Twelve Engine Output Tips - Dandy Dyno Dozen

Just before doing an interview I was asked if I could describe, in one sentence to a less technically inclined audience, the process of generating power. Sure I can: Get it in, keep it cool, squeeze it tight, aggressively light it off, burn it well, expand it with minimum loss and dump it.

Granted, an English major might see that as a sloppy sentence. Grammatical purity aside, will following the simple premises outlined in that sentence actually deliver useful extra power? The answer is most certainly yes, and here are a dozen moves-each costing less than $250, and some less than $10-that prove, within the context of that sentence, that every little trick helps. Making such moves is important to the racer who runs an engine not so much on gasoline but a shoestring.

Let's start with "getting it in"-"it" being, of course, our working medium, air.

Any time performance and cylinder heads are mentioned in one sentence, most shoestring racers start to see large numbers of dollar bills involved. This need not be the case if the class calls for stock production heads, but so often heads sourced from the wrecking yard are past their prime. Worn valve seats and guides can cost more power than you might think. If those valves don't hit the seat and seal up instantly, they leak. The high-pressure gasses which leak back into the intake cause a drop in the engine's breathing efficiency. We are running a near-stock motor here and any losses are unacceptable.

Some years back, I dyno'd a friend's two-barrel Street Stocker 350 Chevy and it was somewhat down on what I expected to see. In an effort to find out why, I grilled him and determined he had not rebuilt the heads as I had told him to do. It's Sunday, the engine is on the dyno, and I had all the gear in the shop to do a guide and seat job. Nothing fancy, just K-Line guide liners and a 30/45/60 valve seat job. If the cheapest oversize stem valves had been used, this job could have been done in a regular machine shop for under $200. This op fixed guides worn to about 0.005 clearance and put a decent set of seats into the equation. Four hours later we re-ran the engine and the results, which will probably surprise many, are shown in Chart 1.

Stock stamped steel rockers wear out at the tips, and although they can be re-dressed, the fix is temporary as it inevitably goes through the hardening. A new set is going to cost about $75, but for a little over $150 you can get a set of Comp Cams Magnum cast stainless rockers. These may not look as racy as a set of aluminum rockers, but they are at least $75 cheaper. In those classes where a change of rocker can be made, these inexpensive rockers can really deliver, especially when a change in ratio is allowed. If the rules call for a restricted intake, then the rate at which the intake is opened and closed becomes even more important to the production of power. Chart 2 shows how the output changes when a set of 1.6:1.5 Magnum rockers for the intake and exhaust, respectively, are used to replace the stock 1.5:1 stamped steel rockers used on both the intake and exhaust. The real gains start at about 4,000 rpm and between 4,500 and 6,000 typically amount to 20 or more hp.

We have looked at a couple of ways to increase intake airflow, so now let's look at an easy way to cut the temperature on an engine regulated to the use of a stock intake. Keeping heat out of the intake is important at any stage of the game but the more the carburetion is restricted the more important it becomes to keep unwanted heat out of the intake charge. A simple move here is to fill the heat cross-over passage in the intake with concrete and then use a performance manifold gasket that has no heat cross-over passage hole. These two simple moves cost very little and typically produce the results seen in Chart 3. At 3,500 rpm, the gains were as much as 8 lb-ft and at the top end 5 lb-ft. Not bad for about $12!