Cam Shaft Selection, and ITB vs Single Throttle Body

[Fly911]

I'm in the process of rebuilding my 1987 Carrera race car, and need advice for cam shaft selection. The engine will be taken out to 3.8L (3,746cc), and every single moving part will be changed out. The car is a dedicated track car (DE, so far...), but I will drive it to/from the track and some week end back road driving. But I don't mind to have a pure-bred race engine, and not a street compromise.

The cam shafts kind of determine everything from here on in the project. I have been looking at DC80 and DC100, bot fairly aggressive cams. Those cams will dictate ITB's, I believe I will have severe reversion problems with a single throttle body.

I have also looked at the RSR 3.8L cams. They have about the same duration as the DC80, but a 109 degree lobe separation angle compared to 100 degrees on the DC80 (and DC100). So I will assume that reversion issues are less with the RSR 3.8 cams (confirmed by my engine builder and ECU manufacturer).

I have equal length headers and a Y-pipe exhaust with straight through resonators, so the exhaust extraction should be fairly good to prevent reversion. I also have a 997 GT3 Cup air intake plenum (with two resonance flaps rather than the usual one flap), and 50mm ID runners for each cylinder.

If I go for a single throttle body rather than ITB's, I believe that this solution will be less restrictive for air flow to the engine. I can have a 100mm throttle body, and the air flow path will be very unrestricted all the way to the cylinders. I believe that ITB's (not slide valves) will cause more restrictions.

So my big question is: Will I get more power (HP and TQ) using the RSR 3.8L cams and a single throttle body, or use DC100 cams and ITB's?

[Steve@Rennsport]

JMHO,.......

Both of your cam selections require some kind of ITB induction system to work; single-throttle systems perform very poorly when using cams with those kinds of duration and LC's narrower than 112 degrees.

3.8 RSR cams offer an excellent range of TQ & HP and would be my choice here, however you REALLY need ITB's for them to work (as well as the GE100's).

If you have the budget, this is what you want:

[al lkosmal]

Steve...........................that is art.

regards,
al

[0396]

Originally Posted by Steve@Rennsport

JMHO,.......

Both of your cam selections require some kind of ITB induction system to work; single-throttle systems perform very poorly when using cams with those kinds of duration and LC's narrower than 112 degrees.

3.8 RSR cams offer an excellent range of TQ & HP and would be my choice here, however you REALLY need ITB's for them to work (as well as the GE100's).

If you have the budget, this is what you want:

Yayaya, if I had the budget. Steve's suggestion would be the choice to take.

[mamut]

You will have less restriction with the Itb's, and much more better idling with them, the DC Rsr cams are an improved version from the OE Rsr's., regarding your question, I will go with the RSR's, but at the end everything rides on the tuning,BTDT many times, cheers, Stef.



QUOTE=Fly911;8014322]I'm in the process of rebuilding my 1987 Carrera race car, and need advice for cam shaft selection. The engine will be taken out to 3.8L (3,746cc), and every single moving part will be changed out. The car is a dedicated track car (DE, so far...), but I will drive it to/from the track and some week end back road driving. But I don't mind to have a pure-bred race engine, and not a street compromise.

The cam shafts kind of determine everything from here on in the project. I have been looking at DC80 and DC100, bot fairly aggressive cams. Those cams will dictate ITB's, I believe I will have severe reversion problems with a single throttle body.

I have also looked at the RSR 3.8L cams. They have about the same duration as the DC80, but a 109 degree lobe separation angle compared to 100 degrees on the DC80 (and DC100). So I will assume that reversion issues are less with the RSR 3.8 cams (confirmed by my engine builder and ECU manufacturer).

I have equal length headers and a Y-pipe exhaust with straight through resonators, so the exhaust extraction should be fairly good to prevent reversion. I also have a 997 GT3 Cup air intake plenum (with two resonance flaps rather than the usual one flap), and 50mm ID runners for each cylinder.

If I go for a single throttle body rather than ITB's, I believe that this solution will be less restrictive for air flow to the engine. I can have a 100mm throttle body, and the air flow path will be very unrestricted all the way to the cylinders. I believe that ITB's (not slide valves) will cause more restrictions.

So my big question is: Will I get more power (HP and TQ) using the RSR 3.8L cams and a single throttle body, or use DC100 cams and ITB's?[/QUOTE]

[Steve@Rennsport]

Originally Posted by al lkosmal

Steve...........................that is art.

regards,
al

Glad you like it.

It works even better than it looks, too!

[jpnovak]

Is that a 997 GT3 muffler? How does it sound on the older 3.6/3.8?

I thought they had two inlets/side. Did you close one up?

[Steve@Rennsport]

Originally Posted by jpnovak

Is that a 997 GT3 muffler? How does it sound on the older 3.6/3.8?

I thought they had two inlets/side. Did you close one up?

Its a 997 Cup muffler which is dual-in, dual-out. Tad too loud for the street without the valves.

JMHO, but these sound far better than ANY other muffler I've ever heard. Plus, it works better than anything else tested, to date (and thats a LOT of mufflers).

[Flat6Heaven]

Fly911, which ITB setup are you considering for your build? PMO, Jenvey, or something like AlKosmal's works of beauty?

TIA, Ben.

[Mahler9th]

I have been running a single throttle set-up with pretty aggressive cams for years with my 3.8L racing engine. The cams are similar to 3.8 RSR. The intake system is from a 993, with a larger 80 mm throttle body and a snorkel with cone filter. My application uses the 993 intake flapper door controlled by an rpm switch. I made some slight geometry changes to the throttle linkage for improved response. Excellent power and decent response (not as good as ITB's, but a fraction of the cost). Idle wasn't great-- adequate for a race car which is occasionally driven on the street. Overall I have been quite pleased with this set up.


