Nicolasticity

About 10 years ago a B&M174 blower was fitted to an HP425 Mercruiser High performance engine. At the time the engine pressures were 130psi measured cold WOT. Engine design as shown in Figure 1 had a design PR of 175psi which was achieved through dome 12cc pistons and forged part to cope with the load. So you could say that the blower made up for a tired engine…


I note that the standard MCM magnum 454 has a dished piston and a lower CR and design pressure of 150psi.as shown in figure 2.

So if I rebuilt the HP425 properly (big ask!) then I would offer to the blower 175psi (un-blown) which I am getting a bit nervous about. I think 175psi basic pressure is too high to be blown. Gas is very expensive in the UK so I want to use regular gas and not use octane enhancers or worry about detonation. I’m more of a cruise guy than a go-fast!

So what to do?

Build as is (nearest equivalent piston is a larger dome 13.8cc Speed pro) and get 8.9:1 or fit a flat piston and have a CR of 8:1?

MILD THUNDER

iTrader: (3)

I know we've touched based on the this compression discussion for your engines before, but....

Do not believe everything you see in the mercruiser manuals. The 425HP 454, and 454 Magnum, used the same pistons. I had a pair of Merc 425's, and they did not have 175psi of cranking pressure, even though, some manuals state that is the spec. They were right around 140psi give or take, which is right where they should be, considering the 8.8:1 static compression, and camshaft they used. The 365HP version, while rated at 8.6:1, also had 150psi cranking, but also had a smaller (earlier closing intake valve) camshaft than the 425HP engine. That spec is normally figured at sea level, with a brand new engine, etc. now, you're numbers of 130ish psi you saw, are probably realistic for a 425 merc with some time on it.

The later the intake valve closing point, the more cylinder pressure is bled off, at low engine speed. This is why, from the old days, when we installed a big old full race cam in a stock low compression big block, it was a turd off the line, and didnt come alive until the upper RPM. Cams and compression ratios go hand and hand.

Now, there are a couple of options IF you are worried about the compression (which really isnt a problem). If you pistons are in good shape, you can use the money that it would cost for new pistons, and have the stock heads worked. Some porting of the chambers, can help airflow esp at low lift on those heads, and will usually add about 3cc of volume to the chamber, which will lower the compression a little, about .2, so say from 8.8, to 8.6. A .050 thou head gasket, will drop it again to say 8.5:1.

Or, you can purchase new heads, with a chamber volume of 121cc, like some Dart 308 Iron eagles, or similar, and while lose a little bit of compression, gain a bunch of free power, from the better flow characteristics over the old Chevy irons. Or, sell the 174 blower, for a 250 blower (significant increase in power, with no reduction in reliability).

With that being said, if it were me, and on a budget, I'd rebuild the 425 to stock specs, run a slightly thicker head gasket, and focus on the tune of the engine. Fuel, timing, water temps (keep low) etc. It should give many years of trouble free service.

Nicolasticity

Spot on - I must just get on with it! Your help really appreciated. One final question.

Stock cam is a Crane 132561, I think its a 'mild' cam and fine for a bit of boost but would I be better with a Cam suited to a blower 134241?

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MILD THUNDER

iTrader: (3)

That 241 cam really wouldnt do anything significantly better, than the 561 cam in this combo, and not worth the change imo.. With your small blower, I'd rather keep the 114LSA, rather than the 112LSA on the 134241.

If your running a decent performance exhaust with the water being introduced further downstream, the 134561 would be my choice. The stock 525sc had 7.5:1 compression, with the 132561 cam. You are over a full point more static compression, and I think the 134561, with some added duration, bigger spread between the int/exhaust, would work nicely on your combo. I have a very similar cam in my blown 468's, just in a hyd roller version. The larger cam will also drop your cranking pressure down a bit, and give you a little more safety from your compression concern standpoint, and run a bit better at wot.

Nicolasticity

I have a standard Gil system which I think is good - would that be OK with a 134561 CAM? Is a performance exhaust required because the exhaust gas temps are up or is a it for flow?

Swamplizard

sorry to wake up an old thread but curious how this worked out

I am looking to bump torque and a bit of HP to bone stock merc 454 330s with Gills. dock fuel down here where I boat is decent non ethanol 91 but occasional bahama runs might drop octane levels so need conservative....

what did this combo end up doing?
if I run 4# of boost will they live?
cruiser will never see 5000rpms

thanks

B38

Yes. What was outcome please. Itthinking on same mod

dereknkathy

best bet is to PM him. he may not have even came on to OSO in the year since this thread was alive. But probably still has same email address.

Nicolasticity

Rebored last year SERDI in London way and now going to build - just not enough time!

Nicolasticity

COVID retirement - engine installed but hey ho still not run. Where does the time go?