SBC 383, 2.02 valve size or go for 2.055?

I'm trying to avoid the bigger is better trap.
Given the same flow numbers would you use a 2.02 or a 2.055 valve in a 4.030 bore?
Street driven, rarely track, 6500-700 rpm max.
Having a hard time choosing between the Brodix Ik200 (2.02) and the Brodix RR200 (2.055)

IK200:
http://www.brodix.com/heads/ik.php
RR200:
http://www.brodix.com/heads/raceritesb.php

If we assume same flow, same cost, is a 2.055 still better in other area's then flow alone or is there too much valve shrouding in a 4.030 bore?

IK 200 :

123 @ 0.2 lift

181 @ 0.3

230 @ 0.4

RR 200 :

125 @ 0.2 lift

182 @ 0.3

227 @ 0.4


We're talking about +2/+1/-3 CFM difference between the heads here.
That is 1.62% difference at 0.2 lift.
Is there such a thing as a linear translation to HP?


Now this is Cubic Feet per Minute so a volume of air.

This is where I get confused; (oversimplified) speed x cross section= flow correct?
So are we talking higher speed x smaller cross section for the 2.02
and lower speed x larger cross section for the 2.05?

So for instance at 0.3 lift its higher speed x smaller cross section = slower speed x larger cross section = 181 CFM?

If the above is the case do you get better cylinder filling (volumetric efficiency) with the smaller valve?
How does an intake with relatively narrow (higher speed) runners tie into this?
Is the better combination a fast intake with a fast (smaller) valve?
Or a fast intake with a slower( larger) valve?
Or a slower intake (larger runners) with a faster(smaller) valve?

My instinct tells me a fast runner with a slow valve and a slow runner with a faster valve should work as they "crutch" each other.
I've been reading the Smokey Yunick book and this shows you again that a little knowledge is dangerous (besides confusing).
Any thoughts?

If you can get the same flow with a 2.02, it'll be cheaper and you won't have to worry about clearance as much right?

I can get either bare for the same price.

Based on everything else you've posted about what you're trying to do, I vote for 2.02. Generally, the bigger valve and port size don't produce as much torque down low. But just as you said, I think you're falling into the trap.
Many years ago I worked for Comp Cams, didn't do anything fancy, just worked in the shipping department. But I asked "Dyno Dean", our main technology prover, the guy who actually built and tested the new cams going on the market, what the biggest fault people run into when building an engine. He told me that people almost without exception over cam an engine. The principle is the same regardless of what you're talking about, whether it's the cam or heads or carb. Chances are, pick what looks best to you and go one or two sizes down and you'll have a good combination.
As for me, it was really hard to do, but when I built my motor a few years back I really, really wanted that 284 roller cam, but instead I heeded the very sage advice from someone (Dean) who knew better. I got the 276 cam instead and I can tell you that I don't think there's a chance the 284 would have worked any better, and probably not as well.

Thanks guys.
Apart from maybe casting/design/shape differences that I don't know about the Brodix IK 200 and the Brodix RR 200 both have 200cc runners.

Maybe not a fair question but everything being equal (including cost)except valve size 2.02 or 2.055?
I can get a set of bare heads with $25 difference in price.

Concerns might be:

• Valve weight (?)
• Valve train (?)
• Flow interference with a 4.030 bore?
• Flow CFM is a volume are there airflow speed differences?
• Valve shrouding?
• Interference with the exhaust valve during overlap?

Give me some technical edumecation pls.

To be on the safe side with high lift, I would go for 2.02. Check valve to piston clearance prior the first start-up. The difference between the 2.02 and the 2.05 will not be signifcant. Added valve weight will not be an issue with less than 6000 rpm. Do not buy bare heads unless you have the skills or have a friend that can help you. If you think you will save money buying bare heads, you may end up paying more than complete heads. Check this out before making up your mind. Always buy springs that meet the cam profile, lift and type of lifters. Anyway good luck! :thumbs:

Arne

clutchdust said:

Based on everything else you've posted about what you're trying to do, I vote for 2.02. Generally, the bigger valve and port size don't produce as much torque down low. But just as you said, I think you're falling into the trap.
Many years ago I worked for Comp Cams, didn't do anything fancy, just worked in the shipping department. But I asked "Dyno Dean", our main technology prover, the guy who actually built and tested the new cams going on the market, what the biggest fault people run into when building an engine. He told me that people almost without exception over cam an engine. The principle is the same regardless of what you're talking about, whether it's the cam or heads or carb. Chances are, pick what looks best to you and go one or two sizes down and you'll have a good combination.
As for me, it was really hard to do, but when I built my motor a few years back I really, really wanted that 284 roller cam, but instead I heeded the very sage advice from someone (Dean) who knew better. I got the 276 cam instead and I can tell you that I don't think there's a chance the 284 would have worked any better, and probably not as well.

Click to expand...

I have talked with Dean Harvey at Comp. for years. When I was looking for about 50 more rwhp on my s/c 'vette, he helped me nail it, where all others failed. I wound up netting 70 more rwhp. I did take the time to call Dean and thank him. He is a sharp cookie!