cubic inches and head selection

[Martin]

All,

I have been trying to learn about head selection and cubic inches.

Taking AFR185 and AFR205 heads as an example.

If you throw them on a 351 you get a high performance engine - with either set of heads. The 185 will do better at the low end and have great throttle response. The 205 will have better top end performance, potentially higher maximum HP/TQ numbers but won't pull through out the rev range as early as the 185.

But if you then stroke the engine to a 393, 408 or 427 - at what point does the 185 become the choker in the system?

Does this question make any sense without considering target hp/tq numbers? If not, assume a number of 450.

Having initially opted for a crate engine or turn key, I'm now having to figure all this out under the threat of my better half. Although I think it needs to be considered as an overall system, I can't get my head around that just yet so I need to think about it in pieces.

Martin

[Bob Cowan]

Unfortunatly, it's not as simple as that. You're on the right track, though. The thing to remember is that the heads, cam, intake, adn induction all have to work together. You could use the 205 heads in one 351 package and get great performance. Or, you could use them on another package and peformance will suffer.

[Martin]

I suspected as much.

Without wanting to know everything - just enough to be able to have a conversation with somebody that will build it - any ideas where I can find out about it?

Martin



Unfortunatly, it's not as simple as that. You're on the right track, though. The thing to remember is that the heads, cam, intake, adn induction all have to work together. You could use the 205 heads in one 351 package and get great performance. Or, you could use them on another package and peformance will suffer.

[AJ Roadster NJ]

Books have been written about this!!

I had to go through the same decision on my 351W buildup, and opted for smaller ports (if you can call 190cc "smaller") in my Dart Pro1 heads. I view large-port heads and intake manifolds as being designed for ONLY pure competition applications, where the motor is going to be run at high-midrange-to-high-range revs all the time, as a matter of course. Those motors can often be extremely difficult to live with on the street, although it's been done on occasion.

Another factor as you build either the straight 351W or one of the stroked variants is whether you'll be using a roller lifter block (or a roller lifter retrofit kit with a small base circle cam, as I did on my older 351W block). If you do that, you'll have to set your rev limiter to no more than 6250 rpm because that is the point at which roller lifters are known to start to float and do extremely nasty things to valves and piston tops. I've got the 6000 limiter plugged into my MSD6AL. My point here is that I can't see going with an induction system designed for high end performance if the true high end gets chopped off by valvetrain decisions.

Going with solid lifters? OK, but now get yourself used to constantly setting valve lash. Not me.

Finally, I don't really see where this car needs a racing induction system to get the power you need. I have not dyno'd my car, but based on the experiences of so many people that have, I expect to see a really nice flat torque curve that jumps up right away and is plateauing by about 2000 rpm and staying thereabouts right through the 6000 rpm limit, with gobs of torque through that whole range.

I don't care about hp numbers at all really, they are a ridiculous mathematical derivation of torque. I've actually seen a show on Spike where the boys were dynoing one new part or another and were bragging about how they gained 6 hp at the top of the curve. Well they reduced the torque up there, which is the only thing that matters. Increasing hp once you get past 5250 rpm (the "magic" number that is part of the torque-to-bhp conversion) is, IMHO, a smoke and mirrors routine. One of these days I'm going to blog that topic.

Now that my rant is over, I'll say that I expect to top out at maybe 420 hp. Stock 351W block and crank, American Eagle rods, KB pistons, Seal Pro rings, Clevite bearings, everything in the valvetrain is from Comp, Weiand Stealth intake manifold, Barry Grant 650 cfm carb (no choke, mechanical secondaries), a very free-breathing K&N filter, and 4-into-4 Hooker headers from FFR.

Plenty of guys have stroked their 351Ws, but for me I felt I could get plenty of power while remaining conservative and reliability-oriented in the build.

Feel free to shout with more questions.

AJ

[Martin]

I have to say, I don't care about the numbers either. However, I'm still trying to understand the relationship between cubic inches and heads (and cam and intake). If the AFR185 head is perfectly tuned for 351cu with great flow rate and velocity there must be a relationship that deteriorates as the cubic inches increase.

I am guessing that at some point the AFR185 heads would limit the engine - so for arguments sake a 500cu engine would not perform well at any rpm range with AFR185 heads.

I've spoken to a few engine builders and I always come away thinking either:
1) I didn't understand the conversation well enough
2) I just need to trust that they will look after my interests - this leaves me feeling uncomfortable
3) They are specifying an engine bigger and more expensive than I want/need

I have strict noise and pollution restrictions to deal with along with a whole bunch of things to consider. Not understanding enough to ask intelligent questions is one of the many issues I am trying to resolve at the moment.

If there have been books written about it - I'd like to know any of the good ones. Most are over 10 years old, most don't even consider fuel injection set ups, and most are aimed at race engines, not high performance engines with street manners.