A TWM ITB system kind of fell into my lap this past winter so I am selling my single throttle induction system.

[Mahler9th]

I think Rothsport has done quite a bit of development with cams and mixed ITB/plenum systems as shown in the photo above. I am sure they have used 993 and GT3 plenums for various applications, along with Jenvey's and their excellent proprietary linkage. Not inexpensive.

This approach is becoming more and more popular in the PRC Spec 911 class with 3L engines that have quite a few other restrictions.

I have briefly pondered a project blending ITB's and a water-cooled factory carbon fiber plenum for my 3.8L, but I cannot justify the development costs.

[Flieger]

Originally Posted by Mahler9th

I have been running a single throttle set-up with pretty aggressive cams for years with my 3.8L racing engine. The cams are similar to 3.8 RSR. The intake system is from a 993, with a larger 80 mm throttle body and a snorkel with cone filter. My application uses the 993 intake flapper door controlled by an rpm switch. I made some slight geometry changes to the throttle linkage for improved response. Excellent power and decent response (not as good as ITB's, but a fraction of the cost). Idle wasn't great-- adequate for a race car which is occasionally driven on the street. Overall I have been quite pleased with this set up.


A TWM ITB system kind of fell into my lap this past winter so I am selling my single throttle induction system.

Is that why Cup cars have so much surging/bucking when driving through the pits?

[Evan Fullerton]

One if the reasons. The other is tall gearing and little inertia. 997 Cups come with a 5.5" triple plate clutch and most 996s have been updated to that.

[al lkosmal]

Originally Posted by Flat6Heaven

Fly911, which ITB setup are you considering for your build? PMO, Jenvey, or something like AlKosmal's works of beauty?

TIA, Ben.

Ben,
Thank you for the kind words............I appreciate the appreciation.

regards,
al

[clutch-monkey]

Originally Posted by Steve@Rennsport

Its a 997 Cup muffler which is dual-in, dual-out. Tad too loud for the street without the valves.

JMHO, but these sound far better than ANY other muffler I've ever heard. Plus, it works better than anything else tested, to date (and thats a LOT of mufflers).

looks great too. not much choice for us modded aircooled chaps when it comes to centre exit mufflers- adapting something watercooled looks like the way to go.

[Mahler9th]

I am not sure about why Cup Cars do what they do. There are likely several contributing factors, weight and gearing not least among them.

Of course, many racing cars are a challenge at idle and light throttle at slow speeds like in the paddock.

I think many (most? all?) Cups have single throttle intakes. And I am pretty sure a variety of plenum designs have been used. The R/RS/RSR family I am pretty sure have mostly (all?) had ITBs with plenums.

There have been quite a few configurations through the years and I haven't tried to keep track of what has gone with what in terms of factory cars. In fact this is true for many parts of factory cars. I went on a shopping spree at FLM's parts department with a friend a couple of years ago and I was astonished with what they had. Stuff many in the Cup world have never and will never see. That friend has quite a few factory car plenums of various configurations in his "inventory."

Rothsport has been a pioneer of combining the 993 and GT3 type plenums with ITBs, the former in quite a few of our Spec 911's and the latter in at least one 3.8L hot rod street car in our area (it is described elsewhere in these forums). Not for the faint of heart in terms of budget.

[Mahler9th]

That being said, I have seen at least one shop combine ITB's with a 993 intake in an approach I suspect is far, far less expensive than what Rothsport has done.

The owner of that shop builds my engines. We started out with mild cams and a 993 plenum with a stock TB and usable flapper door. The car runs EFI. When I upgraded to aggressive cams, I did not want to spend $3-4k more on an ITB set-up, so he suggested a larger single throttle body, and I adapted an 80MM hot rod Ford part. I mated this to the intake and to a 4 inch diameter snorkel and a very large cone filter. And I made a custom carbon fiber airbox that sits under the tailbase. No need to run factory style heat shielding with this approach.

When we dialed in the engine, it made very good power and was plenty torquey. He was a little surprised relative to his own 3.8 with TWM ITB's. Through the years I made mods to the linkage to get more progressive throttle opening and these worked very well.

But, as luck would have it, I just stumbled across a used TWM set up for not too much money, so I am switching over now. My single TB set up is for sale in the classifieds.

There is of course quite a bit of thought that needs to go into cams, of course the induction is part of the equation. The trade-offs between single and ITB with aggressive cams is perhaps more subtle and supportive of more careful consideration of benefits versus $$.

By the way, I am running excellent S Car Go headers and simple "phase 9-like" mufflers, which offer great performance versus $ for my application.

[Fly911]

For my race engine build, I most likely will end up with Jenvey ITB's. I have a 997 GT3 Cup plenum to put on top of the ITB's. Unlike the 996 GT3 plenum on Steve's picture, the 997 plenum has three chambers with two resonance flaps. It looks like the total volume inside of the plenum is much bigger on the 997 GT3 than on the 996 GT3 plenum.
I have been adviced both for and against taking my 3.2L Carrera 930 case out to 3.8L, aparently the cylinder walls will be too thin with 102mm cylinders. I might use 100mm cylinders and 3.6L instead. Also, I have been adviced against LNE pistons and cylinders, but stick to Mahle. My dilemma is that I get very strong, but conflicting opinions from people that really should know what they are talking about. So I don't know what to believe...
We'll see what happens.
As for the cam shafts, I get zero response from John Dougherty... I have emailed him several times. I will try Elgin and Web Cam...
I also get zero response from my ECU manufacturer Canems in the UK. Does anyone have any experience with Canems ECU's? I might have to change to MoTec...
I have also contacted Rothsport on my engine build, and the response from Jeff has been like the response from a good ITB set up; pretty instant! A lot of good information and advice! Sadly, he is way north of my $35k budget...