I suspect that some of you may have seen the dyno results of the difference one point in CR makes, right? 3%/4% according to one mag's back-to-back dyno tests.

For what it's worth, I just ran a couple of sims of engine setups from the late 90s; a Cup engine and a Pro Stock engine. The Cup engine had a DCR mid 7s and the Pro Stock mid 6s.

I don't know if any of that helps, but there it is anyway.

Jake

 

Photo is over 20 years old. It was taken at Great Lakes Dragway. Car owner, Leon, is on right.

I don't know anything about Patterson or the origin of the sticker though.

I just built and maintained the engine; then popped the chute at the top end, LOL.

Jake

 

Huh I wonder if that's my same engine builder. Patterson is rebuilding my engine, his shop is in Oshkosh.

 

Just offering up an easy answer. I didnt know aluminum heads was a requirement. If you build a stroker shortblock, buy aluminum heads an intake and headers, you will spend just as much. Maybe more. Good luck.

 

I think this is a good, conversative answer and in line with the ones I got from most engine builders. But I've also heard of DCR higher than 8.5,,,even up to 8.7ish claiming no issue. Other builders say F*** DCR, they only look at static numbers.

I should have qualified my suggestion of .050 squish by saying that's the most builders (who pay attention to quench) would go. Some say stay between .035 and .045,,,while others say .040" to .050" works well. (I was unable to qualify that further -- based on compression or any other criteria.)

I you look at the kit Red-85 specified, it's pretty clear what parts are being used. Scat9000 crank, 4340 rods, and 5cc flat top forged pistons. He specified the cam, the heads, and the kit is for a 2pc RMS motor -- which means an internally balanced kit can be used. And, he specified the heads too.

The best advice I got, Red was to stay conservative. For someone wanting to build a motor once, try to avoid all problems/possibilities of a redo, choose parts at least 1 step below limits, pick good parts, and work with people you trust.

For the compression thing, you should know/understand that 1pt compression only gets you 10hp improvement (on avg). By contrast, going to a 383 gets at least 3x that amount. Since you're also planning a TPI, it seems apparent that TQ is more important to you than HP -- though what your building should pull hard enough for anything short of racing.

Finally, in the spirit of staying conservative (like Bjanuski suggests), I did too. My suggestion was a bigger dish with the 64cc heads. They're better pistons and the compression would be down where it's hard for anyone to challenge it's viability. For a 1-time build, why not be safe? Do you need every last ounce of power?

 

Kicking around all these differing views and the numbers associated with them makes for an interesting discussion. You know, things like CR, DCR, quench clearances, etc., but it brings to mind something I recently heard a PhD NASA engineer say during an interview.

He said that "Without the science to back up claims, you're just making 'stuff' up." Once you think about it . . . .

Jake

 

 

Your post got my attention, I grabbed my paperwork from the machine shop for a review and I was wrong. My compression is 11.1:1

 

Well, Red....I guess you should get PLRX, WW7, and Jake's phone. Sounds like 11:1 is the right thing to do and there's no way to prove otherwise without trying it yourself.

(Phone numbers are to thank or bitch at them depending on whether they're right or wrong.)

Edit: Seriously Jake? That's all you have to say? Nothing worthwhile? At least I gathered as much feedback/info from (LOTS of) various sites, talked to a dozen engine builders and formed an opinion of what's the right thing to do ESPECIALLY for someone not building on a past design/application. Piping in about the need for science w/o providing your own perspective/reasoning is like standing there with your finger in your nose. (So much for your new ploy to challenge for scientific foundation.)

There is some science posted/listed in the sources from which I draw too, but I'm not going to go back and reference every one. As it is, I think I reference more than most people anyway. For example, go back and follow the link I provided back in post #20. Read all of the connected (linked) material in THAT post & the ones linked from that. Then talk to a dozen builders (including a couple on a national scale), then come back and try to recommend to a person exactly what's safe and what's not. You darn well better believe that those providing more conservative advice while having some point of reference/measure are better contributors than people saying "My LSx, LTx, is setup to get 11:1 compression -- so that means every other car will be fine with that combo".

