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Calling Engine Building Gurus...

2K views 28 replies 4 participants last post by  427L88 
#1 ·
Hey guys... Click this web page and give me some feedback on the beginning of this build-up adventure. Thanks for your help, tips and experiences
...Chuck
 
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#3 ·
Originally posted by 427L88:
Bear with this non-guru amateur reply, this will be a race fuel fed, 3500+ stall, 4.11+ geared car?
Comp Cams tech sheet for this cam says, "Street/strip, 2800+ stall, rough idle. 9.5 compression". The DCR is 8.7:1, so that should put me on pump gas? more comments...
 
G
#4 ·
I think you are confusing the two numbers 9.5 static compression ratio and 8.0 to 1 dynamic compression ratio???

I don't care what CC say's b/c an intake valve closes at it's advertised duration number regardless!

Question guy's?? Does the big .050" duration number coupled with the big advertised duration number allow XE type cams to suck more at AND above .050" which allow's them to not need a smaller advertised duration number to run good at the lower static compression ratio number CC say's ???

The XE284H cam closing at 68 to me really needs the 11 to 1 CR you will be getting to run it's best, IMHO!!!

Now their XE274H cam would do OK at 9.5 to 1, but even then, it would run better at 10 to 1 CR, again, imho....

The way I figure it anything above a schosh under 10 to 1 static compression ration AND 8.0 DCR begins PUMP gas octane problems UNLESS you are building a dedicated low octane gas/high compression ratio motor!!!

You can fudge cam installation intake valve closing timing by installing it either advanced or retarded, modify distributor curve timing and use aluminum heads but it's still going to rear it's ugly head at some time, imho..

Check out Pat Kelley's great Dynamic Compression Ratio Caculator site that you get to through his sig. Please read it about three times and then reconsider..

How far am I off here guy's???? Educate me if I am....

pdq67

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#5 ·
Originally posted by pdq67:
I think you are confusing the two numbers 9.5 static compression ratio and 8.0 to 1 dynamic compression ratio???

I don't care what CC say's b/c an intake valve closes at it's advertised duration number regardless!....
The intake valve closes, but the dynamic compression ratio will change based on overlap...
 
#6 ·
Overlap has no effect whatsoever on DCR. it is controlled by intake closing. Overlap does have an effect on dynamic cylinder pressure, though. 8.7 DCR is way too high for pump gas. 8.25-8.3 is pretty much the max on the street and 8.5 is doable if a number of precautions are taken. Above that performance will suffer as the engine will need de-tuning to keep from detonating on pump gas.
 
#7 ·
Originally posted by Pat Kelley:
Overlap has no effect whatsoever on DCR. it is controlled by intake closing. Overlap does have an effect on dynamic cylinder pressure, though. 8.7 DCR is way too high for pump gas. 8.25-8.3 is pretty much the max on the street and 8.5 is doable if a number of precautions are taken. Above that performance will suffer as the engine will need de-tuning to keep from detonating on pump gas.
Yes...with some number experimentation this morning, i've found that intake closing to be the key and it seems with my pistons I have to have an intake duration of at least 280 along with a .051 head gasket to bring my DCR down to 8.59...CompCams CB280H gives me an intake duration of 280. Is this still putting me out of the pump gas usage? Also, what's your opinions on this particular cam (web page here)...Thanks for all your help so far
 
#8 ·
Looks like you need the intake to close at 74º to get the DCR down to 8.25. A 296º cam will do this but it doesn't match up with your gear ratio at all. A 288º cam installed with zero advance will give you 74º but again this is a pretty big cam.
 
#9 ·
Originally posted by Pat Kelley:
Looks like you need the intake to close at 74º to get the DCR down to 8.25. A 296º cam will do this but it doesn't match up with your gear ratio at all...
As far as the current combo...several other things will be changed also. I have a posi unit ready to install and plan on 3.55s or 3.73s. Does this make the change more feasible?? Thanks and keep the tips coming. This is exactly what i'm looking for
 
#11 ·
Originally posted by Pat Kelley:
The 3.73's would be better. You should use a 3000 rpm stall converter, too. Big cams give up low end torque for higher rpm HP.
Pat...What will that setup do to my vacuum? What about streetability?

