What type & size would bring on the best power in a 454 with a 8-71 blower?
Static compression 7.5:1 TRW forged blower pistons in a LS-7 long block, iron square port heads, Weiand 8-71 with 2 850 cfm Holleys, Stainless marine aluminum exhaust.
Berkeley jet with stainless "A" impeller in a 1975 20' Spectra
With flat hydraulic cam now boat runs 78gps on 91 pump gas making 8psi boost at 5700rpm.
Just looking for better cam choice and considering roller cam.
Any ideas??
I think part of what you need to consider is maint. It is my understanding that you have to take the top of the motor apart and look at the lifters and keep an eye on the bronze gear as it wears. Flat tappet cams you dont have to do that stuff. But I could be wrong.
You could go with a hydraulic roller. The only down fall is the cost. The roller cam and roller lifters are much more expensive than a flat tappet cam and lifters. If you install a hydraulic roller with similar duration and lobe centers as the cam your currently running( the lift on the hydraulic roller will be more than the flat tappet in most cases) you will most likely see around a 25 to 30 horsepower increase sometimes more.
If anybody reads this thats running a blower on a jet boat.... what cam are you using? And hows it workin for ya??
Thanks for the input Jim B, i'm considering a hydraulic roller even though its a little more costly. I'm just wanting to find out witch one to get??
The Crane 741 & 731 cams work great.
Originally posted by Infomaniac
The Crane 741 & 731 cams work great.
Good choices from someone that is in the know. These are pretty mild cams but with a blower you do not need to go real big. Lower lift and a little longer duration will help with reliability. You will want to check around on what manufacturer of lifters you are going to use. They are NOT all made the same. Are you looking at HYD roller or possibly a Solid roller cam? Watch your spring choice and buy the best quality springs you can. Set up is critical for it to live. What HYD cam are you running now? Just my .02
Great info..... One of the cams i was lookin into is the crane hyd. roller 139741 and cranes rollers to. (plus the shorter pushrods)
The cam in there now is a crane 134242
.534 in and .553 ex
duration at .050 is 226 in and 236 ex
This cam seems to work ok but peters out at a top end of 5600 rpm and the motor feels like it wants to go more.
I think i have ruled out the solid roller because of the extensive machine work to the heads for the killer springs they need.
75 Spectra,
The machining to the heads isn't that big of a deal to go to a bigger diameter spring. The solid vs hydraulic debate is more a factor of your desire or lack there of to check valve lash in exchange for a couple more HP. Depending on your valve cover style with the stainless marine manifolds, you probably have to drop the manifold down a tad to get to the lower valve cover bolts. Something to think about when deciding between a cam that requires regular maintenence and one that doesn't.
Having the same hull as you with various engine combinations in it, I know where you're comming from looking for more top speed, but how much are you expecting to gain from the swap? How fast do you ultimately want the boat to go? Since you're setup is running close to 80 right now, perhaps there are some other setup choices in the pump that might get you to th 80 mark without opening up the motor.
It's hard to tell from the pic, but it looks like there isn't shoe/rideplate setup on the pump. That could be another way to go faster without touching the motor. Just givin ya a couple more options.
Also, Crane offers roller cam's with standard distributor gear compatible drives so you don't have to worry about the bronze issue Omega mentioned.
Good Luck,
Chris
Thanks, flat broke.... Thats some good info and exactly what i'm getting from everbody local. If it runs that good why f$#@ with it? I dont really know what i'm after just wanting to try something diffrent i guess? Speed is not really the ultimate goal just having the power available is nice.
Yes the boat does have a rideplate and shoe set up, i added it last year with help from American Turbine with great results. The extended nozzle and trim were leaps and bounds better than the droop snoot (for this hull).
Thanks for the input........
75Spectra,
Post some pics of your Spectra, I would like to see the blower setup up close.
Thanks,
Brent
www.spectramarine.com
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