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measuring Cubic Inches at the track

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measuring Cubic Inches at the track

Postby miniv8 » Sun Sep 09, 2007 7:09 am

In the classes that limit engine size, how are the engines measured?

Ive heard of gadgets that screw into the spark plug hole to measure airflow in the silender and somehow "know" the engine size. How do these devices take into account things as valve timing and piston ring blowby?

how accurate are these things, and how can they be fooled?


I'm not looking into fooling the device, as I will most likely be employing it myself at the track.
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Postby miniv8 » Wed Sep 12, 2007 7:50 am

Ive found the whistler that measure compression ratios, and that CI-tester from mr.gasket.
the mr.gasket thingy only measures up to 500ci and we have rules that limit at 515ci and 560ci.

How accurate it that mr.gasket stuff and is there anything else out there we can use?
What is used at your track?
Have you ever had your engine tested?
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Postby Racer Roy » Wed Sep 12, 2007 8:52 am

We had good luck with the Mr. Gasket tester at our oval track. It proved to be within a few percent every time it was used.
It is very simple to use, you just remove the rockers for the cylinder you are testing, attach the tool to the spark plug hole, set the piston in the tool down, and crank the engine. the air in the engine is moved out into the tool, moving the indicator up, to give a volume reading for that cylinder. you just multiply that by the number of cylinders to get a total displacement. When the track first got it, they did a couple of tests, then removed a head and measured the bore & stroke and found it to be quite accurate. Close enough to find a 383, that's for sure.
Hope this helps

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Re: measuring Cubic Inches at the track

Postby BillyShope » Thu Sep 13, 2007 8:09 am

miniv8 wrote:
how accurate are these things, and how can they be fooled?


Well, I know of one NHRA record (both ends) that was set by a car with an oversized engine and the tester didn't catch it. This was at the Indianapolis Nationals in, I believe, '63. I had accompanied the owner and builder (now deceased) and we were chatting when he mentioned the engine's bore and stroke. I had my pocket slide rule handy (no electronic calculators then) and, for the lack of anything better to do, calculated the displacement. Uh-oh! The engine was too big. I told the owner (he had already won his class and set the records) and he spent the next few hours sweating profusely!

The engine passed! So much for the accuracy of those testers.

(My friend was so dominant in his class that those extra cubes wouldn't have made any difference as far as his elimination win was concerned, but, of course, they made it that much harder for someone else to set new records.)
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Postby Racer Roy » Fri Sep 14, 2007 8:51 am

Well, I know of one NHRA record (both ends) that was set by a car with an oversized engine and the tester didn't catch it.

I knew as soon as I posted my story, someone would have one like this :lol:

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Postby BillyShope » Fri Sep 14, 2007 8:08 pm

Roy, there's always the possibility that the tester was dead on, but the mistake was made by the NHRA officials when they calculated pounds per cube. This was, after all, a class which held little interest. In fact, I think the class was dropped the next year.

The class was DM/SP and the car was an MGA with a 2 liter Coventry Climax engine. Bill Waddill, the owner/builder, had replaced the Coventry Climax engine with a 215 cube aluminum V8 for a wealthy Lotus sportsracer owner in Flint. The Coventry Climax engine had been collecting dust in his shop, so he asked the owner if he'd mind if he put it in an MGA and ran it at the Nationals. The Lotus owner thought it was a great idea and that's how Bill and I ended up at Indianapolis. (Every time the car made a pass at Indy, the announcer declared that it was Chevy powered. He evidently thought the Coventry Climax was some kind of misprint.)

Bill planned on pulling the engine after Indy, but someone suggested that he try to sneak it in as an MGA Twincam at an SCCA race. And, he'd have gotten by with it if he had been a better sandbagger. Of course, the whine from those spur gears driving the cams should have tipped off anyone that it wasn't an engine intended for the street.
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Postby larryjo » Tue Sep 25, 2007 4:07 pm

The gadget that is being referred to is a P&G tube. I worked for a local dirt track a few years ago as the tech inspector. We would spot test some of the top five late models. This thing is pretty accurate as long as you know how to, or take the time to use it right. You have to take a temp reading on the cylinder that you are reading and allow for this when figuring the cubic inches. I've had guys try some strange things to try to beat it. Putting little pieces of rags down in the cylinder, piano wire between the valves, things like this. We had one engine to fire up when the guy spun the engine over. That blew the puck clean out of the tube and about 50 feet into the air. The thing went straight up in the air and I caught is as it came back down. :shock:
I don't miss those days a bit.
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Postby randy331 » Sun Sep 30, 2007 10:22 pm

larryjo wrote: I've had guys try some strange things to try to beat it.


Did any of them work? :idea: I guess if the trick worked, you wouldn't have caught it.

