School me on the 731 head

[acolella76]

the Ebay mid tubes are not as bad as everyone says, BUT you will need to do some extra work. the head flanges need to be checked for being level to each other and planed as needed. you will most likely need to slightly dimple the pass floorboard right in the corner and you will also need to tweek the X Pipe down a little as it runs close to the point of touching the body.
they also are not a direct bolt-on to your cat/mid pipe. if you can't fab up the joining section from the X-pipe to the cat, you need to budget in some exhaust shop work.

the tubing is not as paper thin as it is made out to be, being that they are stainless you can run a thinner wall than mild. they DO transmit more noise than your stock cast mani so be ready for that.

they run a 1-5/8" primary to a 2" crossover back. a nice size improvementover stock.

you will need to extend your o2 sensor by 2 feet in order to have it work alsoas the bung is farther down stream than stock.

as long as you know the facts about them before hand and you can either mod them yourself of budget for the corrections you will have to make, it is hard to beat them for $168 SHIPPED to your door.

they also have excellent ground clearence BTW. no dragging the down pipes over speed bumps is nice.

I don't plan on running a cat :D

I wasn't planning on the ebay headers to fit perfectly, so I don't mind doing any fab work.

[der affe]

in leu of a cat i would put a muffler in it's place like it did.
i used a magna flow dual 2-1/4" in&out as i already had a nice cat back and was not planning on re-doing everything to a single 3".

the combo of the 2 mufflers gives it a nice quiet rumble at idle with the 292* cam and with the ITB's, from 5K-72K it has a wicked scream esp at WOT.

when i get back home i will post up a few picks and the part number of the magna flow muffler. i have a crappy sound bite with my cell phone too.

[acolella76]

sweet, can't wait! What about just gutting out the cat instead of another muffler? I was just going to do straight pipes into one muffler out back.

[der affe]

a gutted cat is pretty f'ing loud in my opinion and does not sound very good. i did glass packs at first, but they were too buzzzzy for me and would buzz when i lifted on the throttle and inbetween shifts.

as promised pics:








and a crappy sound bite from inside the car



the muffler is part number 11378

[eta]

ForcedFirebird's Avatar

Join Date: Feb 2007
Location: Coral Springs, FL
Posts: 700

Quote:
Originally Posted by eta View Post
Well I am taking the data from the post I linked a different one that you have copied. I was not aware they where in conflict. Therefore lets say on avergae a 731 head flows as good as an 885 one at low valve lifts.

I believe you may have been looking at the graph that represents coefficient of flow vs valve size vs bore diameter, not the actual flow numbers themselves.

Well I know what I was looking at and it was a more recent post of the e30 zone than the Marquisrex post that you have referenced. This new data show the 731 head flowing a little better at low valve lifts. At higher valve lift beyond 5 mm the 885 flows alot better. The two posts do show the same data but I believe they are both done at 25 inches of Hg. Different benches different results, different flow bench tester. I think its a little simplistic to say the 885 head flows better than the 731 head, it does but only at higher valve lifts.

Also any 2.7i using eta pistons built with a 731 head gets 90% of peak torque (190 ft.lb peak) at around 2000 rpm about 10.ft.lb more at this rpm than a 2.7i with a 885 head. Also peak power is the same at around 170-180 hp for both variants, indicating that for a simple 2.7i the cylinder head choice is not as important as the flow data would suggest. The extra flow of the 885 head is simply not utilised effectively by a 2.7i with eta pistons unless you built it with 2.5i pistons and a decked block.

[der affe]

i am a little confused by your post.
are you trying to say that because of the smaller ports, the 731 head flows more CFM at lower lifts. this being caused by the smaller ports increasing the port velocity.

yes, the smaller ports will flow "quicker" down low with a low lift cam, but they will fall short in the upper RPMs due to the smaller ports. they designed the ETA this way. that is why they has single springs and a milder cam.

are you saying it is because of the lower compression ratio, you are not making use of the flow abilities of the 885 head?

most people who are building one of these motors are looking to get more power AND RPM out of theur builds. in that case the 885 is better suited for the task. would they benefit more from adding either a 323 cam or an after market cam say in the 288* range? yes

are you saying that it is a direct result of the shorter rod ratio with the 2.7 ETA (130mm) VS the 2.5 i (135mm)? the shorter rod ratio will change the engine dynamics and make it pull harder and earlier down low.

