Retarding cam - help reversion?
 

I ended up pulling my engine for some winter upgrades ( I can't leave anything alone) and in doing so it looks like I am getting some reversion like some on this site had noted I may have with my exhaust (Imco Thunder) and cam. I currently have it installed 2 degrees advanced as the cam card had suggested but I am not running 11:1 (actual around 10:1). I am also looking at increasing my upper RPM range to the 5700+ range. Currently the motor was all done at 5400. Would retarding the cam back to straight up or even 1 degree retarded help my reversion issue. This should help my upper RPM range but not sure how much it would as I have never done this. I am not against having another cam done but seeing if this might work. Attached is my cam card and some exhaust port pics.

the more overlap a camshaft has the greater the chance of reversion,imo 26 deg is the problem.i don,t think retarding the cam will cure the problem but it might help.

I believe I read on one of the posts someone mentioned 24 degrees as the most you want for overlap? I am contemplating on a cam and want to make sure I get everything right.

Is it just one exh port? I remember your build and if I remember right you dyno'd engine. Repost if u would. Meanwhile compare timing events with other MARINE friendly cams. You did a great job with boat. That almost looks like it could be condensation. Hard to tell from my ph driving.

In my opnion with no experience on this (shifting cam around trying to fend off reversion), I would think advancing that cam would be less like to revert.

Reason is overlap after TDC (when piston going down and exhaust valve still open) is when reversion happens...right ? So, retarding cam would put more overlap ATDC.

Again, my thoughts....speaking of which, I would think that cam, no matter where you have it installed, will revert with those exhausts.

It is in every exhaust port some worse than others. What I was hoping for with retarding the cam was to get more rpm out of the engine and hoping that would help the reversion. As discussed before with my exhaust fixing the reversion with this cam will be hard so I would rather buy a cam then some headers. I would like to be closer to 5700 rpm just for personal preference so I think this might be a good time for a new cam compatible with my exhaust.

Ok on a wet exhaust overlap should be kept under 6° according to crane cams.

I can't make out your duration so I can't give advice there.

one thing I can tell you is you are going to want to go with way less overlap a 114 lobe center and shoot for close to 6°overlap.
you will probibly need to reprop the boat by 2 inches or more so you spin it closer to 5000-5200.

They (Crane) say 6* overlap when measured at .050" .

Most measure overlap at .004" or ,006" . So, you'll need to run the #'s at .050" to compare..if you so desire.

Should have been 114 lobe seperation angle or lsa for short.

you are correct I should have explained it better.
Must be tired.

Edit in: His overlap is 26* at .050" .

His ,050" cam timing says Intake Opens at 11*BTDC and Exhaust Closes at 15*ATDC

Dry tailpipes/problem solved

I wish, Lake Tahoe has very specific sound levels and they enforce it. Even with mufflers you should see the looks I get at the dock when I start it up.

With dry pipes he might get 5700 without a big torque drop with a cam that won't stall at the dock etc.
It will be close.

With wet ones he needs to cut back on the overlap and keep in mind they won't flow enough to see 5700 with his cubic inches without a large torque drop off.

He needs to either go dry or change the cam to one with way less overlap and duration and prop it to match a 5000-5400 limit.

How do I know this I've talked to several cam companys and ran many dyno programs to get some ideas for my 550 inch build.

Now I know they are not perfect but give you a good idea of where the torque drops.

Thanks for that information. That is really good to know before jumping into having a new cam ground and not gaining much if anything but no reversion. My last trip out I feel I finally had the tune at its best all season and I was able to push the boat to 57 mph at 5200 rpm with a 20 pitch 3 blade. I could really be fine with just boating as is but I have set a goal to hit at least 60 on Tahoe with the N/A motor I have. The problem is I set this goal before I ever took the boat out or even knew how fast it would go and having no prior boating experience. I know speed is relative to people but what is fast for a single motor 30ft boat? I had it out this season on a lower elevation lake and I hit 62 mph and the boat felt stable and was a hoot to drive so I know I want to go faster. I am finding this to be a challenge but one that has been fun. I have certainly learned a lot about boating performance but I have an appetite for more. You have to pay to play, so maybe some good exhaust is in order to accommodate a decent cam change.

Sent you a pm

thank you... I was thinking that too

Maybe put a call into SLR Cams. You made good power especially with stock heads. You could always exceed your goals by installing crane 651 cam and a 871 blower. There was a great deal on some a while back in the swap shop. Should put you in the mid 70's. Now that sounds like fun. Btw for a single I'd say your pushing that boat pretty well for a 30 footer.

