2.7 to 2.8 upgradr

[911 SLANT]

I recently was having problems with my 1977 2.7 with high leak down numbers on cylinder 5 and blowby from crank case. All the other cylinders were between 1%-2%. I decided to tear down motor and upgrade to a 2.8 with 92mm J&E 9:5.1 pistons. My motor 1977 Mag case with all the upgrades. Stock 70mm crank and 40 carbs, 964 grind cams, ported heads 41/39 and SSi headers with M&K GT3 Sport muffler. 915 transmission with 7:31 R&P. My question is will I have to do any head work for 92mm cylinders? If anyone else is running this set up I would like to hear your input. Hopefully Steve R. Or Henry S. Will chime in on this subject with the experience they have on building 2.8 motors. Thanks!

[rgofast]

I believe you will have to machine a slight camfer at the outer circumference of the cylinder head so the piston does not hit there.

[jpnovak]

The JE pistons usually have a circumferential flat section that prevents the piston from hitting the heads. I wil assume your 2.7 heads already have enough chamfer but you can check.

BTW, I would run way more cam with a 2.8 on webers.

[911 SLANT]

Here's a picture of my heads

[911 SLANT]

Jpnovak what cam would you run?

[911 SLANT]

Should I stay with the SSIs or go with 1.5 I.D. headers?

[spuggy]

Originally Posted by 911 SLANT

Should I stay with the SSIs or go with 1.5 I.D. headers?

Do you spend more time over 5000 RPM than below it? If headers are too big, you give up mid-range in favor of the last 1500 RPM or so.

You building an RS motor for racing, or a hot-rod to drive around?

From a discussion on 3.2 header sizes here: Exhaust/Header Size 3.2 Carrera

Originally Posted by Bill Verburg

what he said

all the copies of the original 911 header are 35mmID, that includes SSI and Dansk

these work well on 2- 2.7 liter engines up to and including 2.7RS, for 3.0 they are better than the smog exhausts used through '89, but would be on the small size for 3.0RS due to the cams and rpm, on a stock 3.2 they will be fine for around town use <6k rpm but somewhat restrictive if all you driving is from 5500 to 7000.

The best compromise for stock 3.2s is usually 1 5/8"OD(1 .5"/38mm ID) it's interesting to note that the 993 headers are 1.5"/38mm ID

[Trackrash]

SSIs will be fine for this motor. Unless of course if it is a true race motor spending most of it's time over 5K. For a street motor you would want response and power now, as when you hit the gas and not have to wait to 5k rpm for it to start to pull.

As far as cams go, your choices should be S, mod S, or perhaps a DC 35 carb type. That would be my choice, a little less power on top, but a lot more in the middle.

You should contact John Dougherty, AKA camgrinder and discuss your goals.

[lvporschepilot]

DC40 or 44 cam is great for this build.

[rgofast]

When we were building our 2.7 short stroke, I consulted with Jerry Woods, who was re building and re curving the distributor for us. He recommended 1.5” headers all the way up to 3 liters. Depends on where you want the torque. Same with cams. We had a set of Crane P206 grind in stock. They are a copy of Elgin Modified S cams. They come on around 3000, and pull beyond 7000. Great all around hot rod cams. I plan on using them on our 2.8-2.9 build also.

[911 SLANT]

Car is more of a street hot rod and canyon Carver. I would like to do some autoX. What was the problem with the 964 grind? 465/406 lift and 304/276 Duration. Everyone always spoke highly and wanted these. What lift and duration should I be looking for on this motor in cams. 3000 to 7000 sounds good to me.

[911 SLANT]

How do my 2.7 heads look? Should they be good to go?

[911 SLANT]

rgofast thanks for your input on the P206 Crane cam, but to qoute Henry Schmidt Crane cams were susceptible to failure in Porsche motors.lobes went flat. I like the 3000 to 7000 rpm range though. Any other choices recommended for my set up?

[Trackrash]

Originally Posted by 911 SLANT

Car is more of a street hot rod and canyon Carver. I would like to do some autoX. What was the problem with the 964 grind? 465/406 lift and 304/276 Duration. Everyone always spoke highly and wanted these. What lift and duration should I be looking for on this motor in cams. 3000 to 7000 sounds good to me.

