Promaxx Heads
04-04-2025 | 08:41 PM
#51
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Joined: Aug 2019
Posts: 1,226
Likes: 416
From: BC
Thanks for all the feedback response guys. It's all good news and info that everyone here can use.
Yeah Tart, if I pull the trigger on those PROMAXX 290 heads then I will probably get some Edelbrock RPM Air-Gap Oval Port dual plane intake manifolds and have either Eric Weingartner go over the heads or even Brian Salter of Salter Racing Engines (he's a very successful NASCAR Cup builder, etc, etc) down in NC.
And also have them port/message the intake manifolds as well. Like I have said, I don't want to swap out the cams even though it looks like they only Peak at 5000rpm.
I'm already running Stellings full length tubular open dry headers...so that greatly helps the equation.
I have to check things out after I get the old rectangle GM cast iron heads off and see what's what in there so as to get an idea on my Comp Ratio, piston crown and Cam Lift, etc.
And keep in mind that I just have these little baby 482cid engines with only a 4.25" cylinder bore.
The ProMaxx 290cc heads might create too much Compression with their 110cc combustion chamber... So I may need to use a different brand of head but I will have to wait and see once I get the old GM heads off.
Yeah Tart, if I pull the trigger on those PROMAXX 290 heads then I will probably get some Edelbrock RPM Air-Gap Oval Port dual plane intake manifolds and have either Eric Weingartner go over the heads or even Brian Salter of Salter Racing Engines (he's a very successful NASCAR Cup builder, etc, etc) down in NC.
And also have them port/message the intake manifolds as well. Like I have said, I don't want to swap out the cams even though it looks like they only Peak at 5000rpm.
I'm already running Stellings full length tubular open dry headers...so that greatly helps the equation.
I have to check things out after I get the old rectangle GM cast iron heads off and see what's what in there so as to get an idea on my Comp Ratio, piston crown and Cam Lift, etc.
And keep in mind that I just have these little baby 482cid engines with only a 4.25" cylinder bore.
The ProMaxx 290cc heads might create too much Compression with their 110cc combustion chamber... So I may need to use a different brand of head but I will have to wait and see once I get the old GM heads off.
The GM RecPorts you have are the 880s? These heads, or the similar castings are really rough in the ex port. The flow as little as a good SBC ex port. Getting a raised ex port head like the 290 Promaxx will help remove the bottleneck of airflow that creates the peak rpm/hp.
My stock FT marine BBC crate cams are 224/[email protected]", with 0.510" lift. Peak RPM is 5200 with the 880 heads. I suspect the HP500 is more aggressive, and will peak much higher.
JaniH's cam peaked pretty high with his Promaxx heads and the 246° ish cam. I think the curve rolled over past 6K rpm.
04-05-2025 | 01:13 AM
#53
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Joined: Jan 2009
Posts: 682
Likes: 177
From: Finland
Eric would be able to ooen up the 110cc chamner to what you would require for optimal CR, seeing as how you're looking to get him to sort them out anyway.
The GM RecPorts you have are the 880s? These heads, or the similar castings are really rough in the ex port. The flow as little as a good SBC ex port. Getting a raised ex port head like the 290 Promaxx will help remove the bottleneck of airflow that creates the peak rpm/hp.
My stock FT marine BBC crate cams are 224/[email protected]", with 0.510" lift. Peak RPM is 5200 with the 880 heads. I suspect the HP500 is more aggressive, and will peak much higher.
JaniH's cam peaked pretty high with his Promaxx heads and the 246° ish cam. I think the curve rolled over past 6K rpm.
The GM RecPorts you have are the 880s? These heads, or the similar castings are really rough in the ex port. The flow as little as a good SBC ex port. Getting a raised ex port head like the 290 Promaxx will help remove the bottleneck of airflow that creates the peak rpm/hp.
My stock FT marine BBC crate cams are 224/[email protected]", with 0.510" lift. Peak RPM is 5200 with the 880 heads. I suspect the HP500 is more aggressive, and will peak much higher.
JaniH's cam peaked pretty high with his Promaxx heads and the 246° ish cam. I think the curve rolled over past 6K rpm.
Last edited by JaniH; 04-05-2025 at 01:25 AM.
