boatn70

i recently bought a 1998 checkmate with a 300hp 454 carb motor. i checked the block casting number today and it says its the vortech 7400 (L29).

so, what exactly does that mean for me? and what would you guys suggest as far as performance mods?
like everyone one else im just trying to milk every mph out of it with the smallest investment.

what should be my first step, second, third etc. eventually i would like to step up to the mag motor or the 502, just not in the checkbook right now. Is there a combination to do now that may be used on the new motor at a later date or is that out of the question.

thanks
kevin

Dennis Moore

Changing over to low restriction exhaust manifolds/risers will help performance now and later. Gil exhaust manifolds and risers would be my choice.
K+N flame arrestor/air filter and synthetic oil couldn't hurt.
Dennis Moore

Kidnova

Dennis, I don't want to steal boatn70's thread here, but I plan on changing the exhaust on my ZZ502/502 from the stock Merc stuff. Thinking of either lightnings with diverters, or Revolution Marine manifolds. Your opinion and or suggestions would be appreciated.

boatn70 might also be interested in your thoughts on this.

Thanks

Dennis Moore

I like the way that the exhaust gases are directed out of the Gil Exhaust manifolds. I also like the large size of the exhaust flange at the riser, the dry flange and the multiple water inlets at the bottom of the manifold.

There are only two ways that you can improve an exhaust manifold:

One, by separating the exhaust gas pulses between cylinders #5 and #7 on the port manifold and cylinders #4 and #8 on the starboard manifold.

Two, by making the internal passages much larger than stock and decreasing back pressure.

Unless you have individual runners for each cylinder (at least 30 inches long) with a collector at the end, you will never create any of the scavenging effects of a header.

Because there is 180 degrees of crankshaft rotation in the firing order between the center cylinders on all V8 engines the two center exhaust ports can be paired together without creating excessive back pressure in each exhaust manifold. This gives a smart manifold manufacturer the ability to design separate exhaust paths for the outer cylinders (Gil Manifolds).

Dennis Moore

Kidnova

I never measured the runners on Lightnings but they don't appear to be anywhere near 30 inches. So it sounds like a properly/well designed set of manifolds would do the job just as well as the lightnings.

I have your BB Chevy book and read the chapter on exhaust, and on cams {as well as the rest of the book}. Great info.

I heard or read {not in your book} that Merc was putting Gills on the hp500, then switched to another set up because of reported reversion problems. Think Merc then said to idle above 750 rpm, and rev the engine a few times before shutting down, after long idle periods. The cam in my ZZ is pretty close to that used in the hp500. The ZZ cam has a little less lift, but actually has a little more duration than the hp500 cam, and same 110* LSA. Because of that Gill's reputed reversion problem would worry me. Any thoughts on that?

Thanks