Here's the scoop: a. I have the block already (402) b. I have the pistons already...is there any other reasonable/affordable setup avenue you can think of for me to pursue with this block/piston combo, or is this previously discussed setup about my only option?? This car is HEAVY...if i go with the big stall/high-end HP, I will loose all my bottom, right?? HELP :confused: ...Thanx, Chuck
 
G
#12 ·
Imho, ebaymotors your pistons!! Then get new pistons better suited for your needs AND a cam to match the static compression ratio you need to run pump gas...

Then put it together for a better matched combination instead of jury-rigging it to just get by..

The low octane gas we have nowadays will eventually get to you..

Heck, I have a brand new set of .100" over KB flat-top pistons and two brand new BB small hy- cams AND returned a dual plane intake before I got settled on a combination after the dreaded, "more power. Scottie", bug bit me!!

He, He!! I went from a 454 to a 475 and finally a 496 so go figure...

Do likewise and be happy with the end result the first time, not build her AND then do it again, AND even possibly again..

pdq67

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#13 ·
Even though I had a set of 820s on a 396, I never pulled them off to check the chamber size. Beleive they're a bit small at 114 or so.

I have a bare set of 049, and those chambers are bathtubs, they have to be all of 118+cc's. If you dont have the 820s yet, get some 049s. They're cheap at $100-200 a set. That should get the compression to 10:1.

240 duration in a hyd cam is too much for a mild stall 402 ci imho. XE 274 if you're stuck on Comp, or the Lunati 302A7 is a much better fit, IHMO as it's done on a dual pattern.

http://www.holley.com/HiOctn/ProdLine/Products/IEC/IECC/Chevy7.html

Check out that HUGE duration, 223/231 ( I'm using .050 durations throughout this post btw). Perfect.
 
#14 ·
If your pistons have solid domes, you could have them cut down. You really are in a situation where you need to run a cam bigger than you should or lower the CR. With a heavy car a smaller cam is highly recommended and you'll need to drop the CR to run one. And a 402 just doesn't have the grunt of a 454.
 
#15 ·
Pat, I didnt catch that he owned the 820 heads yet.

If so, then yes, Pat's right. If they aren't hollow domes, have .100-.200" flycut off them and use a 'matched' cam. Please be sure that they will work with open chamber heads.
 
#16 ·
I don't know anything about matching open/closed chambers and pistons. Or, for that matter, any of the BB casting numbers, they are all pretty much Greek to me
. It's a good thing guys like you, Gene, are around to help with these BB questions.
 
#17 ·
Originally posted by Pat Kelley:
You really are in a situation where you need to run a cam bigger than you should or lower the CR...
I'm running some figures on Lunati cams with over 300 duration and getting DCRs in the 7s! Is the duration or lift what I should focus on for idle qualities, etc? What effect would installing the cam straight up with no advance have? Also, what variable in this equation has the biggest effect on vacuum (brakes, etc)...vacuum is important to me. Thanx, Chuck
 
#18 ·
Duration and lobe separation angle are the two most critical. You'll want to stay in the 270-280 duration range ( seat durations) and no less than 110 LSA. I should think that a 112 lsa is perfect, once again, why I mentioned the Lunati 302A7.

Installing the cam advanced increases DCR, retarding it decreases DCR.
 
#19 ·
Originally posted by 427L88:
...I mentioned the Lunati 302A7.
Thanks for the info 427L88...on your earlier recommendation I went to check out Lunati. I noticed the 302A7 says in the notes, "Canister may be needed for brakes", so I checked some of the other Street/Strip cams...I tried to stay with ones that were described with "smooth" or "good" idle qualities. I also figured the DCR on these with nothing being above 8.72:1 DCR and two cams actually figured in the 7s DCR range. Check this web page and tell me if any of these would fit my needs, leaving me with good vacuum and fair street manners ...Thanks A Bunch Guys
 
#20 ·
Gene mentioned something that slipped my mind, thanks Gene. LSA, if you can find a cam with 286º with a 114 LSA and install it at 111º the DCR is 8.25. This is perfect. The wider LSA will improve low end torque by broadening and flatening the torque curve. Overlap is reduced so vacuum will be better (~63º with this cam, depending on the exhaust duration). You could even use the 284 cam if you get it on a 114 LSA and install it at 112º (overlap ~61º). With both these cams installed at the ICL I suggested, the intake closes at 74º. The engine should make lots of torque with this DCR and the threat of detonation is pushed far away. Either of these will get along better than the larger cams with your chassis setup. If you can't find one off the shelf, custom grinding adds only a small amount to the cost. I think this might be the solution to your problem.

