Print

Email

Surfacing Equipment Can Increase Horsepower

By David Vizard

If your shop is in the business of selling performance, especially to street and/or moderately budget-constrained clientele, then some of the best free advertising you can get is by word of mouth from satisfied customers. Often this can be no further away than taking advantage of a few simple machining operations and explaining to your customer the potential gains versus the cost.

Your head/block surfacing equipment is a good example of machinery that performs a simple operation that can significantly contribute to an engine’s performance. One of those jobs can be the optimization of the block deck height in relation to the piston height at TDC.

A common move when building a relatively serious performance motor is to deck the block so that the pistons make a closer approach to the face of the cylinder head. This increases the quish effect thus increasing the mixture motion, which in turn creates a faster more desirable burn. Often customers will avoid this operation because they see it as an increase in compression and therefore assume detonation will be more likely to occur. The truth is that even though the compression ratio (CR) has gone up the engine’s resistance to detonation has also increased. Here a case example will serve to show how much the excessive quench clearance can adversely affect power.

Some 17 years ago a good friend of mine in the blower industry was developing a kit for small block Chevys. This was a positive displacement type blower and he brought over a built test engine to run on my dyno. The intent was to produce a 600 hp engine with totally street drivable manners that would run premium pump fuel.

This motor sported a set of ported aluminum heads and a solid street roller cam. At a little over 7 lbs. of boost it made 528 hp and some 520 ft.lbs. of thrust. Any attempt at an increase in boost to achieve the target output produced detonation. After grilling my friend on the buildup, I found that since he had used a new block he had not decked it, especially since it produced the 8.5:1 CR he was looking for.

I advised him to take the motor back to his shop, strip it and deck the block so that the net clearance between the pistons and the head face was .030˝. This would mean the pistons had to be .008˝ out of the hole with the .038˝ Fel-Pro head gasket used. This move also raised the CR from 8.5 to 9.07:1.

Back on the dyno the motor, with all else the same, made some 540 hp and 538 ft.lbs. and no sign of detonation. This was encouraging so, by means of pulley changes, the boost was increased until detonation was seen. At 9 lbs. of boost, almost 2 more than we could initially run, the motor was once again just short of detonation. At this point the motor made just under 580 hp and just over 580 lbs.ft. Admittedly, that was still short of the target but it was a lot closer, and for a pure street motor with a 650 rpm, glass smooth idle it certainly was nothing to make excuses over.

The effect of an operative quench action on a supercharged engine seems adequately proved at least for power plants of similar valve and chamber layout to a small block(SB) Chevy. I knew that minimizing the quench area clearance was also good for normally aspirated engines as well, and had it in mind to do some tests that would establish just what it may be worth. These got put off time-after-time because it is a time consuming and relatively costly experiment for a one-man business to do. Fortunately, while lecturing at the SuperFlow Advanced Engine Technology Conference in 1998 I met a gentleman who had actually done these tests.

As I remember it, the 350 SB Chevy test engine was in the 380-400 hp range and started with a CR of just 9.9:1. After baselining with the piston-to-head quench at .065˝ (.040˝ for the head gasket and .025˝ down the hole for the piston) the quench clearance was reduced in steps of .010˝ until the quench clearance was .035˝. When you are looking at potentially small changes in power data, scatter is a consideration. However, by averaging out the numbers the results appeared to consistently indicate that each .010˝ reduction in quench clearance was worth 6-7 hp or 5.5 to 6.5 ft.lbs. of torque.

The .030˝ change in quench clearance increased the CR from 9.9 to 10.6. So I made some relatively sophisticated calculations to establish what the increase in output from this factor alone would be. The answers I came up with indicate that, at best, the increases seen were only 60 percent attributable to the increased CR. Other issues of note were that the engine needed less total timing to make the power seen. Along with this, the ignition swings in the advance direction indicated it to be no more prone to detonate even with the higher compression.

The point of our discussion is that whatever equipment you may use to deck blocks can usefully be used to generate additional performance in the form of increased torque throughout the entire rpm range. Plus an increase in mileage at a cost that is attractive to the customer. Those extra ft.lbs. are just like adding cubic inches, but without the fuel consumption penalty. Since most street customers judge the performance of their engines by the amount of torque available right off idle, this move will make your engines look good.

So how close can the quench clearance be run? Good question. A friend of mine who ran a machine and dyno shop until recently has, using good stiff race bottom end parts, run down to .022˝ before contact was seen. I have run a SB Chevy with stock crank and rods down to .025˝ where it showed that contact had just occurred. This was a motor that peaked at 5750 rpm. In practice I usually build a typical domestic V8 for a customer with .035˝ total quench clearance and that seems to be safe so long as the pistons are close fitting in the bores. For my own motors I typically go to .028˝ when using a known combination of good bottom end parts.

Although I would hesitate to say it is the last word, my experience to date has shown that attention to minimizing the quench clearance also pays dividends on almost any engine with limited quench area. This usually is the case for multi-valve engines.

As far as failing to produce positive results there has been one notable exception. During the mid-’70s I was competing in the fiercely fought British Touring Car Championship with a 1600 cc Chrysler UK car. The car concerned had a very tough engine designed with emission compliance very high on the priority list. As such the combustion chamber was formed by the piston stopping short of the top of the block by some quarter of an inch.

In stock form the head itself had virtually no chamber. Each time a new motor was built, which in effect was one for each race, I sunk the chamber into the head deeper and brought the piston further up the bore while maintaining the same mandated CR.

At each stage the motor made more power. When the piston was .080˝ down the bore (for .120˝ total quench clearance) I decided to make the move to minimize the quench the rest of the way in one go. With the pistons out of the block by .010˝, and the chamber all in the head, the motor was loaded on the dyno. Result – a drop of 8 hp over our previous best. This just goes to show nothing is sure-fire in the world of automotive technology.