extending stock risers
 

I've seen mention of it several times here on various threads, but never a discussion devoted to extending stock exhaust risers to increase the water introduction point. Does anyone have experience with this? It seems like an inner pipe extension could be welded to the riser elbow with a piece of rubber tubing over to create the longer water jacket. A baffle (reversion ring) could be fabricated on the new outlet end to further reduce reversion chances. I have GLM manifolds (small block chevy) and would like to go more aggressive with cam if I can find a solution for exhaust. I'm eliminating my thru prop, removing the Y-pipe, and installing side exit tips due to a low swim platform so I will have a 90 degree elbow close behind the riser. Does Imco have a riser that would solve this without buying a complete exhaust?

Corsa for 90's or whatever turns / bends and Merc or GLM extension riser blocks that fit in between your riser and manifold to raise the waterline height to aid against reversion.

First I don't see anyone welding to cast iron, next the flexible rubber tubing will not hold up and have other issues. To help with reversion adding riser extensions will help
http://www.mercruiserparts.com/bam/s...31954/7585/180

You need something like this but you can only do it the SS risers (BBC).

http://imageshack.com/a/img923/8082/atA9Fm.jpg

Yes, that is exactly what I'm trying to do. Are those the 6" extra tall SS risers from IMCO? where did you get your elbows from?

Some of the elbow/riser combos from Merc for small blocks were stainless steel. Not all were cast iron. Second, a vertical component in the exhaust prior to water introduction does not aid much in regards to reversion. Water introduction as far away from the highest point in the exhaust will be the most benefit.

I have a guy I think I met on here that will take my 496 SS risers and makes them any size I want. The last set I had shortened so a BBC would fit under the hood of a Donzi 18. The picture is from my classic race boat where the front motor sits real deep in the hull. Had to use two 6" extensions to make the height right so it wouldn't back flow into the motor (the rear motor is almost the same). I hated the extensions so I had those risers extended accordingly and now have six 6" risers and two 3" risers sitting on a shelf where they should be.
PS, the work I had done on both sets in the race boat was cheaper then one set of 6" extensions for the same exhaust and looks much cleaner.

Here is the short set for the Donzi

http://imageshack.com/a/img924/4509/DQQhWo.jpg

http://imageshack.com/a/img922/2676/4IqOxz.jpg

Trash, who makes the longest tail riser stock off the shelf that will bolt up with a GLM cast manifold? I can't go dry thru hull straight back due to transom shape / swim deck so I need enough room to elbow sideways.

I wish I knew. It is problematic for SBC folk. The only thing I can tell you is that I've gone as far as a 221/230 @.050" on a 114 LSA roller cam and it doesn't revert with GLM manifolds and stock Merc stainless steel elbow/riser.

I'm not recommending you go dry either but the solution you seek with the stock manifold is elusive. I wish that weren't the case.

I'm just thinking out loud at this point but does anyone know if the Stainless Marine riser/elbows would bolt up? I don't think so but I haven't tried it.

Trash, I'm wanting to go to the Comp XM-276HR which is 224/230 @.050" but I think its a 110 LSA. But the 4 degrees less LSA combined with 3 degrees more duration means more overlap correct? EMI has some complete manifolds and risers that I can make work, I just don't want to spend that much to get the job done.

I've extended the inner pipe on the riser on a pair of factory style GLM center riser exhaust by welding 2.5" mild steel exhaust pipe into the 2.5" pressed in place pipe. It kept the water and exhaust seperated for another 6"

The only thing I would add is if using it in saltwater you may want to go with stainless tube.

You'd have to look at the intake and exhaust overlap with data off the cam card. I'll try take a look at it later or perhaps SB is motivated to look ;)

I will guess you are close to reversion. When I went to Crower and got my cam Jerry (at Crower) said don't go any bigger.

