custom cam grind choice help

[curtis73]

I'm finding several cams that are close to something I want but I don't feel confident with them. I called Comp about their marine grinds and told them about my concern of reversion with too much overlap and the tech said its ramp speed that causes reversion, not overlap. I gently set the phone down and ran out of my garage crying.
So anyway, I started fooling around with some cams and I have some general overlap questions. Forgive me, but I only have overlap numbers @ .050"
A stock flat tappet chevy Mercruiser cam specs out to about 202/213 on a 110 LSA. That makes -12.5* of overlap, and since its the stock cam I assume that is adequate. I have a Vortec block so I'll be doing a roller, but I'm just speaking of overlap right now. Comp does a 212/218 roller on a 112 LSA that specs out with -9* overlap. A little more, but they don't seem to think that reversion is an issue.
So, as long as I'm not increasing overlap, the LSA is not a factor in reversion, right? I'm looking at two grinds that I concocted myself: a 204/214 on 110 (which makes -11*) and a 200/210 on 108 (which also makes -11*).
The reason I ask is that on the simulation, a 200/210 on 108 retarded 4* makes the exact same power with a bit more torque than the 212/218 on 112, and has a little less overlap despite its 108* LSA.
The real question is... I assume the main factor in reversion is overlap regardless of other events, right? So if I am reducing duration along with LSA and keeping the same overlap, am I still OK?
Dyno sims attached showing a 212/218-112 installed straight up and a 200/210-108 installed 4* retarded. Same basic curves, 12* less duration. Sounds like a winner. Thoughts?

[Jetaholic]

I'm finding several cams that are close to something I want but I don't feel confident with them. I called Comp about their marine grinds and told them about my concern of reversion with too much overlap and the tech said its ramp speed that causes reversion, not overlap. I gently set the phone down and ran out of my garage crying.
So anyway, I started fooling around with some cams and I have some general overlap questions. Forgive me, but I only have overlap numbers @ .050"
A stock flat tappet chevy Mercruiser cam specs out to about 202/213 on a 110 LSA. That makes -12.5* of overlap, and since its the stock cam I assume that is adequate. I have a Vortec block so I'll be doing a roller, but I'm just speaking of overlap right now. Comp does a 212/218 roller on a 112 LSA that specs out with -9* overlap. A little more, but they don't seem to think that reversion is an issue.
So, as long as I'm not increasing overlap, the LSA is not a factor in reversion, right? I'm looking at two grinds that I concocted myself: a 204/214 on 110 (which makes -11*) and a 200/210 on 108 (which also makes -11*).
The reason I ask is that on the simulation, a 200/210 on 108 retarded 4* makes the exact same power with a bit more torque than the 212/218 on 112, and has a little less overlap despite its 108* LSA.
The real question is... I assume the main factor in reversion is overlap regardless of other events, right? So if I am reducing duration along with LSA and keeping the same overlap, am I still OK?
Dyno sims attached showing a 212/218-112 installed straight up and a 200/210-108 installed 4* retarded. Same basic curves, 12* less duration. Sounds like a winner. Thoughts?
How is it that overlap doesn't cause reversion? If the exhaust valve is still open when the piston is on its way down on the intake stroke, you can bet there will in fact be reversion in the works.

[241Libby]

Not an expert on this but my 460 stock 340hp OMC came with a 216/220 at .05 and 112 CL. and I did a complete rebuild last year and installed a 226/236 at 112 CL and I had some minor reversion with the stock OMC thru hull exhaust and as a result will be installing Lightening headers to hopefully solve the reversion issue.

[curtis73]

How is it that overlap doesn't cause reversion? If the exhaust valve is still open when the piston is on its way down on the intake stroke, you can bet there will in fact be reversion in the works.
Did you read my next sentence? --> "I gently set the phone down and ran out of my garage crying" :D
I have no faith in what cam tech lines say anymore these days.

[curtis73]

Not an expert on this but my 460 stock 340hp OMC came with a 216/220 at .05 and 112 CL. and I did a complete rebuild last year and installed a 226/236 at 112 CL and I had some minor reversion with the stock OMC thru hull exhaust and as a result will be installing Lightening headers to hopefully solve the reversion issue.
Ok, that's good info. The stock cam specs out to -6* overlap and the one you installed later specs out to +7. So, that tells me that as much as -6* seems to be kosher, at least on a big block 460. That might mean nothing on a 350 :)

[241Libby]

Help out here. is +7 bad? Lightening recommends no more duration than 236 on the exhaust. Last thing I want to do is spend 2K on exhaust and still have issues.

