Compression
 

Have a 400 30 over, eagle stroker crank, h beam rods, torker II, with iron heads that need a rebuild, so thinking about tossing them. I have set of speed master 72cc heads I would like to use instead of the iron heads.

I ran this calc off butler’s site, assuming I use their copper .093 head gasket. Are my figures correct?

If so is 11.18 too high for pump gas, and would a 108 LSA cam help this deal be more pump gas friendly? (have not selected a cam yet)

Have a 3.36 rear and continental converter “should flash 2800-3200”.

Thanks all

a thick head gasket is a bad idea. better to run a dished piston.

Assuming your stroker kit with the .030" over Bore gives you about 455 cid you will need a total of 98.2 CCs above the top of the piston to have the needed 10.5 compression that you want for a Aluminum headed street or street strip motor.

There are many ways to do this, but one way you should not do this is with a .093" head gasket!

The KRE heads can be had in a 85 CC chamber so you would only need a small 13 CC dish in your Pistons to have a 10.5 compression.

If I had your combo I would be looking into a KRE D port Aluminum head since its power band will be far more productive for you needs.

Definitely some things wrong in those figures using Bulter’s compression calculator. The piston dish or dome volume would not be zero. I assume the engine had valve reliefs, likely 4.5 cc or more. You may be assuming the engine has zero piston deck to height clearance. Unless you mill the block deck, the piston to deck distance is usually .025-.015 below the deck on most longer stroke aftermarket rotating assemblies.

You don’t want a narrow LSA like a 108. It needs to be wider, preferably at least 114. It is certainly not ideal, but with a well chosen cam, you can use a thick head gasket to drop the compression.

Your compression is likely mid 11s with stock parts and SM’s. I helped someone do a similar combo with speedmasters, I think he compression was just over 12:1. I think it needed 94 or 95 octane with the parts he had. He was going to run 93 and spike the fuel some. I don’t think he has it running yet though, so I don’t know how he got along. He was making short trips to the track running more of a drag car.

You can likely make a flat top stroker speedmaster combo work, but a lot more details are needed if you want more help.

My guess is your better off rebuilding the iron heads. The SM’s don’t fit the combo very well.

Diamond Pistons has a more sophisticated static compression ratio calculator you may find useful.

https://diamondracing.net/p-10-compr...alculator.html

Is e85 readily available in your area? Since it sounds like you already have the speedmaster heads, the way I would run this deal is with E85. Realistically I'd actually want a bit more compression with this fuel type.

Imputing the following information into Wallace Racing's compression calculator I come up with 11.46:1 compression ratio.

4.15 bore
4.25 stroke
72 CC heads
6.6 cc piston valve reliefs
.039" head gasket crush (typical of FelPro 1016 gasket)
4.3" head gasket bore
0.010 deck height.

E85 would be quite safe on this combination. You'd just need to setup the fuel system and carburetor to run it. If the block needs decked and you can get that down to about .005 or even to 0, adding some compression would work. All of this doesn't mean much if you don't have E85 readily available in your area though.

If that's the case, I wouldn't even try to use the speedmaster heads. I'd sell those to fund a KRE or Edelbrock D port head. At 85/87cc's you're below 10.5:1 compression with a standard FelPro 1016 head gasket, unless your pistons have a dome. With a cam in the 230ish range on 112 lsa, it would be pump gas friendly with enough room to forgive poor gas or a tune that isn't spot on. The mid 11:1 ratio from the speedmaster heads should be possible on pump gas, but it's going to take extreme attention to detail in cam selection, tuning and heat management, with little room for error if you get a tank of crap gas. For a street car, you don't want to deal with that I don't think.

Did you cc the heads? Or are you going off the advertised chamber size? If so then personally I would verify the combustion chamber size before making any decisions. It could be 1 to 5 cc's off in either direction.

The block was 0 decked and currently has flat top pistons. I don’t have access to E85 here in NJ.
Not sure which way to go now……

You still have to account for any valve reliefs in the piston face. It may not be a "dished" piston, but I can almost guarantee it has valve reliefs. Check on that based on your ordering info for the rotating assembly.

At any rate, if the deck height is already at zero, that 72 cc head is going to create a lot of compression. Going to a thick headgasket will lower the compression marginally while making the quench distance pretty terrible, likely causing more propensity to detonate.

If you don't have E85 available, your options become, rebuild your current heads which should be fine if you were using them prior, replace the pistons with something that has enough dish to get you down to around 10.5:1, or sell your speedmaster heads and buy a KRE/Edelbrock head.