I discussed this on another thread but unfortunately I have no space for any blower pulleys. I only have a few inches of space with the stock crank 3 rib v-pulley. Again me not even paying attention to this when I bought the boat because I have no experience.

Been around for many years but stil a great cam and I bet would work well with your current build / heads. I'm not sure what your plan is to do but thought I'd post the cam card anyway.

Just wanted to make note I'm not certain timing events are the same for gen VI cam 169651 as one listed above. One above is 139651 non step nose. I would imagine they are but...

That would be a good cam for his application if it wont revert.
still is going to have to prop the boat for 4800-5000 rpm.
And both the numbers have the exact same timing events just one is a step nose and the other is not.

sent you a pm

same cam except for nose.

651 should pull peak 5,700-5,800 easily in a 540.

I assumed they were but you know what assumption can be remember? The mother of all fk ups. Mike you've used the 651 in several builds. What's typical peak hp in a 540 +/-? Also is there known reversion issues with wet exhaust. I wouldn't think it would but I'm not certain as I've never used it.

Better yet Mike whats the torque difference between 5000 and 5700.and please take into consideration the 088 heads and wet exhaust.
And yes please educate us on if it will revert.

i never used a 651 in a n/a engine but in my blown 548s if i remember correctly they peaked around 6400 rpm.

i think the torque curve was pretty flat,they made good torque from 4000 on up.i am to lazy to go to the shop to look at the dyno sheets,also i was not worried about reversion due to dry exhaust.

You don't prop to peak torque if that's what your refeering to... well you can but will be a total turd.

I wasnt propping for peak torque.
The dyno program I'm using shows a drop of 135 foot pounds from 5000 to 6000 rpms.
With the exhaust.Heads Cam etc all programmed.
If he had better flowing heads and exhaust I don't think it would drop off like it shows.
I know it's not exact but they usually are accurate to show a pattern and when torque drops.

So of you think that he will have a higher top speed with that kinda drop off then i dont know what to tell you.
I personally do not.
Ps with that cam he will have around 580 foot pounds at 2000 rpms so I doubt it would be a turd.

Did you input what intake he's using also. Dual plane, single plane big dif in torque curve. I understand what your asking however you completely lost me when saying he'd still have to prop for 4800-5000 with the 651 cam. More like what he's looking for if I had to guess. 5,700.

Yea I understand. Not everyone likes dry pipes. I've had several nasty comments concerning my exhausts but even more complements and there are places I simply have to avoid.

thats funny. Some people just like to complain because while others dig it. I don't like waking up to fishing boats leaving at 5:00 AM nor sailboats noisy mast clanking all night long either. I don't complain about that. See how they like my twin 900 SC plus dry pipes. Lol... From past experience I have to agree with you though, the higher percenter like it.

Tried both and torque still dropped big time between 5000 and 6000.

You just keep concentrating on the cam, cubic inches and hp numbers.
The torque is what drive the boat to accelerate.
you can have a higher Hp number because you are spinning the engine faster and still be dropping torque like a rock.
If the boat was making 450 lbs torque when it topped out at 5400 then it needs more then 450 to keep accelerating.
Which means you would need to prop it to a point where it had more then 450 or it's not going to go any faster.

The dyno program showed his current cam dropping off from 5000 and being down considerably by 5400.
Just like it is doing in real life.
And the 651 is smaller and for a lower rpm range then his current cam.
So I really don't know how you think it will make the boat have a higher top speed.

After reading it looks like the 651 will revert so I would recommend the 771 cam
or something close in a different brand.

So it seems to me from the information given I need new exhaust if I want to get any kind of RPM and a decent cam. Since I already have it down to a short block I will probably look for heads or have a pair of those promax 317's done. My question on headers is I see a lot of used ones on this site and I really don't think I am going to buy new headers for this boat as they would be more than I paid for the whole thing. Would any name brand header be OK or should I be looking for a specific brand model for my engine size as I have seen some that look to be aimed at a 496?

I think that's an excellent idea.
I don't know near as much about good headers versus bad ones.
So I'm going to bow out.

You might want to start a new topic about quality used headers etc

I'd go with Stainless Marine manifolds with long tailpipes, (all the way through the transom). Water exits at the very end of the pipe. That's what I run except I welded up the water exit holes and welded in a bung near the end of each pipe and dump the water out the side of the hull. They are super reliable, relatively inexpensive compared to headers and no worries with water crawling that distance back up the tailpipe.

We docked at a restaurant back in the summer that had their wine glasses handing in a rack. When we walked in the bartender was like, "dude, your boat is so loud the damn wine glasses were clinking together."