I don't know if Red is going to build this motor himself or have help from a builder. If the later, I think that's who should be providing most of the advice/recommendation. OTOH, lots of builders get crap thrown at them and get told to assemble it. So, often, they just build. Two of the best contributors to this forum told me to go looking for a local with a good rep around town -- and have him advise the build. I think that's great advice -- especially if the builder will stand behind the combo being built.

Otherwise, if you take advice from forum's like this (without any wiggle room if you're wrong), then it'll probably end up differently that you hoped. OTOH, if you check/double-check with multiple sources until you to get to a reasonable truth you can believe in, you'll probably be O.K. So, I that's my recommendation.


Footnote: For what it's worth, my sources include Vizzard, GrumpyVette (and the sources he quotes), TPIS, CNC-Motorsports, the online performance mags, Bullet Cams (and Comp), The Engine Store, two of the most experience forum members (who don't want to be named), Alvin (PCM4Less), another tuner, four local builders, and the one I'm using.

Only 1 out of this list feel it's safe to go above 10.5SCR and 8.5DCR -- if you want to be guaranteed "detonation-free". All of them said tune would be critical as you get close to that "boundary". Most said staying 1/2 pt under that boundary was the safe thing for a person not needing every last 1/10th in the 1/4 mile. A couple said "DCR, What's DCR? I don't pay attention to no stinkin' DCR!". All of them said it's complicated with lots of variables like: quench, static compression, water temp, gasoline quality, electronic timing, dynamic compression, piston hotspots (or lack thereof), head material, etc..., etc....

The ones who said they recommend 10:1 with a DCR of 8:1 were the ones who'd built the most motors. They're the ones who have to stand behind what they build. And, they are the ones who I personally feel are the safest to follow. Again, that doesn't mean you can't get away with a lot more. But, you better be WELL prepared to understand tuning, go to a good tuner, and/or do the right things to make sure you don't damage you one-time build.

Other things to consider include (as mentioned above) gas quality. How is it near your home/work? Are you prepared if it starts to degrade in quality? Does that matter? How much timing are you willing to take out -- if you have problems? What things can you improve on -- if cooling isn't adequate? Will you have any loss of HP thru oil drag? There are more questions/issues that you can imagine -- unless you've been doing this for years of course.

BTW Red,,,,Don't let all of this scare you off. These are issue you'll have to address whether you do a 383 or 355. It's just part of the game.

 

What's your DCR? Where do you keep your water temps? How does the reverse cooling of an LTx compare to a gen1 motor? Are you SURE it's the same environment -- because I think differently. I believe -- just as alum vs iron headed cars -- that's there's a difference in how much compression those two types of motors can handle.

What's your quench, what type of piston (shape), etc... etc....

And, the biggey,,,,will you pay for Red's engine IF YOU'RE WRONG?

 

Gregg, I agree with alot of what your saying,I agree its better to be to low on compression then to high , to low won't cause problems.. There have been literally millions of small block Chevy engines built in the last 40 years. If I was Red, I would just go with a known build that has been done thousands of times with good results..(This really isn't as hard as its being made out to be).. The 383 build Pete K is doing for me now is going to be a fairly strong running engine that has the potential for more power down the road if thats the way I want to go later on, but is one that has been done many many times with no problems. Its not like any of us is going to invent a new way to put a small block Chevy together, or come up with new parts to put in that motor that is going to give much greater performance then the builds we have available now. So in conclusion: My suggestion to Red would be to pick a known build that will give the hp and torque that is close to what he wants , get the parts , have a good machine shop do what they do , don't go over 10.5 to 1 compression (just to be on the safe side), install the motor,get a good tune, and drive the car.... Very easy!!!......WW

 

[QUOTE=GREGGPENN;1573047244]What's your DCR? Where do you keep your water temps? How does the reverse cooling of an LTx compare to a gen1 motor? Are you SURE it's the same environment -- because I think differently. I believe -- just as alum vs iron headed cars -- that's there's a difference in how much compression those two types of motors can handle.

What's your quench, what type of piston (shape), etc... etc....

