Cam and speed question: Merc 370 HP-454 in Scarab III 34'

[tomcat]

A friend of mine is rebuilding his engines from a 1985 Scarab III 34'. The engines have:

- Qjet on GM high rise dual plane
- Rectangular port heads
- Gold rocker arms
- Log style exhaust manifolds
- Borg Warner transmissions
- TRS drives

I'm pretty sure these are the specs for Merc 370 HP.
When he bought the boat nine years ago the engines had just been rebuilt. The gold rocker arms were added at that time. Tearing them down we see pistons in good condition, they are .030" over with small dome pistons.

The strange thing is the cam. It's a Federal Mogul CS-1005 with only 208 duration and .461" lift on the intake. Catalog gives the power range as 1500-4000. This is the same cam specs as a 300 HP peanut port 7.4L.

What is this cam doing in these engines? My friend has never been able to spin 19" Mirage props up to 5000 RPM. Maybe this is why!

What cam should he have? And how fast should he be going with 370 HP engines in this boat?

[38KV]

I'v just checked the Merc service manual from 1987 (i don't have the -85) and it's kind of confusing.

It says the MCM 370, MCM 420 and HP 420 use the same cam.

The specs are:

Lobe lift Intake .294
Lobe lift Exh .294
Dur Intake adv 356
Dur Exh adv 352

[38KV]

Found the valve lift specs!

In/out .500

BTW, this is strange. The 400 cyclone seems to have a "hotter" cam compared to the 420 since the book says 370 and 420 use the same cam.

400 cyclone specs are:

Valve lift In .530
Valve lift exh .551
Dur in @.050 298
Dur exh@.050 306

400 and 420 both have 8.8:1 CR.
The use the same heads and intake.
Both have roller rockers.

What I can figure out from this book is that the only differences between those engines is:

Carbs 400= Rochester/420= Holley
Ign 400= Mallory /420= Thunderbolt
Exh 400= Iron crap / 420= GIL
Cam "Hotter" in the 400

Can somebody explain this?

Sorry tomcat, I'm out of topic, back to the 34'

[PatriYacht]

The engine could be either a 370 or a 400. Because it has roller rockers, I'll guess it's a 400 Cyclone. The cam is a Crane 132561. It's the same cam used in the 400, 420, 425 and 525sc. Crane says it's applicable to the 370 with some modifications. It's a good performance cam.See it at www.cranecams.com.

[38KV]

The valve covers will tell if it is a 370 or 400.
370's have the same as 330/365
400's have the same as 420/425 (cast alu)

Am I blind or is my book wrong, now your'e saying the 400,420 and 425
use the same cam,which of course makes more sense.
I have to read that chapter again.

[blue thunder]

Originally posted by 38KV
Found the valve lift specs!

400 cyclone specs are:

Valve lift In .530
Valve lift exh .551
Dur in @.050 298
Dur exh@.050 306

KV are you sure those duration specs aren't seat to seat?

BT

[formula31]

The cam specs in my book (90-14499--1 887) are wrong and meaningless for "advertised" duration. Their "note" references are wrong too. While that cam does seem small, be careful going too big . That boat is heavy and needs torque.

[38KV]

I'm not sure of anything anymore

I think my book must be missing here.

It is the Mercruiser service manual nr 9 from Brunswick printed 1987.

The engine specifications says "Note *2: Duration @ .050 (1.27) cam lift"
But I agree, these values doesn't make sense.

This is even more confusing:

MCM 370 400 440 460 MCM
MCM 420 cyclone cyclone cyclone 575
HP 420 HP 575
------------------------------------------------------------------------
lobe lift in .294 .312 .323 .363
lobe lift exh .294 .324 .328 .375

dur in 356 *1 298 *2 288 *2 292 *2
dur exh 352 *1 306 *2 296 *2 300 *2

Note *1 Duration give is "advertised duration"
Note *2 Duration @ .050 (1.27) cam lift.

I can't take this for truth even if the book is a real pro manual for authorized work shops, not some cheap aftermarket crap .What do you think BT?

[38KV]

Oh chit! something funny happend when I posted that,
I was supposed to be a schedule.
I'll try to edit the damn post.

[blue thunder]

I have often thought it would be a great value added feature if OSO would put together a concise factual database of specs on all popular production engines. They could also add to it a section on 330 on roids specs. It would really answer a lot of conflicting data.

BT