[curtis73]

Not sure... You had just said your 226/236 cam caused reversion issues through the hub, and it specs out to +7. Not sure about the rest... that's what I'm trying to learn here :)

[steelcomp]

Curtis,
If you really want to know what's going on, talk to Chris Straub...CStraub69 here on the boards. There isn't a better cam designer in the country.

[paradigm shift]

There are many factors besides the cam in your reversion question. Exhaust has a lot to with how big a cam you can run without reversion along with how well your heads flow. Maybe I missed it but more information would help. Type of exhaust, where the water dumps in and I missed what motor 350 or BBC and type of boat and drive set up. Many people I think run too much cam for a boat. Most cam companies over the phone reccomned too much cam for me.
Infomaniac here on the boards is a good resource if you give enough details on build. He has done a few motors. :D Choosing a cam can drive you crazy and you will get opinion from one end of the scale to the other. Look for proven packages close to your build and better yet from someone you trust.
Don't stress over the choices and remember to have fun with it!! Good luck with your build. Keep us informed as it comes together.

[curtis73]

Vortec 350 that I'm building for an Alpha in a 19' Baja SunSport. Plan on 5000 rpm, 320-ish hp, replace my rear riser exhaust with center 4" risers to a stock Y pipe and thru-hub exhuast.

[SmokinLowriderSS]

Not really sure where this will apply since I have a 454, not a 350, but I am running Lightning headers, have 1 full summer on them, and no reversion troubles. At least, I certainly had no hydro-lock trouble, and no water-in-oil trouble.
Cam is a custom grind of an Isky 280H, 232* duration at .050", 110* LSA, ground for faster ramp speeds and ground 4* advanced, installed straight up (so is running 4* advanced).

[cfm]

Vortec 350 that I'm building for an Alpha in a 19' Baja SunSport. Plan on 5000 rpm, 320-ish hp, replace my rear riser exhaust with center 4" risers to a stock Y pipe and thru-hub exhuast.
Ton of people running the XM264HR and XM270HR hydraulic rollers with Merc center rise exhaust and thru hull exhaust with no reversion issues on 5.7's and stock thru aftmkt heads.
However, I have no info on build-ups with thru hub exhaust, so take my info above with a grain of salt if staying thru hub.
Most Stock Merc flat tappet and HR cams for 5.7 are as follows:
Stock flat tappet cam:
.004 " 270, 284
.006" 258, 270
.050" 200, 212
.400", .410" with 1.5
110 LSA
===============
Stock hydr roller
GM#14097395/ Merc#431-811658
.004" 258, 272
.006" 249, 264
.050" 197, 207
.430", .450
109 LSA

[SmokinLowriderSS]

My biggest concern over duration has less to do with reversion and more to do with the RPM range of the HP the engine is delivering. Longer duration pushes the torque and HP peaks into a higher RPM range. It also beyond a point, starts hurting the lower RPM torque. Both these effects are detremental to a performance boat engine (less so but still IMO detremental to a car engine). You can cover a lot of sins with gears, torque converters, and clutch slipping (kinda like ketchup on marginal food) untill you can get the rev's up.
I have dealt very little with small blocks, but on a big block, a need for muscle in the 5-6,000 RPM range causes a need to avoid .050" durations over 240*, and 230* area is better yet (a number hard to get as lifts get tall tho).
My concerns would be worse about reversion if I was running a wet (injected) exhaust. There is a lot more leeway from a dry (jacketed) exhaust with it keeping the water out for a couple feet away from the heads instead of a couple inches.

[MACHINEHEAD]

Only have done thru hull with 218 @ .05 on 350. To be real safe with the y-pipe, run the xm264. Get a chain and gear set with adjustable cam timing. Install it on a 104 intake centerline to get that exhaust valve opening a little sooner. Or offset the price of the chain set and have comp grind one up with an extra 4-6 degrees ICL. Turn around is very fast, maybe an extra day or two, and only about another $40!

[MACHINEHEAD]

SS you are correct. Plenty of real dyno tests have proved that a 230/236 +/- 2 deg. cam with a vortec headed 350/383 shows the best average HP/TQ numbers. This cam size is a comprimise in terms of HP, but these motors continue to make more TQ without a upward shift in the TQ rpm range. At about 235 @.050 the TQ curve begins to shift upward and begins to narrow

[SmokinLowriderSS]

Then those duration numbers are where I would be looking to build arround. Thx for the mouse-motor knowledge. :) My 454 is working very well on 232*, and about as fast-opening a hydraulic grind as you can get made.

