nailsandrails
VIP Member
I did the add .030 base gasket and cut the heads .030 on a 99 SRX. BEfore my compression was 125 on all three..now it is 120 on all three. I haven't checked the squish yet, but I was expecting my compression to go up with the .010 difference. The only thing I can think of is the fact that the port timing is different. Anyone know if this is right?
You gotta cut the heads 10 thou more then the thicker base gasket you added. You should have added 20 thou more (2 stock base gaskets) for a 30 thou head cut.
Junior
New member
port timing is different forsure, your static compression will be abit lower since more of your intake charge is escaping, at RPM it'll be higher tho, since the expansion chambers are getting more back into the cylinder.
still seems abit low tho
still seems abit low tho
nailsandrails
VIP Member
Hmm... I removed the stock gasket and bought a single .030 base gasket from Maxximum perf. I also had the heads cut .030. I was assuming with me adding .020 on the bottom and cutting .030 on the top I would have gained compression. I had done this to another SRX and I thought it went up 5-10 PSI. I need to get some solder to check the squish. Maybe that will tell me more.
ModMMax
New member
I don't pretend to understand the effects of "decking" on port timing and static compression. I had engine mods, including decking, done years ago by a company in New Mexico. They convinced me that decking needed to include head work. What kind I don't know. Maybe related to squish.
Your powervalves keep the main exhaust ports closed anyway so any diff is minimal.Different guages, altitude, temps can affect your readings.
nailsandrails
VIP Member
Thanks for the replies. I'm not too worried about it. I will check the squish and see what I have there. I have the receipts from the dealer the first year it was serviced and it had 125psi on all three. When I checked it before I did this it still had 125 on all three. I'll check the squish just to be sure the head got cut .030
simplified answer
If we take the head cut out of the equation, actually what has happened is you have now gained a additional .019-.020" deck clearance, the stock base gaskets are .010-.011" thick. Now by adding more shim thickness, means the piston now sits down inside the bore more at top dead center (increased deck clearance). This lowers the compression a considerable amount because now the entire cylinder volume is added to the head volume. Of course by machining the head you can gain some of this back but you will need to take off more then you added at the base to get back to where you were,because of the added deck clearance. You can never gain back the the correct cylinder deck clearance.(unless you remove material from bottom of cylinder) You not only raise the exhaust port but you also raise the transfer ports, this will require a higher rpm to gain peak hp, and will lose torq. output, not to mention the engine will not be nearly as efficent as it was before because the mixture will now be more stagnent in the chamber, the compressing of the fuel charge will not nearly be as tight. The mixture is not compressed to the center and is allowed to remain around the edges of the dome of the piston, this leads to hot spots and piston pinging on low octane fuel. It would be alot more benefical to just run the stock base gasket, raise the exh port to 30.5mm, open up the triple ports and do a piston mod. Cut the heads so you have around .055-.051" squish, it will not only run better but will have more grunt on low end where you need it. This way you could still run her at 84-8500rpm and it will have alot more torq, and you need torq. to get out of the hole.
If we take the head cut out of the equation, actually what has happened is you have now gained a additional .019-.020" deck clearance, the stock base gaskets are .010-.011" thick. Now by adding more shim thickness, means the piston now sits down inside the bore more at top dead center (increased deck clearance). This lowers the compression a considerable amount because now the entire cylinder volume is added to the head volume. Of course by machining the head you can gain some of this back but you will need to take off more then you added at the base to get back to where you were,because of the added deck clearance. You can never gain back the the correct cylinder deck clearance.(unless you remove material from bottom of cylinder) You not only raise the exhaust port but you also raise the transfer ports, this will require a higher rpm to gain peak hp, and will lose torq. output, not to mention the engine will not be nearly as efficent as it was before because the mixture will now be more stagnent in the chamber, the compressing of the fuel charge will not nearly be as tight. The mixture is not compressed to the center and is allowed to remain around the edges of the dome of the piston, this leads to hot spots and piston pinging on low octane fuel. It would be alot more benefical to just run the stock base gasket, raise the exh port to 30.5mm, open up the triple ports and do a piston mod. Cut the heads so you have around .055-.051" squish, it will not only run better but will have more grunt on low end where you need it. This way you could still run her at 84-8500rpm and it will have alot more torq, and you need torq. to get out of the hole.
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Junior
New member
Turk said:
good call, hadn't thought of that.
thanks for jumpin in Mr. Viper, good clean, concise tech post, as always
nailsandrails
VIP Member
Thanks for the reply Mr. Viper. Unfortunately I was looking to get a few more HP as cheap as possible. I was able to get the heads cut for free, and the base gasket for 30 bucks. For my situation this mods works because it is only 30 invested and I'll gain a few HP. I only run premium fuel. My last SRX was ported and the gains were probably worth the money. I just decided on this one I am going to get back into it cheap. I just don't go enough to spend the extra cash. If I was going to spend the money, you would have three cylinders coming your way. A lot of positive posts about your work!