SBC Base circle questions ?

[Monza355]

I going to build a 434 street engine. I did ask a few questions about it. Members here recommended looking at lowering compression ratio for pump gas friendly engine..

After doing some research i’we seen engine builds delivering up to and over 600 hp with hydraulic roller cams.. some builds even list off the shelf hydraulic roller cams which up on investigation aren’t even small base circle cams..

What dictates the size of the base circle you can use in a 4” stroke sbc ? Duration and lift ? I’we read that if the lobe lift gets above .420” cams are often ground on small base circle..

I spoke to some technician at comp cams and he said they could only grind their cast core roller cams at 1.050” base circle but could go to 0.900” base circle with their billet core. But then again they can not use for example their xtreme energy solid roller lobes on a billet core with 0.900” base circle which he said was the only size cam you could use in a 4” stroke small block chev.

[Ericnova]

A lot of the available clearance depends on the crank journal diameter, like using the 2.0 small journal or the even smaller Honda journal size instead of the common 2.1" size....along with the shape and size of the big end of the connecting rod.

[Monza355]

The rotating assembly is just standard 400 main and rod journal. The rods are H-Beam but have been ground for clearance at the bolt shoulder

[R.Olds]

I just finished a 427, I used the scat "stroker" H beams and worked with Bullet for my cam. They ground mine on a 1.030 small base circle. This is a mild street engine and only has .530 lift. When I checked clearance I had a bunch (over .110)

[geraldtson]

What dictates cam base circle on 4" stroke SBC. Mostly rod to cam clearance.Comp is correct that you should go with .900 base circle for this combo anything else will hit without rod grinding which you've already done. I have ran into dead solid cam to rod contact with so called "Callies Stroker rods that were advertised in every hot rod magazine to clear .660 lift on 4" stroke. They wouldn't even clear 3.875 stroke with .638 lift. Piece of advise stay away from true .900 small base circle or smaller base they are hell on valve train parts especially if your after durability. you'll have to go with taller lifter and it sits way to far down in the hole which causes lifter side loading in bottom part of lifter bore.You'll need more spring pressure for smaller base circle no matter what others tell you and everything in valve train wears out quicker. Taller pushrods and so on. It will work if you don't mind constant maintenance.If you've already ground the rod go with bigger base circle this will save many headache. If you decide to go with small base circle cam i would call bullet. They will listen to you and give you what you want and need not what there trying to sell you. Last bullet .900 base circle came in at exactly .900 most others what they call small base circle will come in .930-.940.950 and so on sometimes even bigger. Regardless go with the 8620 billet steel and you don't want no pressed on iron distributor gear. Yes less lift will help clearance also and easier on valve train. Many other options such as going with raised cam bore block it just gets more and more expensive. I'd just grind rods and check for clearance. I like .060.minimum. A small base circle cam is a weak link in a race motor. Street motor with less lift and rpm's and you can get by with one!

[R.Olds]

I dont want to derail the op's thread....but what is your reason for NO pressed on iron gear?

[geraldtson]

I dont want to derail the op's thread....but what is your reason for NO pressed on iron gear?

Many people use them,i just think there weaker under big spring pressure situations. Have seen several post of them breaking at rear of cam.If you have one don't worry about it just saying if i were gonna purchase a new one.Bronze distributor gears work fine and by giving them a little extra oil feed they are rarely an issue. If its not an all out race motor the iron gear may be the way to go nowadays.

[Geoff2]

Melonized steel dist gears are even better; Lunati, among others, sell them.

[rfoll]

"434 street engine". If it is truly a "street" engine, you are going to be much happier with the 3.875 crank. The engine will be easier and less expensive to build and will be much more durable. The tires don't care if its 420 or 434 cubic inches when they are spinning, and unless it goes into a 4 wheel drive truck, they will be spinning a lot.