I have done a lot of reading and know that if you get too big a port on too small of an engine you will lose lowend, but when does too big become too big. It seems that everyones perception of big is different. Some say 200cc is pushing the limit on a 350, while I have heard that some people think that that is just right. Is there a cc/ci rule of thumb? I just asking bcuz I am thinking of putting together a build 355, and some people think that they would rather have a larger port then norm that flowed a little better and then compensated for the larger then norm port with a smaller cam. I understand that you can only use the airflow that the cam lifts too.

I guess what I really want to know is something like a 210cc port too larger for a 355 that it would cause the above problems? Didnt GM use their Fastburn heads, which are 210cc intake I think, on their ZZ430?

Thanks
Jeremy

 

man, you ask a very broad question.

you have to look at the car and what it's purpose is. is it a nitrous motor, is it turbo, n/a, supercharged, gasoline or methanol? what is the car weight, what are the tire limitations? does it need to power out of a corner on a track with little to no straightaway or does it need long straightaway power? is it a drag motor? what is the rpm range, the transmission gearing and rpm spread? this list of questions could go on for a few pages but they are all important in determining the right cylinder head for the job.

an runner can be larger for a turbo car, with a smaller higher velocity exhaust port. it can be larger for short rod motors. it should be larger for methanol motors. it should be larger for high rpm motors. it might be better to be larger for small tire drag cars so as to tune the suspension with a softer bottom end hit. so it depends.

you want as much cfm as is necessary with the smallest port possible. in other words, for a heavy street car with a 3 speed auto the small head is probably going to be the quicker at the track. 4-6 speed trannies in the same weight and you may be faster with a larger port but it depends.

it would help to narrow the question down with some specifics. you could be talking about a 66 impala with a 2 speed glide or a 2,800 lb car with a 5 speed tranny spending all of its time at 4,500 rpm and up. so let's get more specific.

and the fastburn head is a good one but i'm not into talking about engines as an independent element. the engine has to go in something and they have to work together. otherwise we'd just race dynos.

JB

 

Ditto

 

Sorry, wrote it late last night forgot about the important things. Lets say this: daily driven, n/a, 355 in a camaro, 6sp with 4.10s, max rpms would be 6500-7k rpms.

Would running more overlap, through scavaging, help the part throttle or low end with running too big of a head on a 355?

 

No the part throttle will suffer even more with more overlap.

Bret

 

Exactly... Mainly, and someone correct me if I am wrong.. You need more port volume if you run more overlap, less separation, as you will increase the speed of the gasses, more so on longer stroke engines, where you really need velocity to fill up the engine and get a higher power band.. That is why, whenever you want to rev you go to a higher port intake runner.

THat is why LS1s have pretty big (200ccs and 220 IIRC for the ls6) since they are happy revving engines...

a 350 will not need that big of a runner and will loose a bit of low end...

Its the same thing as a bigger cam on a smaller displacement engine or smaller stroke... I think you need 4x the area for the stroke of the engine or something like that???

 

i think one of the problems people run into is that they try to overanalyze things.
large port vs small port, doesnt matter. one is right for one app, and one is right for the other. it all depends on other things and most of those other things aren't in the engine. we're talking about an engine yes... but that engine has to work with everything else. people forget this, and that's why they can't get their cars to ET.
the three main things to look at are the engine size, the hp goal, the rpm needed to reach the goal. now we put everything together and we have to keep in mind that the converter, transmission, car weight, rod/stroke ratio, port size, all has to be in harmony to make the most power in the revs we've decided to work with.

JB

 

Hmmm,

I don't know what to say here other than what has been said already.

One thing we really have to look at here is that the heads are not just a port size but also a minimum cross sectional area.

For example you could take a 220cc port and fill in the area before the short side and lose a good 15cc from that port, and you will have the same basic cylinder head with less volume and it's probably going to make more power in a carb setup due to a more constant cross sectional area. It has a lot more to do than that but that's the basic jist of it. Now when a guy sees that the 205cc setup with a 2.08 intake valve makes all sorts of HP at 7000rpm in a 350 then he thinks that the magic runner volume is 205cc, when in reality that head flow and valve size are a lot closer to a 220cc port. Then you would miss the fact that the better mixture quality of the port helps that all out a ton and adds to the HP number.