[396_WAYS_TO_SPIT]

Mr Straub will be in here tomorrow to clear things up;)
_Spitter

[curtis73]

===============
Stock hydr roller
GM#14097395/ Merc#431-811658
.004" 258, 272
.006" 249, 264
.050" 197, 207
.430", .450
109 LSA
Whoa... this may be a moot point. I was going with a custom cam because I thought the factory roller was 194/198. If its 197/207 at 109, that's darn close to my 200/210 at 108 imaginary cam.
I give up some top end, but only about 15 hp, which translates to, what... 2-3 mph? Here is how they both spec out, installed 4 retarded:

[paradigm shift]

C73 it looks like you have some more direction on choices. You lost me at exhaust thru the hub sorry. I think no matter what manifolds you go to your restriction will be the Y-pipe thru the hub.

[502schiada]

Give George a call at Clay Smith. He will get ya squared away.
As for the reversion, my boat with stock merc manifolds and a smaller cam use to suck water like crazy. Rebuilt the motor put in a bigger clay smith cam 256 deg @.050, int. and exh. .647 lift and put on a set of lightning headers and doesnt suck water anymore.:D

[curtis73]

C73 it looks like you have some more direction on choices. You lost me at exhaust thru the hub sorry. I think no matter what manifolds you go to your restriction will be the Y-pipe thru the hub.
I would think so as well, but (literally) dozens of folks have told me that they gained absolutely nothing going thru-transom and no one, zero, nobody has told me that they DID gain anything. I'm tempted to believe that the stock Y and hub can support 350+ with no issues.
I'm sure at higher hp levels its cause for concern, but I want this thing to be QUIET. I use it a lot for fishing at 5am on a resort lake and I don't see the need to make it loud especially if it doesn't offer any speed. I might go with a switchable exhuast if I rob a bank and just want to spend $1000 on making it sound good :)

[BLOWN JOB]

doug herbert in anaheim he is one of the best tell him your specks he set you on the right path

[SmokinLowriderSS]

I would think so as well, but (literally) dozens of folks have told me that they gained absolutely nothing going thru-transom and no one, zero, nobody has told me that they DID gain anything. I'm tempted to believe that the stock Y and hub can support 350+ with no issues.
I'm sure at higher hp levels its cause for concern, but I want this thing to be QUIET. I use it a lot for fishing at 5am on a resort lake and I don't see the need to make it loud especially if it doesn't offer any speed. I might go with a switchable exhuast if I rob a bank and just want to spend $1000 on making it sound good :)
I have not heard from anyone making performance gains from going thru-hull on a small-block. It's not the HP level, it's the exhaust gas volume. The stock system has enough flow capacity to not be a big restriction to the smaller motor. To make up the 100 CID difference a 350 has to turn about 1/4 turn farther every revolution (or 25% faster) to match output on a 454. Volume on a 454 at 4,000RPM takes a 350 5,000 rpm.

[cfm]

The stock GM/Merc Hyd roller is only good for approx 295-300hp with good 4bbl carb, intake, and with dry basic dyno headers.
Overlap is one thing we must consider but that only involves exhaust closing and intake opening points. Tightening LSA opens/closes exhaust later and opens/closes intake earlier. Widening LSA opens/closes exhaust earlier and opens/closes intake later.
If 325hp or so is your goal I believe we are going to have to run some more duration.

[Ryan00TJ]

I would have no problems going with the Comp XM264HR with stock exhaust. With 212/218 @ .050" you'll have -9* overlap @ .050". Even the larger XM270HR 218/224 has negative overlap of -3*. However with your 5,000rpm powerband I would stick with the XM264HR. You said you plan to switch manifolds anyway, why not pick up a set of those Moore/GLM HP manifolds I see all the time for $110 a piece. While not a EMI or SM they will guarantee no reversion and only add power over a stock rear exit stock manifold.
As to the no power gained from thru hull on a SBC, in most cases it's probably true. A stock 5.7L is not going to gain much with thru hull exhaust if you keep the stock manifolds and risers. My experience has been a bit different. Stock my Challenger went 60 on radar with a 5.7L 260. Picked up 2 mph by adding HP manifolds and 4" smooth risers thru hull on stock 5.7 260. Added AFR 190 heads, Solid 257/269 cam, Vic Jr, Holley 750, roller valvetrain, 21p labbed cleaver and gained 14mph spinning the alpha to 6500rpms.
As this will illustrate it's all about combo. The more power making mods you perform, the more you stand to gain from thru hull.

