I've been reading that I guess that at .500 mach index is where VE peaks and at .550-.600 it plateaus and above .600 it drops rapidly. I want my 273 to make peak power about 6600 so do I try to match .500 mach there and hope .600 is at or above the 7200 redline I want or make sure .600 is above the redline or try to get .500 Closer to peak torque? Any ideas?
Question, Intake Valve Mach Index
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moper
Well-Known Member
Your "red line" to me doesn't make sense to me. If you need power at 7K, you'll have to make it there. If your at the end of a straight and your peak rpm is 7200, you need to make power to 7200 or the car will not be accelerating much. For the racing you plan to do - with the gearing and transmission you plan to use - What do you need for horsepower and what max rpm?
The short answer on the mach number - I think you're overthinking it...lol. You are right that there is a point where going too fast will cause turbulence and the port stalls. But mach number (and the associated percentages of it) change with temperature and humidity and will be very localized within the port. By that I mean that without a lot of testing you will never know what your exact package is potentially doing. So unless this is a spare-no-expense endeavor and you won't be thoroughly flow testing and recording airspeeds through the port - I would limit your concern to the normal trouble spot. The biggest issue will be the smallest area of the port - inevitably in a factory head that's the pushrod pinch. To calculate the limiting port velocity of that spot:
LPV=(.00353*RPM*S*B2)/CA
Where:
S = stroke (in)
B = bore (in)
CA = minimum port cross sectional area in sq./in.s
RPM = peak power rpm
LPV = limiting port velocity
The short answer on the mach number - I think you're overthinking it...lol. You are right that there is a point where going too fast will cause turbulence and the port stalls. But mach number (and the associated percentages of it) change with temperature and humidity and will be very localized within the port. By that I mean that without a lot of testing you will never know what your exact package is potentially doing. So unless this is a spare-no-expense endeavor and you won't be thoroughly flow testing and recording airspeeds through the port - I would limit your concern to the normal trouble spot. The biggest issue will be the smallest area of the port - inevitably in a factory head that's the pushrod pinch. To calculate the limiting port velocity of that spot:
LPV=(.00353*RPM*S*B2)/CA
Where:
S = stroke (in)
B = bore (in)
CA = minimum port cross sectional area in sq./in.s
RPM = peak power rpm
LPV = limiting port velocity
Moper I was just reading on Mach Index wanted see where my engine stood and it seem ok, Its just for theory so is Mach .500 suppose to be at peak HP? This isn't a Max effort race engine it's just gonna be 3.662 x 3.58 273/302 with ported stocker with under cut hollow stem ford 1.85-1.55 valves with beehives and titanium retainers and .525 cam, I want peak power about 6600 and the redline 7000-7300. Right now the Mach .500 @ 6200, .550 @ 6700, and .600 @ 7200 which seems reasonable if I went with a .560" cam the would put Mach .500 @ 6600 but I don't want to go that high with lift.
moper
Well-Known Member
As far as I know there is little correlation between a potential power peak and a mach number. I'm sure at some level there will be, but it's nothing I've considered for what I do. It seems that only when you exceed what seems to be the "magic" speed does power fall off, and fall off quick. From what I read that top speed is higher than .5. Somewhere between .6 and .7. But again, theoretically speaking, ".5 mach" is a moving target. It's closely related to temp and humidity and if you've ever seen a non-heated manifold on an engine running methanol get ice on it, you know there's some big changes in temps throughout the port once fuel is added to the airstream. There's a lot of talk about keeping the port volume smaller to keep velocity up. IMO - in a factory type head that's sized properly to the displacement, that's a solid approach because the factory design is such a compromise and was never designed for high rpm operation. Look at the difference between a 360LA port, a Gen3 Hemi or LS port, and a DOHC intake port. There are drastic differences in basic shape. Once you get a head with a better designed port or you need to rev the heck out of it the "too fast" becomes a distinct possiblity and you may need to affect the air column within the port because of "too fast". Again - without a lot of testing it's really a moot point and as far as I'm concerned all conjecture. Because the MSA may be in other spots than the pinch - like just above the short turn, especially taking into account the guide boss - you're looking at making a template and "counting squares" or making a mold and doing the same trying to find where the smallest point is so you can calculate more accurately.
I'm still confused by your "redline" comment. For purposes of this discussion I'll take the "shift point" rpm out of play and figure you need to make peak power at 7K.
Using the cam lift and specs you gave - I calculate a max rpm potential of 6200 for that valve and valve lift. So temporarilly removing the flow ability of the port from the discussion - IMO you need a larger cam and/or larger valve if you need to make power above 7K.
I'm still confused by your "redline" comment. For purposes of this discussion I'll take the "shift point" rpm out of play and figure you need to make peak power at 7K.
Using the cam lift and specs you gave - I calculate a max rpm potential of 6200 for that valve and valve lift. So temporarilly removing the flow ability of the port from the discussion - IMO you need a larger cam and/or larger valve if you need to make power above 7K.
Moper Thanks for all that info, what I mean about line redline is I want to choose valve spring that will work to about 7000-7300 with lightest valvetrain I can afford this is a street car that will see racing. I'm looking to build peak horsepower about 6600 (hoping for 375-400hp) as for the cam I want to get a custom grind once I do the heads and get them flowed I'll know more to as what cam I need but right now I'm thinking 280ish and around .525 lift.
moper
Well-Known Member
Am I correct in thinking you're using the 273 heads?
moper
Well-Known Member
Call this constructive criticism butI think that is a mistake... Once you make the valve big enough to support your plan the port will really need some work to support the valve. Again - bearing in mind there's a difference between revving to 7200 and making power while revving to 7200.
