At what power level would big valves start to matter?

[timk225]

I'm thinking of plans for rebuilding my 225 later this year. Nothing too extreme. A mildly cleaned up head, Holley 350 2 bbl, ported stock exhaust manifold, TTI 2.5" exhaust. No turbos, no EFI, no nitrous. The rest of my 1973 Duster won't be too exciting performance wise. 904, stock converter, 2.76 or maybe eventually 2.94 gears.

I might put in a Dutra RV10RDP cam, a baffle in the oil pan, and double roller timing chain, I see Rollmasters are available online.

I'm just wondering at what HP level would it be worth the cost and machine work to upgrade to the 1.70/1.44 valves. This engine will rarely if ever get over about 4500 rpm, except on a speed run on the highway, or a rare dragstrip or dyno run, if at all.

Given how bad the stock head is, I'm sure it would help even on a stock build, but I don't want to dump $1000 into valves and machine work if I'm only going to pick up an extra 5hp or so.

 

[Murray]

Sorry- no mild /6 is worth the work of larger valves.

 

[volaredon]

well Im gonna try that theory. I had the head on mine machined for the larger valves along with a polish and port. It cant hurt. I used teh SI 1.7/1.44 valves. My engine has been run but not yet driven.

 

[dddDuster]

well Im gonna try that theory. I had the head on mine machined for the larger valves along with a polish and port. It cant hurt. I used teh SI 1.7/1.44 valves. My engine has been run but not yet driven.

@PROSTOCKTOM do you sell SI’s valves?

 

[slantsixdan]

Sorry- no mild /6 is worth the work of larger valves.

Did you know this forum has whole, entire sections dedicated to things other than Slant-6s? It's true! Go and see for yourself, it's pretty cool; that way you can spend whole, entire sessions not being forced to read and comment about things that don't interest you.

 

[dddDuster]

Sorry- no mild /6 is worth the work of larger valves.

I’ve been thinking about this all day. I think this is just flat wrong. The complaint has always been about the head on these engines, seems to me almost everything I’ve ever seen or read indicates bigger valves and some pocket porting with flash cleanup helps these engines every time.

 

[halfafish]

I’ve been thinking about this all day. I think this is just flat wrong. The complaint has always been about the head on these engines, seems to me almost everything I’ve ever seen or read indicates bigger valves and some pocket porting with flash cleanup helps these engines every time.

This is correct. The head on a slant is the choke point for power. My truck got a stock bottom end, 2106 cam from OCG, Clifford shorties, Offy intake with an AVS2 500, and a pocket port with the big SI valves. It runs surprisingly strong.

To the OP, bigger valves will help even a moderate slant.

 

[Killer6]

From experience, and I'm not saying either of these were 100% optimized, but the 'Killer6' base was; .060" over 225 w/head & deck milled to achieve ~9.25:1, DC 6-1 header w/2.5" exh. out the back, Super6 2bbl. intake.
V1.0 ; '72 318 BBD, Crane 280° solid .440" lift 218°@.050". Stock valves. MPH in the 1/4 only 77.
V2.0 ; '70's 360/400 Holley model 2245 (272cfm@1.5"Hg), DC PurpleShaft 276° solid .490" lift 220°@.050". Intake valve 1.74" Exhaust valve 1.41". MPH in the 1/4 jumped to 86.
Even tho' a friend of mine coined the nickname 'Killer6' for My '72 Swinger, I hardly look at it as a killer build, it is a mild build by any standard.
Picking up 9 MPH in the 1/4 is pretty substantial on anything, & on an engine this size, I'm certain the carb & cam didn't do that on their own. Oddly, My best ET slip didn't have My best MPH, that happens in a lot of fast cars' runs.....but a Slanty only runnin' 16 flat??? Lol..
EDIT; The weight/power math said HP was ~190 flywheel 160's rear wheels.

 

[PROSTOCKTOM]

@PROSTOCKTOM do you sell SI’s valves?

@dddDuster

Yes I do.

Thanks,
Tom

[FOR SALE] - 1.70" / 1.44" Slant 6, SI Valves, 3/8" 1- Piece Stems, Stainless Steel Portflow Valves, Single Groove, Full set of 12, New $235

 

[dddDuster]

@dddDuster

Yes I do.

Thanks,
Tom

[FOR SALE] - 1.70" / 1.44" Slant 6, SI Valves, 3/8" 1- Piece Stems, Stainless Steel Portflow Valves, Single Groove, Full set of 12, New $235

Thanks Tom, I just took the head in to get checked. If the machine shop doesn’t go south on me because I want to buy my own parts, I assume you can get the locks, retainers, etc. too?

 

[PROSTOCKTOM]

PM sent

Thanks,
Tom

 

[Jim Kueneman]

My buddy owns a machine shop and adding bigger valves was not much more than stock seats. The Performance Trends model says it would give you a bit more with the RV10 grind (Oregon Cams 2106 is basically the same) at high RPM as the head flow is the bottleneck. I did it for fun and have something to talk about at shows. Decking the block/ head to get 9:1 to 9.5:1 with that cam is the magic.

 

[MidnightSwinger]

IMO if you are going to put money into the head, even a stock slant six would benefit. When are they necessary? Anything more than a super six swap I’d think. Once you go over 350 cfm on the carburetor and swap out the cam for more lift, the head, specifically the intake side becomes the bottleneck.

 

[pishta]

For $1000, I would put that towards a better 2bbl intake, a better exhaust system and taller rear gears. 2.76 is great for Kansas highways, 3.23 is a whole lot funner in town. My 65 had 3.55 gears on a 7.25 SG and I beat on that for 6 months with a mild 340 before the rear gave up. That light car didnt put alot of stress on that rear especially with the hard plastic Pos-a-traction tires I ran.

 

[PROSTOCKTOM]

There used to be a guy that ran a Slant 6 at the local drag strip here. He called it the, "Hillside Strangler" and it sounded like the engine was being strangled all the way down

the track. He had a Clifford's intake (4 barrel), Cliffords header, and I am sure a lot of head work. Reading this thread made me think of that guy over 35 years ago.

Tom

 

[PROSTOCKTOM]

As long and steady as they made the Slant 6 there has to be thousands that have been upgraded/ improved on over the years.

I remember a lot of them running around when I was younger. Sadly, I rarely see any old cars driving around here much anymore.

In the summer time I see a dark blue Beck's 356 Speedster running around on my end of town regularly. That's about it.

Tom