heads you can afford, stay away from out of the box "world" heads as they barely flow more than stock rectangle ports, Smitty

Here's one I dyno'd for a customer, built by an OSO advertiser.

500EFI, AZM monoblade, the same merlin heads with a little clean up and port matching, all accessories, full wet exhaust with factory tails, stock cam. 300rpm/sec. standard correction. I don't think the heads did anything for it in this situation.

My dyno is generally very conservative but I think you'd need a hell of a camshaft to make 600 with this one. I don't even know if RMbuilder could pull that one out of his hat, and I've seen him come up with some incredible results. :drink:

I've seen around 30hp on a 600hp engine from wet to dry, but the biggest problem I find is the difference in tune up from dyno headers and an airflow turbine to wet exhaust and a flame arrestor is huge. With a variable load dyno and the engine in complete trim you can really get close on your fuel map before it goes in the boat.

I can do a controlled back to back, wet vs dry on the next one on the dyno if anyone wants?

I have a great deal of respect for both Bob at Full throttle and Eddie Young as engine builders and testers so please all take may comments here as not the only or last word but as just to what I have seen and done with Merc EFI engines. Both are being as honest and candid as their situations and test facilites can provide and we can all agree that almost no two dynos in seperate ownership are going to provide the same test outputs all based on correction factors, calibration, machine type, wet versus dry exhaust, and propshaft versus crankshaft measurements.
My expierenes would lead me to say that based on the construction and design of the World iron heads in question here and the cost of a full new set of heads that money might be better spent upgrading the flow characteristics and performance of the stock GM iron heads. (Remember here that Eddie stated the stock iron heads were rusted up beyond good reuse) I think we must also realize here in this case that in a triple engine boat even adding 75-80Hp for three engines to achieves 225-240 HP increase for the boat is going to produce some nice added boat performance in an efficient hull type.
What we always seem to find in both the HP500efi and HP525efi engines is that its really more the intake manifold and in the case of the HP500efi the throttle body also that tend to be the power limiting parts. We did some pretty good experimenting on heads, head flow to power development, intake manifold conditions and restrictions as well as manifold modifications to determine where addtional power was possibly hidden in these engines. what we ultimatly found was that the use of an good (optimized) aluminum head with its resulting better compression ratio changes allowed, a rework to the intake base manifold and our plenum redesign along with the monoblade throttle body made the best power increases and volumetric efficiences. I think on a HP500 efi with its E-top headers and a good cylinder head, reworked intake manifold and throttlebody and the correct camshaft selection 580-600HP is very achievable on a good corrected dyno test. To get to the 675-700HP we achieved on the Raylar HP675efi engine package we had to do special cnc and valve size upgrades to the stock Merc/Edlebrock aluminum heads to increase the very poor stock flow numbers, take the engine to 540 cubic inches, design a new camshaft profile, rework the intake manifold base, design and build a new intake plenum to accept all the stock HP525efi hardware and fitment, upgrade injector size to 55lb/per hour injectors, change the firing order to the new 4/7-2/3 swap (18726542) and do some reporgramming on the stock HP525efi ECM and keep compression ratios at 9.5 to 1 for 89 pump gas operation.
I think this thread is good infor for those out there with HP500 efi engines as it gives them some possibilities for upgrading their engines based on budget and power requirements.
My advise is if you are going to have to rebuild an HP500efi I would start by adding cubic inches with a healthy bore size increase, having a good shop or head expert massage the stock heads for better flows, select a good camshaft from a recommended camshaft expert like Bob at RMBuilders, rework the stock intake for better flow and add the monoblade throttle body and DO NOT FORGET TO PUT CORRECT PROGAMMING OF THE ECM TO MAKE THE WHOLE THING RIGHT!
I am attaching a few pics of the Raylar HP675EFI engine so you can see the new plenum, just remeber its whats inside the plenum that you can't see that really makes it work.

Best Regards,
Ray @ Raylar

Nice looking piece, needs more Raylar stickers. J/K. :drink:
what kinda torque does that bad boy make?

Nice looking piece Ray. Is it casted?

One other thing I forgot to mention is that these 3 500's I did were 516 ci. One had been rebuilt and was already at 4.500. It was poorly finished and too large to go back at that bore. So, in order to keep them all the same, I bored them all to 4.530 (516 ci).
Eddie

Ray is incorrect. Black motors are always rated at the prop(and usually over rated) and blue motors are always at the crank(and usually under rated).

The HP500 was rated at 500 hp at the crank and 470 at the prop and usually dynoed a couple ticks over 500.

The 502 mag was 415 hp at the prop and usually dynoed about 430 at the crank(should be 445 using merc's math of 30hp loss). 496HO was rated at 425 at the prop. Both engines were over rated.

The 525 usually dynos at 550hp just because it is an under rated motor. It is not rated at the prop.