a 2.7i motor would benefit from the higher comp of the euro ETA pistons, along with the 885 head and a cam. you are going to loose some HP/torque from the chamber to piston mis- match, but if you start out with the 11.2:1 pistons and add the 885 you will drop you compression ratio to 10.2:1 and add a cam to extend the effective RPM range you will end up with a good all around motor that will pull well from down low (short rod ratio and increased stroke) and still make plenty of power into the upper RPMs ( cam and head flow abilities)

[eta]

I agree the 885 head is better from a flow standpoint but the results from both BASIC 2.7i builds are remarkably similar (I am at no point assuming a rebuilt motor in anyway or even an after market cam). For some reason the better flow capabilites of the 885 are not fully realised. It's not just my car car I am comparing either in which I have had both variants but from the experience of other in the U.K and the dyno plots that have posted. Both varients make in the 170-180 hp range.

"yes, the smaller ports will flow "quicker" down low with a low lift cam, but they will fall short in the upper RPMs due to the smaller ports. they designed the ETA this way. that is why they has single springs and a milder cam."

I believe I have already said that. The 731 head flow at least as well as the 885 head at low valve lifts. So at low RPM you get similar or better gas flow in a 731 head equipped 2.7i as with 885 equiped 2.7i. At higher rpm the 885 should make such an equipped engine perform better but it does not seem to (unless you have 2.5i pistons, a decked block and a decent cam profile), at least there is no clear differance between the dyno plots I have seen. The different camshaft in the 731 head is partly responsible for this.

As for the CR, U.S 2.7i builds using low compression pistions and U.K builds using 10.2:1 pistons with 885 heads produce very similar numbers for an identical top end spec. Given the differance in CR of the resultant engine is a whole point (9.35:1 U.K and 8.35:1 U.S) the CR does not seem to make much differance.

My comments are based on taking a stock eta bottom end and fitting a stock 885 or 731 cylinder head. Bolt up the E30 exhaust, 325i intake system and wire in motronic 1.3. If you do that either head is a good as each other.

If you rebuild the engine with high compression pistons/2.5i (early euro) psitons, high lift/longer duration camshaft and other trick bits then yes you can get a more powerful engine for which you will need the 885 head. But I was never talking about such an engine, I was mearly trying to point if the OP wants to do basic build it doesnot matter too much which cylinder he uses as they both give similar and resonable results.

[ForcedFirebird]

I think we understand your point that the 731 head is "almost" as good as the 885, but the fact of the matter is, that every head porter will tell you the same thing: "Make the head flow as good as possible without sacrificing minimal cross sectional area." and the rule I follow when I am standing in front of my flow bench. If the head only spends a nano-second at that 2mm lift, compared to the longer duration at let's say 6mm, then it's a mute point.

I will gather all this info in my own way soon enough. I need to build a head adapter for one other motor before the m20 stuff makes it's way to my test station and bench, but until now, I am not convinced that the 731 head "is as good" as the 885. I make my living on producing better flowing heads for various cars, one of which I actually adapted late model heads that right out of the factory box will outflow anything anyone has ported on the earlier stuff. Albeit, the ports are larger, and the powerband is raised, even the stock cam compensates for that (just like the different cams offered in BMW stock format).

If the idea wasn't to have the best flowing heads on your car, then perhaps I entered the wrong business lol.

[eta]

I never said the 731 head is as good as the 885, it simpley isn't. It only performs as well at low valve lift. If you are planning a to build a 2.7i properly then the 885 head is the way to go but thats what I have been saying all along.

If you plan to slap an "i" onto an stock eta block with no modications in any way, it does not matter which one you chosse so long as you use the 325i intake. I have repeated this again but the details of my post seem to be missed.

[ForcedFirebird]

I never said the 731 head is as good as the 885, it simpley isn't. It only performs as well at low valve lift. If you are planning a to build a 2.7i properly then the 885 head is the way to go but thats what I have been saying all along.