Any engine I've had pumped, always pumped a few inches smaller than it was. The most recent was a 331", it pumped 329".


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Postby BillyShope » Mon Oct 01, 2007 4:49 am

That's like saying a shaft has a 1 inch diameter +/- 0.006 inches. I'd say that's pretty good accuracy.

Sometimes, manufacturers dazzle you with precision. A wheel scale, for instance, might have a digital readout to a hundredths of a pound. But, for all you know, the accuracy might not be any better than +/- two pounds. I'm not saying that's the case with wheel scales, but, when checking out a product, always ask for the accuracy, NOT the precision.
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Postby Bubstr » Tue Oct 02, 2007 2:52 pm

I've been checked a couple times, all within an inch or two. Unless it was a very small displacement rule like a go kart, they aren't going to get away with anything that will make a difference.

I believe the key is to have them drive under power to the check point, then back off and do the test. If for any reason this wasn't acceptable, it would be a failed test. Never test the same cylinder all the time either. Do it randomly. Draw a number or something. If your going to give a margin for heat or some mecanical discrepancy state that in the rules.

There will always be that illegal competitor that says you going to believe me or your lieing eyes. Make sure your machine is working well and say the Ayes have it.

Unless it's a weight to cubic inch thing, most cubic inch rules are to discourage the buying of expensive stroker cranks. With this intent in mind, most times there will be a more than ten cubic inch difference, so you could actually give a 5 cubic inch margin just in case the machine read wrong and still be with in the intent of the rule. Just state that before the first test.
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Postby BillyShope » Tue Oct 02, 2007 5:01 pm

A bit off subject, but, back in the days of the flatheads, I heard of an oval track competitor who was bone stock under one head and ported and relieved and full race under the other. He was finally caught when someone noticed that he always tore down the same side.
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Postby Tuner » Tue Oct 02, 2007 6:21 pm

Oh yeah….We had a family of Flathead Ford drag racers who were famous for that trick. The `49 to `53 engines that have the distributor on the right head always got the left head removed at teardown time to avoid the hassle of dealing with the timing, 3/16” over on the right side.

One time I was there when the patriarch of the clan was going to get the full body cavity search and he pulled the dipstick and showed the water in the oil while saying, “Why not do it next week when I have the new motor in?” He put the water in for just that purpose and claimed it was faster than plain oil anyway. Since Flatheads are famous for cracking whether they’re bored big or not, he got away with it.

I know someone who spent hours 35 years ago finding what size monofilament fishing line clamped in the header and in the exhaust valve seat would pump the right size for some circle racing when they were pumping them before they went on the track. The tech inspector couldn’t deal with the temperature compensation “problem” so he did them cold at the beginning of the day.
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Postby Bubstr » Wed Oct 03, 2007 8:55 pm

You have intrigued my interest. What kind of racing is this and what are the cubic inch limitations?

I was there for the Flat head days. I built a half inch stroker once and was afraid to use it. 322 inches of flat head. Finally ran it at a outlaw track and it was the biggest bust I ever built. My smaller ones out preformed it. Now I would have really felt bad if I cheated and lost.

I believe the most stringent rules and most well enforced, are in Go Karts. I got involved with them with my kids. We had a good time but if you was in the top 5 you could count on two hours and going home with your engine in a basket. They have measurements for everything and go threw them with a fine tooth comb. I seen one get DQed for porting because the guy used a ball hone to freshen it up, rounding one side edge of the port ever so slightly. I've seen one that had a loose key way get DQ for being worn out throwing the timing off. Now they use a cerate and transition fluid to check first combustion chamber volume, then running the piston to BDC to check cylinder volume. They check fuel two ways , electronically them the water test. If you ever has Gasohol in your storage container it would not pass, no matter how many times you you flushed it. Even with all of this there was areas that you could fudge. Not necessarily cheat but all chambers don't have to be shaped the same to get the same CC and ports that can't be ported come out right on the best hight and width and still look as cast. But if these guys didn't do that the factory teams that had access to thousands of engines would have the best reserved.
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Postby icebluecuda » Wed Oct 03, 2007 10:33 pm

A friend of mine was saying that one of Yunick's tricks was to bore out one cylinder and leave the rest legal. One in eight chance. :D
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Postby MadBill » Fri Oct 05, 2007 1:25 pm

My favorite was a guy who had a really nice set of mics, calipers, etc. and always let the inspectors use them on his engine, 'cause their own were kinda s****y.
Finally someone happened to use their own stuff on a teardown, and everything checked out way oversize! Turned out he had a special set of tools which were built for mold makers, and had the shrinkage factor of molten to cold iron 'built in' for convenience! :shock:
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