[QUOTE=articfriends;3546581]Lets get a few things straight: a STOCK hp 500 makes 505-510 hp at the crankshaft and that is using the sae J-1349 dyno correction factor. Every performance shop except for most oems uses j-607 which will typically give you hp numbers that are more generous to the tune of 5 %, to the average guy this sounds real technical and confusing BUT what it really means is that 510 hp that merc gets magically turns into 535 hp on most dynos using the older, more generous J-607. Add some dry headers which should be worth 25 hp that hp number now becomes 560 hp, so a fresh stock merc hp 500 with dry headers on a dyno is now a "560 hp" motor even though NOTHING has been really changed. Add a better cam, bigger throttle body and a set of Merlin heads , which by the way ONLY flow about 1.3% better at .600 lift intake/4% better at .600 lift exhaust (un-ported): (merlin Grumpy Jenkins 310's vs GM 990 317's))
http://users.erols.com/srweiss/table...hevy_Big_Block
and I wouldn't be surprised if that 560hp could become 600-610 hp. Now, if a guy had some high hour hp 500's and they weren't putting out the hp like they did when they were new , yes, the boat would pick up a decent amount of speed but without investing in a seriously performing head don't expect a ACTUAL 100 plus hp change, Smitty


I agree with you. I didn't claim to have added 100 hp, just that these made 599 hp with dyno headers and all accessories. By the way, these were the 325 Merlins, not the 310 Grumpy's, for what it's worth. We actually weren't even looking to make that much. I didn't think they would. It was mainly just a freshen up with a few needed changes. These were some of the best ones that I had done with a 500 intake. God knows I did a few wrong ones where the combo just didn't work.
I expected more out of the one with aluminum heads. I was very dissappointed that it only made 550 hp. It took quite a bit of playing and poking to even get that. I considered that one a failure.
I had done a few in the past that were more inches, but didn't make any more power. It seemed that the smaller inch engines did better with that intake. I did some 555's with 345 aluminum heads, 10:1, fairly aggressive cam, etc. and it barely broke 600 hp. It was a tub of chit. I don't know if the combo was that bad or if the intake just couldn't support that many inches. Either way, it pulled to about 4500 and fell flat on it's face. I couldn't stop it from going pig rich past that and just puffing black smoke out the pipes. I played with it for 2 days on the dyno and nothing made it any better. I think the best it did was 610 hp. I was embaressed. I ended up pulling the intake and using the Holley setup. I swapped the intake on the dyno and the first pull it made an additional 85 hp. It ended up at 710 hp. Big difference from the 600 hp with the 500 intake. I learned a lot from those mistakes.
Anyway, the customer is happy and the boat is running faster than it ever has, so that's all the matters in the end. It doesn't matter what I think. I have had some that made good power and other that didn't when it comes to the 500 intake.
Eddie

If you read my post I was essentially saying that the hp 500made 505-510 hp stock at the crank and used Raylar for reference. As far as what they make at the prop, I didn't really care in this instance and should have just left that off. My contention is it isn't real easy to make 600 hp from a hp 500 by bolting Merlin heads on,swapping the cam and bolting on a better throttle body unless you are running dry headers in that case you started at 550-560 hp anyways between the j607 correction factor and everything else, Smitty

The engines are in my boat. It is substantially quicker and faster. I do have to change the props, it currently runs 28 pitch props. I don't feel that the boat was that down on power to start with, two of the three had never been out of the boat.(11 years) They may have been down on power, but they were not leaking or blowing oil out of the breathers. The boat would plane and run just fine. They were in the 500 hour range.The Mercury tech that installed the engines was very impressed on the sea trial. His personal boat is a 35 Fountian with 575's. Eddie is comming in the spring to do the in water tune. I am going to get a set of stock untouched 32 pitch props from another 45 Sonic. It has 575 untouched engines. We will see if it will pull them , and if so we will post the numbers. The boat does have GPS. The best numbers ever run in this boat by the previous ower was 83 GPS. Reguardless of the out come I would ask for the same combo package from Eddie. I was there during the dyno session. I AM VERY HAPPY WITH THE OUTCOME. Happy customers are what any bussiness would want. Young performance has one with me.

A friend of mine had his refreshed a couple years ago, they did Dyno them, they made 521 hp. ( I have a copy of the dyno sheet somewhere)

I watched one with approx 500 hrs being dyno'ed, to
get a baseline before rebuild, 3 pulls, 525, 531, 545, as I recall, this was december, no humidity.

We rebuilt and did some upgrades on a pair this spring,
Dart Pro One's, bigger cam, mono blade, ecm, 575 is
all we could get out of them.

We have slight ECM tuning issue to hash out, but I doubt
were going to find 25 more hp out of them.

