BEWARE THE FLOW TEST

by Lee Wickstrom

Quoting from the SuperFlow operators manual: " In it's simplest form, flow testing consists of blowing or sucking air through a cylinder head at a constant pressure. Then the flow rate is measured at various valve lifts. A change can be made and then the head can be re-tested. Greater air flow indicates a flow improvement. If the tests are made under the same conditions (emphasis ours), no corrections for atmospheric conditions or machine variations are required. The test results may be compared directly."

"Under the same conditions"; therein lies the rub. Variations include, but are not limited to:

1. Brand and size of flowbench -Variations between brand of flowbench can be a problem when comparing data. There is probably no good rule of thumb for comparing results from different brands of flow bench. Even when using the same brand of flow bench differences will be seen between benches of different capacity. (for instance a SuperFlow 110 vs. a SuperFlow 600)

2. Test pressure used - SuperFlow has a chart in their operators manual for converting flow figures from commonly used test pressures to other commonly used test pressures. Flow figures taken at 10" test pressure must be converted in order to be compared to flow figures taken at 25" test pressure.

3. With or without intake manifold on intake port - Obviously a flow test done with a restrictive stock manifold on the head will not show as much cfm as one with a very unrestrictive manifold.

4. With or without carb on intake - Same idea as with the manifold, in order to compare apples to apples the flow figures you are comparing need to be taken through carbs that flow the same.

5. Type and design of radius entry on intake - If testing is done without a carb or manifold a radius entry should be used at the port entrance. The shape and size of this radius entry can effect flow figures. If an intake manifold is used on the head, it too should have a radius entry.

6. With or without pipe on exhaust - Adding a piece of pipe to the exhaust when using a SuperFlow 110 does not seem to effect the results much, while on the SuperFlow 600 with its higher test pressures it makes a large difference.

7. Design of pipe on exhaust - flow figures will increase from having a "bell mouth" or "flare" on the exit end of it.

8. Operator methodology - The way in which the operator does the test can also play a big part in results obtained. The angle that you read the manometers from will affect the results, as well as how quickly you take your reading after turning the machine on, to name just a couple variables.

All of these variables can be minimized to the point of being negligible if you are comparing only heads that are checked on the same flow bench. In other words, "Cousin Roy's Backyard Porting Shop" (fictitious name, I hope) with his SuperFlow 110 may do just as good a porting job as Mr. Hi-Tech in LA (who uses a SuperFlow 600 with all the bells and whistles), if he has done his homework and learned from his testing, porting and re-testing. The trouble is, it may be hard to know how his work compares without testing Mr. Hi-Tech's heads on Cousin Roy's bench or visa versa.

How then can the average consumer pick the best cylinder head shop to spend his money with? Well, here are some guidelines that we would suggest:

1. Spend your money with a shop you can trust. Ask around. If possible talk to someone who has used their services before.

2. Is the shop's reputation based on advertising or on results? Anyone can claim to be the best cylinder head porter in the business. The truth be known, any GOOD porting shop is capable of putting out quality work. There is a limit to how much any valve size is physically capable of flowing. Most talented cylinder head porters should not be too far apart on their flow figures for the same head and valve size. Much of the difference in their flow figures may be attributable to the items listed previously with the heading "under the same conditions."

3. Does the shop keep records of their flow testing? If you are paying for a porting job and flow test you should get a copy of the results. If you are not getting a flow test with your porting job, the price should reflect that. Some misunderstanding can be eliminated by using correct terminology. A flow test is only the actual measurement of the airflow. Porting is the actual modification to the head to increase the airflow. A "ported" head has not necessarily ever been on a flow bench. A "flowed" head is for the most part a meaningless term, since you can put a stock head on a flow bench and test it. Usually when a person uses the term "flowed head" what they really mean is "ported" or "ported and flowed". It is not illegitimate to sell a porting package without flow testing. Remember though, without a flow test you are at the mercy of the experience of the person doing the porting. If they do a lot of the particular head and valve size you are using, then they can probably be fairly consistent without checking every head. Stock evo heads are pretty easy to be consistent on, but if you only port one every 4 months you may not realize that you have done something different and lost flow unless you test the head. Shovel heads are at the other end of the spectrum. The large differences in port core from one head to the next make it very difficult to get consistent results without testing and rework. CNC ported heads are a different matter altogether. Due to the repeatability of the machine, the minimal difference in flow from one head to the next should only be because of core shift in the head casting. In this case a flow test is not really needed. All heads should flow reasonably close to the CNC Port Master (the head which was ported by hand and then digitized for CNC)

4. You get what you pay for... sometimes. A mediocre porting job will probably flow about halfway between a stock head and a well ported head. Hopefully the price will reflect that difference. Don't expect top notch work for a dirt cheap price. On the other hand there will always be people out there who are more than willing to charge premium prices for mediocre work and make up for it with bravado. See # 1 above. SuperFlow says there is only a 1% difference in the flow loss due to wall friction between a sand cast and a polished surface. Shine does not necessarily equal airflow.