Martin




Books have been written about this!!

I had to go through the same decision on my 351W buildup, and opted for smaller ports (if you can call 190cc "smaller") in my Dart Pro1 heads. I view large-port heads and intake manifolds as being designed for ONLY pure competition applications, where the motor is going to be run at high-midrange-to-high-range revs all the time, as a matter of course. Those motors can often be extremely difficult to live with on the street, although it's been done on occasion.

Another factor as you build either the straight 351W or one of the stroked variants is whether you'll be using a roller lifter block (or a roller lifter retrofit kit with a small base circle cam, as I did on my older 351W block). If you do that, you'll have to set your rev limiter to no more than 6250 rpm because that is the point at which roller lifters are known to start to float and do extremely nasty things to valves and piston tops. I've got the 6000 limiter plugged into my MSD6AL. My point here is that I can't see going with an induction system designed for high end performance if the true high end gets chopped off by valvetrain decisions.

Going with solid lifters? OK, but now get yourself used to constantly setting valve lash. Not me.

Finally, I don't really see where this car needs a racing induction system to get the power you need. I have not dyno'd my car, but based on the experiences of so many people that have, I expect to see a really nice flat torque curve that jumps up right away and is plateauing by about 2000 rpm and staying thereabouts right through the 6000 rpm limit, with gobs of torque through that whole range.

I don't care about hp numbers at all really, they are a ridiculous mathematical derivation of torque. I've actually seen a show on Spike where the boys were dynoing one new part or another and were bragging about how they gained 6 hp at the top of the curve. Well they reduced the torque up there, which is the only thing that matters. Increasing hp once you get past 5250 rpm (the "magic" number that is part of the torque-to-bhp conversion) is, IMHO, a smoke and mirrors routine. One of these days I'm going to blog that topic.

Now that my rant is over, I'll say that I expect to top out at maybe 420 hp. Stock 351W block and crank, American Eagle rods, KB pistons, Seal Pro rings, Clevite bearings, everything in the valvetrain is from Comp, Weiand Stealth intake manifold, Barry Grant 650 cfm carb (no choke, mechanical secondaries), a very free-breathing K&N filter, and 4-into-4 Hooker headers from FFR.

Plenty of guys have stroked their 351Ws, but for me I felt I could get plenty of power while remaining conservative and reliability-oriented in the build.

Feel free to shout with more questions.

AJ

[Hankl]

Martin,

I basically breaks down to what the expected HP is, RPM range, and CID of the engine. If your shooting for 450 HP, with a displacement of 393 CID or less, with a operating range not to exceed 5500-6000 RPM, then the 185's would be a sane choice. Once you change any two of the parameters, then you have to start moving up to the larger heads. As you said, the smaller head will give you a more responsive motor, and that very often, is more usable than excess HP. HTH

Hank :cool:

[AJ Roadster NJ]

I understand your question, Martin, but there are no easy answers. I definitely agree with what Hank says, but he's giving you some specific criteria behind the 185 recommendation.

What do you want to do with the car? Any drag racing? Any track days, and if so, how would you rate your "demand split" on the motor? (For example, if the use is 50 - 50 street and road course, do you want a motor optimized for one of those, but capable of running on the other? Or do you want a motor that is perfect for neither but comfortable compromise that is fine for both?)

These are questions that your engine builder is going to ask. Unfortunately there is no simple pat answer for what size the intake runners should be for a given cubic inch capacity motor.

AJ

[Bob Cowan]

Here are some points to ponder. I didn't learn these all from a book, or series of books. Most of them I learned in my garage.

- Solid lifters need to be adjusted once in a while. How often depends on how well you do it the first time, and how many miles you drive. If you need to do it more than once a year, there's something wrong.

- A stock hydraulic roller valve train will limit your rpm's to about 5,600-5,800. The appropriate parts selection will raise it to about 6,700. My rev limiter is set at 6,600. And that's about the limit of a stockish windsor bottom end, too.

- Solid lifters will extend the rpm range up to 10,000 if you want to. It's not just the lifters that do it, though. You also have to have special push rods, rocker arms, retainer, springs, etc, to get you there. The down side is that the ultra high spring pressures and high rpm's put a lot of strain on those components. They need to be replaced often.

- The easiset way to make power, is to increase cylinder pressure. That's what power adders do. Use facters like static compression, intake valve closing, altitude, etc, to get the max pressure you can live with. Your dynamic compression ratio should be a smidgen above 8:1.

- In the olden days, you had a choice between high rpm power, or low end torque. You couldn't get both. That meant an engine could make good torque below about 4,000, or hp abve 3,500. You couldn't get an engine to make good torque down low and still make good power at the top end.

- The same was pretty much true for the heads. Before choosing heads, you had to decide where you wanted your power to be.