04-05-2025 | 04:27 AM
#54
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Joined: Oct 2000
Posts: 4,541
Likes: 107
From: Western Michigan
Ski Doc --- how's your project coming along, brother❓
Well guys I'm kinda thinking I may be hitting the built in Rev Limiter on those Merc Thunderbolt distributors.
It's been a while now, but I had bought a bunch of Used Merc HP500 parts from someone here on OSO back around 2007/2008 including the GM rectangle port cast iron heads, Holley/Merc 800cfm carbs, Merc Thunderbolt distributors, the stock Merc/Dart intake manifolds and if I remember correctly even the stock Merc HP500 cams ---- it all came as a USED package deal from a guy in Texas back then.
So I believe those stock Merc HP500 hydraulic roller cams are what's in my little 482cid baby engines now with all new blocks, pistons, Eagle cranks and H-beam rods, and valve train.
Ended up Not being able to reuse the intake manifolds that I had gotten in the package deal because the water jackets were too eaten up by salt water corrosion.
Anyway, from info that I have gleaned from over the years is that a cam with a tighter LSA tends to peak a little sooner in the RPM range which can be beneficial for a Marine engine application other than causing a possible reversion issue.
And it looks like my Merc HP500 ("731") cams were ground on a 110° LSA. That all being said, my Cam LSA of 110° and the Merc Thunderbolt distributors w/their internal Rev Limiter chip may be what's holding my RPM down to only peaking @5000. And BOTH engines Dyno tested at exactly 510hp each.. ---- which I think is possibly hitting the Rev Limiter --- I am Not exactly sure, but it's a bit suspect. ... and I maybe wrong, but I think it may possibly be holding my peak down by 200rpm or so ---- yeah❓🤔
According to the Merc specifications on their HP500 engine with that "731" Cam, if am correct the HP500 engine peaks about @5200rpm or so. Anyone have any quick data on that❓
Anyway, I think I am getting a bit of short end of the stick RPM wise.
By the way, thanks to SB and Griff for their input on the Cam reference info. Both are a wealth of knowledge 👍
And JanH you made me absolutely crack up when you said you were sorry about posting more about all the Bore Notching you have done and that you practically bore notch everything engine you do in all your past years ---- especially bore notching a 4.500" cylinder bore with only a 2.06" Intake valve ----- I mean man dude, I thought I was a bit extreme and "over the top" ---- but brother you are out of this world...like --- OUT THERE!!! 😳 But don't get me wrong, I love it, I love it.
That 2.06 valve on a 4.500 bore notch comment was absolutely hilarious. 😂🤣 That's what I call some really deep experimental commitment and dedication. I enjoy reading your posts. 👍
I have to tell you guys this hilarious story ... I have to add lib a little bit as I tell the story just to set it up for you.
But I don't think I have laughed so hard since the time maybe around 2002 or so a guy here on OSO had said he just bought a new single engine 26' boat and had also just met and had been dating this new hot looking country girlfriend of his and was really being kind and humble with her...he really liked her a lot and was just trying to put his best foot forward with her, etc.
After going out with her after several weeks and spending time with her and taking her along on his new boat a few times, he was told her he was looking forward to a certain weekend and meeting up and hanging out with all these other boaters at a particular hot spot hang out with his new hot looking boat .... so she asked him if she could invite a couple of her girlfriends along with them on the his boat for the day during the weekend. He was more than happy and willing to say yes to his new hot girlfriend and her hot girlfriends and he was eager to show off his new boat, his hot looking girlfriend and her friends.
He tells his girlfriend to have her friends meet them at the boat launch. On that day, he was just about to back the boat in the water when here come the girlfriends --- and Not just 2 girlfriends.... more like 6-8 girlfriends ----- like BIG cornfed, Cowgirl girlfriends ----- in bikinis with towels wrapped around their waists ... with lipstick sunglasses and sunhats like they were headed for a Miss piggy contest or something. ----- and they did NOT look anything like his hot looking new girlfriend.
I think the guy said at that point he was terrified to put the boat in the water, but he really liked his new girlfriend so he just smiled 😁
Once he got the boat in the water, and helping all those girls get in the boat one by one and when they were all in the boat he noticed how obvious and abnormally deep his new boat sat in the water and having an embarrassing list to one side.