We use these - Standard height thru transom stainless steel riser pipes fit stock Mercruiser center rise manifolds, as well as Glenwood Magnaflow manifolds. (Sold in pairs $1130.00) All kits include all necessary mounting hardware and gaskets. Weld on or Bolt on brackets available for Shift, ECM or EFI

On my 383s I had some custom ones made from Hardin Marine. I had some pretty nasty cams running in to GLM manifolds then the Hardin Marine risers. Worked very well.

have stainless extensions welded on to your current exhaust portion of your risers. Tack weld extensions with the riser mounted in place to the current angle of your tips. Then pull the riser and fully weld. If done right the extension should not touch your exhaust hose. Bring the extensions to about 1 inch off of your current tips. Nothing else in your setup will have to change. I've done this with great success combating reversion.

I ran the xm276hr on a 355ci. With stock length risers it will revert water. Found small puddles in the center manifold runners. I added 8" extensions to my Hardin stainless risers to fix the problem. Was running GLM alum manifolds. Xm276hr is 224/230 on a 112+2.

GLM sbc setup. Water hoses are covering riser extensions

http://i77.photobucket.com/albums/j7...imagejpg22.jpg

I've done the same thing to my 496 setup.

http://i77.photobucket.com/albums/j7...416_173901.jpg

Ryan, did you like that cam for a small block build? Did you run it straight up or advance it any? Mine will be a 383. Are those risers the Hardin 620-32410 part number that you started with? Thanks for the help.

I ran 1.6 rockers on intake with that cam for more lift/duration. Installed straight up as it has 2* advance ground in. With my AFR 190R heads it pulled very nice to 5600rpms. With it in a 383 it will be all in at 5000-5200 roughly. Mine were the 620-32420. If adding extensions these are what you want. The inner exhaust pipe is extended past the outer water jacket making it easier to weld too vs the 32410.

Ryan, did you ever dyno that engine? Just curious as those are the heads I am using for this engine. Do you think there was anything to gain with putting 1.6's on the exhaust or did the grind have enough duration even with wet exhaust? I'm going to start with the 600 edelbrock carb I have from my current 383 setup. Depending on where it drops off on top end, I'm putting together a 750 DP Holley to try as well.

Time will be better spent with the Holley.


Ryan's 1984 Hammond Challenger MKII V2150 ran 80.5mph with that motor. :ernaehrung004:
http://www.speedwake.com/forums/107-...tml#post681072

I know SB, but I only have a 30 gal. fuel tank :) My boat is very close in weight and dimension to that Challenger and I would hope a more efficient hull. Wow! Wasnt expecting those kind of numbers!!! I'm at 62 now with 340 hp.

Never dyno'd. I always liked the GPS dyno. The 1.'6s on the intake were per Bob M's advice. The XM276HR is perfect for a Marine 350. In a 383 with AFR heads it will idle like a dream and have great midrange punch. IMO, if your after top end power it and a 600 carb will be disappointing. If set on the XM276 then 1.6's all around will not hurt. My HP750 and 234/240 cam made the 383 a real top end screamer.

Ryan, reviving this post for a little more advice. Maybe Trash and SB can chime in as well please. Finally ordering parts for motor so I can get it to the machine shop. Definitely going with the 276 comp cam. I'm needing advice on static compression ratio with this cam. My choices in d dish pistons put me at 9.35:1 or 9.77:1 with close to ideal quench. Do I play it safe or will that result in a lazy engine in the lower rpms?

That engine will not be lazy with the lower of the two compression's in your 383 with that cam.

I agree. Truthfully, I don't believe you would feel the power difference between the two ratios on the water. Running alum heads, I always shot for 10-1 static. When tuned right they ran great on 89 marina fuel. A 9.7-1 alum head SBC tuned right should be fine on marina 89 fuel. If you can't source 89, shoot for the lower compression and go boating.


I ran AC FR2LS @ .050"gap in mine. There a step colder than what AFR recommends. Holley 750HP 78P, 88S 2.5PV, 87-89 octane, 36* Timing 10.18-1 static.

Great info. As a note, those ^^^^ plugs are no longer.