I’m almost finished putting together the motor (picking components). I've done a lot of reading and talked to many different companies about different combos etc. I've decided to go with the SDPC 383 long block which consists of all forged components with CNC heads, and the GM 847 CAM. I can give a more detail parts list if necessary. The last decision I have to make is the static compression ratio for the motor. 10.4, 11.2, or 11.8. From reading and talking to people I've concluded there is a 3% increase for every full point of compression. Correct?? I should see a 20 to 35HP gain by going with the 11.2 over the 10.4. My current 350 is running at 11.4 compression and if I put anything less then 93 octane the car pings like crazy. If I get bad gas the car pings. I can't decide if I should go with the safer 10.4 or go with the 11.2. What are you thoughts? Pros and cons?

Thanks
Robert

 

are you going to be cruzin' alot?, road trips etc???

 

Yes, Not to many road trips but driving to different club events and shows.

 

Bluewasp, No Go has a VERY similar combo to yours (heads, displacement, and same cam), he would be a great source of information. I beleive he runs pretty high compression.....

As for answering your question, you have done it yourself. Do you want to worry about bad gas or not having 93 octane in it and getting detonation? It sounds like you may want to stay under 11:1 if you are worried about it , especially if it sees much street or daily driving.

Let us know how it goes.....

 

I'd go with the 11.2:1 CR. It's a littler "safer" if you get bad fuel, and not so high that the computer won't be able to compensate as long as you are not in WOT for too long.

I understand the dilemma. My new 427 SBC is at 13.5:1, so it is racing-fuel only. The plus is that it will generate nearly 800 FWHP

 

I'm running 11.6:1 and have had no problems with 93 octane. It will somewhat depend on your tune also. I'd go with the 11.2. 10.4 seems a bit low unless you have plans for boost in the future (and 10.4 may even be a bit too much for that). If you don't go overly aggresive with timing, you should be just fine if you run into bad gas occasionally.

 

3% is 15hp increase if the motor makes 500hp at the flywheel. Some say its 4% but i used 3% because 10.4 to 11.2 is only .8 of an increase.

Would it be safer to go with the 10.4 and possibly be able to put a blower on it later on down the road. HMmm... don't know.

Stock Lt4 compression is 10.8 and LT1 is 9.6??

robert

 

LT1 is 10.5 or 10.2 depending on who you believe. BTW with the reverse flow cooling of the LTx style engines you can easily run 12:1 on pump gas.

 

The percentage increase for one point in CR is not linear. That is, going from 10:1 to 11:1 may show a 3 percent increase, but going from 8:1 to 9:1 may show 4 percent. It's not a static percent number.

Using a percentage figure is convenient but can be a little misleading. The percentage depends on the power the engine was making before the CR change compared to what it makes after the change.

For example 3% of 400 HP is 12 HP, but 3% of 500 HP is 15 HP. The percentage figure is best used as a rule of thumb - like so many other things.

So it depends on what the CR was before the change AND what the power was before and after the change.

Jake

 

out of curiosity, what is the CR for a stock L98?

 

I'd keep it at 10.5 or so, I wouldnt want to worry about ever driving it on the street and having problems with ping/knock.

 

I'd go for the 11.8 CR.

I'm currently running an actual 11.9 CR with .016 piston in the hole, .039 FEL PRO head gasket, and the GM 847 camshaft. This equates to a ~225 psi cranking compression. I have no knock even with 37 degrees of timing at max rpm (6800). I run almost 100% of the time on 92 octane. Sometimes 91 and sometimes 94. As stated above...LT1/4's seem like they are in another league when it comes to static compression ratios. Normally 10.5 is the limit for iron block aluminum head SBCs, but I'm convinced these are different.

I seriously looked at the Scoggins shortblocks, but figured I could build my own for cheaper, but not everyone is in that situation. I would be very interested at the actual CR they provide. Quoting numbers is one think, but are the blocks zero decked, etc? Closer to .040 quench is supposed to minimize combustion issues such as knock. I'm .054 so certainly not optimal...but if I zero decked the block I would end up with something around 12.5 CR. Wow.

I mention the cranking compression just for comparison. Previously, I had a HOT cam, angle milled LT4 heads, and stock shortblock. This equated to 11.2 CR and the same cranking compression. I never did a stock cranking compression test unfortunately.

The GM847 cam is quite large...296/304 advertised duration and frankly I'm super impressed that it will work with the factory computer (with LT1 Edit), but disappointed at some mild drivability issues below 2000 rpm. I'm almost certain that it is a lean miss issue-factory computer demands 14.7 AFR other than WOT-which is great until the big cams want richer mixture due to their nature.

If you decide on this camshaft, then you almost need the higher CR to make up for the loss in pressure due to overlap, etc. If you go smaller then the lower static CR is probably preferred.

Also as a side note. I'm seriously thinking of going with a similar GM847 camshaft, but custom for better drivability even with all my max performance exploits...

Although, I sure do love that 775 RPM staccato this thing makes thru the Corsa system. Downright radical...cool.

 

The quoted numbers are with a 58cc head. I'll call on monday and more specific info...

My plan is to use the 847 while I'm in europe and swap it out for something else when I return to the states. If I move to an area that requires emissions testing.

With the "mild drivability issues below 2000 rpm" problem. There isn't a way to compensate for the lean with tuning?

Do you have a dyno results from your car that you can post?

Last question, who tuned your car? I'm trying to find someone that can do a mail order tune. I've been told PCMforLess is the best, then Hypertech, and finally Fastchip.

A friend of mine races super gas drag cars suggested I go with the 11.2 because the dynamic compression would be lower with the overlap. basically said the samething you did.

 

My Static CR is 10:97, no pings ever on 92-93 octane, I would shoot for close to 11.1, if lower CR gives you peace of mind then, thats your choice.

 

Yes, the LT1 is much more forgiving with higher Crs. I'm running about 11.3:1 on my new engine and will probably increase it some more with shaving the heads when I do some porting. Except for the idle and when you stab it, you'd think you were still driving a stock LT1. I always run premium because "The Critter" deserves the best!

 

My heads were shaved when the engine was rebuilt, how much is my CR? god only knows

The only thing I know is the guy who rebuilt the engine told me to put premium gas because the heads were shaved.

 

Compression Ratio is a fine line.

Too much and you have knock or reduced spark timing that kills power.

Too little and your engine is a stone.

Just right and you have a good running engine.

Again, just basing the cranking compression comparison between the traditional SBC and the LT1/4's...the older non reverse cooled were really pushing knock with anything over 200 for pump gas. I'm running over 220 with no problems. I would shoot for 11.8 with the GM 847...just make sure you cover you bases with the calculated CR...you still have head gasket, YOUR cylinder head, valve notches, etc.

At 6900 rpm thru the Corsa's...it sounds pretty good! Autobahn would be ideal...

 

Running 10.4 myself and running 91 oct.Running great.

 

If you've had them shaved, and especially if you don't know how much, the only way to know for sure is to have the combustion chambers checked for CCs and then do the math. You also need gasket thickness and it's quench area, piston specs, bore, etc.

 

Depends on the year.

My '90 is 9.5:1, '88 - '91 were the same CR (aluminum heads).

I think the '85-'87 have less CR (iron heads).