[mtnrat]

I would have no problems going with the Comp XM264HR with stock exhaust. With 212/218 @ .050" you'll have -9* overlap @ .050". Even the larger XM270HR 218/224 has negative overlap of -3*. However with your 5,000rpm powerband I would stick with the XM264HR. You said you plan to switch manifolds anyway, why not pick up a set of those Moore/GLM HP manifolds I see all the time for $110 a piece. While not a EMI or SM they will guarantee no reversion and only add power over a stock rear exit stock manifold.
As to the no power gained from thru hull on a SBC, in most cases it's probably true. A stock 5.7L is not going to gain much with thru hull exhaust if you keep the stock manifolds and risers. My experience has been a bit different. Stock my Challenger went 60 on radar with a 5.7L 260. Picked up 2 mph by adding HP manifolds and 4" smooth risers thru hull on stock 5.7 260. Added AFR 190 heads, Solid 257/269 cam, Vic Jr, Holley 750, roller valvetrain, 21p labbed cleaver and gained 14mph spinning the alpha to 6500rpms.
As this will illustrate it's all about combo. The more power making mods you perform, the more you stand to gain from thru hull.
I have decided to go the XM264HR and did pick up a set of Moore/GLM manifolds and risers. The pieces I am now are sourcing are a balanced rotating assembly with quench pistons and chevys new small-port vortec bow-tie heads.

[cfm]

I have seen no killer deals on those heads. Everywhere I've called say 'same ol' same ol' $1000-$1100 for pair assembled. Let us know if you do find a better deal.
What are you going to be using for an intake ?

[mtnrat]

Yep, I think I will have to just cough up. Now the intake is a good question. I am FWC so it will likely be aluminum and likely a performer vortec.

[Ryan00TJ]

I have seen no killer deals on those heads. Everywhere I've called say 'same ol' same ol' $1000-$1100 for pair assembled. Let us know if you do find a better deal.
What are you going to be using for an intake ?
$1100 for those heads is kinda steep or am I off base here? I mean $300 more and you can have a set of aluminum AFR Eliminators!!!

[cfm]

Those Bowtie Vortecs are pretty bad azz. Run right with the Dart Iron Eagles.
I think bang for the buck right now is the Brodix Iron Killer's. Usually found for $1G-$1100 even. You can even get springs for .575 lift HR's for that price too.
As far as iron, I seriously like the Bowtie Vortec's.
Speaking of intakes - with that cam and those heads, a regular performer will step on the airflow a good amt. I'd go RPM or AirGap Rpm. No doubt. Oh, and if you decide later to throw more cam at it for way more performance, you won't have to buy another intake.

[SmokinLowriderSS]

Speaking of intakes - with that cam and those heads, a regular performer will step on the airflow a good amt. I'd go RPM or AirGap Rpm. No doubt. Oh, and if you decide later to throw more cam at it for way more performance, you won't have to buy another intake.
I agree, get the RPM Air Gap if you can find one, drop the extra change. I have one on my 454, the low-RPM opperation is flawless after a couple min to warm up the first fire of the day, and there is plenty of headroom on the intake airflow to deal with higher RPM, especially if you increase the "visibility" of both sides of the carb with a short spacer plate (like the nitrous plate on mine).
I don't know how much the "air gap" helps on a covered boat, but it gets the hot oul off the bottom of the runners, and I'm pretty sure the fuel mixture running down the intake cools it a bunch more than a standard performer would.

[cstraub]

It seems to be a trend in our industry to find a reason to let go good knowledgeable people from the sales and tech rooms of performance companies and replace them with a body that can answer phones, read from a prepared questionaire, and then suggest a part in stock.
Anyway, reversion is based on overlap, as Scott said Intake Opening and Exh. Closing and the time (degrees of duration) that the valve on each is open (overlap).
In this situation that you need a quiet fishing boat with some extra grunt I would take the advice of others and just put in the Comp 264. Your after a little more power then you have now and marine friendly. That cam should do the job. Your intended RPM is Peak is in a range that spring pressue needs to be addressed. I would run a minimum of 130# on the seat pressure.