Outside of that, you'll need really great cam selection, really great tuning and really great heat management. It can be done, people like Cliff have proven it, but it won't happen with a "throw it together" deal.

I've been through this game trying to get my 455 with #62's to perform and behave on pump gas. You end up hating the car, and it becomes expensive to drive as you hobble it with low timing and expensive octane boosters or high octane fuel. Then, because the tune is so neutered to keep it from pinging, it becomes a dog and it'll likely run hot.

You'll end up rebuilding the heads you have, or buying the KRE/Edelbrock D ports anyway. Save yourself the time, aggravation and added expense, just bite the bullet now.

I re-ran your info about your motor again since I had assumed a 4.210” stroke and not a 4.250”.

You need 99 CCs of total volume above your pistons for a 10.5 compression.

The usage of a Fel Pro 1016 gasket with its 9.4 CC volume knocks down that 99 CCs to 89.6 CCs.

If your pistons only have two valve notches then let’s call that 5 CCs.

Now for all practical purposes your right at the 85 CCs chamber volume that you can get that are made.
.
Now you have your 10.5 compression ratio .

You need 85cc or 87cc heads ideally.... or no stroker crank and you would be fine

Put some dished pistons in it or change the heads.

Two choices.

If the short block is already balanced and assembled then a head swap is likely the best option. Swapping to the correct heads or swapping to dished pistons will be a wash either way at this point.

This is a street car....not a race car. Lower the compression! Pistons or heads....your choice. See post above!

I have two pump-gas motors without any issues at all sreet-strip cars
One motor is 11:98:1 comp with a 106 LSA and a 11:7:1 comp 108 LSA
First one is Eddy-heads the second is Speedmaster-heads with a tunnel ram.
I drive these cars alot and race almost every weekend. I posted the
dyno sheets on another page I think in the race section.

GT

To add to Blueghost’s post, on these engines when the cam gets up past 280* @.050” with head porting and the exh and induction too match (like I imagine Blueghost runs on his engines) you can narrow the LSA up to 108 or less, and run a lot of compression on pump gas. Unfortunately those things don’t play very nice with a street engine with 3.36 gears and a 2800 too 3200 stall.

Hello, out of curiosity has anybody tried opening up the chambers on these heads? I see suppliers advertising the cnc chamber volume of your choice. Are they using the same castings and valve lengths for both 72cc and 87cc?

I have not heard of anyone offering a CNC recut of the Speedmaster chambers but I expect it could be done. By hand for that matter. Leaving the valve seats alone and opening it up.

I believe Edelbrock still uses the cheap method like Pontiac did and sinks the valves and chambers into a common casting. So shorter valves and short side radius.

so whats the final compression of this motor? i see 11.18, 11.46 & 10.5 mentioned. myself & many others run above 10.5 on aluminum head street/strip engines on pump gas with no problems.

can the flat top pistons be dished? many piston manufacturers dish pistons, might be worth checking with some or butler to see if the current pistons can be dished some to get closer to 10.5-11:1 to run on pump gas with a good cam selection.

The old TRW pistons we dished had top thickness of over 3/8" of an inch and it was no big deal to take a .200" cut out of them. Most new flat top pistons are pretty thin on top.

I come up with 11.7:1 using the Wallace calculator and a standard .039" thickness FelPro head gasket.

The following is only for speculation and I sure don't want to be responsible for results if it is tried.

Staying under .050" piston to deck clearance is only the first part of the no-man's land. Usually a FelPro head gasket is .039" so the danger area is anything over .011" piston to head since the total of the two would be right at the .050" limit of the danger zone for detonation. The actual high detonation area is usually stated to be between .050" and .080" where chance of detonation is highest. I have read that the flame front doesn't like that defined area and once the clearance is greater than .080" the risk of detonation falls to normal levels. So zero deck and a .090" Cometic MLS gasket might work. Would be really interesting to have someone take the risk and report back. I show 10.42:1 with the thick gasket.

Also, I've had terrible luck with copper head gaskets without the fire ring insets. Normal machining seems to always have a dip or rise with a failure to fully seal and the copper just doesn't conform to any imperfection.

My 400 is 11.5:1 comp with a 108LSA solid roller camshaft with E-Heads and it runs on pump fuel without issue. I’m probably going to start running it in e10 ethanol as it really likes e85. Don’t be too scared about compression with the right cam and todays fuels. This is a street car too.


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