Backcutting the intake valve - worth it?

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M_Body_Coupe

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Folks,

I'm about to pull the heads off of my 360 motor...a bit of a CR bump, and since it's all coming apart and every single book/post implies back-cutting the intake valve is worth it I'm curious if anyone has actually done a before & after check?

Say, head flow before and after? Or perhaps a real track time compare?

The "word" on the street is that back-cutting usually ipmroves the low to mid-lift flow.
 
If it is the tulip shaped shape intake valve, there is a big ski jump that is remove and cause a better flow.

Nail head valves.......back cutting..........not so much.
 
I generally replace with back cut performance nail head valves. But, the last time I did have some testing done, back cutting the intakes was worth about 12cfm at low lifts, and around 20 at peak on a 2.02 intake. It's nothing to do it during the valve job. My machinist would do it for nothing as part of the valve job.
 
Well, I'm running a 11/32 stem Manley Intake valve, part# 11826. That is a nailhead (I believe) Severe Duty valve with the under-cut stem.

Thanks for the feedback so far guys! I should have the heads off tomorrow so I'm actually looking forward to taking a peek at the pistons tops and head chambers. Plugs came out looking very clean, which I was thrilled to see because previously (prior to the most recent valve stem seal replacements) I was using up a bit of oil.
 
It has always been my understanding that back cutting the exhaust valve had a bigger impact. Makes it easier to get it out so to speak.
 
M_Body_Coupe said:
Folks,

I'm about to pull the heads off of my 360 motor...a bit of a CR bump, and since it's all coming apart and every single book/post implies back-cutting the intake valve is worth it I'm curious if anyone has actually done a before & after check?

Say, head flow before and after? Or perhaps a real track time compare?

The "word" on the street is that back-cutting usually ipmroves the low to mid-lift flow.
Click to expand...

What makes you think you want to improve low lift flow? I almost never back cut anymore. Without a flow bench, and a very clear understanding of what is happening during overlap, should be your considerations.

I have lost flow and made a bunch more power. The shape of the flow curve, the width of the margin, seat width, top cut, especially the top all need to be considered.

I will say this and then let it go...you must flow the head at higher test pressure and in reverse before you ever bless a valve job or port work. If you are just standing there and flowing at 28 inches you can hurt power with higher flow numbers.
 
yellow rose said:
What makes you think you want to improve low lift flow? I almost never back cut anymore. Without a flow bench, and a very clear understanding of what is happening during overlap, should be your considerations.

I have lost flow and made a bunch more power. The shape of the flow curve, the width of the margin, seat width, top cut, especially the top all need to be considered.

I will say this and then let it go...you must flow the head at higher test pressure and in reverse before you ever bless a valve job or port work. If you are just standing there and flowing at 28 inches you can hurt power with higher flow numbers.
Click to expand...

from what I`ve seen at a buddies shop, back cutting kind of depends on the heads and valve jobs. some it`s worth doing, and a few it doesn`t help enough to do.
 
Rose - I agree 100%. However, find a performance valve that has no back cut. I'd be interested to see it. I took the post as working with stock tulip type valves, no porting, and a moderate cam. In this instance, the back cut IMO would be well worth it.
Low lift flow is not always one's best friend because the flow goes both ways. Most successful porters swap low lift flow for mid or high because strong low lift flow with a cam that has decent overlap, can lead to worse issues than a few less cfm.
 
Head is the factory iron '596 casting, having gone through a DIY porting effort but with a 3 angle valve job by a competent machine shop.

Here are the flow numbers (flowed on SF-600 @ 28" water):

LIFT I E

0.100 67 57
0.150 99 79
0.200 130 104
0.250 162 125
0.300 189 140
0.350 215 152
0.400 235 160
0.450 253 167
0.500 262 172
0.550 261 175
0.600 248 177

I'm using Hughes Engines cam, the HE3844AL piece, it specs out as follows: .535/.540 lift, 237/243 dur. @ .050", 286/290 adv. dur., 108 LSA.
 
The numbers are fine. It's a moot point anyway - that Manley valve is designed with the head smoothed and the approach to the seat angle flat. It basically is back cut.
 
yellow rose said:
What makes you think you want to improve low lift flow? I almost never back cut anymore. Without a flow bench, and a very clear understanding of what is happening during overlap, should be your considerations.

I have lost flow and made a bunch more power. The shape of the flow curve, the width of the margin, seat width, top cut, especially the top all need to be considered.

I will say this and then let it go...you must flow the head at higher test pressure and in reverse before you ever bless a valve job or port work. If you are just standing there and flowing at 28 inches you can hurt power with higher flow numbers.
Click to expand...

All of this ^^^^^^^^^^^^^ They always flow more backwards!

I do not backcut anymore unless it is a very very oddball combination. J.Rob
 
moper said:
Rose - I agree 100%. However, find a performance valve that has no back cut. I'd be interested to see it. I took the post as working with stock tulip type valves, no porting, and a moderate cam. In this instance, the back cut IMO would be well worth it.
Most successful porters swap low lift flow for mid or high because strong low lift flow with a cam that has decent overlap, can lead to worse issues than a few less cfm.
Click to expand...

Low lift flow is not always one's best friend because the flow goes both ways

X2
 
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