And, the biggey,,,,will you pay for Red's engine IF YOU'RE WRONG?[/QUOTE]

I'm not pay for anything buddy. I posted my compression numbers and that we only have 91 in California. We have our choices, AC spark plugs were recommended by Jim and I chose AC spark plugs. If a plug blows up, Jim would have to pay?...please get real

like Neo said,

"The problem is choice"

 

That's what I thought, you don't know.

 

If the choices are so difficult, why not let your engine builder make them. They probably have warranties on what they make verses making one to what specs you want and aren't sure will work.

Good reading here, I'm rebuilding mine to.

 

Here's the deal....

The reason DCR is so important is because it's a better measure of the compression. Without a cam, the valve's would never open/shut. So, it's the cam that controls how compression builds.

As the piston is coming up the bore, it won't start building compression until the valve closes. DCR is a better measure of when that door actually closes and compression starts to build. Take two builds for example, If both are 10:1 compression but build #1 closes the valve 20% up the bore while the other doesn't close until 30% up, compression would be 8:1 vs 7:1. Obviously, that's a huge difference.

The difference in adv vs .050" lift on Red's cam is extremely narrow. (There, I tried not to call it small! LOL). If you compare it to something like the commonly-used HOTCAM, you'd see about 20 degrees difference in adv closing events vs only 5-degrees in the .050 events. Red's chosen cam will simply hold more compression.

Running the figures on the DCR show the hotcam displaying the same 11:1 compression as Voodoo cam (using the 64cc head scenario). With the hotcam, DCR is 8.6. With the Voodoo cam, it's 9.3! Because the voodoo cam gets closed 12 degrees SOONER, it generates almost 3/4 of a point more compression! That's not small potatoes people.

It's not until people really start to look at the actual cam events that they can understand how to compare cams. Personally, I don't believe a "known build" has to be used to accomplish Red's goals. I just think some common sense has to be applied.

Instead of comparing SCR to SCR and thinking "Well, if I lower my compression to only 10:1 vs 11:1, my car's going to suck. Dude, it's not! You've picked a cam with a lower lift and shorter duration. It's operating range is 1000-5500 rpms. That's where it's power will be. Considering you're going to use a TPI, that's PERFECT! And, because you chose that cam, torque is going to leave your nuts and the last stoplight.

Real compression will be just as high as the person running an 10.75:1 motor. Kapish?

 

PLRX: What this means to you is you'd have to raise your compression another full point -- before I believe you'd be justified in assuring Red he won't have detonation issues.

Why not 7/10th...you might ask. Because I also believe the reverse cooling of an LTx motor makes the heads effectively run cooler. As such, they can tolerate 1/4 to 1/2 more point of compression IMO.

So, go on PLRX, raise your compression to 12.1:1. You'll be fine! (That's exactly what your trying to post in this thread you know!)

 

Long ago I came to realize there's no universally acceptable answer to this issue. We could beat on this 'til times got better' and the result would be the same.

This issue has been kicked around for so many years and with so many differing views and recommendations that, for me, it all boils down to whose advice to heed.

I'm sure that many of you have read about this issue as much as I have. I've concluded it's all over the map, covering both ends of the spectrum and everything in between. Many of those who post on this offer lengthy explanations supporting their position and are absolutely convinced they're right. Then, along comes other guys who have differing views and are equally convinced they're right.

At one point I simply said "Enough's enough".

Essentially, what I did was put my confidence in the GM engineers; what they came up with in designing the engine. I have confidence that they already did the necessary homework on temperatures, gasoline quality, redline, etc. That eliminated a lot of the suppositions, variables and assumptions so often made when this issue is discussed.

As an example, I took a look at the LT4's CR and the factory installed cam for that engine. I crunched the numbers (hence the science) and used the results as a ball-park.

To me, there's no need to re-invent the wheel and it was time to move on. Yet, you guys are free to have at it, LOL.

Jake

 

Hey, that's my compression ratio,


Mike

 

And, you probably have a big-azz cam. Says nothing -- just like PLRX's input.

I think Red wants a concrete answer about HIS setup, not someone else's, or what GM did.

 

...I was responding to this post... I was letting PLRX know that you were correct, 12.1:1 can be fine on pump gas in a reliable DAILY DRIVEN all weather LT1 C4.


Mike