billla

If you don't need it right away, I'm going to do a FAQ on doing this as part of a general top FAQ. Otherwise I'll send via PM to keep from going OT - just let me know

I have to admit that the numbers aren't adding up from my perspective I would tend towards making another choice; the L98 heads really aren't great from a flow perspective, and all this monkey business seems like a lot of work and expense to make something work vs. picking the right parts for the application.

I'm not a big fan of using DCR as an offset to a too-high SCR; from my perspective this is a racing design approach that doesn't work well on a street-driven engine as in the end it's a "too big" cam to bleed off CR at low RPM. It does work, but at the cost of low-speed throttle response and fuel economy.

But...I'm a pretty conservative guy and virtually all of my builds are < 1.2 HP/CID

jordan89

Do you think that the jegs cam will work?
http://www.jegs.com/i/JEGS-Performan...ductId=1215318

Is there a cam(flatt tappet hydraulic) that will work?

GS977

Intake Exhaust
Advertised Duration 292° 302°
Duration @ .050" 234° 244°
Valve Lift w/ 1.5 Rocker Arms 0.488 0.510
Valve Lift @ Cam 0.325 0.340
Max Lift Angle 109° 119°
Lobe Separation 114°
Cam Timing @ .050" - Opens 8° ATDC 61° BBDC
Cam Timing @ .050" - Closes 46° ABDC 3° BTDC

thats alot of duration. do you have a stall convertor. @2000-2500 would work. that cam also has a wide seperation which builds cylinder pressure.
my motor has 11.1:1 comp. but my cam has 107 seperation. lots of overlap. maybe it helps bleed of some cylinder pressure. and my my cam is alot smaller. 227/241@.050 with @ the same lift as that one. the l98 heads don't flow that well after .500 lift. not much gain going past that.
billa would know more about this, but you can advance or retard the cam to increase or decrease cylinder pressure. i don't know if i would go with that cam. and 64 or larger heads might be a better choice if you can't get 93 gas there.

jordan89

It's a manual transmission. I'm going to give these heads a try. I see people here running them on their motors with a similar setup. I just don't have the budget for any other aluminum heads.

pricesgarage

polish the combustion chambers and round the edges off it will reduces detonation youll be around 10.1 but alum heads let heat out quickly so 91 should be ok if carbs tuned correctlty going with a better cam would be good one with split duration i like the luniti voodoo cams if u dont wanna do the cam swap 1.6 rockers would be good to do it will in crease your lift by .30

63mako

It might work but I would look to a bigger chamber head. I don't see a lot of upside for the work and hastle and a good chance you could have detonation issues. Not much power gain and could actually lose power if you have to dial your timing back to avoid detonation once it is together.

jordan89

I will take all into consideration and let you guys know what I decide. Thank you everybody for being great help.

jordan89

Oh, and Billa I will be looking for that write up for the procedure. Thank you.

cardo0

I looked all over but nothing seemed match the pistons in my "Hecho in Mexico" crate GEN I motor. And i even tried to measure piston volume with short block in the car - lost the seal every try. But heck of coarse if u have the engine out of the car then measure the piston volume.

I think your easiest measurement would be compression. It should be something like 160psi or less while cold for a stock L82. Once u install the L98 head u could test compression again and anything less than 200psi cold will work on 91 pump gas, maybe even 210psi.

Just one caution is u need a smoother deck finish on the block for aluminum heads. Some thing like Ra 25-30 where iron heads need more like 50-60 Ra finish on the block deck surface. This allows the alum heads to slide around more due to the greater expansion and contraction rates. The GEN I sb motors came with some pretty rough looking decks. U can search here on the forum to find others running the L98 head having gasket leaks.

Hope this helps,
cardo0

7t9l82

anyone have experience putting bigger valves in these heads? in looking at them that's a big restriction.

billla

Umm...did you try to cc the bore with the engine in the car, i.e. plexi plate, etc.?

This isn't really about the heads, but the gaskets - steel shim gaskets commonly used with aluminum heads require a very smooth deck. This isn't really an issue any more now that we have the rubber-coated Multi-Layer Steel (MLS) gaskets from Fel-Pro and Cometic. They're not cheap, but they'll seal on just about any deck

jordan89

What do you think about the composite gasket? Do you have a part number for the MLS gasket?

billla

The first question is how thick you want the gasket for quench.

The thicker the gasket the more options you have...and the less you'll pay for the gasket.

Any P/N would be specific to compressed thickness

Here's a PDF link to the marketing sheet for the Fel-Pro:

http://www.jegs.com/PDFs/375mls.pdf

And the FAQ for Cometic:

http://www.cometic.com/faq.aspx

jordan89

Thank you Billa. I'm going to do a bit of research.

cardo0

U need to read the gasket manufacturers recommendations for which ever gasket is chosen. Last time i looked several required a smoother deck surface for aluminum heads on steel block. FelPro is difficult to get the specific gasket info i recall but for others its an easy google. Try it yourself sometime - before posting your opinions rather than facts. Search here for turtlevette and his gasket leaks with L98 heads.


cardo0

billla

That's good advice My facts are just fine; as an example from the Cometic link I posted above:

What surface finish is required to us an MLS head gasket?
A surface finish of 50 RA...

These gaskets are specifically intended to offer the benefits of a steel shim gasket while sealing to a stock-type finish on the block and head deck - hence the rubber-coated MLS.

AirBusPilot

It's been nearly 20 years, but I had 2.02/1.66 valves installed in my L98 heads, along with porting. This was for my 90' corvette.

I also seem to remember the combustion chamber having some sort of square obtrusion right on the edge of the chamber. My porter/machinist had no idea what it was there for, but we decided to leave it. The car ran great otherwise.

jordan89

I'm going to to get the edges rounded out and have them smoother the chambers out.

billla

How much are you paying for this and any other work on the heads?

cardo0

oK, here it is. For aluminum heads using MLS the deck should be in the 20-50 Ra range. I found even the FelPro MLS wants max 60 Ra from here: http://www.felpro-only.com/tec_notes...ayer%20MLS.pdf. But the real problem is the stocks block is somewhere like 60 to 120 Ra range if your lucky. Many stock blocks look like the deck was cut with a chain saw. In fact from just the google hits i read Jordan would be better off using a graphite gasket rather than an MLS.
I don't know how you can recommened aluminum heads on a stock '74 deck surface. Let me try this one more time. Max roughness for a FelPro MLS is 60 Ra. Stock '74 blocks are decked for iron heads with a surface greater than 60 Ra.
Try it for yourself and let us know but to recommend that for someone else is a brainless set-up. Unless your trying to ruin someones elses build - then it would be very deceiving.

cardo0