If you plan to slap an "i" onto an stock eta block with no modications in any way, it does not matter which one you chosse so long as you use the 325i intake. I have repeated this again but the details of my post seem to be missed.

Well agreed. The "i" manifold and throttle body are an improvement even on a stock "us spec" eta, along with any euro eta.

During some flow testing, there was much surprise that how much of an impact a manifold can have. Porting a manifold much larger than the head, there was as much as a 3% gain, even though there was a "step" created in the wrong direction! With a cam such as designated for the eta, whats' the concern for reversion anyways? hehe

I personally do miss the lazy shifting in the full eta, any movement at all could call for third, even taking off at a light could easily be second, now with the 885 top end, torque was decreased significantly, but all-around usefulness has increased.

Let's not talk gas mileage, lol, could be better, but the 885 head and 2.7 just asks for rpm.

[digger]

I agree the 885 head is better from a flow standpoint but the results from both BASIC 2.7i builds are remarkably similar (I am at no point assuming a rebuilt motor in anyway or even an after market cam). For some reason the better flow capabilites of the 885 are not fully realised. It's not just my car car I am comparing either in which I have had both variants but from the experience of other in the U.K and the dyno plots that have posted. Both varients make in the 170-180 hp range.

It would be good if you could post some dyno plots showing this, I imagine it would be hard to find any genuine scientific comparison i.e. where only the head is different

"yes, the smaller ports will flow "quicker" down low with a low lift cam, but they will fall short in the upper RPMs due to the smaller ports. they designed the ETA this way. that is why they has single springs and a milder cam."

I believe I have already said that. The 731 head flow at least as well as the 885 head at low valve lifts. So at low RPM you get similar or better gas flow in a 731 head equipped 2.7i as with 885 equiped 2.7i. At higher rpm the 885 should make such an equipped engine perform better but it does not seem to (unless you have 2.5i pistons, a decked block and a decent cam profile), at least there is no clear differance between the dyno plots I have seen. The different camshaft in the 731 head is partly responsible for this.

One source says the 731 flows less than the 885 at low lifts and the other says a few % more so this can’t be the reason for more lowend (if that is indeed true). The reason for any difference would be in the flow velocity not the low lift flows. At every RPM the valve cycles through high and low so why wouldn’t the low lift numbers also show trumps for the 731 at higher rpms.

As for the CR, U.S 2.7i builds using low compression pistions and U.K builds using 10.2:1 pistons with 885 heads produce very similar numbers for an identical top end spec. Given the differance in CR of the resultant engine is a whole point (9.35:1 U.K and 8.35:1 U.S) the CR does not seem to make much differance.

So if CR makes little difference why would you use the 731 then if at best it’s a few % higher at low lifts but hugely inferior at the mid and higher lifts. One thing I have noticed is that those who have studied and done work with the M20 heads almost all say the 885 is far superior stock for stock and it seems BMW also though the 731 was not good enough for the M20B25.

[eta]

I know what you mean by saying the 885 head is superior to the 731 head. It is but when used on an engine it all depends on the spec if that extra flow is relaised. Just because the 885 head can flow more air does not mean that every other part of the engine involved in gas flow can keep up. In fact the most recent flow data on 885, 731 and 325i intakes manifold I have seen sugests that the 325i maifold flows less than the 885 head at same pressure. I will post this data from the e30zone as well. I will post dyno plots of various cominations using the 731 and 885 head but not unitl Sunday as I am of to work soon and I am also away for the weekend pursing my other passion cycling.

Also my 2.7i has plenty of torque in fact more than the eta had. If you want to know if those number you have Forcefirebird are real give me the 30-70 time in 3rd gear, I will use your RR data (I presume those are wheel figures or those numbers are depressing, if there not post the wheel numbers please) and input them into my spreadsheet which will factor in air resistance, rolling resistance and it will then calculate the acceleration in any gear and top speed. I can also tell you the precise difference difference diff ratios will make including shifting points. I presume you have a manual car. I will get on this on Sunday as weel. It won't take long. I will do a long post on Sunday.

[acolella76]

could some of the torque lost by swapping heads be gained by a good tune? And with a tune how much more power do you think it could make?

[eta]

Well here are some figures.