And once those girls were on board they began to remove all their towel wraps from around their waists and he said he had never seen so many "hail damaged asses" in his entire life all at one time and all on his boat. He said all he could do was smile 😁 and wait for the day to end.
SB or Griff or anyone of you older members do you guys remember that story from that guy here on OSO from way back then❓
Just remember y'all ---- I myself have many shortcomings, but I love my neighbor ---- my fellow man. 👍🇺🇲
It had everyone on OSO rolling back then.
Well guys I'm kinda thinking I may be hitting the built in Rev Limiter on those Merc Thunderbolt distributors.
It's been a while now, but I had bought a bunch of Used Merc HP500 parts from someone here on OSO back around 2007/2008 including the GM rectangle port cast iron heads, Holley/Merc 800cfm carbs, Merc Thunderbolt distributors, the stock Merc/Dart intake manifolds and if I remember correctly even the stock Merc HP500 cams ---- it all came as a USED package deal from a guy in Texas back then.
So I believe those stock Merc HP500 hydraulic roller cams are what's in my little 482cid baby engines now with all new blocks, pistons, Eagle cranks and H-beam rods, and valve train.
Ended up Not being able to reuse the intake manifolds that I had gotten in the package deal because the water jackets were too eaten up by salt water corrosion.
Anyway, from info that I have gleaned from over the years is that a cam with a tighter LSA tends to peak a little sooner in the RPM range which can be beneficial for a Marine engine application other than causing a possible reversion issue.
And it looks like my Merc HP500 ("731") cams were ground on a 110° LSA. That all being said, my Cam LSA of 110° and the Merc Thunderbolt distributors w/their internal Rev Limiter chip may be what's holding my RPM down to only peaking @5000. And BOTH engines Dyno tested at exactly 510hp each.. ---- which I think is possibly hitting the Rev Limiter --- I am Not exactly sure, but it's a bit suspect. ... and I maybe wrong, but I think it may possibly be holding my peak down by 200rpm or so ---- yeah❓🤔
According to the Merc specifications on their HP500 engine with that "731" Cam, if am correct the HP500 engine peaks about @5200rpm or so. Anyone have any quick data on that❓
Anyway, I think I am getting a bit of short end of the stick RPM wise.
By the way, thanks to SB and Griff for their input on the Cam reference info. Both are a wealth of knowledge 👍
And JanH you made me absolutely crack up when you said you were sorry about posting more about all the Bore Notching you have done and that you practically bore notch everything engine you do in all your past years ---- especially bore notching a 4.500" cylinder bore with only a 2.06" Intake valve ----- I mean man dude, I thought I was a bit extreme and "over the top" ---- but brother you are out of this world...like --- OUT THERE!!! 😳 But don't get me wrong, I love it, I love it.
That 2.06 valve on a 4.500 bore notch comment was absolutely hilarious. 😂🤣 That's what I call some really deep experimental commitment and dedication. I enjoy reading your posts. 👍
I have to tell you guys this hilarious story ... I have to add lib a little bit as I tell the story just to set it up for you.
But I don't think I have laughed so hard since the time maybe around 2002 or so a guy here on OSO had said he just bought a new single engine 26' boat and had also just met and had been dating this new hot looking country girlfriend of his and was really being kind and humble with her...he really liked her a lot and was just trying to put his best foot forward with her, etc.
After going out with her after several weeks and spending time with her and taking her along on his new boat a few times, he was told her he was looking forward to a certain weekend and meeting up and hanging out with all these other boaters at a particular hot spot hang out with his new hot looking boat .... so she asked him if she could invite a couple of her girlfriends along with them on the his boat for the day during the weekend. He was more than happy and willing to say yes to his new hot girlfriend and her hot girlfriends and he was eager to show off his new boat, his hot looking girlfriend and her friends.
He tells his girlfriend to have her friends meet them at the boat launch. On that day, he was just about to back the boat in the water when here come the girlfriends --- and Not just 2 girlfriends.... more like 6-8 girlfriends ----- like BIG cornfed, Cowgirl girlfriends ----- in bikinis with towels wrapped around their waists ... with lipstick sunglasses and sunhats like they were headed for a Miss piggy contest or something. ----- and they did NOT look anything like his hot looking new girlfriend.
