Fiat TC 'Waffle' IDF Manifold flow

[vcg]

Hello Guy,

I was wondering, how much can a stock Fiat IDF manifold flow?

It is designed for 40IDF with 32 chokes as on 1608 spiders and coupes, will it be restrictive if used with 34 or 36 chokes on larger and/or modified engines?

If indeed this manifold is restrictive, then what are the ways of impoving it?

PS. I've "heard" around that inlet manifolds should go down from carb size at entry to carb choke size at head mating face. Does this hold true?

Thanks,

[Guy Croft]

Standard factory 40 IDF inlet manifold has quite a rough surface finish with average diameter at narrowest point of 32.4mm (31.8 horizontal by vertical 33mm) and flows around 103cfm at 10".
Nicely cleaned up with av dia 34mm at narrowest point (H33.3 by V34mm) you can get 119-121cfm.
You can go bigger, with a 44 intake and get 125cfm plus.

It is designed for 40IDF with 32 chokes as on 1608 Spiders and Coupes, will it be restrictive if used with 34 or 36 chokes on larger and/or modified engines?

However much they both flow - and the manifold must always outflow the port - there will generally be a loss of flow of 4-7% with the IDF manifold fitted to the head.

If indeed this manifold is restrictive, then what are the ways of improving it?

Enlarge it and get rid of the rough surface, take it to 80 or 120 grit. It's the horizontal section that has to be bigger for a given carb, bigger, smoother and with the best radii internally. The bigger the carb (given that the diameter of the carb mounting face on the manifold is the same bore) - the lower the loss with it fitted.

PS. I've "heard" around that inlet manifolds should go down from carb size at entry to carb choke size at head mating face. Does this hold true?

No it's doesn't, or you'd be using for example an inlet manifold with a 40mm bore on 40mm chokes, way too big. You do the head first, maximise the flow potential of the ports and valve setup, then try to minimise the loss with the manifold fitted. The manifold bore will follow the size of the finished head not the other way round.

You have to remember with flow data there is no such thing as a 'exact' benchmark figure, every item produces different flow. You always test before and after to see if you've improved it. Don't assume that if I put another factory IDF manifold on test like I have done here for this thread and it will flow the same, it may not. And if the short-side radius profile is wrong it won't flow well however big you make the outer port region and manifold - hence flowbenches.

GC

[vcg]

I couldn't get a more detailed answer.

Guy I would like to take this opportunity and thank you sincerely for spreading solid and dependable knowledge and revealing info that for most professionals in your field is considered gold.

I actually believe that by doing so, you assist in building a momentum for the glorious Fiat TC among young people that hardly can recall the 124 Rallye, 131 Rallye, 037, S4, Integrale years. Today Fiat TCs are very popular only in Egypt and Turkey as midle-low class everyday transport and cheap modified sports cars. For the rest of the developed world the TCs are almost 'extinct' and can't be found even in scrapyards anymore (with the exception of Spiders and Integrales, that have some 'collector's item' value).

I am convinced that in the years to come the Fiat TC engine will become the most respected historic modifiable engine. Escorts and Alfas have a much bigger following that Fiat TCs at the moment and dominate historic racing. The reason is support. I am so jealous of the people that race Escorts because they can find anything, anywhere. I have to dig for knowledge.

Your contribution in helping the Fiat TC engine to get the following and support it deserves is enormous. You are actually the ONLY person I know of who spreads his knowledge. I tried to contact a popular among racers spanish Fiat TC tuner a couple of years ago, that two greek historic racers buy engines from. The only thing he told me (and he speaks perfect english) was that he doesn't sell parts and only delivers 180bhp 1995cc complete Fiat TC engines based on used parts for an X amount of Euro, and the only technical info on his engine he could reveal was his account number for a 25 percent deposit in case I wanted to buy one... I want to be fair to him, and admit that his engines do really well in historic racing, but his attitude doesn't bring any new people in the Fiat TC world.

Thanks so much & all the best,