NVetro

I have the bug and I have it really bad....i just finished a 383 stroker, full spohn suspension, 12 bolt, built t56, etc..etc..etc.. and put down the numbers in the sig bellow...I haven't even run it at the track yet and i'm already thinking of what to do to get more rwhp!

Anyway this post is mostly how in the future when i get away from the stock ported lt1 heads/intake how I can raise my compression as the car is turning into a full drag car...my heads are flowing 276in/248exh @ .600 lift, 2.00" intake valves and 1.56" Exhaust valves (Exhaust flowed with a pipe) (stock lt1 ported) and are 54cc after being milled...currently i have 11:1 compression using a -16 cc JE 2816 nitrous piston and a .040 cometic MLS gasket, 5.7" rod....

In the future if I go with an AFR 227cc head would I be able to mill them down to lets say 50cc and use a .027 head gasket or will I be coming REALLY close to my piston to valve clearence? My shop zero decked the block.

I know i'm screwing myself with those dished pistons but I was eventually going to do a turbo and wanted to plan ahead...what do you guys think i can do? I want 12:1+ compression

Thanks in advance

Z-RATED94

Why not see what it runs first and then worry about new engine mods? You might find that you need to spend some money to tweak the whole combo, although it looks like you have a solid foundation.
Ever think about adding some spray? A good nitrous kit with a first gear lockout might be fun. Looks like you have the motor for it.

cnorton

Don't forget the other clearance issue. At .027" of total deck you're in the danger zone of piston/head contact. That will destroy parts just as quickly as piston/valve contact. Get some professional advice if you're considering anything closer than .030". Piston/valve clearance can be tweaked by moving the camshaft ICL or cutting deeper valve reliefs. Piston/head clearance is an absolute.

AutoRoc

I don't see a reason to plan so much and worry about a point of compression. When you're ready to make it a track car/race gas only, you will probably look to change the piston and shoot for 14-15:1 compression and a lot more cam to go along with a lot more cyl. head. A swap to VP Q16 fuel is your best bet also when that time comes, it's worth a good bit of power.

AFR227's/Carb intake/15:1 comp/Nasty cam/Q16 fuel/ You're probably looking at an easy 100hp more than your current combo. See how it runs as it sits! goodluck

NVetro

Thanks for the input guys...as for the spray mentioned...I play to eventually spray a 250 progressive shot from a NX wet kit with a deticated fuel cell...The motor was built for nitrous, and the cam was spec'ed for nitrous use as well.

I am going to run her for the first time prob in March sometime....last time I ran her was SLP day in englishtown in 2003 when the car was pretty much stock....had a:

* Gutted cat
* K&N filter
* MSD 6AL ignition
* MSD Coil

I think I ran a 14.1 @ 102

Should be a large difference :-D

Steve in Seattle

Completely agree. 12:1 SCR is under 9:1 DCR... that's pump gas territory for a well thought out engine. If you're looking at race fuel you can either drop to 9:1 static and turbo the sucker, or up the compression to 14:1 using flat-tops and a small compression chamber.


NEVER try to adjust SCR/DCR by playing with the quench distance... that is just asking for trouble. Going from 0.050" to 0.040" quench can get you gains in knock resistance, but getting below 0.035" gets to be a hit or miss issue (literally). Aluminum rods need at least 0.060" for thermal expansion, but even steel rods expand when hot, and a little over 1" of the distance above the rod is aluminum piston... which will also expand.

If you're contemplating a 0.025" quench, you'll gain very little if anything in knock resistance over a 0.040", but you'll be flirting with piston-head contact. The set up must be EXACT (remember those cyclinder-to-cylinder varriations?), the cooling must be excellent to keep temperatures down, and you'd best consider some light pistons, wrist pins, and con rods (to reduce the forces).

It's not "impossible", particually in a large displacement engine with a lower redline (say 6500rpm?) but it is a big risk that you really don't need to accept unless you're racing for millions or have a class limitation that makes it worth the minimal knock resistance or power gains.

Don't nuke a $10000 engine in search of 5hp from a 0.1 DCR gain. It just isn't worth it.

By the time you get new heads you'll probably need new pistons anyway. I've thought about a turbo V8 with a slight dish in the pistons so the quench "jets" charge into the center of the chamber (instead of pushing it accross the floor of the chamber/piston top), but for NA you'd need a REALLY small head to keep SCR up.

If you want to up the DCR, deck the heads a touch or get new -5cc pistons. That'll do it right.

AdioSS

you should pick up noticeable gains by switching to a solid roller valvetrain.