I think the guy said at that point he was terrified to put the boat in the water, but he really liked his new girlfriend so he just smiled 😁
Once he got the boat in the water, and helping all those girls get in the boat one by one and when they were all in the boat he noticed how obvious and abnormally deep his new boat sat in the water and having an embarrassing list to one side.
And once those girls were on board they began to remove all their towel wraps from around their waists and he said he had never seen so many "hail damaged asses" in his entire life all at one time and all on his boat. He said all he could do was smile 😁 and wait for the day to end.
SB or Griff or anyone of you older members do you guys remember that story from that guy here on OSO from way back then❓
Just remember y'all ---- I myself have many shortcomings, but I love my neighbor ---- my fellow man. 👍🇺🇲
It had everyone on OSO rolling back then.
04-06-2025 | 03:26 PM
#55
Registered
Joined: Aug 2019
Posts: 1,226
Likes: 416
From: BC
Have seen few stock carbed hp500 dyno sheets here, peak hp @ 5300rpm ÷- 100rpm. I think my tunnelram intake makes my combo to peak few hundred rpms higher, vs single or dual plane intake. Ron sporll has nice looking 230/235 @050" 114lsa, 0.646/0.646" lift grind, wich i think would fall in the 5500rpm peak with ops longer 4.25" stroke with single carb intake.
Important to factor in, peak HP is a relative term. Bigger exh duration will hold the tq curve up longer...giving a very shallow drop off of the HP.
I prefer to shoot for a flat HP curve, and prop my peak rpm a little past the peak HP rpm. Not losing much 'power'...maybe 15 hp...if that. But not my cam is far more efficient where most boats do 90% of their running.
04-25-2025 | 05:37 AM
#59
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Registered
Joined: Oct 2002
Posts: 2,056
Likes: 92
From: Bowling Green, Kentucky
First start went a little rocky. I tried to find someone to dyno the engine for me with no luck. I contacted Eddie Young who built a pair of engines for me but he was too busy and most of the other Dyno people around here just do the chassis dyno.
Of all things I broke the positive terminal off the starter tightening the battery cable. I was able to superglue the plastic back together and it worked. Then I wired the positive and negative coil posts to the battery and found out that coils in fact will burn. Then I remembered that distributors work by grounding and had a spare coil lying around.
I had set the initial timing to 12 degrees base and had run the oil pump with a drill. The engine fired up really well sounded great and had good pressure. No leaks, noises or other problems. I intended on doing the initial break in but I couldn’t keep enough water supplied so I guess I’ll put it in the boat and tune and break in there.
I had the transmission on and it seemed to shift without load well.
I was very happy and as far as I can tell everything is good. I apologize for not being able to post dyno data as this would have been great info for the thread.
Of all things I broke the positive terminal off the starter tightening the battery cable. I was able to superglue the plastic back together and it worked. Then I wired the positive and negative coil posts to the battery and found out that coils in fact will burn. Then I remembered that distributors work by grounding and had a spare coil lying around.
I had set the initial timing to 12 degrees base and had run the oil pump with a drill. The engine fired up really well sounded great and had good pressure. No leaks, noises or other problems. I intended on doing the initial break in but I couldn’t keep enough water supplied so I guess I’ll put it in the boat and tune and break in there.
I had the transmission on and it seemed to shift without load well.
I was very happy and as far as I can tell everything is good. I apologize for not being able to post dyno data as this would have been great info for the thread.
04-27-2025 | 02:07 PM
#60
Registered
Joined: Aug 2019
Posts: 1,226
Likes: 416
From: BC
Yeah, good knowledge on onnthese builds is invaluable. You'll find out soon enough how it performs.
It's the small sruff that often gets overlooked.
Mr.Murphy always has a vote.
IF you're having issues woth rpm etc and water feed, always best to do it in the boat. Modern rings with plateau'd bores don't need as much break in compared to the old chrome rings.
Looking forward to the vids and pics.
It's the small sruff that often gets overlooked.
Mr.Murphy always has a vote.
IF you're having issues woth rpm etc and water feed, always best to do it in the boat. Modern rings with plateau'd bores don't need as much break in compared to the old chrome rings.
Looking forward to the vids and pics.