Now this leads us to look at different types of heads on the same engine, like a 23 deg LT1 head vs. a SB2.2 head. A large well-ported LT1 head might flow 290cfm and be 208cc, and the Sb2.2 head is 270cc and flow 400cfm. Both will have different valves and both will have different minimum cross sectional areas, but the runner sizes of the 2 can almost never be compared to each other. Now we can compare a 23 deg head to another 23 deg head and compare port sizes almost all of the time, especially if they are right out of the box from AFR, Brodix, Dart, All Pro, Pro Action etc.... Looking at a 250cc 23 deg head and saying that a 260cc SB2.2 is pretty close but flows more is not a good comparison. The Sb2.2 head will be much better suited to a bigger engine or a higher RPM range most likely.

O.k. now onto the actual question that started this all. Intake Volume (or Cross Sectional Area) vs. displacement. Displacement will want more intake runner, just like other fuels, nitrous or boost. RPM also wants more volume. Now remember that this is a general statement, meaning that I'm comparing all 23 deg heads here, it doesn't matter that they are 23 deg heads, they all could be Canted Valve Brodix heads, just as long as we are talking about the same rough dimensions here.

RPM range will eat up volume about as fast as displacement will. Looking at a guy going from a stock 350 that doesn't really do much over 6000rpm and then him saying I want a 383 drag motor that runs 7500 is completely different setups. You can tweak a certain combo to make it better but it will not be optimum. The reason being that a 7500rpm 383 will need a hole a lot of airflow and too much port velocity up there will kill power and not hurt it. So for example a 7500rpm motor for a certain car might want 240cc of intake volume on a 23deg headed motor, while the stock 350 only has 170cc. A LT1 head can go out to 210cc or so, but it should also have some larger valves in it to work with that, but as you can see it's far from optimum. Now the thing I think that's funny is that guys want these crazy flowing small ports and large cams to maintain the low end TQ, that's not going to work. TQ is a product of a lot of things, one thing being that you don't bleed off the cylinder pressure at too low of a RPM with a big cam on a street motor. One reason why I will take a larger better flowing port on a street motor and "under cam" it. I'd rather have the TQ that's produced from the higher cylinder pressure and hold that TQ peak out longer (but it also comes in earlier) which will help most street engines on heavy, automatic cars. There are a lot of other tricks to this to get the big heads to work and get the TQ up, but why would I tell everything, that's not the question we are trying to answer here.

Now one thing you might have picked up earlier is that the valve size has something to do with it. Well most times the valve will have some effect on the cross sectional area, but then again the valve job will also.

To clear up something else said here. The port does care about the bore it's going into but it doesn't really care about the stroke. If you have 377 cubes, the large bore setup will flow more air, but the stroked setup will still need the same amount of area. IMHO you don't try to correlate the stroke and the port size.

The overlap portion of the same statement is also kind of fuzzy. I don't really see a hug connection with the port volume and the overlap. I see more of a connection with the total overlap area and the RPM range, and the valve job and it's effect on head flow in relationship to the overlap area.



You want airflow to achieve a certain average velocity at HP peak, but not have too much velocity. If you get the air fuel mixture moving too fast it's not very efficient and you will lose volumetric efficiency. So having a little larger minimum cross sectional area will allow the HP to hang on past peak a little longer. High velocity is not always the best thing, having a lot of airflow going thru the size of the port is.

Getting good peak numbers is one thing on engines, but building them the right way will get you peak numbers and more HP past it's peak and more TQ before it's peak, that makes it one large flat curve that will do much better. The amount you pull it up and hold it on before and after will have to do some with the engine/car combination.

Let me say that I'm not a big fan of scavenging as the 5th part of the cam system. I think that the intake has a lot more power in it than the exhaust system does. I'd rather have the exhaust system just get the stuff out of there as fast as possible and not hold anything up. There are definitely some good tuning effects there, but most of the time on a streetcar getting those effects is going to kill the streetablity of the motor. There is power there, but it's not worth it on a daily driver. Run a slightly larger head, run a little less cam and then you can run a smaller LSA, but it doesn't keep the overlap area up to much.