Ouch..wow..that going to be quite a feat..I did a set of 273 heads..never again..
I could have ported 3 sets of x heads in the time I spent on those 273 heads.Getting the seats in without them falling out after a couple of heat cycles will be challenging.
But if your guy has the experience I'm sure he'll get the job done.
Those 273 heads I did went 265,but I had a bench so I could trim and lay back and toss it on the bench to check.
But all in all that little 277 makes some onions...I'd bet it makes 310-325 at the wheels..and all that at 6000.
So it can be done with that 920 casting.
I could have ported 3 sets of x heads in the time I spent on those 273 heads.Getting the seats in without them falling out after a couple of heat cycles will be challenging.
But if your guy has the experience I'm sure he'll get the job done.
Those 273 heads I did went 265,but I had a bench so I could trim and lay back and toss it on the bench to check.
But all in all that little 277 makes some onions...I'd bet it makes 310-325 at the wheels..and all that at 6000.
So it can be done with that 920 casting.
MopaR&D
Nerd Member
Someone correct me if I'm wrong but I feel like a stock 273 bore with a 3.58 stroke and small-port 273 heads is going to be tough to hit 7000+ RPM with. If you want it to rev that high IMO you should stick with stock stroke and run some 360 heads (at least). 3.58" is the same stroke as a 360 (which are known to be better for torque than high revs) but with the tiny bore it will be even harder to get enough flow to make power above 6000 RPM let alone 7000...
MopaR&D
Nerd Member
Interesting I'm sure he knows more than me then lol... What kind of cam is he running? I know 308 heads are some of the best-flowing LA heads from the factory...
That is going to be no SMALL feat..it's going to take more then waving a dremel around inside those pockets to get anywhere close to 200..
I'm sure if you use a 4" fixture on the bench you will see 200 but use a 3.75 like I did and those numbers come VERY SLOWLY..
My numbers are with a fixture that was .100 larger than the bore so they are not 100% correct..
For what it's worth..finish your heads first..!! before you even start to think about what cam to use,you need to know what you have to work with.IMHO
Porting and getting my heads flowed will be one of the first things I do so I get the right cam for my combo, I've been somewhat conservative on my head flow on my Dyno Simulator with a peak of 195cfms, haven't been able to find any numbers for 273 bore but have found comparison between 350 vs 305 bore sizes with Chev heads and don't seem to lose too much flow even though 273 is a .100" smaller. Do you have the flow numbers on the heads you did with the 3.75" bore?
moper
Well-Known Member
Test bore size is an easy way to get numbers the tester is shooting for - and a quick way to be disappointed. You can do yourself a favor by locating or ordering the test bore size closest to or smaller than the 3.66 because that .100" is a lot of flow the engine won't have.
273..
I don't know if I kept the numbers,this job started out as a favor and snowballed into a fight between those hard as glass heads and myself.
I remember having 4 different roof variations on one head to see which gave the best results.I would just slide the head down the bench and test each one.That small fixture makes a difference from a 4" or 4.100..
Now the ex side had it's own unique problems..I used a different center line on the ex lobe to help in the end.
My only regret is that I didn't use the Cam Motion lobes that were a little faster ..
Ifitsgotwheels might have the sheets from the heads..
On a different note..what carb are you going to use...Please don't say Edelbrock.
I don't know if I kept the numbers,this job started out as a favor and snowballed into a fight between those hard as glass heads and myself.
I remember having 4 different roof variations on one head to see which gave the best results.I would just slide the head down the bench and test each one.That small fixture makes a difference from a 4" or 4.100..
Now the ex side had it's own unique problems..I used a different center line on the ex lobe to help in the end.
My only regret is that I didn't use the Cam Motion lobes that were a little faster ..
Ifitsgotwheels might have the sheets from the heads..
On a different note..what carb are you going to use...Please don't say Edelbrock.
RobR thinking 650 DP from Demon or someone similar, My uncle got a bunch of holleys kicking around I was gonna try a few see which cfm gives me the best throttle responds vs best WOT vacuum reading.
P.S. you guy are making me worried that valve shrouding gonna be a bigger issue than I thought its was gonna be doesn't notching help restore some of the flow lost to the bore size?
P.S. you guy are making me worried that valve shrouding gonna be a bigger issue than I thought its was gonna be doesn't notching help restore some of the flow lost to the bore size?
Not even going to comment on the throttle response and vacuum reading at WOT.you need to do more research on carbs.. :yawinkle:
Now the valve shrouding..work those chambers..free power.
Scribe the cyl bore on the head and move the wall right up to the line.
I don't have a picture of those 920's completely finished but this shows them at about 90%.maybe you can get an idea of what I'm talking about.
Now the valve shrouding..work those chambers..free power.
Scribe the cyl bore on the head and move the wall right up to the line.
I don't have a picture of those 920's completely finished but this shows them at about 90%.maybe you can get an idea of what I'm talking about.
moper
Well-Known Member
I'd suggest a couple things for this build in terms of locating the true edges of the bore: First - bolt the heads on w/o the gasket on the block, and use the bore to scribe the circle - mark the heads left and right so they go back on thier respective sides. Second - plan on having aCometic gasket made up - there's too much wasted space in using the std LA gasket on a 273 bore.
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