First is the new flow data. This show the stock 885 head (first in key), then the best flow from all the 885 porting attempts, alpina 2.7i head flow data, stock 731 head, 731 head with 42mm/36mm valves, and 885 inlet manifold flow data. All done at 25" vacuum. Vertical axis is CFM, horozontal axis is valve lift. The previous data posted by forcefirebird from the E30zone as well (OP was Marquisrex) was gathered at 25" inches vacuum as well. [IMG]file:///C:/Users/Hal/AppData/Local/Temp/moz-screenshot-7.jpg[/IMG]I've posted both for comparison.

Forcefirebirds data sourced from e30 zone by marquisrex

Lastest data from e30 zone.

Hense the comment I made about the 731 head out flowing the 885 head at low valve lifts. The difference is not great but its there. The two sets of data are obviously different so which is right I don't know.

Dyno plots.

First a E30 327i using an 10.2:1 CR eta bottom end with 731 head (giving 10.2:1 CR) port matched to an 325i inlet manifold with a 325i cam installed, 325i inlet manifold, 325i TB, 082 AFM, motronic 1.3, 173 ECU with a chip from A-tech in the U.K, hottuning exhaust maniold on 325i exhaust system. A powerful motor, flywheel fiqures shown. Wheel figures not posted but dynodymanics RR here seem to record 22% drive train loss for these cars so wheel fiqures would be~ 138whp/148wtq. The owner of this has finally fitted the 731 cam and his peak power has moved nearly 500 rpm to the right (I can't find the RR plot atm) but no loss/gain in power, he also has a Miller MAF conversion on it as well.

Second
E30 327i.

Spec, 10.2:1 eta bottom end, stock 885 head (ging a CR of 9.34:1), 325i intake manifold, BBTB, motronic 1.3 with A-techs chip, alpina 6 branch exhaust manifold on a scorpian E30 exhaust system. Again no wheel fiqures so est wheel figure are ~135whp/148wtq. This is done on Dynodynamics RR as well.

Third. E28 527i


This is my car. The spec when it was put on the RR was;
Eta bottom end, 885 head (9.34:1 CR), 325i intake, eta TB (the garage that done the work fitted this by mistake oops), motronic 1.3, 173 ECU with A-tech chip, stock eta exhaust maniold and eta exhaust system. On the day it ran rich (AFR 11.5:1 at WOT). This did not help. With the proper TB fitted and a chip retune it feels like it has a bit more power but no RR data on the new set up. However I believe the main differance between the my engine and the above is the exhaust system. The reason why I think mine is down on power and therefore why your Force firebird is also down on power is because of our eta exhaust systems. Some evidence coming up.

An E28 527i from the U.S

U.S eta bootom end, 885 head (8.35:1 CR I believe), 325i inlet mainfold, 325i TB and motronic 1.3 (unkown chip). It makes similar numbers to mine with the eta TB showing the TB is not a big restriction. Wheel numbers shown here.

Another E28 527i.

U.S eta bottom end with 9:1 forged pistons, 885 head, 325i intake mainfold, 325i TB, 3.0 bar FPR, motronic 1.0 running some sort of Dinnan chip, headers (unkown brand) and 2.5" exhaust.

So a decent exhaust on the 731 head equipped 2.7i, the second E30 885 head equipped 327i, and this last 2.7i seem to make 135-140whp and plenty of torque.

Mine makes less hp but torque is good and normal compared to the other plots. So a 731 equipped 2.7i head on the same type of RR (dynodynamics) makes more than 885 equipped 2.7i! Although the RR are run by different opperators I stand by my asseration for a simple 2.7i build using stock eta bottom ends, 325i intake manifolds, 325i TB, motronic 1.3 it does not matter much with cylinder head you pick 885 or 731, either will do as the high flow potential of the 885 head is not realised. It is only realised if you 325i pistons and deck the block to achieve sensible CR. With the stock 325i intake system, TB e.t.c builds like this can push out c.210 fhp and 210+ftq. A-tech build them all the time.

I hope this settles it for everyone and the data is useful to people.

[matrix]

Looks like the "731" is the better head overall. 885 is perfect for turbo only. So 731 will smoke a 885 on the line all day, but the 885 eventually will catch up.