Smitty, I wasn't disagreeing with your post, just pointing out that black motors are rated at the prop and blue at the crank. That's why I only quoted that part. :ernaehrung004:

Did you get my last PM?

This is exactly opposite of what I was told by a Merc rep. Black at the crank, blue at the prop. Dyno testing of both types of motors tends to back this up.

496HO was listed at 425 propshaft hp. 502 mag was listed at 415 propshaft hp. HP500 was 500hp at the crank and merc rated it at 470 at the prop. If they rated it at the prop it would have been called a HP470. :D

Now, I totally agree that dyno numbers don't back this up as the blue motors always have more hp than rated and the black less. I think the confusion really started with the 525 since it put out so much more hp than advertised.

If you go to Merc sterndrives specs right now they are all listed with prop hp ratings, the new 8.2L HO is listed at 430 propshaft hp. If you go to Merc Racing sterndrives they give crank hp.

Didn't mean to start any kind of conflict, just discussion. I'm not sure how Merc rates what. It's all marketing anyway. I know that Merc advertised prop hp for several years on all the engines, blue and black. Then they quit saying prop hp altogether. Maybe because the black motors were not very close to those numbers. Here are some I've actually dyno'd at the prop. 496 mag 353-365, Mag HO 387-392, 502 Mag - 390, Raylar 525/496 - 492, 525 efi 475-508, Teague 800 720
This is using "standard" correction and a weather station to get atmospherics. Using the SAE correction that Merc uses would have resulted in 3-4% lower readings.

Bob Lloyd
Full Throttle Marine

Not arguing with anyone. Just stating what I was told and pointing out there is conflicting information out there on the topic. And yes, it was the debut of the 525 where this topic came up for discussion at a manufacture's rally where I was speaking with the Merc rep.

I agree it is confusing as it does seem reversed. As Bob's dyno numbers point out the black motor numbers are always low.

Anyway I am taking this thread off topic so carry on.

Engine Improvement 201
 

Obvoiusly there is so much conflicting information on what all Mercury engines make in HP at the crankshaft and at the prop and the mis-information that comes even from Merc marketing materials, sales reps versus Merc Racing techs, etc. that I think its worth saying that this is really a moving target and the only for approximate reference and marketing.
Raylar as an engine builder is always conservative on our horsepower ratings on our kits and engines and we do measure only at the crankshaft, using SAE J-1349 specs and with water exit in the exhaust headers on a fully calibrated dyno with full corrections to try to create a HP condition that the customer will usually achieve in the boat with a proper installation and similar conditions.
I realize that many builders and shops do not always have the ability or equipment to dyno test in the exact same fashion.
Besides this, we can always assume that no two dynos will test the same exact numbers even with corrections and sometimes an engine will vary from one pull to the next on the same day on the same dyno sometimes only minutes apart.

In any event, back to the real thread inception question and request. As a fairly knowledgeable engine builder I will reaffirm the statement: "Engines are really just air pumps that are burning fuel to make horspower and torque" To properly and efficiently add power and torque to an exisitng engine design or state of original engine specs. and build it is imperative to look at three simple aspects to get the best efficiency and power and torque production. One is air in either pulled or pushed from normal aspiration or boosted input. Two, the compression of the fuel and air charges with their subsequent ignition and burn and Three the exit of spent burned air and fuel mixture (exhaust) thru the exhaust.
A good and general way to achieve this measurement of an engines efficiency as an air pump is to measure and calculate the engines volumetric efficiency at various rpms and load to establish this efficiency.
Generally the higher the volumetric efficiency (air pumping efficiency) at various rpms the higher the power (HP) and torque that may be produced at those rpms.
This is why it is so important to look at a lot of variables in increasing power and torque in an engine thru rework or upgrades. Simply, if one does not have an efficient intake system, and efficient air/fuel cylinder filling and combustion system and an efficient exhaust system, all three being created in the best equality and balance the engines overall efficency and power output will not be optimized.
What this tends to mean for the average performance boater, novice and professional engine builder is that you cannot just put a a selection of various intake configurations, camshaft specs., cylinder head types and configurations and exhaust configurations and end up with the even good or better power and torque production without a lot of good testing and verification to achieve the best results.
This is why we really use the dyno. As a test only tool and not as a solution for the goal of best efficiency and power production.