Steve in Seattle

I dunno about that. He says his heads only flow 278/248cfm at 0.600", and Bret's already put him on a pretty big cam for hydralics (23x/24x 110). For comparison purposes, the GM847 (arguably the largest catalog-hydraulic-cam most people buy) is only 234/242 .539/.558 with a 112 LSA.

Knowing how Bret manages to get hp decent numbers with smaller than normal durations I'd imagine this engine is near it's limit already. Sure you could solid roller the cam and try reving it through the roof, but I doubt you'll see any gains below 6000 rpms.

bigger heads, or more displacement will change that of course... but I think those heads are already a major choking point before trying to cam it further.

NVetro

I definitly agree....the heads and intake are my limiting factor....I think later on down the road the best thing to do will be afr 227cc and a single plane intake + solid roller....should be close to 500rwhp with that setup I would think.....my question is this though...

As I stated...I have the block/pistons zero decked....and currently there is 54cc heads + .040" commetic head gasket...if I am going to do that afr 227cc setup how much more room do i have before piston to valve clearence will be an issue?

I want to chop some cc off the heads or go with a smaller gasket...if I keep the afr 227cc at 54cc..and go with a .027" gasket...will i have an issue? I just wnat a few more points in compression.

Steve in Seattle

I'll say it again... never adjust your gasket thickness in search of more compression. Gasket thickness is THE determination of your quench (since the block is 0-decked).

Compression should be adjusted with piston height, dish/dome size, and combustion chamber size.

NVetro

ok with the deck zero decked and a .040" gasket how much lower below 54cc heads can I go to increase compression without piston to valve clearence being an issue?

blue 79 Z/28

its not about the piston to valve, its about the piston to HEAD. if you got a good engine builder i would keep it no lower then a .035 dangerous min. for the quench.

Steve in Seattle

Exactly. Gasket is NOT something you change to adjust SCR/DCR or even valve-to-piston clearance.

Valve clearances change with different cam lobe lift, rocker ratio, cam lobe profile/timing, piston height (over/under deck), dome/dish height and shape. Way too many factors to give general guidelines over. You need to assemble the engine (cam, rockers, gaskets, etc...) and either scope it through the sparkplug hole or use the "putty test" to be sure. Turn the engine over SLOWLY and listen/feel for any resisitance that indicates valve-to-piston touching. Then tear off the heads and check the putty for the gap it shows. Even if it clears you still need some room to account for thermal expansion of the rod/piston (which will expand upwards) and the valve (which will expand downards), as well as inertia (7000 rpms not only can close a quench gap, it can close valve clearances as well... especially if the chain stretches later in life and the timing events drag a degree or two!).

Super-tight gapping is never easy nor advisable for a street engine with thousands of miles between tear downs.

If Bret put you on a custom cam I'd make sure to ask him about decking the head further, he may have used a similar timing/lift on a similar engine. (Make sure you tell him about any piston or rocker changes).

That said, unless your lift is at or over 0.600" I doubt you have anything to worry about there... especially with dished pistons.

Quench is a totally different animal.

Steve in Seattle

Yep, i belive it was Smokey that said he found 0 gains for quenches tighter than 0.025". He went lower (0.020" I belive), but even at 0.025" the heads showed signs of light "tapping" from the pistons after a full rpm pull (~8000+ for his engines) at operating temperature. He also mentioned 0.060" min for aluminum rods due to heat expansion AND STRETCH.

GM quench in the LT1s varried a bit depending who you talk to, and how accurate they measured the real gap, but I think 0.055"-0.060" is a fair estimate. There are gains there for those who look.

Just remember though... EVERY piston should be 0-deck if you want to push the limits. If one piston "presents the crown" even 0.005", you could have major issues depending on the gasket you go with. a FelPro 0.035" would be ideal here... but I'd stay away from the GM-Impala or other thin-gaskets which may pass fine upon assembly, but once hot and revving the tolerances may close up and give you a nasty suprise on a WOT run!

Check, then double-check.

blue 79 Z/28

i got the impala .028 gaskets for my build up but i am gonna keep the pistons in the hole 10 thou. .038 is as low as ill go, Advanced Induction recommended high 30's for detonation resistance because im gonna be running about 12.4:1 on 94 pump gas, custom HR cam by them etc. motor wont rev past 6800rpm so thats another cushion. be sure to check the clearance on multiple holes, corner to corner, stacking tolerances are what can really bite you. rods a few thou longer then spec, piston variances, crank throw variances etc. it all adds up, next thing you know is you got no room and the piston is getting intimate with the cyl head.