To backcut valves or not to

[Guest]

So, my valves are arriving tomorrow, (8 exhaust @ $30 US ea) so I called my machine shop to ask about backcutting my valves. They want $120 to do all 16. He said he would do a little lap on each valve to see where the seat is, then backcut right to the seat. I read somewhere someone saying a 37 degree cut is best and he said it is worth 20% flow. Is it worth it? I have spent waaay more on this thing that I intended already, and its hard to swallow. Can I just do the intakes? Seems to me the intakes would benefit more.

[posplayr]

No expert

Just recently was considering the same.

but I think you would want a radius seat cut (in the head) as well . Are you planning on cutting the seats?

For me if it was the difference between $120 and cut back valves and radius seats I would go for it. Otherwise a hard decision.

I have a low milage head, I can either lap in the valves or spend at least $450+ for back cut valves and radius seats. (For a 1100ED head maybe 10 cfm better flow and 10 hp) It will be noticeably faster but money is stacking up here.

Other can advise on flow rate increases.

Pos

[gearhead13]

Just recently was considering the same.

but I think you would want a radius seat cut (in the head) as well . Are you planning on cutting the seats?

For me if it was the difference between $120 and cut back valves and radius seats I would go for it. Otherwise a hard decision.

I have a low milage head, I can either lap in the valves or spend at least $450+ for back cut valves and radius seats. (For a 1100ED head maybe 10 cfm better flow and 10 hp) It will be noticeably faster but money is stacking up here.

Other can advise on flow rate increases.

Pos

The seats are being cut already($250) so about 370 + tax after backcut.
10 HP? if it was that much increase i would do it for sure

[Nessism]

Not an expert here but 10 hp sounds optimistic on an otherwise stock head.

[rapidray]

10 Horse gain by back cutting is VERY optimistic! I would say for what you are doing to only do the intakes. If it were MY motor, I would have spent 25 each on stainless valves that are designed for more airflow & don't NEED back cut. Ray.

[posplayr]

Ray

10 Horse gain by back cutting is VERY optimistic!

Ray
I guess I was figuring backcut valves, radius ed seats and 1mm over on intake with the 1100 to add 10hp peak. Figure 30% less for the 750. About Right?

Pos

[headsbikesmopars]

$120.00 !!! Anyone needing valves back cut, lap them in, send them to me I will back cut any set for $25.00 for the whole set 16/8, any set. Just pay shipping I will be glad to help. $120. BUCKS, My GOD that is too high in my opinion. Takes about 10 min. tops. Paying more than $30. IS OUTRAGEOUS. Ben doing that for years and yes it picks up low lift flow just about 20% under .250 lift. equals TORQUE... I hate it when I hear this type of thing. PM me if interested.

Terry

[duaneage]

In terms of HP gains it is a tough pill to swallow. Only you know if it is worth it or not.

Removing metal from the exhaust valve is a questionable practice since it needs to get rid of the heat from the gases. Intake is not affected by the heat as much and is cooled by the inlet charge.

I would not cut the exhaust valves.

[headsbikesmopars]

Duaneage, I think you are missing something here. Back cutting the valves has nothing at all to do with the seats on the head or on the valves themselves. And has no adverse effect on heat transfer from the valve to the head. All back cutting includes is the removal of the un needed part of the valve margine that touches nothing. By removing this 'angle' it uncovers or 'unshrouds' more of valve area at lower lifts. Just like unshrouding the valves in the chamber promotes more low lift flow thru the port,..back cutting helps to get the 'moving' column of air moving in the port sooner in the lift cycle. Also, it lightens the valve slightly, very slightly but it does. As far as being worth it ..you must decide that for your self, but having almost the same flow @.050 as you had @.100 or @.100 as you did @ .150..well... that is your call. I think it is really cheap horsepower. Like I said earlier...anything over $30. bucks or so and you are really being over charged for such a quick an easy and simple mod. Just trying to help out a fellow GS owner here. Gearhead13, he did ask.

Terry

[duaneage]

Sorry, I thought he was going to cut swirls into the valve head. That was another technique entirely.

Difference in terminology.

[headsbikesmopars]

Duanage, you are right, it is a very misleading term. It sounds like the narrowing of the seat itself which would do exactly what you said , over heating of the exhaust valves. Not anything we would want to promote.
Good call Duanage, you guys are on your toes out there. LOL,LOL

Terry

[larrycavan]

10HP from backcutting the valves.....

[trippivot]

there is a better way

A very deep 60 cut in the seat- seat/head material will get you more low valve lift flow in place of trying to make a clunky stock valve flow like a smooth performance valve.

Work with what you have in a smart way when dollars count.


even on a excellent Duane Williams head ( known fo the special poppet valve work he did) the valves alone difference were less that 0.3 C.F.M. at 10" on a super flow 110.Truth is-- it was the seat cutting that made the most difference on the same head... 3 angle valve job 30, 45v/46s, and 60. The simplest good valve job makes more power and stays cooler.


back cutting stock valves in a head is not worth 10 HP Titanium racing valves are.

radius seat or 7 angle valve jobs are exotic sales pitches NOT worth my money or time.

[posplayr]

Terry did my stock 1100 head . Only parts change was to go with 28mm 1150 intake valves v.s stock 27mm. Exhaust remained stock.

Back cuit valves, Serdi valve job, "street" port work.

In this link there is an attachment. You can see teh flow number gains comparing before /after. (sorry the vertical and horizontal axiues need reversing).

Intake side went from about 78 to 95 CFM at my 0.340" lift

Terry went to town to create a wonderful street head. He pushed the CFM up from stock about 78 CFM to about 95 cfm at my usable lift. He used 28mm 1150 intakes, backcut the valves and ported as shown in the pics. Bill (Chef) thought it was an aggressive port (I don't know), but the bike runs great. Idles very strong, pulls very strong and has lots of low end grunt.

http://www.thegsresources.com/_forum/showpost.php?p=999223&postcount=29

[rapidray]

radius seat or 7 angle valve jobs are exotic sales pitches NOT worth my money or time.

A radiused valve job is worth 5-7 HP over a 3 angle on a low compression 1395 street GS motor in back to back dyno runs I did back in 92. I dynoed it with the 3 angle valve job first, pulled the head, recut the seats with my Serdi radiused cutter, put it all back together, made the 2nd pull 5 hours later when it was 15 degrees hotter & still gained 5-7 almost straight across the rev range! Don't tell ME a radiused valve job is a sales gimmick! I KNOW better! Ray.