Planning my 2.5SS - Question on CR and single plug

[jpnovak]

I have finally collected all the necessary parts (sans cams) to build a 2.5SS for my 71 targa.

Build will be based on a 2.7 74 7R case with the usual updates
90mm Nikasil cylinders
90mm JE pistons (10.5:1 on 70.4 crank)
66mm non-coutnerweighted crank that will be cross drilled (modified bearing included)
DC-30 Mod-Solex cam

The engine is designed to replace my 2.2E. I love the engine, it has a perfect power band yet needs just a little bit more. More torque in the power band (displacement increase_) and more top-end (cam choice). I want to maintain or increase how fast it spools. Ultimate HP is not my goal. Highly responsive backroad/street engine is my goal. Here is my question.

The pistons should CC about 9.9:1 on the short crank. I have not measured these out since I have not built out the case. I can use a late 2.7 head with slightly more dome volume (plug hole relief) to drop this slightly. Does anyone have real world numbers for such a combination?

Is nominally 10:1 acceptable on a 90mm bore with single plug? (consider 93 gas for your thought process) I do not mind twin plug and have built a few in the past. I just think this might be added expense I might not need.

I realize that twin plug will help the motor in many ways. Just wanted to hear thoughts on the CR.

[Danimal16]

I have the same motor you desire in my 71 t. If you are going to run pump gas the general opinion from the many experts I consulted is to keep the CR under 9.5. I hope others chime in here but mine is under 9.5 and I run Pump gas. Another tidbit, based on the many drivers I consulted, I switched to Chevron (I am in California) and it does run better than the 76 I ran before. If you build that motor you are gonna love it.

[Steve@Rennsport]

Originally Posted by Danimal16

I have the same motor you desire in my 71 t. If you are going to run pump gas the general opinion from the many experts I consulted is to keep the CR under 9.5.

Thats good advice, especially considering your summer temperatures and the need to run decent ignition advance.

JMHO, but I'd run twin-ignition to take full advantage of the torque and throttle response potential of that engine.

[jpnovak]

Dan and Steve,

Thank you for confirming my thoughts.

I guess I was swayed by the CR of the 2.2S which (published value) is not far below my targets. I do agree that twin plug is better in so many ways.

yes, I am sure I will love this motor. It should be exactly what I am looking for.

[Vintage914Racer]

Good timing. I'm hoping to build a similar engine this winter. Will you be adding updates to the thread as you build the engine up?

[911 tweaks]

Hey JP...I am curious as to why the 2.5SS engine size...?? Did you consider the 2.8SS engine that porsche never built that Henry/Supertech feels very strongly about as the best engine size that porsche never built...??

When you build this engine, please continue here or put a link here so I don't miss it as you do great work & fun to follow...!!

Take care... Bob

[jpnovak]

Yes, I will post about the build once it starts.

Bob, To answer your question. The 2.5 fits perfect with the plans for my SWT. I don't need an 8500 rpm 300Hp motor in a targa that just cruises the backroads. However, I find that the 2.2E is slightly lacking from a power standpoint. Since I do not plan to change gearing I want to maintain torque through the power curve. I find that sometimes I need just a little more OOOMMPPH (so to speak) to rotate the car on some of the great driving roads around Central TX.

I am building the motor to address these direct needs.

Besides, the parts to build a 2.5 are readily available. Not so much when you need an early 930/C3.0 case.

[Plavan]

I know its extra cash out of pocket, but I'm with Steve- Twin Plug it.

My back up 2.5L will be getting twin pluged, but thats for racing.

[YTNUKLR]

Jamie
You sound more planned / knowledgable of what you want than most . You know you want to get a little more but not go crazy. Very good first step.

Now, this combo is cool, but if you changed nothing else I'd say you are just a tad high on this C/R for your cam. I would knock it down a pinch (9.5:1) unless I was using S cams... Don't go crazy on the C/R. Make it up with timing and keeping your motor cool (fan) at high revs.

Yes you could twin plug it...that's great...but for the cost of twin plug stick a 70mm crank in it..Now you have a 2.7L...AND make some custom pistons and put some money back in your pocket... A 2.7L is going to do some damage on a 2.5...even a twinplugged one.. This should get your targa to rotate..

Just MHO...

[BURN-BROS]

Originally Posted by jpnovak

I have finally collected all the necessary parts (sans cams) to build a 2.5SS for my 71 targa.

Build will be based on a 2.7 74 7R case with the usual updates
90mm Nikasil cylinders
90mm JE pistons (10.5:1 on 70.4 crank)
66mm non-coutnerweighted crank that will be cross drilled (modified bearing included)
DC-30 Mod-Solex cam

Hi Jamie

Like Scott suggested, why not a 2.7 crank/9.5/stock 35mm port 2.7 heads/solex build? It's a well rounded motor with good torque.

That said, the 2.5 is not a bad build. I recently finished a similar 2.5 build but with 9/1 and the car had very good maners with decent top end. Customer was very satisfied with the results.
The car cruised well in 5th gear and you were able to pass without downshifting. Customer originally had a T in his 71.

BTW I found the dizzy parts you were asking about.

[swade]

Jaime,

I have all the parts now for a similar motor and am going with twin-plug and 10.5:1 CR. I have not made any decision yet on ignition system, but have read many posts and other forums (MicroSquirt). What method will you use?

Regards,

W

[jpnovak]

Scott and Aaron, You are correct. I could build a high CR 2.7. However, I like the snappy response of the short crank. To me, there is a difference. Most notably heel-toe downshifts and any area where throttle blips come into play. I realize teh 2.5 will not have the same torque curve and I am comfortable with that.

Besides, I already have the 66mm crank and pistons on the shelf.

Wade, I was tempted to install EFI on the car. However, in the interest of cost, I will just stick with the carbs that are currently on the car. There is the possibility that I could hide an electronic ignition control on the car but I have not decided. My targa is going to stay as close to period correct as possible.

The ST and the 914 are for modern experimentation.

[Danimal16]

Originally Posted by BURN-BROS

Hi Jamie

Like Scott suggested, why not a 2.7 crank/9.5/stock 35mm port 2.7 heads/solex build? It's a well rounded motor with good torque.

That said, the 2.5 is not a bad build. I recently finished a similar 2.5 build but with 9/1 and the car had very good maners with decent top end. Customer was very satisfied with the results.
The car cruised well in 5th gear and you were able to pass without downshifting. Customer originally had a T in his 71.

BTW I found the dizzy parts you were asking about.

Yep,

Aaron did a fine job and I love the car!!!

[jpnovak]

Yes, Aaron does great work!