72 2.4T - Crank and Rebuild info needed

GotaT · 10:46 AM

I have a Carbed 72 2.4T engine 5R case with reground T to E spec cams (DC05). The heads were resurfaced/shaved when the reground cams were installed and I am currently in the process of rebuilding the bottom end and replacing the Piston/Cylinders.

I'd like to know some best recommendations for more power without modifying the bottom end. So 2.7 is out of the question.

I've read somewhere about using 2.2 heads? with 2.4 piston/cyl for increased compression ??

Parts at my disposal for the build :

A mag 7R 2.4 CIS engine that I believe has usable heads and bottom end (Pist/cyl are bad)

Also have a long block 2.2T with bad (P/C's)

GotaT · 09-05-2017, 06:26 PM

Hoping someone can share some valuable experience here.

Trackrash · Trackrash's Garage

You might want to post some pics.

A 2,4 has the same crank as a 2,7. Difference is the bore.

2,2 and 2,4 heads are the same.

Compression is controlled by the piston. Although 2,2s had more compression per engine type than the 2,4s. Some have replaced 2,4 pistons with 2,2 pistons for more compression.

You need to decide what piston and cylinders you will be using. There are a number of possibilities.

Catorce · 09-05-2017, 06:51 PM

Not to sound too basic about it but you really have 2 options - displacement and compression. Those are your two biggest power adders at this point.

Go as big as possible on the cylinders, and as high as possible on the compression (cough...twin plug...cough).

GotaT · 09-06-2017, 05:13 AM

So from what I have read I'm thinking of going to 86 mm cylinders I just don't know the differences between piston choices. I was considering a JE PC Choice from AA anyone have testimonial of these?

I'm also a little confused as to using the largest cylinder size, say 86 mm with a 2.2 piston. Aren't the 2.2 Pistons 82 mm? So how would this work

Trackrash · Trackrash's Garage

Once you decide on your bore, you order pistons to match. JE and others make pistons for any bore, stroke and compression you want.

JE make a good piston as do others. There are a lot of us running JE pistons, which is a good choice for these motors.

You should contact JE or some other experienced 911 engine builder before ordering. There are some differences in the designs depending on which cylinders you will be using.

GotaT · 09-12-2017, 05:35 AM

Thanks for all the input. I believe I'm going to land on 86 Biral and JE from AA, since our host doesn't offer, with 9.5:1 (low compression).
I'll continue to use my Doughtery DC05 E spec cams and T heads that were on the engine which have been decked/shaved (Can't remember how much at this time)

I want to continue having good low end torque since 98% of my driving is on the street. I measured my Intake and Exhaust ports to be I= 30 E= 33 and my Intake Manifold (1R) to Head being 32

Should I port to any of these for more overall power?

What are some recommendation setups for my Weber 40's?

Trackrash · Trackrash's Garage

I would go with 32 to 34mm intake ports. I ran 35mm intakes on my SS2,5 w/ E cams and it was a good match.

GotaT · 09-12-2017, 09:52 AM

Quote:

Originally Posted by Trackrash

I would go with 32 to 34mm intake ports. I ran 35mm intakes on my SS2,5 w/ E cams and it was a good match.

Does a SS (short stroke) 2.4/5 use 2.2 rods? What are the benefits of SS motors?

Just to clarify, are you referring to intake ports on heads 32-35?
Will the larger size create less power in lower RPM and more at higher?

Trackrash · Trackrash's Garage

My motor used a 2,2 T crank, 66 mm stroke, and 90mm bore.

I used 2,2 T heads ported out to 35mm on the intake and exhaust. No loss of low end power over the smaller original ports (32mm). Huge difference above 4K rpm.

I don't think there is an advantage to the sort stroke version. I started with a 2,2 T, stayed with the original crank and rods, just went big on the bore. If I had it to do over, I would probably have upgraded the crank as well and made mine a 2,7.

GotaT · 09-12-2017, 12:03 PM

Should I match the bottom of my Intake Manifolds port size to the Head size?

Also should I match the top part of my Intake Manifold to the Carb size?

Is there a way for me to port these myself?

... Also got me wondering if it would be a good idea to smooth/polish the dome in the head or any other flow locations? Would this have to be done at a machine shop?

Trackrash · Trackrash's Garage

You should definitely match the ports, manifolds and the carbs.

You might want to do some research on port size, porting, and port velocity. I have ported the heads on most of my cars over the years. It is all about having all the elements of your motor working together in harmony.

There is a lot of how-to info out there on head work.

You mean polish like a mirror? The benefits of that are debatable.

There are a lot of shops that will take your money and do porting and polishing. I doubt that you would get your moneys worth going that route.

304065 · 09-15-2017, 04:48 AM

You have to know the combustion chamber size-- if the heads were decked/shaved the combustion chamber will be smaller than stock, meaning your compression will be higher with the same pistons-- the engine will also be narrower, which means you could have chain issues.

Years ago (10) I posted photos of how to do this with a glass burrette.