Where can I buy a Type-IV "stroker" crank? Options

[Charles Deutsch]

Scat quoted me a price of $1499 for a 78 mm, one piece chromoly billet, T-4 crankshaft and I can buy one from Aircooled.Net for $500 but are there any other places that I can check for prices?

Thanks.

[tat2dphreak]

AC.net is usually the cheapest place... most other places don't have t4 strokers... you can always check cbperformance and mofoco too... but don't expect much...

[john rogers]

Two sources are Pauter in Chula Vista CA and Autocraft. Both have websites. I had a Pauter 78mm stroker in my big four and it worked very well.

[Brad Roberts]

The Scat is ONE piece and not a stock welded crank. The SCAT is a NEW crank. You get what you pay for. AC has good core stock 914 cranks that they weld the counterwieghts too and then offset grind the crank.. Scat forges a whole new crank. I have seen plenty of cranks break from offset grinding to get the desired stroke... the inside material ends up becoming too thin.


B

[Charles Deutsch]

How good are Demello cranks? I see they sell 78 mm T-4 cranks for $350.

[Brad Roberts]

Damn it.. I knew I should have bought that used Demello at the German Autofest. They are welded cranks also (all of them in the under 800$ range are offset ground/welded cranks.) I have not heard of a Demello failing.



B

[tat2dphreak]

I would also ask Jake... see where he gets his(or if he makes them?)... he may make them... I haven't seen a 78.4mm crank anywhere else...

[bernbomb914]

Is there a reason that porsche limits its stroke to 75.4 mm in there engines? what would a 96x75.4 4 produce? I would like to build a bigger engine than stock but it would be a daily driver and need to dependable & cool easily.

Bernie (IMG:style_emoticons/default/driving.gif)

[Gary]

Formula for displacement is piXradiusXradiusXstroke X number of cylinders.

I get 2183cc's.

for 78mm stroke, you get 2258

for 78.4mm, you get 2270.

[Charles Deutsch]

Next week I'll probably order the Engine Analyzer program and "design" a big-four motor. It will be interesting to look at the hp and torque curves for the different bore/stoke options and to see the difference between the output of WebCam's 86a and 494 camshafts .

[Aaron Cox]

berman vw sells them. they have some w/ vw journals and some w/ chebbies....

look in a hotvw's mag or

Here

[tat2dphreak]

read shoptalk before buying anything bergmann... if you have a type-1 bergman is good! but the consensus is that for type 4... bernie bergmann BLOWS!

[Korijo]

Demello. Thats what I'm getting. 78 w/ Chebbie journs...

[john rogers]

An item that is easily overlooked is the rod length if you are building a stroker engine. If the stroke goes up the rods have to get shorter unless you move the barrels and heads away the same amount as the stroke increase. The "short rod" problem can result in much higher loads on the small end and can result in even the end of forged rods pulling loose. Seems this used to happen to off-roaders for years until the problem was solved. Good luck.

[Brad Roberts]

John..

Have you played with Rod ratios for the type 4 ?? I'm wondering what is optimum.


B

[Brett W]

Check out DPR I have heard nothing but good stuff from them. For street use the welded stroker cranks are not a problem. If you want to race get the new forging from SCAT or HTC. Moldex might do one for you as well.

DPR can be reached here:

DPR Machine Shop, Inc.
2121 S. Susan Street, #B
Santa Ana, CA 92704
(714)979-7441
(714)979-7442 Fax


AS far a rod ratio goes, 1.70-1.80 is generally considered optimal. The shorter the rod the higher the piston speed and side loading on the cylinder walls, but it also sucks harder through the intake ports, so bigger intake ports can work well on the street. For durability stick with a rod ratio close to what porsche designed into the engine. IF you change the rod ratio you need to develope the rest of the engine to take advantage of such a change. Meaning heads, compression, exhaust design, cam design, etc...

[Charles Deutsch]

Thanks for the great info.

[Charles Deutsch]

An item that is easily overlooked is the rod length if you are building a stroker engine. If the stroke goes up the rods have to get shorter unless you move the barrels and heads away the same amount as the stroke increase. The "short rod" problem can result in  much higher loads on the small end and can result in even the end of forged rods pulling loose. Seems this used to happen to off-roaders for years until the problem was solved. Good luck.

I keep reading this but it's not intuitively obvious to me that piston speed decreases as the length of the connecting rod increases so I tried to do the math. The equation I get for the position of the connecting rod small end as a function of the angle of rotation of the crank, a, the crankshaft throw, T, and the connecting rod length L is:

x = T* Cos[a] + L*Cos[ArcSin[T*Sin[a]/L]]

then dx/da and d^2x/da would be

[Jeff Bonanno]

meaning there is a rod length dependance on piston 'velocity' (if velocity is defined wrt to time constant rotation).

i had trouble with the dimensional analysis - shouldn't the units be mm squared per degree?

[LvSteveH]

I could be wrong, but I believe the main issue is the stress generated by an excessively short rod, largely due to the added acceleration (speed) generated by being more closely connected to the crank. As the crank passes 90 degrees from TDC, the short rod has a greater moment of acceleration based the steep angle it has in relation to the wrist pin and crank journal. A longer rod would have a less severe angle and have more even acceleration and less stress. I’m not a math guy… so I can’t back this up with any figures, but it seems to make sense…. At least in my own little world.