SMTCapeCod

Boring it will give you greater flow, potentiall at the top end. But that is only if the cams and heads can make good use of it. In the mean time, the additional diameter can slow the velocity of the incoming air charge, particularly at the lower end of the spectrum. Enough to notice? Well, I don't know about that, but too much capacity/air can diminish low end responsiveness.

gruffalo

What do you mean "ever reach redilne" ? I have the same engine as you and it revs smoothly right into the rev limiter, no problem. The torque curve drops some above 5k, but mine feels almost linear into the rev limiter as I said.

A bigger throttle body will not fit the other parts you have (plenum above, and K-Jet thing below). You will need the complete S setup, and then again, you will probably need the heads and cams, and there you go down the slippery slope...

the flyin' scotsman

Very true...............Porsche engine designers did what they did with good reason. Engine mods including air/fuel input and exhaust output takes serious engineering and they have to 'flow' together.

Increase the air/fuel, change the cams and piston compression, open up the exhaust, lighten the car all sounds good; making all these changes work in harmony........tough.

danglerb

Bah Humbug, nothing before the plenum chamber is anything but a restriction, and not all the designers work was toward goals we might like, such as making sure the upgraded model in 3 years sells well.

Velocity through the throttle is nothing but a restriction, intake runners and exhaust headers are tuned, ditto ports on the heads, and the cam should "know" about them, but its not rocket science.

danglerb

I was just looking at a picture of the US 83 throttle body, the "in" part looks ok, but then its this ridiculously narrow U with a small looking outlet.


That is quite a restriction.

GRTWHT

Thats good to hear about the 4.5L engine.
I should get out and check the ignition timing and mixture strength and go for a blast to see if it redlines.
Car drives fine, I just dont see the point of heading towards redline once the torque drops off at just over 5000 or so like you said?
Maybe it's just me??

Glenn
'81 928

BC

The USA 81s have ALOT of potential in both Throttle body size, Timing, and exhaust. Even just exhaust and some advance on the single distributor will make that thing want to hit redline. And the early 2Vs sound great with headers.

GRTWHT

Yeh BrendanC, I have had the exhaust system modified to be alot more free flowing than the origanal system that was on the car. It's now 2 1/4" out of the manifolds into a Y connector and from their on to the diff it is 2 1/2" all mandrel bends. From the diff back it's still quite restrictive (non standard), so thats next to do.
This alone made a big difference to throttle response. So I figured that these 4.5L engines are choked up quite a bit, thats what made me think of going for a larger throttle body.
I'll check the timing soon and reset it, see how it goes from their.

Cheers,
Glenn
'81 928

BC

78 cams would be a great addition to your motor with an Euro LH intake and throttle body. Port match the heads and you should have a nice mouse motor.

GRTWHT

Sounds like a plan

Glenn
'81 928

BC

Jeez, I am making others plans for taking apart their cars. Just ignore.

Sharks

Be very careful removing material from your throttle body ....

When I installed my supercharger I removed and cleaned everything. After cleaning the throttle body (car only had 56K miles on it at the time) I found a crack in the casting that went about 3/4 the way around the butterfly spring post. I took the throttle body to a buddy that builds race cars and he was able to weld it back up and grind down the weld. The spring tension on the butterfly is so tight that I when I put it back together I took two turns out of it and the throttle responcse is much better now (not the production cable routing).

Rich9928p

The intake is a system - ALL the components need to meet the needs of the engine for airflow. Just honing out the throttle housing will not improve the system.

The 300 HP Euro S motors were higher compression, they produced the power at higher RPMs, so the valves were larger, cams were more radical, throttle housing larger, intake runners larger, and in CIS form were able to do this with the same size fuel distributor, and in 1984 went immediately to LH with the hot wire mass flow sensor and pumped out about 310 HP.

The 234 HP US 928s from 1980 - 1984 were low compression (will run on regular grade fuel - NO need for premium) and used L-Jet fuel injection. The throttle housing matches the size of the L-Jet airflow meter. There will be some performance improvement by installing a better flowing intake, but the real power comes from higher compression.

danglerb

Compression is not a treasure chest of HP, you get about 1% gain for each .5 increase in compression, flow is where you find HP. LH and MAF allow a LOT of flexibility, but 320 rwhp is doable with the factory L setup. Thats not me talking, but what smart people have told me and I believe is correct.

Its Kenne Bell data, but its fairly basic stuff,
http://www.kennebell.net/techinfo/ge...perf-guide.pdf

GRTWHT

Thats exactally what my engine is i believe. 10.0:1 compression 240H.P 4.5L (m28/10) K Jetronic
60 H.P down from the 4.7 S engine.
I'm still learning about these family of engines, but could'nt believe that for .2 of a litre thats all that took for the extra 60 H.P.
As I'm finding out its all the breathing equipment thats bolted up to them. Has someone done a horsepower comparison with a 4.5L standard and a 4.5L with heavy breathing 'S' intakes and exhaust?

Glenn
'81 928