imq707s

I've got a Jasper Marine sbc 350 in my 21' scarab. I'm trying to find a few simple way to get a little extra power out of the motor. One thing that has worked on my street car has been to upgrade the stock 1.5 ratio rocker arms to 1.6 ratio in order to get some more valve lift. Depending on the cylinder heads, sometimes you'll get more power and torque from just changing the exhaust rocker arms and leaving the intakes at the stock ratio. Has anyone ever experimented with this in a marine application? Is it worth messing with? Also, what would be a good cam to put in that motor? I know that CompCams make a few Xtreme cams that are for marine use.

jr

I have 1.6's on my 409's. I'm also using and CompCam Extreme Marine cam. I only have 6 hours on the engines and they need some tuning, but I'm pretty happy with the cam. There are cams out there that have more lift, but the 1.6's may get you there. I would becareful about putting a high lift cam and 1.6's in. Or using anything over stock before you clay the piston to make sure you dont smash a valve into the piston, or have too little clearance.

I know of a person here that is using an Extreme marine cam in a 383 that is getting about 380 hp, with vortech heads. He is using the same intake and cam I am. But I'm using World Torquer Heads. I'm getting, according to Desktop Dyno, about 425 HP. Desktop Dyno is alittle optomistic but I've done other things to compensate.

imq707s

Thanks for the info JR, how do you like that cam? Does it relly suck down the gas? Does anyone know what kind of HP the high performance roller cam Jasper 350's put out?

jr

I like the cam alot. Idle is pretty good, and the power out of the hole is great. Mid range snap is very good. As for gas, well, they burn alot, but I'm also way to rich. My plugs come out black, wet and smell like gas. I need to rejet my carb and see where its at on economy. Keep in mind, that the Comp Cam is a flat tappet hydraulic cam, not a roller. You'll give up some performance to the hydraulic flat lifter. I went this route so I didnt have to adjust solid lifters as part of my maintainance. Also I would have had to had the block preped for roller lifters.

The engines now are turning a 15 1/2 x 22 inch 4 blade props. I need to reprop to at least a 24. I spin up to 5000 rpms before I back them off, and the secondaries (Mechanical) haven't even opened yet.

I looked at Jaspers website. Their 350 Marine engine puts out 325 HP.

[ 01-15-2002: Message edited by: jr ]

jafo

I went with 1.6's (CompCams Magnums) on my 355's- on the intake only. Used an Isky cam (flat tappet hydraulic). After talking with Bruce about the benefits of rollers vs. flat tappets in a SBC, the HP gains just aren't quite worth it for the large cost of a roller set-up. The issue of 1.6's are covered in just about every SBC build-up book, including Dennis Moore's, and the consensus is to use them intake only if at all.

imq707s

I've always heard that you'll get more power by adding the 1.6 rockers just on the exhaust valves because of the restrictive exhaust ports on most SBC heads. Am I wrong? Is it different for every motor/cam setup?

BajaRunner

Im interested to hear about this one. I have thought about going to the 1.6's.

I have 350 mags, with hot cams and wonder if it would help.

good thread

Jason

imq707s

All you're really doing is adding more lift to the valves without adding any more duration. I've messed around with this concept on my 350 sbc in my street/strip camaro. You can gain a little power, but nothing huge. Maybe 20HP? I think it really depends on your engine combo. If your heads are really lacking in exhasut port volume and you put the 1.6 rockers on and add more lift to your exhaust valves, you might gain some power. I think it's all about testing and tuning.

jafo

If you can find it (Amazon.com?) I highly recommend a book called "How To Build & Modify Chevrolet Small-Block V-8 Camshafts & Valvetrains" by David Vizard (Motorbooks International 1992).
The topic of rocker ratios, including many dyno runs using different ratios on both single and dual pattern cams are covered extensively in this book.
There are many dyno runs and graphs showing that using 1.6's on the exhaust only will lose power, not gain. The exhaust side on the SBC is shown to be more responsive to increases in duration, not lift. Adding 1.6's on the intake only can gain up to 17-20 HP.
Take note that a stock Chevy SBC rocker arm actually works out to around 1.42 ratio, not 1.5- good aftermarket rockers (CompCams, Crane etc.) are true 1.5 or 1.6 and can add performance when coupled to the right cam.
I read a great deal of this book (and others) and made a helluva lot of phone calls before I even started on my engines. Research never hurts!
Jim

GopherBaroque

Note that you can get a similar effect as 1.5 to 1.6 rockers by swapping out the OEM Mercruiser for:
Crane Cams HR-260-2-12 IG (109811)
Isky 201262/270-12 (262/270 HL)
(by specification, not accounting for jafo's reminder about stock rockers being 1.42)