back cut large valves or not? on a 402 CID..

[wfot]

Hello all,
building a 402, 4 x 4 square motor.
I have a set of large valve heads. I think they are 90s. It appears that the porting was done by someone who knows what they are doing. they came with a package of parts that were all gathered for a 402/427 build.
Ports at the mouth are 37mm. Valves are stainless steel, 40.2mm and 36.7mm. planning on stage 3 cams.

The valves have a slight lip above the seat.... my question is should this lip be blended into the back of the valve for a smooth transition or left alone?
I think blended, this has always worked for me in past for other, non LT5 engines but I know this motor is particular with certain things.... hence want to ask before I do..

thank you in advance for any thoughts on this.
John

[Jagdpanzer]

Can you show us a photo?

[wfot]

sure can. I tried to angle the valve to be able to see the edge. It is there, you can feel it...

[Jagdpanzer]

Check put this Speed Talk discussion on back cutting valves
https://www.speed-talk.com/forum/viewtopic.php?t=46843

[Jagdpanzer]

Another reference
http://www.ws6transam.org/afrflowdata.htm


Sent from my iPhone using ZR-1 Net Registry

[wfot]

thank you. excellent info.
I do have to keep in mid at what lifts the flow numbers improved/got worse. I am planning on stage 3 cams and they should be about .44 lift, so they would be nothing like the .6 lift numbers.

I am now thinking of just blending over the intakes...


another thought is a Siamesed runner plenum or a reduced runner plenum. I want the torque but the HPs as well...

[Paul Workman]

My valves were back cut, and I "siamesed" the plenum runners (somewhat) internally, leaving only ~ 1.5" of a septum between the runners at the plenum outlet leading to the IH; leaving a leading "razors edge" on the septum (to reduce reversion leading into the septum).

I did fully port the runners, tapering from 37.5mm from the plenum into the IH and into both runners of the heads, and tapered and shaped the head passages (avoiding entering coolant or oil passages) and bowls. And, the exhaust ports got some special attention as well.

I have yet to port the THROTTLE BODY, which Marc says will likely give me perhaps another 4-6HP. And, with some special tweaking (phasing) done by Pete Polatsidis on my stock cams, AND dyno tuning by Marc Haibeck, I was pleasantly surprised by the 'before n after' results (graphed below).

[wfot]

Impressive. the curve stays much flatter after the work. do you have any pictures of the bowl area?

The block I have, i believe, was done by Krichner (sic), I believe he block was used in Kirt white's 415. 12mm studs on all mains. The heads are probably done by krichner as well, the work is top quality. I did find a few variations on the ports mouth at the head, most are 37+ a few are under. I will be correcting this..

my understanding is that most builders, not lt5s, SBC, back cut the intake and not the exhaust. If you compare before and after flow numbers, the back cut does hinder some flow over .500 inch. but under .500 there is almost always a flow improvement. I feel that the multi valve engines can benefit from better low lift improvement and will not suffer the high lift problems of 2 valve motors...
I also heard there was an exhaust port trick on the LT5 that takes away the point just where the ports meet in the middle... if that shape edge is removed, it is worth about 10 hp... this has been done on my heads.

I appreciate the input/comments

[Paul Workman]

Originally Posted by wfot

Impressive. the curve stays much flatter after the work. do you have any pictures of the bowl area?

The block I have, i believe, was done by Krichner (sic), I believe he block was used in Kirt white's 415. 12mm studs on all mains. The heads are probably done by krichner as well, the work is top quality. I did find a few variations on the ports mouth at the head, most are 37+ a few are under. I will be correcting this..

my understanding is that most builders, not lt5s, SBC, back cut the intake and not the exhaust. If you compare before and after flow numbers, the back cut does hinder some flow over .500 inch. but under .500 there is almost always a flow improvement. I feel that the multi valve engines can benefit from better low lift improvement and will not suffer the high lift problems of 2 valve motors...
I also heard there was an exhaust port trick on the LT5 that takes away the point just where the ports meet in the middle... if that shape edge is removed, it is worth about 10 hp... this has been done on my heads.

I appreciate the input/comments

Yes, Ryan (a builder now in Phoenix, last I heard) told be about this trick too. Easy enough to do, so I removed that sharp junction in my exhaust ports as well.

No pix on the bowls OR the exahust ports (sorry). But I followed Ryan and Pete Polatsidis's advice and enlarged (slightly) the intake bowls AND removed the tights and all castings resulting in what would be an impediment to smooth air flow (in and out of the cylinders). NO pictures, as I promised Ryan and Pete I would not divulge specifics. But, there are dozens of books written on the subject and lots of examples (photos) in the public record.

One of my favorite examples that clearly demonstrates the principals of what good porting should look like is captured in this picture of a pair of Webber velocity stacks...says it all. This is not from an LT5, mind. But, it does say volumes to those with the least bit of curiosity about what good porting should endeavor to accomplish in the end...hint, hint!

.

[XfireZ51]

Paul, didn?t you also shave the heads a bit?