[curtis73]

I heard you might chime in.
I'm going to shamelessly pick your brain now that you've opened the can of worms :)
Do me a favor and take a peek at the picture linked below. Its a dyno simulation that compares the XE264HR to a theoretical cam that I made up that specs out to 200/210 on 108 LSA. -11* overlap (compared to the comp's -9*)
The basic summary is that they make darn near identical curves, but the cam I concocted does it with 12* less duration on the intake and 2* less overlap. Since the cam I made up has a touch less overlap (despite its narrow LSA) I assume that reversion is a non-issue.
Questions (if you feel compelled to answer)
- I know that reduced duration can offer a reduced idle speed, but does is that strictly because of reduced duration, or is it because of the implied reduction of overlap? When comparing these two cams in the picture, would the 200/210 cam have a lower idle because of its lower duration, or would they have similar idles because the overlap is very similar? Another way to ask the question might be... does the similarity in the curves imply a similarity in idle speed?
- The cam I made up is simulated at 4* retarded to close the intake later and shift the hp up. Does retarding the cam (closing the intake later) affect idle speed?
- As a combination of the questions above; you recommend the comp 264 which is 212/218 on 112. Since the 200/210 on 108 gives similar tq/hp, is it a better choice if it gives a lower RPM.
I hate to second guess cam manufacturers, but I doubt they place as much importance on idle speed as I do. I think most sport boat owners don't care as much about idle speed as long as they can dock safely, but I like to think I'm a little more informed than the average boat owner.
Sorry to spring this on you, but the others foretold of your contributions :)

[cstraub]

I heard you might chime in.
I'm going to shamelessly pick your brain now that you've opened the can of worms :)
Do me a favor and take a peek at the picture linked below. Its a dyno simulation that compares the XE264HR to a theoretical cam that I made up that specs out to 200/210 on 108 LSA. -11* overlap (compared to the comp's -9*)
The basic summary is that they make darn near identical curves, but the cam I concocted does it with 12* less duration on the intake and 2* less overlap. Since the cam I made up has a touch less overlap (despite its narrow LSA) I assume that reversion is a non-issue.
Questions (if you feel compelled to answer)
- I know that reduced duration can offer a reduced idle speed, but does is that strictly because of reduced duration, or is it because of the implied reduction of overlap?
Both duration ( how long are we open) and overlap (the time in which the intake opening and exhaust closing each have valves open) effect idle with the valve events having, IMO, a larger effect on it. Reason I side this way is if you properly cam an engine which in simple terms let the air in and out at the right times I have seen "lope" deminish and throttle response will increase.
When comparing these two cams in the picture, would the 200/210 cam have a lower idle because of its lower duration, or would they have similar idles because the overlap is very similar?
Would have to put them on a dyno or run them back to back. Without knowing the duration at seat, .100, .200 to get an idea of lobe area I would not want to guess.
Another way to ask the question might be... does the similarity in the curves imply a similarity in idle speed?
- The cam I made up is simulated at 4* retarded to close the intake later and shift the hp up. Does retarding the cam (closing the intake later) affect idle speed?
- As a combination of the questions above; you recommend the comp 264 which is 212/218 on 112. Since the 200/210 on 108 gives similar tq/hp, is it a better choice if it gives a lower RPM.
Valve events are the most critical in camshaft selection. When an engine is properly cammed with the cam that it needs you should be at max power with the way the cam is ground. "If you have to advance or retard a cam to gain power you have the wrong cam " Exact words from John Reed to me many years ago, God rest his soul." Your simulation shows similar power but the real test is in the boat. As far as which is a better choice without knowing the particulars of the combo, the intended max RPM, the flow numbers on the heads I can't comment on what is best. Will either of them do the job you want? Yes, your not after a max effort engine, you seem to want a little more performance for the money and if this is the case bang for the buck would be the shelf stock Comp because a custom is going to run you more money.
I hate to second guess cam manufacturers, but I doubt they place as much importance on idle speed as I do. I think most sport boat owners don't care as much about idle speed as long as they can dock safely, but I like to think I'm a little more informed than the average boat owner.
Overall boating manners is up to the owner/driver. IMO I would not want something that was hell to dock, not interested. Also overall width of the powerband. Boat and heavy street cars are 2 performance applications that need wide powerbands with boats not having the luxury of multiple gears, converters, or clutches. So breadth of power under the curve is what is most important. A marine engine that has a powerband of 1000 as compared to an engine with a 2400 rpm power band: The 1000 is going to be a slug to get on plane if it is an I/O, nasty docking manners because with the peaky powerband it is over cammed, be slow to gain rpms getting to WOT, but at peak WOT should and will run its best. The second with the 2400 power band will be quick to get on plane, good docking manners, and accerate like a raped ape through the rpm range, but may be a little slower at top end then the first boat.
So we have to decide how we boat
Sorry to spring this on you, but the others foretold of your contributions :)
If you have read this and still have questions you can PM me, post more on the thread, or call the TN Redneck and talk. 423 854 0007
Chris

[curtis73]

Ok, I'll chew on that for a while and let you know if I have any questions.
Thanks to all for the excellent info and help.