I think that covers it enough for me right now.

Bret

 

I always thought the higher the velocity the better the mixture was going to be...

I had the idea correctly (I think) since you need velocity to fill up the displacement... And specially on stroker engines you need velocity to achieve a better filling since piston speed is greater...

 

So you would want to look for a head with a certain avg cross section that has a x amount of port velocity, but not too much, and will support a vavle size that will match up with the avg cross section?

 

From my experience, I think that compression ratio and displacement both have a great deal to do with chosing an intake runner size. I had a friend who had a 327 with under 10:1 who switched from a set of Edelbrock Performer RPM heads (180'ish cc) to a set of Brodix track 1's (220cc) and lost almost a full second in the 1/4 mile.

 

A 327 would most likely not like a port as big as 220cc, unless you were spining it very high. That is a good example of to big of a head port on a combination.

I don't see how the compression ratio plays into that. The camshaft and CR are linked thru DCR but I don't see how the larger head port is.

Bret

 

A higher velocity will help the mixture stay together below HP peak, but it doesn't help it after that. The constant cross section and shape of the system (not to tight or radius for the air/fuel to go around) probably has more to do with it.

It's the same thing as spinning the motor faster if you are looking at increased piston speed. RPM and displacement both need more runner volume, higher velocity is not going to do all of the cylinder filling, especially at high rpm.

Now some things can confuse what people see in this respect. If I had a large bore 377 (4.155 x 3.48) vs a small bore (4.00 x 3.75) most guys would tell me that the larger stroke motor would have more torque down low, due to the mechanical advantage of the longer arm. I don't belive that to be the case, but people can think what they want. The longer stroke will have higher piston speeds eariler in the RPM range, but it will be sucking in the same amount of air/fuel as the large bore example based purely on volume. Now it will actually have more velocity as a 350 in the ports while it was operating if it had the same heads, but it will need a much larger cam to achive the same HP peak. That is the case for either engine (big bore or big stroke) On the big bore motor the engine will have a better operating head flow (just check that on a flow bench, a 4.00" test bore vs a 4.155" test bore) so it will actually flow much more air into the engine, but that shouldn't change the HP peak too much, but there will be much more power up to and past that peak. Either 377 will have HP peak pulled down at least 500rpm using the same heads that were on the 350 with the same cam, due to the runner being too small and choking off the setup. So you can either go to more cam or more head port to gain that back or extend it even more. Now the interesting thing is that the large bore 377 will have the same piston speed at peaks as the 350 does, but the average port velocities will be higher so the peaks will both be lower. Funny how the volume of the cylinder means more than the speed of the piston.

Some of these things you said are happening, but it's not the area that one should focus on when looking at a eninge, there are much more important things.

Now I'm not saying that we want low velocity in a cylinder head, but we don't want too much. About the only things that you can't have too much of in a motor is head flow and cubes, sometimes RPM but that can cause problems if the motor is not designed and built to handle it.

Bret

 

Yep, all of that. The valve size has a lot to do with the throught size of the port. That is also likely to be the smallest part of your port cross section. A larger bore will also allow a larger intake valve which when doen right will help you gain useable RPM.

For the basic example the RPM range and displacement are what will deterimine the head port volume. The more of either, the more head port volume you need.

Bret

 

I am having trouble imagining this, maybe because of the "misconceptions" i had on stroke and velocity???

Why is the velocity higher?? because there is more flow overall or more power is being made, and thus you need more velocity??? Or is it the velocity is because of the distance needed to travel through the bore and stroke (which would equal displacement) which is more important than piston speed???

If you have higher piston speed, wouldn't you need more velocity or better cam timing so the air comes in faster than what the piston is moving up and achieving a better fill?? On the bigger bore you dont have that problem of a faster moving piston and a better fill can be achieved easier...

This is awesome info... clearing some read stuff out