gonetoofar

well ive finally made the call and im building a 3.8 RSR motor for my race car. but i find as i speak to more people about throttle body size i keep getting different answers. so i decied to "ask the audiance". any help you might have would be appreciated as i place an order to TWM. i will be using motec, rsr grind cams and JE 12.5:1 pistons.

what size tb's did the factory 993RSR run?

TWM offer 50,52 and 54mm anyone out there with current experience and what have you found works best?

Geoffrey

The factory RSR uses 50mm throttle bodies with a 45mm intake port. You are looking at the TWM 3006 ITBs and personally, I think there are much better ITBs out there. The TWM tend to leak, go out of adjustment easily, and the linkages are not as easy to adjust as they should be. Further, the middle butterfly will move when at WOT.

On my RSR engine I run 50mm ITBs, although I think I'd like to try 48mm. I have run 48mm on a similar engine and have made similar power. So, depending on your port size, camshaft design, etc. 48-50mm will be the size that you want. I have tuned a MoTeC ECU engine with 54mm 3006 ITBs and it was the lowest producing 3.8 engine I have tuned to date. They were too large and the engine very unresponsive.

gonetoofar

Racetek, thks for your reply and the insight. actually the 54's with 49mm intake ports were part of the conversation and it did sound like wishful overkill to me.

which ITB's have you found to be better than TWM? some folks on the west coast are running Jenveys.

Geoffrey

I like the Jenveys, however, that is only a small part of the intake design. The manifolds and intake trumpets are the key in allowing the engine to breathe properly. So, basically people take the Jenveys and make their own manifolds and select the sizing for the intake trumpet length as they see fit...some better than others...

From an engine design perspective, the larger ports, valves, and intakes are the wrong way to go with these engines since the velocity is lost and cylinder filling is affected. I have a lot of flow bench data from various combinations of heads and intakes. My engine for instance has 41mm ports, 50mm intake valves, 50mm ITBs, runs 12:1 compression with 93 octane street fuel and produces more power that a PMNA Sprint engine using racing fuel and a lot more torque under the curve.

gonetoofar

since i have exisiting 45mm ports and 51.5 intake with 42.5 exhaust valves i am thinking to stick with those heads. based on what you said im inclined to go with the 50's since i do have the larger intake port.

are you running the 993RSR cam or another grind?

Geoffrey

I am running a custom cam ground to my specs based on flow data from my heads, intake, and exhaust. Factory 993RSR cam is on the small size.

You probably actually have 43.5mm exhaust valves which are the stock size for 993RSR heads as is the 45mm port.

gonetoofar

thks and i think you are right about the exhaust valve size. what i have are 993 heads that were made to RS spec.

so now you have triggered another consideration. i was told the RSR cam profile was not all that aggressive. im thinking i should consider other grinds but i dont want to go nuts here. i want to have something that is relatively proven rather than become a pioneer of previously charted territory.

that's where the RSR "spec" approach came from. do you have any experience with dougherty cams? they have a DC 100 with quite a bit more intake lift (0.045") and more duration. webcam has something similar but only 0.025" more intake lift than the original RSR grind.

Geoffrey

If you want something proven specifically for the RSR head, I would suggest getting a copy of the Thierlet RSR grind which PMNA offered at one point in time and several cam grinders can make them for you. Just be sure to use chill cast core billet blanks, not welded regrind camshafts. The 45mm ports are too large for the 3.8l engine and the port velocity slows greatly compared to 41 or 43mm which are a better choice. You need to have a camshaft designed to be used with the 45mm ports otherwise you will not be happy and the engine will feel unresponsive.

Ok, so this really is not an RSR engine, but really a 3.8l racing engine that have ports designed similar to the street 993RS? These engines are sensitive to intake manifold and exhaust manifold selection. I have seen switching exhaust change power from 305 to 335 to 345 on three different exhaust tests. Just be sure you are connected with someone who is familiar with this engine to help you out.

Red9

Sorry to butt in-- where does that Cam sit in performance range? Where would you go up(??) from there or down? What suppliers would have or be able to supply a Cam to this type of spec?