Back to the HP500 or for that matter any 4 cycle BBC V-8 engine as discussed here.
I as many professional engine builders in this industry hear and see all the time is that many times because of condition of the starting engines, budget of the customer and the need from the new engine configuration many customers want to stage or add upgrades or rework in stages. They will only want to add a new intake system and exhaust system at this point or change a camshaft only, or make other block increases in cubic inches or compression at one time an expect better or best results! Most of the time this is the wrong approach and generally results will be much lower improvements than if they or the builder took the approach that all these three systems have profound effect on each other and the ultimate increases will be how well this overall efficiency in these simple three systems work well with one another to give the final result.
This is somewhat well exampled by Eddie's results when his customer or his choices resulted in less than expected or poor results from trying to mix and the subsequent mismatch of intakes, head specs and sizes and possible exhaust differences . Eddie was using the dyno as a tool to identify what the combination would actually produce and found out that these choices were not well matched for overall efficiency and power production, hence the combo became a "pig" and serious "underachiever."
The HP500 base engine platform as produced by Mercury Racing can and does respond with additional power and torque when the total of these three systems is increased in efficiency and size increases. You just cannot make an across the board upgrade reccommendation without stressing and applying the overall efficiency model to the upgrade or rework.

If an HP500 engine receives good upgrades and choices in intake changes or improvements, cylinder head improvements thru rework or replacement, possiblecamshaft changes and potential exhaust improvements it nis very possible and not to hard to get this engine in a size between 502 -530 cubic inches up to a 600HP crankshaft output in a normally aspirated configuration. This is a 1.14 to 1.2 horsepower per cubic inch ratio and if overall efficencies are improved with these most or all of these three areas it is very reachable.
What does your HP500 need? A carefull blend of improved intake air flow( very critical in HP-efi engines as the stock intake and throttlebodies can and will become a limiting factor), improved cylinder head flow in all lift ranges from either well massaged stock iron heads, new iron or aluminum heads and potential compression increases from the use of aluminum head,improved camshaft profiles (in terms of valve event timing and some lift changes,) and potential exhaust system improvements (and that can mean in the header used and from the header back to the outlet also) Obviously good accurate ECM re-tuning and fuel system modifications will be needed to support this higher power level.
And again don't live by power (HP )alone, the additions to torque and where in the rpm bands these torque increases can be made will have a profound effect on how these power increases are really seen in the added performance of the actual boat! Remember that torque is what makes the prop keep turning and for the most part how much addtional pitch you can carry with the new added power at that rpm! I feel in boating with propellor driven boats torque is really more important to consider in most recrational uses and how and where it occurs than just talking about peak HP numbers!
And lastly keep the phrase in the headlights "There Is No Replacement for Displacement" in an average rpm N/A recreational use BBC engine. If you are reworking the engine with new internals and budget will allow ,use addtional bore first and stroke second to gain any reasonable increase in cubic inches as this is somewhat free power and torque when you are buying things like pistons and such! Just make sure the added air flow needed to support more cubic inches is supported by efficencies in all those three parts of the Air Pump!
This post will probably bore the hell out of most readers and I know I can be a wordie! I have always tried to never stop reading , listening and learning from those who might be more knowledgable than me! No one knows everything and I don't know nearly enough!
Never stop trying to improve! it makes everything better!

Best Regards,
Ray @ Raylar

To the OP's question, Brad Smith of Smith Power in Joplin, Mo worked a pair of 2003 500EFI's for a friend of mine this spring. Dyno'd right at 600 HP. You could contact Brad to get the details of the build and how the motors were dyno'd.

guys thanks a lot for all of the info I have received on this thread!!!it will be instumental on my build !!!

I would love to see that comparison just out of curiosity.

Is the higher hp using dry exhaust because no water is being mixed with the exhaust gasses, or because the automotive style headers have longer primaries? Or a combo of both?

Heavy Water
 

Raylar, as Bob at Full Throttle and others builders who have run engines like this on the dyno's with both dry dyno header exhaust, dry marine header exhaust with tails and marine headers with tails and full wet exhaust, we all have pretty much seen about 25-30HP less with a wet marine exhaust versus dyno headers and the dry marine exhausts with tails still tend to make about 10HP less than long tube dyno headers. This is caused by the fact that it is harder for an engine to pump exhaust gases out when they are full of water and water vapor as this mixture is heavier than just dry exhaust gas and there is more friction in the collectors and tips from the water on the surface of the tubes and tips.
This obviously is on normally aspirated engines of mild builds and does not apply equally to supercharged or turbo equipped engines.

Best Regards,
Ray @ Raylar

So when comparing marine exhaust systems(wet and dry) to dry dyno headers you usually find:

Dry marine exhaust: -10hp
Wet marine exhaust: -25 to 30hp

YES, thats what we tend to find.

Best Regards,
Ray @ Raylar

So, basically you lose 15-20hp doing from a marine dry to a marine wet. So, does that transfer into the real world? Meaning would you see a 15-20hp increase going from full wet exhaust to dry or close to dry?

Most people go dry or close to dry because of reversion from big cams. But is there power to be picked up as well on engines that do not require dry exhaust because of reversion?

In realty I know it's not worth it to most to go to dry exhaust for 15-20hp. The cost and noise wouldn't be worth it to me. I am just curious if someone did want to if they would see a 10-15hp increase.