Cam selection for a small bore engine?

[Bernhard]

What can be done to aid breathing through cam design?
Small bore long stroke Oldsmobile 400 G
Class limited engine
Cam lift .474
stock intake
stock heads
headers
Bore 3.87 stock 3.930 with .060 over
4.25 stroke

Thanks
Bernhard

[PackardV8]

Hi, Bernard,
More than the cam, it's what heads and what size valves will clear the smaller bore. When you say "stock heads" do you mean the heads which came on that engine or any Oldsmoble part number heads?

[Bernhard]

Original heads no porting of heads stock valve size. The seats receive a multi angle valve grind. The valves are back cut old school or more common a valve with improved flow characteristics is used.
Some use under cover porting to extract even more performance from the heads and intake manifold.
There are well known machinist that perform this type of head work so stock is a loose term.
Most cams are designed for the larger bore 455 or larger bore E 400. I was just wondering if a camshaft was tailored to the small bore size what it might look like. Class racers usually work with a camshaft manufacturers in maximizing there combination, I'm quite away off from ordering a custom camshaft.
I thought I would post the question here, given the level of discussion that takes place on this forum.

[CamKing]

Small Bore = undersized valves.
When you don't have enough valve area, you need to increase the lift as much as possible, to gain area under the curve.
You will also want to widen the LSA to help extend the power past peak.
If it's a drag racing application, you can add duration too, and just increase the stall to deal with the torque loss from the added duration.

[Walter R. Malik]

Small Bore = undersized valves.
When you don't have enough valve area, you need to increase the lift as much as possible, to gain area under the curve.
You will also want to widen the LSA to help extend the power past peak.
If it's a drag racing application, you can add duration too, and just increase the stall to deal with the torque loss from the added duration.

Yep, and the higher percentage of mean piston speed can usually tolerate a bit more duration at any time.

[mag2555]

You want to get to that .474” valve lift limit as fast as you can with those stock heads .

Running a can lobe of lesser lift with a higher ratio rocker to get to .474” would be the way to go .

Note that most Olds heads that I have had on my bench in terms of the exh side will loose high lift flow when a 3 angle valve job is applied if you do not near fully blend off the bottom cut by hand around the short turn.
On the exh side you will also have( and hear ) turbulence in the port with just appling a common 3 angle valve job .
This is due to the lack of short turn height these heads have.

In fact what I would be inclined to to on the exh side is take the seat OD out to within .004” of the OD of the valve , then apply a .030” wide top cut that butts right up to the 45 seat and then put a 30 degree back cut on the rear of the valve and skip the full 3 angle job.

Do the rules call for stock valve stem dimensions?

Are you limited to exh manifolds?

[Bernhard]

Stock size valve stem and valve size original valve angle + or - 1 degree
stock rocker ratio.
Headers yes

Thanks for all the reply's
Bernhard

[mag2555]

Can you post up a picture of the back side of both the Intake and Exh valve?

You do not want them to beTulip on the back side as that defeat's running a Cam with the needed faster opening rate!

With aTulip valve more flow is seen during a static test on a flow bench, but in practice it cuts into power on a running motor.

You want valves with a 25degree under head angle after the 45 degree seat for those ports , and in my opinion it's worth it to get them made if you can't find anything close enough to get cut down to the head diameter's you need.

[Elroy]

Can you post up a picture of the back side of both the Intake and Exh valve?

You do not want them to beTulip on the back side as that defeat's running a Cam with the needed faster opening rate!

With aTulip valve more flow is seen during a static test on a flow bench, but in practice it cuts into power on a running motor.

What kind of testing did you do to make these determinations? Im assuming some exhaust flow tests on a flow bench?

[mag2555]

Yes on my superflow bench.

[Bernhard]

IMG_0011.JPG

IMG_0012.JPG

IMG_0014.JPG

Here are a few of the intake port and valve
The intake valve has been back cut.
My valve spring compressor struggled to compress the springs so I will have to upgrade before removing more valves.

[BCjohnny]

The intake seat is running out, maybe the valves too ....... next time you get them done take them to someone who can put them in right

Failing specific information on that particular head and very generally ......

You need at least four angles on the intake seat, 37/45/57/69 would be the starting point, and keep the throat percentage conservative at around 88%, 30* backcut on the valve

The exhaust seat straight into a throat radius, minimal topcut, depending on the shape of the valve a 30/35* backcut may help

If others are running optimised 'undercover porting' you need to consider doing the same, or risk running towards the back of the class

For the given capacity the earlier big bore engine should run harder, but you likely can't run that, as would a weaker 403 or stronger overbored diesel block

By no means an Olds expert so JMO

[Bernhard]

Thanks for the informative reply.

[mt-engines]

You need at least four angles on the intake seat, 37/45/57/69 would be the starting point, and keep the throat percentage conservative at around 88%, 30* backcut on the valve

Why keep the throat percentage so small? Especially if it's way under valved?

[BCjohnny]

I don't consider it 'small', just conservative, and I also don't totally buy into the 'under valved large throat' philosophy, not under all circumstances anyway

I get the impression it's a modestly funded 'suck it and see' deal ..... no harm in that, not everything's high dollar ...... so it's easier to take a bit more out if needed than try to put it back in, on a purely practical level

What would you advise re angles and values ?