ALFAGTV6.COM

Barry · Post by **Barry** » Mon Jun 06, 2005 9:25 pm

mATS SAID STATIC OR SWEEP-oops-havnt heard that before in dyno talk..

junglejustice · Post by **junglejustice** » Tue Jun 07, 2005 10:55 am

While this can not be by any stretch considered to be a scientific method of calculating drive-line loss, or altitude loss, it is a pretty reliable method of knowing what in the real-world that number looks like for a given make and model...

We'll take Alfa.

Stock SA model 147 GTA rated by the factory at 250 crank horses; On Dawie's dyno at 6000 foot elevation - 178 horses at the wheels (stock - not the semi-tuned, open exhaust, open electronics racer...)

Stock fresh SA model GTV6 2.5 V6 L-Jet rated at 154 horses at the crank; On Dawie's dyno - 103 to 105 horses at the wheels.

One more; Stock 3.0 24 valve 164 Q rated at 230 - 156 at the wheels.

Now, we are dealing with altitude and drive-line fudge here...

So, here are some ROUGH, FAST, HEAD-CALC. numbers:

A 3.5 12 valve big-bore on Dawie's dyno = 180 horses at the wheels. If you take that number and surmise that it is about 63% of the real crank-power (given a driveline loss of about 20% and an altitude handicap of about 17%), you are looking at a combined loss of about 37%. With that you are probably looking at about 285 horses at the crank before the 37% loss... (180 divided by .63, which is the same as 285 times .63, or deduct 285 times .37 from 285... Either way - that calculation produces about a 285 crank-horsepower number!

An exact copy of that motor (pistons, rods, cams, specs and all) went to the UK for Alek Jupe at sea-level and produced 220 horses at the wheels! Now this was already at sea-level so we are now “only” looking at the rear-wheel driveline loss of say 20% again, but now to get to about a 275 horsepower number (220 at the wheels divided by .80, or 275 times .80, or 275 times .20 deducted from 275 - either way... You end up with 228 horses (but it only made 220) if you try to use the full 37%...

We see some differences there.... Maybe the altitude number is more in the 20% range and a good driveline loss number for that engine in a 75/GTV6 is closer to 17%

Micke · Post by **Micke** » Thu Jun 09, 2005 12:18 pm

Back to the original topic.

This doesn't have much to do with this case but I was struggling with something similar.
My 2.0 didn't rev as it should some 10 years ago. Everything was supposed to be OK.
Tried different carbs (45 instead of my 48 )
Tried tuning cam timing and ignition.
Swapped the ANSA extractor for a Supersprint. This helped a bit (very tiny one). The hint made me build a new extractor and whoops, the engine was like new. Some 1500 rpm more and 20 hp more power.

Post by **Zamani** » Thu Jun 09, 2005 1:48 pm

I suspect my exhaust system too.

It has a 2" outlet at the back. Do you think 2" is too small for the tail box? Not only 2", but 2" with crush bends (two crush bends actually to make an S bend). So maybe 1.75" where the bends are.

But the SZ headers are probably not much help either. They're probably a little better than stock but will not match the CSC, PACE, liverpool exhaust or Beninca's. Beninca headers are HUGE (almost 1.75" primaries).

Barry · Post by **Barry** » Thu Jun 09, 2005 8:23 pm

Zman,Crush bends are not as restrictive as you might think-Ive done a lot of research into airflow with my turbo setups.You will be amazed at how effiecntly Crush bends flow.Its got to do with the mass and inertia of the gasses flowing in that bend.
Is the 2" outlet on your 3.0l??A bit small imo.On the 3.8l Im running 76mm all the way from the y collector..............

Post by **Zamani** » Fri Jun 10, 2005 12:18 am

Baz,

I'll try a bigger exhaust. I thought 2" at the rearbox is not an issue. But I guess it could be the source of the restriction. Will go with 2.5" (62.5mm) and re-dyno.

Micke · Post by **Micke** » Fri Jun 10, 2005 12:36 am

IMO the rear outlet is mainly for street credibility.
Of course less restriction there doesn't hurt.
The critical part in the 4-banger are the headers.

Jim K · Post by **Jim K** » Fri Jun 10, 2005 12:57 am

Despite exhaust design being a 'black art' (even with all formulas you're no better than +-10% accurate) there's a definitive way to learn about too big or too small and I've mentioned it a few times: back-pressure measurement!!! But nobody listens, although its something soooo simple to do! One little fitting after the headers and one before the second muffler (assuming you only have two) will give you the answers you want. In na engines, going too big can negate much of the extraction effect.
Jim K.

Micke · Post by **Micke** » Fri Jun 10, 2005 1:10 am

Jim,
I listen 4 sure.
However, I'm with Smokey in this matter. Back pressure can't really help.
Only in case of too wild cams, too big ports or similar, which you want to "detune" by backpressure, I suppose.
I, once again, admit I can be wrong. Please educate me and explain why backpressure can be beneficial.

Mats · Post by **Mats** » Fri Jun 10, 2005 2:28 am

Backpressure is not beneficial, gas velocity however is something you would wanna keep high otherwise you will loose the scavenging effect.

Building several systems varying diameter and mufflers/bends and measuring backpressure in different parts of the system and keeping the diameter the smallest possible without pressure buildup sounds reasonable (using my spider-sense here) but dyno or preferably actual laptimes would naturally be the best way to see which way to go.

Talk about time consuming...

Jim K · Post by **Jim K** » Fri Jun 10, 2005 6:15 am

Micke, Mats answered on the spot! Where did I say BP is good?

Exactly as Mats said, gas speed is good for scavenging the spent gases out of the chamber. Measuring BP is good assurance you don't have any!Then, you don't have to go overboard with needlessly huge pipes!
Jim K.

Mats · Post by **Mats** » Fri Jun 10, 2005 9:58 am

Yeah, big pipes are heavy and scrape the ground...

Jim K · Post by **Jim K** » Fri Jun 10, 2005 10:35 am

This ground scraping problem for a lot of people, can be reduced to a very good degree by copying the solution AR adopted in the 75Turbo: In the lowest part of the car, use a length of pipe of larger diameter which, when partly squashed, presents the same cross-sectional area with the rest of the piping. This way, you can increase ground clearance and have the required exhaust bore size, too!

Jim K.

Mats · Post by **Mats** » Fri Jun 10, 2005 11:15 am

Dude, that part is the first to go for any kind of power upgrading.

Jim K · Post by **Jim K** » Fri Jun 10, 2005 11:20 am

I ain't talking about the damn Turbo, just about the philosophy and reasoning for the job which is a valid proposition in engineering terms!
Jim K.

If I may...