- Fortunatly, this is not the olden days. With modern advances in heads, cams, and (especially) EFI, you can have it all. And it can all be done with off the shelf parts. For example, the Dart 195 heads can make well over 600 hp, where some other 205 heads would struggle to match that.

- I would say the induction system is the key component. You choose your other components based on that. A carb will have very differant requiremants than a multipoint EFI. And an 8 stack EFI has differant requirements than a multipoint. The greatest flexibilty and broadest performance range will come from multiport EFI with a largish central plenum.

Here's my engine: 427W, 13.2 static compression, huge hydraulic roller cam, 8 stack EFI, Dart pro-1 195 heads with 2.02" intake valves, and matching components to make it all work. Makes around 500hp on the chassis dyno. More importantly, it has a very broad flat torque curve, is easy to drive on the street, and gets almost 20mpg if I can keep my foot out of it.

The down side is the 8 stack EFI system limits max power. Without a common plenum, I can't use head ports or valves any bigger than what I have. I tried it, and lost a lot of power.

[Martin]

I'm starting to think that posting a thread about my specific engine needs is going to give me an answer, but not necessarily the understanding I'm looking for.

Looking for a street car that can go to the track and provide lots of entertainment - not worried about the fastest lap. At the same time, don't want an engine that might blow up on me. Probably do 3-4 track days a year which means 4 30 minute sessions at each track day.

Wayne Presley built a car ("Black Widow"?) that had an engine basically the same as what I was thinking of, although it may have been a bit bigger than what I was looking for.

Martin



I understand your question, Martin, but there are no easy answers. I definitely agree with what Hank says, but he's giving you some specific criteria behind the 185 recommendation.

What do you want to do with the car? Any drag racing? Any track days, and if so, how would you rate your "demand split" on the motor? (For example, if the use is 50 - 50 street and road course, do you want a motor optimized for one of those, but capable of running on the other? Or do you want a motor that is perfect for neither but comfortable compromise that is fine for both?)

These are questions that your engine builder is going to ask. Unfortunately there is no simple pat answer for what size the intake runners should be for a given cubic inch capacity motor.

AJ

[Martin]

Bob - some good rule of thumbs there which is what I am after as a starting point.

Our choice of engine bits so far is Dart Iron Eagle 351, stroked to either 408 or 427, Iglese XFI stack induction and AFR185 or AFR205 heads. Need it to run on pump gas (which means 93 or 97 octane over here). Cam was looking like the compcam 282HR but I wasn't sure if it was too aggressive for our needs so started looking at the 274HR.

Unfortunately, the three decisions around 408/427, 185/205, 282/274 heads are fairly big decisions that impact a lot of other choices.

Martin



Here are some points to ponder. I didn't learn these all from a book, or series of books. Most of them I learned in my garage.

- Solid lifters need to be adjusted once in a while. How often depends on how well you do it the first time, and how many miles you drive. If you need to do it more than once a year, there's something wrong.

- A stock hydraulic roller valve train will limit your rpm's to about 5,600-5,800. The appropriate parts selection will raise it to about 6,700. My rev limiter is set at 6,600. And that's about the limit of a stockish windsor bottom end, too.

- Solid lifters will extend the rpm range up to 10,000 if you want to. It's not just the lifters that do it, though. You also have to have special push rods, rocker arms, retainer, springs, etc, to get you there. The down side is that the ultra high spring pressures and high rpm's put a lot of strain on those components. They need to be replaced often.

- The easiset way to make power, is to increase cylinder pressure. That's what power adders do. Use facters like static compression, intake valve closing, altitude, etc, to get the max pressure you can live with. Your dynamic compression ratio should be a smidgen above 8:1.

- In the olden days, you had a choice between high rpm power, or low end torque. You couldn't get both. That meant an engine could make good torque below about 4,000, or hp abve 3,500. You couldn't get an engine to make good torque down low and still make good power at the top end.

- The same was pretty much true for the heads. Before choosing heads, you had to decide where you wanted your power to be.

- Fortunatly, this is not the olden days. With modern advances in heads, cams, and (especially) EFI, you can have it all. And it can all be done with off the shelf parts. For example, the Dart 195 heads can make well over 600 hp, where some other 205 heads would struggle to match that.

- I would say the induction system is the key component. You choose your other components based on that. A carb will have very differant requiremants than a multipoint EFI. And an 8 stack EFI has differant requirements than a multipoint. The greatest flexibilty and broadest performance range will come from multiport EFI with a largish central plenum.

Here's my engine: 427W, 13.2 static compression, huge hydraulic roller cam, 8 stack EFI, Dart pro-1 195 heads with 2.02" intake valves, and matching components to make it all work. Makes around 500hp on the chassis dyno. More importantly, it has a very broad flat torque curve, is easy to drive on the street, and gets almost 20mpg if I can keep my foot out of it.