A 280-114 installed at 114 works, too. Overlap is about 57º. Vacuum should be very good.
 
#21 ·
If this is a pump gas street ride, lower your compression. Plain and simple. If you dont want to do that run a cam with an advertised duration in the 290-300 range but its going to be a dog unless you have the rest of the car to match. I run 11:1 compression on 89 octane with an advertised duration of 302 on the intake but my car is MUCH lighter and i have alot of gear.
 
#22 ·
Originally posted by Nickel333:
If this is a pump gas street ride, lower your compression. Plain and simple. If you dont want to do that run a cam with an advertised duration in the 290-300 range but its going to be a dog unless you have the rest of the car to match. I run 11:1 compression on 89 octane with an advertised duration of 302 on the intake but my car is MUCH lighter and i have alot of gear.
I can lower the DCR with the Lunati cam part#30218 and keep the duration at 274/284. According to the Desktop Dyno torque peaks out around 4500RPMs and HP peaks at around 5500RPMs. This sounds like a reasonable choice for my setup, right? Chuck
 
#23 ·
Even installing that cam at 114 the DCR is too high (8.49). You need to be below 8.3, preferably below 8.25.

What about the 30219? Install it at 114 and the DCR is 8.29. A bit higher than I'd want but should work if the temps are kept under control (no more than 180-190) and you don't let the engine go lean. If the top ring crevice is taken into account, the DCR is 8.22 which is excellent.
 
#24 ·
Trouble with a 114 lsa is that they don't mate well, imho, with a low torque motor. You'll miss the mid range punch of a 112 or 110 lsa cam. BTDT. Wide LSAs are nice for big motors, as they tend to give very broad , flat torque curves. You'll wnat something with a nice midrange peak to it.

Look, at age 16-17 I was the young apprentice in a production/engine shop. I made the jig and I flycut pistons. Its really not a complicated operation ( obviously!), so I'd calculate how much material you want dusted off and have it done AFTER you pick the exact cam matching the rest of the combo. Then you can dial it in perfectly and enjoy the benefits of a well matched combo. Well worht the cost of flycutting 8 slugs, which can't be more than $20 a pop, maybe less.

BBMike runs the 302A7 in a 402, and I think it idles at 12" with a slight lope. Certainly enough to run brakes. Mike, pipe in here and clarify.

Comp XE274 would be my second choice although I'm not enamored with Comp and would prefer to use another maufacturer. The biggst is not ususally the best, but many users here run Comps successfully.
 
#25 ·
Originally posted by Pat Kelley:
...should work if the temps are kept under control (no more than 180-190) and you don't let the engine go lean...
I'll put a 180 degree thermostat on and what do you think about an Edelbrock AirGap?? would it help cooling??

Just for my own learning...what made you pick the 30219 over the 30218? the five degrees extra intake duration?

All things considered so far, what kind of idle and vacuum do you think this setup will yield with this particular cam?

...If the top ring crevice is taken into account, the DCR is 8.22 which is excellent.
"top ring crevice"...that's a new one for me, what exactly are you referring to?

Thanks Pat (and everyone), you've been a tremendous help so far
...Chuck
 
#26 ·
Originally posted by 427L88:
Trouble with a 114 lsa is that they don't mate well, imho, with a low torque motor. You'll miss the mid range punch of a 112 or 110 lsa cam...
What's your opinion on the second cam listed on this web page (the part #00020)? If I take out the 4 degrees of advance and install straight up, I get a SCR of 10.88:1 and a DCR of 8.16:1 - this seems to fall in the "magic zone"...it does not exceed the 280° intake lift limit that ruins vacuum and has a decent lift at .501" .527"...what do you think?? Chuck
 
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