High compression 2.4
 

I am back to worrking on my next racing engine. It has to be under 2.5 liters so I am using a 70.4 mm crank, 86mm pistons, DC 70 cams, 40mm Zeniths. The heads are ported.

The question I have been pondering is how high can I go on compression based on using 108 octane racing fuel. In the past we have run as high as 14:1 in a water cooled Nissan. I don't want to deal with detonation, but I need the HP.

I plan on using Biral cylinders because they are what I can afford.

Any thoughts on the upper limit?

Thanks,
neil

Are you going twin-plug?

Your upper compression limit will be different, depending on single or twin plug.

Well I'll say this from a design perspective: having a longer stoke will allow you to run a comparatively smaller piston dome than you would on a shorter stroke motor with the equivalent displacement. The smaller the dome, I think the better the flame propagation in the cylinder and the less likely the onset of detonation, all other things being equal. This isn't a panacea, however, as lots of turbo motors with completely flat pistons detonate and blow up.

I assume you are going twin plug with modern engine management.

The factory used 12,3 to 1. I've heard guys at the track claim 13,0 to 1. Who knows if that is track talk.

Was it Smokey Yunick who had an 18 to 1 compression ratio, that you couldn't use full throttle below 5000 rpm, because it would blow the engine? With throttle-by-wire and knock sensing you could accomplish the same thing.

I think there's probably a point of diminishing returns in terms of thermal efficiency. I would personally be a lot more worried about finishing the race-- and so would stick with an honest 12,3 to 1. After a successful season of not blowing up, you could sneak the compression up gradually by reducing the deck height. Gotta tear the motor down anyway for a maintenance rebuild. Grady Clay has many threads about this.

I do plan on running twin plugs. with two MSD's. 12.3:1 wouldn't be too bad I was thinking 12.5:1 just to round it up.

Or 40mm Zeniths......

Quite right, I failed to note that above. Zeniths, while underappreciated by the Porsche community due to the lack of off the shelf go-fast parts, could prove problematic. If you are going to the ragged edge of detonation with very high compression, very precise control over AFR will be required.

All the more reason to keep it to 12,3.

I have given this more thought and was thinking why did the factory use 12.3:1 and not 12.4:1 or 12.2:1? I was thinking they may have made pistons to 12.5:1 and by the time they rounded all the corners and were able to get good piston to valve clearance, you ended up with 12.3:1. It is not a metric thing since it is a simple ratio.

Its very hard to get much higher than 12;1 on a small bore motor and a 'heavy cam'. Even with the longer stroke. Its even worse if your stuck using 2.0 heads.

110 octane will support just about any CR you can acutally build.

Any reason your limiting your cam? Utilizing modern lighter weight rods (550 g vs 700 stock), lighter pistons, and a 5.5" tilton on an aluminum flywheel will let you spin out past 8k without the dreaded flywheel problem. That and overtighten the bolts.

If you have your heart set on birals, you might want to consider getting them nickie plated. Nickie on steel is supposed to reduce friction and oil temps. The 356 people who don't go with LN engineerings products swear by it.

t

12.5 is fine for sprint applications.How long are your sessions?

you can get 12.5/1 with the 66mm stroke cranks and 2.2-2.7 heads with no problems.

However, I would lower your cam selection as that cam needs approx 41mm-42mm of port size. Will you be boring the carbs out? Can you bore them?

Neil,

Are you planning to use your 2.0L heads? If you manage to get above 12:1 using 2.0L heads on a 2.5L race motor, I'd think the chances of detonation would be much higher than if you used 2.2/2.4/2.7 heads. 2.0 heads have a steeper dome than the later heads, which I'd think would inhibit flame propagation (cause detonation), especially with the high dome pistons you'd have to use in order to get a high compression ratio.

Scott

What are the DC 40 cams equvilant to ? S, E, other?

Just getting back from the ARRC at Road Atlanta. Car ran good, as good as 126HP will go. The engine is a stock 2.0 running on pump gas. I picked up 3 seconds fom last year, but I have 11 seconds more to get to the front. The ARRC Champion in SPU again this year was Jim Kellog in his Prochallenge car with an Esslinger sprite car motor. Just dang fast.

Our races are usually 15 laps aound RA, 20 laps for the ARRC.

So you can see why I need this 2.4, I hope to make a little more power, 250 hp I believe whould put me in the hunt.

I am going to use 2.4 liter heads.

I have already bought the cams. I plan on using the Zeniths until I can get a fuel injection setup working using the 46mm Polaris thottle bodies. I was planning to use 36mm intake ports but I can go bigger.

I plan on using stock rods and I aleady have ARP bolts for them. The rods are being tumble polished right now. I have a stainless steel flywheel that is 7.25" in diameter and run a twin Tilton clutch. I don't have any trouble spinning the 2.0 it has seen 8500 rpm on the track.

A 2.4 with DC40 and 10.5 compression is a 230 hp engine.
All that additional compress will only generate head aches.
The DC40 is a mod S cam and really isn't going to produce high RPM power. Speaking in general terms, small engines need to spin to produce more power.
If you really want more power look to a higher RPM cam and port to match.

I am running DC70 cams, not DC40. I do plan on spinning this 7500 rpm. My ports are currently 38mm intake and exhaust. This is for sprint racing not the street.

the DC 40 cam is a compromise between the E and S cam. It breaths better at high RPM's (more lift) but retains most of torque associated with the E cams. It has a lot of overlap as well which reduces compression but improves volumetric efficiency.

I use one with approx. 9.5/1 compression and it's great. Engine is so smooth and makes power past redline.

Chris

73 911 E