Ask Away! with Jeff Smith: Can I Put a Blower on a 383 with 10:1 Compression?

blown 383 - Steve's Nova Site

(Image/Steve’s Nova Site)

Can I put a blower on my balanced and blueprinted 383 c.i.d. stocker motor that has 10:1 compression? I just want it for looks, not to race. – G.H.

Jeff Smith: At first, my gut feeling was to answer no—it’s not a good idea.

The only way this will really work is to set up the blower to spin very slowly and make less than 5 psi of boost—which defeats the whole purpose of having a supercharger in the first place other than the visuals.

But then I thought about it more, and there are several approaches to this question that would make it feasible.

While you say this is strictly for looks, we both know that once you bolt a blower on the engine—you’re going to run it with your foot buried in the throttle. There’s no way to avoid that!

The idea is to avoid running into detonation problems from excessive cylinder pressure. There are ways to avoid detonation, but require making sacrifices or investing in additional systems to make it work. On the plus side, with 10:1 compression—that motor will be crisp!

You didn’t mention whether this engine is carbureted or runs EFI.

We’ll assume it’s carbureted but that really isn’t critical.

Any engine with a 10:1 static compression ratio is right on the edge of working with 91 to 93 octane pump gasoline, assuming an aggressive timing curve intended to maximize power.

If we now add a supercharger even with just a small amount of boost (let’s say 7 psi), this is roughly equivalent to raising the static compression ratio to 14:1 at full boost.

Conventional wisdom holds you can’t run an engine on pump gas with a 14:1 compression ratio. Having said that, this is the effective compression ratio at that amount of boost.

At part throttle running around town, you probably could get away with running a blower very slowly and the engine might not detonate. But you’d never be able to go to wide-open-throttle (WOT) because the engine would detonate badly unless the timing was severely retarded.

At that point, it might actually make less power than it would normally aspirated because of the combination of retarded timing and the power it takes to turn the blower.

Consider an E85 Conversion if Fuel is Readily Available

One option would be to convert to E85 fuel. This is a pump gas—but its availability is rather limited outside the Midwest. This fuel is 85 percent ethanol—essentially grain alcohol. The other 15 percent is a cheap grade gas—likely something not quite as good as base grade gasoline. Combined, these two fuels are worth an octane rating somewhere in the neighborhood of 100 to perhaps 105 which is equivalent to a mild race gasoline.

The downside to E85 is you can’t just pump it in the tank and run. It’s not that simple. Ethanol creates about 70 percent of the heat per pound of fuel compared to gasoline. Which means you would have to burn more fuel to make the same power.

This means an E85-dedicated carburetor, or—if the car is fuel injected—a dramatic 30- to 50-percent increase in the injector size.

Of course, you could run your engine on 100-octane race gasoline and accomplish the same thing with no conversion costs, but at $7 to $8.50 per gallon that quickly gets prohibitively expensive.

We have run a 10:1 compression 4.8L (293 c.i.d.) LS engine using a blow-through carburetor and a Vortech centrifugal supercharger.

With 6 lbs. of boost, we made 500 rear wheel horsepower. But this was using 100-octane race gasoline.

It’s possible that you could also mix pump gas 50/50 with 100-octane fuel to produce a 95- to 96-octane fuel that might allow the engine to run cleanly but still with retarded timing.

Another idea would be to convert this engine over to run on E85. This fuel not only increases the octane but also adds inlet air cooling benefits that would likely offer benefits that would allow us to make even more power.

A major problem is that E85 is possibly not available where you live. Next, this conversion, even if you have an EFI engine still requires some sophisticated tuning techniques to get it right. From your question, we’re going to guess this may not fit into our plans.

Consider Part Changes to Lower Static Compression Ratio

Another alternative if you still want to run a blower on your 383 c.id. engine, would be to change parts to lower the static compression ratio. You should be able to run a 7-8 psi blower with a 9:1 static compression ratio and the engine might run fine on 91 octane fuel.

Changing pistons is the best way to affect this change, but it might be possible to reduce compression ratio by switching heads and adding a larger combustion chamber.

It depends upon the existing heads’ chamber size. If the chambers are already large, then you may have to go to lower-compression pistons.

Water/Methanol Injection Could Work Too

As yet a final option would be water injection. Running a water/methanol injection system like that offered by Snow Performance, combined with keeping the timing conservative, might reduce the inlet air temperature sufficiently to keep the engine out of detonation.

Water does a great job of reducing the inlet air temperature. The engine benefits because for every 25 degrees of reduced inlet air temperature, the engine’s octane requirements are reduced by one full point—from 96 to 95 octane, for example.

Methanol combined with the water further reduces inlet air temperature but also adds its own octane to the mixture, so this type of system has some real advantages. The key is to add the water into the airstream as far upstream as possible because it gives the water the time needed to reduce the temperature.

Plus, water also tends to slightly reduce cylinder temperatures during combustion. Tests have shown that water injection alone will reduce inlet air temperature roughly 50 degrees, which is equal to reducing the octane requirement by two full points.

All of this combines to improve the chances that the engine won’t detonate.

The bottom line here is—you could get a blower to work. But it will take more work than simply bolting one on your engine and driving away.

It would take professional-level tuning skills and careful attention to detail, but it can be done if you’re conservative.

The key to making this a successful venture would to become fanatical about preventing detonation.

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  1. Buddu Noland says:

    What else do you want to know? That was pretty mind boggling!

  2. Mark Eitemiller says:

    Very nice job of looking at this from many different angles or perspectives. My initial answer is no. But the difference you nicely pointed out is what could be done and what should be done.

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