The down side is the 8 stack EFI system limits max power. Without a common plenum, I can't use head ports or valves any bigger than what I have. I tried it, and lost a lot of power.

[Bob Cowan]

Bob - some good rule of thumbs there which is what I am after as a starting point.

Our choice of engine bits so far is Dart Iron Eagle 351, stroked to either 408 or 427, Iglese XFI stack induction and AFR185 or AFR205 heads. Need it to run on pump gas (which means 93 or 97 octane over here). Cam was looking like the compcam 282HR but I wasn't sure if it was too aggressive for our needs so started looking at the 274HR.

Unfortunately, the three decisions around 408/427, 185/205, 282/274 heads are fairly big decisions that impact a lot of other choices.

Martin

Since you're using a Dart block, go for the 427. Cost is the same, and you still have cylider left for wear or damage. You could even use a slightly longer stroke and get around 450 cubes.

Since you're using an 8 stack system, you head and cam choices are very differant than what would be considered "normal". First, the cam needs to have a wide LSA, like 114-116*. Mine is from Comp Cams. They make some specificaly for use with Webers (CC now owns Inglese), and they're pretty mild. They also make a couple cams specifically for stroker engines - the XFI series.

That's what I use. Remember that more cubes will "soften" the cam a bit. A 5.0 cam made for power in the extreme upper range (4500+) will be much "smaller" in a 427. Mine is a huge cam, which would be almost undrivable in a street 5.0. But in a 427 it works really well. http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=1069&sb=2

Cam selection will also help you choose compression ratio. I have 13.2:1 static ompression, and I run in 91 octane pump gas. That works because the intake valve dosen't close until 86* ABDC, and I live at 6,400' ASL. On the track I run 110 octane leaded Sunoco race fuel. Expensive, but keeps the cylinder temps down and prevents detonation. I tried alcohol, but the computer couldn't handle it.

Head selection is critical. If intake ports and valves are larger than the throttle body bores, you'll lose a lot of power. My throttle bodies have a 52mm bore (2.05"). I tried a set of Brodix heads with bigger ports and valves, and lost a ton of power. Find out what size those throttle bodies are before choosing the heads.

As I said before, choose your induction system, and then the rest of the parts will flow from that.

Comp Cams has a free download program that allows you to play with various parameters and see what will happen. Desktop Dyno is not free, but it's a very helpfull tool. It's doesn't seem to be very accurate for IR systems, though. Still, you can see what happenes when you change cam specs, compression, etc.

[Martin]

Thanks Bob.

I installed the comp cams program last week and it killed my laptop. Probably coincidental, but I'm not going to rush out and reinstall it again.

Martin




Since you're using a Dart block, go for the 427. Cost is the same, and you still have cylider left for wear or damage. You could even use a slightly longer stroke and get around 450 cubes.

Since you're using an 8 stack system, you head and cam choices are very differant than what would be considered "normal". First, the cam needs to have a wide LSA, like 114-116*. Mine is from Comp Cams. They make some specificaly for use with Webers (CC now owns Inglese), and they're pretty mild. They also make a couple cams specifically for stroker engines - the XFI series.

That's what I use. Remember that more cubes will "soften" the cam a bit. A 5.0 cam made for power in the extreme upper range (4500+) will be much "smaller" in a 427. Mine is a huge cam, which would be almost undrivable in a street 5.0. But in a 427 it works really well. http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=1069&sb=2

Cam selection will also help you choose compression ratio. I have 13.2:1 static ompression, and I run in 91 octane pump gas. That works because the intake valve dosen't close until 86* ABDC, and I live at 6,400' ASL. On the track I run 110 octane leaded Sunoco race fuel. Expensive, but keeps the cylinder temps down and prevents detonation. I tried alcohol, but the computer couldn't handle it.

Head selection is critical. If intake ports and valves are larger than the throttle body bores, you'll lose a lot of power. My throttle bodies have a 52mm bore (2.05"). I tried a set of Brodix heads with bigger ports and valves, and lost a ton of power. Find out what size those throttle bodies are before choosing the heads.

As I said before, choose your induction system, and then the rest of the parts will flow from that.

Comp Cams has a free download program that allows you to play with various parameters and see what will happen. Desktop Dyno is not free, but it's a very helpfull tool. It's doesn't seem to be very accurate for IR systems, though. Still, you can see what happenes when you change cam specs, compression, etc.

[Waiting4Kids2Bgone]

Thanks Bob.

I installed the comp cams program last week and it killed my laptop.
Martin

I hope you didn't lose all your data AGAIN! I still have all the data you sent me if you did. I can forward it to you.

Tommy

[Martin]

I had a back up that was only a day old.

fool me once...

Martin