The 964 cam is designed for a smog/fuel injection motor. People running FI will need a mild cam like that so it will work with the FI. I speaking of stock FI systems of course.

On my motor, I chose to use a GT2-102. It is like a mod solex but with way more lift. My motor starts pulling around 3K, will hit my limiter at 7,200, but shifts at 6500 are best. Perfect for autocrossing where I can easily pull out of those slow corners on the power. My old motor was very frustrating since it was gutless below 4K.

In your case with 9.5 CR, you really don't want to go to radical with your cams, IMO, as you will loose too much bottom end power.

Go to John D's web site and you can see the specs for all the cams. Dougherty Racing Cams Porsche 911, 930 and 964 camshaft profiles

[911 SLANT]

Performance Parameters:
Street/AutoX
Pistons-J&E 92mm 9:5.1
Heads- Single plug, valves 46/40
RPM Range-3000/7500
Fuel- 91
Trans- 7:31
Induction- 40 carburetors
Port size- 41/39
Exhaust- SSi w/ M&K GT3 Sport

[Trackrash]

Originally Posted by 911 SLANT

How do my 2.7 heads look? Should they be good to go?

As far as your heads go. The chamfer mentioned by rgofast may be an issue. You will have to do a trial assembly to be certain.

Personally, if it were me, I would carefully check the exhaust valve stems and guides for any wear. You are probably OK, but IMO, worth checking since you are in there and they can be a wear item.

[911 SLANT]

Originally Posted by 911 SLANT

Performance Parameters:
Street/AutoX
RPM Range-3000/7500
Fuel- 91
Trans- 7:31
Induction- 40 carburetors
Port size- 41/39
Exhaust- SSi w/ M&K GT3 Sport

Single plug heads

[jpnovak]

Yes, you should chamfer the heads. You can see evidence of carbon buildup in the edge squish zone. That is the 3mm ring of stuff at the transition from the chamber edge towards perimeter of sealing surface. You can also see where the cylinder CE (sealing) ring resides as well as the cylinder mating surfaces. Looks like the head in the top picture was moving around alot. Is the sealing surface damaged or is that just combustion crud?

From your description a Mod-S cam would work great. I have run one in a 2.8 and it is fantastic.

The challenge you will have with narrow lobe center cams (like a true Mod_S) is that your port size is HUGE. The heads are ported too large IMO. They are more set for full WOT conditions that need max airflow. These ports run the risk of stall in the mid-range where airflow can not overcome the reversion pulse. In other words, you will have a big flat spot.

To overcome this ask for a cam with wider lobe center. John Dougherty can grind a DC-40-108 that would probably work much better. Think of it as a mid point for your 964 cams and DC-40.

For a street engine I would favor much smaller ports to drive mid-range intake charge velocity.

To really optimize for your engine you would check the cylinder head flow. Then match the cam lift to the top of the knee on your flow rate vs lift curve. Then pick a duration of cam that matches your target rpm. Calculate flow velocity based on airflow and port diameter. Then select lobe center based on induction system and dynamic compression ratio.

Since you are building the engine you want it to perform as an optimal system. That means carefully checking your parts so they seamlessly work together.

[911 SLANT]

Can my 964 cams be ground to a S cam spec? Or should I sell my 964 cams or trade for some 2.2 S cams? I also have the stock 2.7 CIS cams. Could I do mod S grind on them?

[Trackrash]

You know, after looking at those heads more closely, I noticed a couple of things. First the date of manufacture is ground off. Not that it may matter, but I wonder why.

Then the head seating surface looks rather wide, like the heads have been flycut to the extreme. Also the distance from the edge of the seating surface to the valves looks very tight. I wonder if there is enough space left to do a chamfer cut.

Do you have knowledge of the head's machining? How much was removed? It sometimes is stamped on the head.

You are definitely going to need to do some checking as far as head to piston clearance and by measuring the HEAD VOLUME. If they have been machined much it will affect your compression ratio.

Yea, as Jaime mentioned those 41mm ports are race car size. You really want something like 36-38mm.

You can cheat a little by using intake manifolds that "neck" down to something like 38mm to give you a venturi effect to get your intake velocity up to help with mid-range torque.

And BTW, I don't believe 964 cams can be reground to S specs. If you look carefully at the link I gave it should be on John's website.