IRL engine spec changing at Indy 3.5 down to 3 liters - The Technical Forum Archive (2024)

canon1753

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The IRL has a maximum bore of 93mm. To get to 3 liters the stroke would be shortened by 9.2 mm. (55.3mm i think). How difficult and how expensive will this change be with the same rev limit?

McGuire

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Originally posted by canon1753
The IRL has a maximum bore of 93mm. To get to 3 liters the stroke would be shortened by 9.2 mm. (55.3mm i think). How difficult and how expensive will this change be with the same rev limit?

Not very, as the IRL will not allow a reduction in deck height to go with the reduction in stroke. The block must remain the same, as must the heads. So essentially we are talking about a new crank and a change in pin height (rod, piston, or both). Also, since the shorter stroke will decrease piston acceleration off TDC a bit, that would theoretically allow a little more intake valve rate. On the other hand these engines are already freakishly efficient volumetrically [due to the rev limit] and besides, at 3.0 liters there is now 62cc less cylinder volume to fill. So they will be taking a look at camshafts too, but that is an ongoing thing anyway.

In some ways a displacement reduction may be easier/cheaper on the engine suppliers than a change in the rev limit. These engines are very sensitive to harmonics in this rpm range...especially the cam drives and valvetrain.

marion5drsn

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Since a reduction is the stroke is going to get the stroke to bore ratio down to 59.35 percent and these engines have 180-degree crankshaft an increase in the rod length is going to somewhat help the Secondary Shake problem, I don’t believe it is going to be any worry in that particular area. The real problem is; why are they doing it? If they are doing it for the speed problem why did they allow 180-degree cranks in the first place. 90-degree cranks would have held the speed down automatically. No one, to my knowledge has solved the exhaust/intake manifold problem on a 90-degree V-8 to my remembrance. Close, maybe, but no cigars on that one.
M.L. Anderson

McGuire

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Originally posted by marion5drsn
Since a reduction is the stroke is going to get the stroke to bore ratio down to 59.35 percent and these engines have 180-degree crankshaft an increase in the rod length is going to somewhat help the Secondary Shake problem, I don’t believe it is going to be any worry in that particular area. The real problem is; why are they doing it? If they are doing it for the speed problem why did they allow 180-degree cranks in the first place. 90-degree cranks would have held the speed down automatically. No one, to my knowledge has solved the exhaust/intake manifold problem on a 90-degree V-8 to my remembrance. Close, maybe, but no cigars on that one.
M.L. Anderson

IRL engines don't have intake manifolds as such, but individual throttle valves for each cylinder. There is an airbox, but it runs at ambient pressure above the throttle plates. (Just as in F1.)

Meanwhile the dual-plane V8 intake manifold has fallen totally out of favor in racing applications which still use carburetors. For example, all NASCAR V8s use single-plane intake manifolds with their 90 degree cranks. The dual-plane manifold generally makes more torque until around 4000 rpm, due to its better velocity and more even distribution -- especially on the two adjacent-firing cylinders on the off-bank (#5 and #7, using Chevrolet numbering). Above that the single-plane manifold has the decided edge, due to its greater total air potential: every cylinder has the carburetor's total capacity available.

Thus it would seem the only remaining volumetric or "breathing" advantage of the flat crank is on the exhaust side. However, that is mainly a packaging issue: if the room is available, there is no reason why exhaust pipes cannot be paired on opposite banks. This was done on the 427 LeMans Ford engines: two outside pipes on one bank joined two inside pipes on the other, above the bell housing. This is still done on some stockblock-based V8 race engines today. On Ford's DOHC 255 CID Indy engine, the exhaust ports were routed out the top of the cylinder heads, which made the scavenge matches easier... or at least shorter. This engine used a 90 degree crank, thus the "basket of snakes" exhaust plumbing atop the engine.

In 2000 the IRL rules allowed both 90 and 180 degree crankshafts, allowing a direct A-B comparison, albeit with a 10,700 rpm rev limit. The pole and race were won with a conventional 90 degree crank. The winning engine builder, Doug Peterson of Comptech, told me he could see no clear performance difference on the dyno, and opted for the 90 degree combination as it was more thoroughly developed at that time.

GM did suffer some harmonic problems with the introduction of the 180 crank for 2000, mostly in the cam drive, which was solved by changing from a roller chain to a Morse-type steel link belt. In 2003 GM ran into cam drive harmonics again (among its other troubles) when it switched to gear drive, despite the use of pendulum dampers on the camshafts. The vibration problem was reportedly cured by enlarging the axle diameters of the train gears. However, the engine's real trouble -- the use of bucket cam followers -- could apparently not be fixed and it was retired in favor of a Cosworth-built engine. To me that was a shameful day for what was once the greatest engineering company in the world. Kettering and Cole must be looking down from above and shaking their heads.

In my opinion the one true advantage of the 180 crankshaft lies in the crank itself. A flat crank requires less counterweighting, for a lighter, stiffer crank and a more compact crankcase. The main problem of 180 crankshafts -- the secondary reciprocating imbalance -- seems to be of relatively little concern anymore, except for its effect on cam drive and valvetrain harmonics. That must be due in part to the trend toward shorter strokes, which of course reduces the magnitude of the reciprocating force.

Here is something interesting: ever since the introduction of the Ford flathead V8, tuners of American stockblock V8s have been fooling around with 180 degree cranks in search of a power advantage. It's been tried many times, and takes some effort, involving a new crankshaft blank and camshaft among other tasks. But no one has been able to produce a convincing power gain... in these particular engines, for whatever it's worth.

BRIAN GLOVER

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Very interesting. Thanks Bill.

Maserati will be entering tghe GT class in PLM. It will be interesting to see the difference in performance between that and the F360. Basically they have the same engine except for the Ferrari's flat crank.

Originally posted by McGuire

In my opinion the one true advantage of the 180 crankshaft lies in the crank itself. A flat crank requires less counterweighting, for a lighter, stiffer crank and a more compact crankcase. The main problem of 180 crankshafts -- the secondary reciprocating imbalance -- seems to be of relatively little concern anymore, except for its effect on cam drive and valvetrain harmonics. That must be due in part to the trend toward shorter strokes, which of course reduces the magnitude of the reciprocating force.
But no one has been able to produce a convincing power gain... in these particular engines, for whatever it's worth.