Great info to add to our library, thanks for the effort Louie..

 

Louie,
The calculator I've been playing around with shows a total intact tract length of 15.27" from the plenum to the back of the intake valve (tip of the intake horn through the throttle body, through the intake port to the back of the intake valve). Any chance you could plug that number in to your stroker configuration and see what the crystal ball says? This is assuming 50mm ID intake horns, with the rest of the motor configuration similar to your own.

 

Hi Adam,
Here are the results. Your 11.27" (15.27" total) as compared to my approx 7.5" (11.5" total length) runners. In real life, I don't see the torque dip between 3000 and 4000 rpm. I don't know what causes that in the EAP program. Don't spend a lot of time trying to get the length perfect to the last bit.

https://rennlist.com/forums/attachme...1&d=1232331764

 

Thanks Louie. A couple points in response to your effort and work.

1) Thanks!
2) I found similar things. I tried to program in my cut cams, but do not have full enough specs or do not understand how to get all that info from the durarion and lift specs I have.
3) I found the same issue with boost. What I did, and its based in real fact, is change the fuel to alcohol Rich. This allows much higher boost. I will be running E85, so I did not feel this would skew my results in any way. I had to change injectors and I think something else, but it worked very well. I could focus on 10:1 compression (which the motor actually is) and the real boost I will be obliged to shove down the runners (over 20, hopefully 30 or more). These two things are important for me to keep real.

When I did this, I found exactly the same issues/things as you. So it really made no difference. It was just at a higher level of power. Very little loss for shorter runners, and a much bigger loss above 3500 if you DON'T run short runners.

I should try playing with turbos.

Thanks again Louie. It really seems as long as its over 20psi, and its bringing in boost at least by 3500, that shorter runners are okay.

Full Circle I guess.

 

I can live with that power band. I was shooting to having the torque plateau extend to 5500rpm, but hey 5K is fine for where I would be running the motor.

Got time for one more Louie? I was anticipating having to cut the M5 horns down to optimum length, but now you have me curious. I have a measured length from the back of the intake valve to the top of the ITB of 8.5" With the adapter plate to make the M5 intake horns work, and the horns themselves adds another 10.25" for a grand total intake tract of 18.75".

Any chance you could plug in the 18.75" number and add it to the graph?
If I can keep a somewhat similar powerband to what I've calculated as optimum, and not have to modify the intake horns that would be a huge bonus.

 

Good Job Louie. Brendan, looks like you are on the right track. Have you considered making the intake out of plastic? I was just working on a friend's E46 BMW last night and the entire intake was made of plastic, kinda like the 993's. I wonder how much it would cost for a plastics manufacturer to make a run of a custom plastic intake...

Dan
'91 928GT S/C 475hp/460lb.ft

 

Hi Adam,
Here is the results of the longer runners. It's as you'd expect moving the torque peak down in RPM and increasing the value a little. That comes at the expense of higher end torque which affects HP. Still, it has usable power to 6000 RPM even though it's past the peak HP. This isn't going to show exact numbers, but the effect of the change should be generally proper.
https://rennlist.com/forums/attachme...1&d=1232480219

 

Thanks Louie,

I'll start a dedicated thread once the rest of the intake horns come in. Fed Ex says that'll be today.

 

Seems to me that since each shift after 1st lands you at 4k, you would not want the longer runner.

 

I did a plot using runners 2 " longer than I have which would be 14" from the valve head. That seems to give a good compromise of more tq than my short (12" total length) runners between 3200 to 4800 RPM with a max of 30 lb ft more tq @ 3800. The loss isn't much up top either. It has about 15 hp less at 6000, but still good out to 6400. To get that 2" extra length, you'd need a curve in the runner and that'll throw in some loss. I have no way to tell what it would really do without experimenting.

 

This idea is still in process. It has been slowed by the fact that I will be installing a rebuilt 4.5L in the car to learn some things like tuning and how to work with ethanol, etc, before I put the "more expensive" 5L in. So I have more time to figure out that intake manifold.

I am now at a point where I may attempt an intake manifold on the 2V as well with what I have learned from the exercise for the 4V.

One main issue is that the port surface is again flat, but its angled differently. Almost as though it would be better to go STRAIGHT in (straight down) into the port, if not a bit angled from the cam tower side. Interesting.

The flange is easier, since the bolt holes are in more normal places.

The curved runner thing keeps poping up. If I stay straight, I would need large balance pipes. If I can curve them a bit, the two plenums could be welded together to create one structure.

 

Louie
Great program......... Looking at the modeling software and your real curves....it does follow the 12" runner quite closely.....

Do you have a graph for your engine with the shorter runner? The 8" total length...it would be interesting to see the differences in how it lowers peak torque and shifts the torque band higher in the RPM range..... Or is just too short and looses peak power?

 

Hi Brian,
It does as you'd expect. There is less peak torque for even more driveline preservation and the torque peak is broader. Less tuned effect. The hp peak moves from 585 at around 6000 RPM to 600 at 7000 RPM and only drops about 20 hp down at 7250. At 7250 the longer runners are down 70 hp. The hit comes in torque below about 5800 rpm. There is a 50 lb ft loss at 4000 to 4800. Below 3600 they are about the same since, I guess, neither are close to resonance at all. The shorter runners would be good if the engine was for track use and you had really light pistons and Ti rods. For a stroker with normal components, even 6500 RPM is pushing it. Piston speed is 4052 ft/sec and at TDC piston Gs are 2960. Think of the rod bolts.

 

Louie
It would take some properly designed internals to get the gain for the shorter runners.....but only 15whp 1000rpm higher......thats not worth it....even for a track engine... Honestly the powerband you have now would work just fine for most race 928's....sure shifting the torque a shade later would be nice.....but you have more than 500ftlbs from 4K to almost 6k with HP climbing to 6600....thats plenty of power with a nice wide power curve...using the typical race car powerband of 4-6600k you have nearly 500ftlb average and 500hp average...

So if you shifted at 3500rpm.....I might have a chance!!

 

If you were to be extreme with the shorter runners, you could shift shift well past the peak because the hp doesn't drop so fast after the peak as it does with longer runners. Then you'd drop RPMs not so far on the low rpm side of the peak. The average tq/hp would be better. Probably more of a drag race practice than road race. You have hit exactly on why I have the runner length I do have. It's a hard compromise to beat. You have far better chance than you imagine no matter where I shift. Remember, it's me driving. {:^)