Runner Length

[ERA Chas]

Originally Posted by bobcowan

The valves were 2.10 x 1.90, with a 225 runner. Yes, they were custom made, from Gordon Levy.

I could probably make a little more top end power by using a 2.05" valve. But I'm not after every last bit of power. It's for fun, not for money.

Thanks Bob, I love stuff like that. I agree-fun not cash.

I'm guessing also that the chamber size is north of 75cc by the exhaust valve size? What does compression come in at? BTW, not trying to steal your speed secrets, I'm an FE guy...

[bobcowan]

The chamber size was pretty small, they'de been milled. Something like 65cc or so. Static compression was a little bit >10:1. I can't seem to find the spec sheet anywhere, so I'm going off memory.

I'd don't have any speed secrets. It's all fun and games to me. I'll happily share any knowledge I'v picked up along the way. And believe me, it has been a learning experiance.

[vector1]

bob, if you have different specs for the heads or any other data, that is important to the sim ware.

[bobcowan]

I'v never done any flow bench testing. So the basic specs as above are the best I can do. The numbers from the sim don't have to be real accurate. I would just like to know what happens when the length is decrased by 1".

[vector1]

alright bob, here is what i came up with preliminarily. i used afr205's, library didn't have the darts. estimated a lot of the cam data and made some assumptions on other stuff of course.

initial tq with your present setup showed 494@5000, 507 @5500. your present tb's flow 2779 cfm, best guess. this is using 2.25" length on tb's.

the preferred dia. of the tb's would be at 1.75", or 44mm, lesser so 46mm. length of the tb's was 8.25-9.25 using eap, i don't have pipemax here to give a preferred wave tuning but can do later. cfm of this setup is 1793cfm. using this setup tq was 570 at 4500, hp was 556 at 5500.

i'm not sure how the tb length is measured but have a question in to find out for sure.

[vector1]

Originally Posted by vector1

alright bob, here is what i came up with preliminarily. i used afr205's, library didn't have the darts. estimated a lot of the cam data and made some assumptions on other stuff of course.

initial tq with your present setup showed 494@5000, 507 @5500. your present tb's flow 2779 cfm, best guess. this is using 2.25" length on tb's.

the preferred dia. of the tb's would be at 1.75", or 44mm, lesser so 46mm. length of the tb's was 8.25-9.25 using eap, i don't have pipemax here to give a preferred wave tuning but can do later. cfm of this setup is 1793cfm. using this setup tq was 570 at 4500, hp was 556 at 5500.

i'm not sure how the tb length is measured but have a question in to find out for sure.

i'll go do a 1" shorter setup shortly.

[vector1]

in addition

the stack height is measured from the head mounting surface to the first abrupt enlargement--plenum, open to atmosphere.

checked a few numbers and here is what i came up with using 1.75" bore, 2.05" bore would be down in power

11.75" pk tq 569@4500
12.25" pk hp 514@5500
14.25" avg tq 429
11.25" avg hp 325

after looking at the graph with pk numbers, there is a wave that moves with each .5" increment change in stack length along the peak of the curves. successive waves i might add, these are not small either. if the length is held to a max of 9.75 there is no wave, although the peak and avg numbers are lower. this might account for flat spots or lean/rich problems guys are having at certain rpm's.

i'll play with the numbers again at 2.05" throttle diameter and if the numbers are significantly different i'll let you know.

hth

let me know what your exact length is at present and i can let you know what 1" shorter will do.

[bobcowan]

I don't get home until 2200 tonight. But they're the same height as a weber carb. The velocity stacks are 2.25" tall. The manifold is a pretty standard piece. Measured across the center, I'd say about 4" from flange to flange (WAG).

[vector1]

this is using the information you gave me using pipemax program. note--these lengths are from valve face to atmosphere. previous figures are from head mounting surface to atmosphere, so they are pretty close numbers it appears. the distance from valve face to intake manifold mounting surface distance is 5". so subtract 5" from the below numbers to tune for a specific harmonic, this would be the length of the throttle body from mounting surface to opening at the bell.

- Induction System Tuned Lengths - ( Cylinder Head Port + Manifold Runner )
1st Harmonic= 39.072 (usually this Length is never used)
2nd Harmonic= 22.176 (some Sprint Engines and Factory OEM's w/Injectors)
3rd Harmonic= 15.482 (ProStock or Comp SheetMetal Intake)
4th Harmonic= 12.186 (Single-plane Intakes , less Torque)
5th Harmonic= 9.887 (Torque is reduced, even though Tuned Length)
6th Harmonic= 8.318 (Torque is reduced, even though Tuned Length)
7th Harmonic= 7.179 (Torque is greatly reduced, even though Tuned Length)
8th Harmonic= 6.314 (Torque is greatly reduced, even though Tuned Length)
Note> 2nd and 3rd Harmonics typically create the most Peak Torque
4th Harmonic is used to package Induction System underneath Hood

[vector1]

changing length of throttle body only

5" 518/524 396/305
6" 525/530 401/308
7" 532/532 405/312

pktq/pkhp avgtq/avghp

i guess that's it in a nutshell, all relative of course.

[bobcowan]

Oh, well that kind of surprised me. But exactly the information I was looking for. Longer = more horse power and more torque. I guess I should leave the spacers in place.

[*13*]

Interesting stuff, for sure!

[vector1]

Weber carbs are designed for maximum efficiency at 235 ft./sec. port velocity (28" depression). That equates to around 470 ft./sec. in a live running engine or .417 Mach. Webers aren't designed to meter fuel at port velocities much faster than that.

found this interesting data.

[Gaz64]

Adding spacers to runner length lowers the engine speed at which peak torque occurs at, and also the power peak rpm.

The engine could make more torque across the whole rev range, and if it does you'll make more peak rpm power as well, you don't know until you try.

[markha]

Ran across this today. May be interesting. Only work in Internet Explorer (IE). So if your a Firefox user, you'll have to cut and paste the link. Interesting stuff though!!!

http://www.velocity-of-sound.com/vel...alculator2.htm

[bobcowan]

Originally Posted by Gaz64

Adding spacers to runner length lowers the engine speed at which peak torque occurs at, and also the power peak rpm.

The engine could make more torque across the whole rev range, and if it does you'll make more peak rpm power as well, you don't know until you try.

That confuses me a little. I remember the old Dodge cross ram. It had runners about 2.5 feet long. I thought that was to increase top end power. Was it for low end torque?

[Gaz64]

Originally Posted by bobcowan

That confuses me a little. I remember the old Dodge cross ram. It had runners about 2.5 feet long. I thought that was to increase top end power. Was it for low end torque?

The long runners with the carbs above the exhaust manifolds boosted the midrange torque by around 10% at 2800.
It would have made more power as well because it made more torque over the whole useable band.

The point of the exercise was to maximise torque in the 50-80 mph zone.

From:Scientific Design of Exhaust and Intake systems, Smith and Morrison

[bobcowan]

Ah, I see. I thought they did that for differant reasons.